You would not want to miss this informative, knowledge-packed conversation with Dr. Kunal Shah. Dr, Kunal generously gives us all a crash course on surface finishes and understanding different types of materials that could affect signal integrity, reliability and electronic shelf life. He will also tackle in detail the pros and cons of various types of nickel-free finishes.
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Transcript:
Dr. Kunal Shah:
... One of the things people have started looking elsewhere from ENIG is because nickel is what is causing signal integrity loss or insertion loss, what they call, in the high frequency application.
Zach Peterson:
Hello everybody and welcome to the LTM on track podcast. I am your host, Zach Peterson, and I'm very happy to be interviewing Kunal Shah, president of LiloTree. Kunal is someone who I have seen out there in some other podcasts and LiloTree is a very interesting company and I thought it would be very great to talk to him on the podcast today. Kunal, thank you so much for joining us.
Dr. Kunal Shah:
Zach. Thanks for having me. This is always exciting. I think when you describe the symbiosis between the designer and the manufacturing concept, it's always a great bridge for better products, I believe.
Zach Peterson:
I agree. And for myself, I kind of made it my mission over the last year to try and talk more to people who are involved in manufacturing, so I can learn all of those little nuances. I mean, I think we rely so heavily on just the design rules to ensure that we don't screw something up or make something that can't be manufacturable, but we can't rely on that.
Dr. Kunal Shah:
Yeah.
Zach Peterson:
I think you need to know some of those nuances.
Dr. Kunal Shah:
Yeah. One of the things we always sort of harp upon is, as we are going into a next generation technology with the RF and the high frequencies and everything, lot of times everything looks great on paper, but when you actually manufacture it, you don't get that performance or the reliability or what you expect. And what it turns out to be is understanding the type of material that need to be included or used to be manufacturing, and the manufacturing process is sort of very critical. So if the designer knows that, "Okay, I need to be aware of these things," sort of everything becomes much smoother to what you get once everything is manufactured, from the design perspective all the way to the final manufacturing.
Zach Peterson:
Yeah, I totally agree. Maybe before we jump into that a bit deeper, you can tell us what LiloTree does and how you got involved in the electronics industry and how maybe you got involved a little more on the manufacturing side.
Dr. Kunal Shah:
Absolutely. So again, I start off with LiloTree. LiloTree is an innovative sort of a company which actually uses innovation in materials development, especially in the electronic industry. And our main focus, our main sort of set of products that we offer in the market is the surface finishes. And not getting too much details about all the products and everything. In nutshell, basically the surface finishes are, usually the conventional surface finishes you see or have seen, are usually, their goal is to just protect copper by any means. And then it's the solder materials' responsibility to form a strong solder joints and realize the reliability.
However, the unique aspect of the products that LiloTree has developed is actually the surface finishes that actually creates a symbiosis with solder to form a very robust solder joint. So it's actually participating in forming a robust solder joint, so ensuring a better and robust reliability of this electronic assembly. So, that is the main difference. But we do have special products for RF, microwave, high frequency that has been very popular in the market. We call it as Nickel-less ENIG Premium that is specifically designed for high frequency applications that is running anywhere from two, three gigahertz all the way to a 100+ gigahertz.
But again, coming back to my personal background. Again, I'm president and the founder of the company. But prior to that, I did my PhD in material science and engineering, so the developing material is my background. I worked for Intel Corporation as a senior research scientist, developed interlayer dielectric material which is a low dielectric constant, a polymer material to realize or reduce the dielectric losses that you see at high frequencies. And then moved on to start LiloTree along the way. And we work with lot of medical, electronics, aviation, aerospace, semiconductor, consumer electronics.
And also we receive supports from Federal Government, National Science Foundation to develop these products and commercialize these products for better reliability. Our solutions are ecofriendly. We have gotten rid of cyanide which is one of the most toxic chemical that is present in gold solution. So our solutions are ecofriendly and enhance reliability and offer better performance in terms of signal integrity, so that's in nutshell, me and LiloTree.
Zach Peterson:
So, when I think when you say a surface finish there, I think most designers who are not chemists or have not done anything with maybe electrochemistry, may not know how you actually get the finish onto the copper. I mean, I've done some electrochemistry, but it was metal oxide, so it was not metal surface finishes. Can you maybe describe the process and then where the LiloTree material fits in? Is it just a solution?
Dr. Kunal Shah:
So basically, surface finishes are as the designers know that bare copper plating happens and there are different ways of including copper in the printed circuit board, so I'm not going there. But once you have these bare copper pads, before it goes into assembly, this copper pad, copper sort of instantaneously gets corroded, and that surface has to be protected. Hence the name surface finish is basically you have to include these plating solution on top of copper and that is what the surface finishes role is to protect copper, right? Traditionally there have been several surface finishes in the market. Back in the day, the emergent tin, where you just put tin, a material which is light, so basically, and then the solder is also mainly tin nowadays, especially with lead-free solder.
It used to be leaded solder, but now it's lead-free solder. So light attracts light, so it used to solder much better, but however, with the RoHS compliance with lead got it rid off from these chemistries. The popularity of these lead-based emergent tin has gone down. Also, there is a lot of surface planarity issues, a lot of other corrosion based dendrites issues and stuff like that. That has the popularity of these surface issues have gone down. The another one that came about almost couple of decades ago is the ENIG - Electroless nickel immersion gold, where you put nickel layer on top of copper and then put immersion gold. That is a gold layer on top, very thin goal layer, almost two micro inches or 15 nanometers of gold layer on top of nickel, it's been the most prevalent surface finish across the world.
You'll see ENIG everywhere, right? The only concern that people have started looking the alternative. It has its own reliability issues, but that's not sort of a limiting factor. People are still using it. However, one of the things people have started looking elsewhere from ENIG is because nickel is what is causing signal integrity loss, or insertion loss, what they call in the high frequency application. So something that people have started looking. Oh, I need to be looking for a nickel-free surface finish because otherwise I'm getting a lot of insertion loss, or I'm getting my signal integrity compromised. So that's when the nickel-free surface finishes have started becoming more popular, but along the way, there are immersion silver, OSP, ENEPIG. So ENEPIG involves the palladium in the Electroless Nickel/ Electroless Palladium Immersion Gold.
There is immersion silver in the market. There is OSP, that is organic based surface finish in the market. So every surface finish has its own pros and cons. And I believe that designers should sort of slightly understand what are the pros and cons and what are the selection criteria for my application, from a signal integrity perspective. And also what is the application, like what sort of an environment, what is the end application of these devices? Because the choice of surface finish affects the overall performance in terms of signal integrity and the overall reliability in terms of the application of these products.
Zach Peterson:
You know, I'm going to be honest with you. When I first started manufacturing anything with PCBs, I would go onto the quote form from the manufacturer and immersion tin was the default choice. So in my first boards, I just kind of went with that, you know?
Dr. Kunal Shah:
Yeah.
Zach Peterson:
And then later I realized, oh, most people are using ENIG. I should probably do that too. I, and so then I just started going with that. Yeah. And it wasn't until you brought up some of the issues with, with nickel-less finishes for particular applications that I realized, okay, on this particular design, it's going to be better to go with something that is nickel-less.
Dr. Kunal Shah:
Yeah.
Zach Peterson:
Is it correct that the issue with nickel-less is the rough interface that it creates between the outer surface metallic and then the copper, or is there some other issue on top of that with the nickel based platings that creates problems that in these more advanced designs?
Dr. Kunal Shah:
Yeah. So, in terms of high frequency and then advanced design as you said, the frequencies are going nowadays as you are more aware from almost 2.4 Gigahertz into 2.5 Gigahertz into 5G to millimeter wave, to RF and microwave in 200 plus Gigahertz. So what happens is with the nickel, there are sort of two issues with nickel. One is, the conductivity of nickel is almost one-fourth the conductivity of copper. Now, for example, like from a designer perspective, what do you expect that, okay, signal integrity or insertion loss will be, bare minimum would be the loss that copper would entail. So copper will always have its own signal or insertion loss, right? You do not want a surface finish adding to this loss. And that is why these surface finishes adding to this loss is called lossy surface finish.
You do not want a surface finish that is adding what you are already encountering from the copper, right? So from a surface finish perspective or the design perspective, you should be selecting a surface finish, which is a non-lossy surface finish, right? With nickel, instead of the signal passing through copper. Now the signal is passing through nickel and the conductivity of nickel is now one-fourth the conductivity of copper. So right there, you already have insertion loss because signal is not passing as quickly as it could have passed from copper. And second is also, it's a ferromagnetic material. So it does create this electromagnetic interferences at high frequencies. So it actually adds even more insertion loss. And lot of times, people are saying, oh, instead of thicker nickel, use thin nickel. But again, you are still living with these insertional losses.
And, and also, with the signal integrity engineering, at a particular frequency, you have a skin depth, meaning at what depth these signals are passing. So if you sort of try to engineer that, oh, I'll try to make sure that it only passes through copper and not nickel or some of nickel and some copper, it becomes very complicated. So instead, everybody is now looking for, or alright, instead of going through this complication, let me just look for nickel-free surface finishes, and there are some alternatives out there. And so that actually makes a designer, the manufacturer, as well as the assembly houses as well as the final OEM's life much easier because there is no nickel involved, so I don't have to live with this insertion losses associated with it.
Zach Peterson:
So, in terms of the nickel-less options available, and maybe you can tell me kind of how LiloTree operates in this area.
Dr. Kunal Shah:
Yeah.
Zach Peterson:
Are those options only limited to, I think, the immersion based like immersion tin, immersion, silver, OSP, or is there something else that's probably better like maybe hard gold, and then like, what does LiloTree do within those different areas? Are you licensing a solution? Are you licensing just a process? I'm kind of curious on that.
Dr. Kunal Shah:
No, absolutely. So to answer your first question, so there are other alternative out there immersion tin does have higher insertion loss. So, it may not be the best nickel-free option. Immersion silver is a very viable option. It does have a very good signal integrity performance because the silver conductivity is slightly higher than copper. So with everything said and done, the insertion loss performance is very much similar to copper. So it's a good performance. However, there are a lot of reliability issues with immersion silver, one being that it gets tarnished, doesn't have a long shelf life. Also, it does create a lot of corrosion, not lot of, but yeah, it does create corrosion with the sulfur based environment. So if it's out there in the open, the environmental effects would kick and it'll have corrosion issues on silver surfaces and also multiple reflow is a concern.
So that is from a silver perspective. OSP also does provide, it's an organic-based surface finish. It does provide good signal integrity, but again, similar to immersion silver, it has low shelf life issues, the reliability issues, it gets sort of exposed and then the copper would be oxidized. And also multiple reflow is a issue with OSP as well. So even though as a designer, you have to look at, oh, signal integrity and the reliability sort of go hand in hand, because if you sort of take care of the signal integrity, if the reliability is not handled, then the overall assembly will have its own issues. So you have to look at like a whole package that I have to look at the signal integrity aspect along with the reliability aspect. I'll just add one more thing.
When I harp on shelf life, because nowadays, with all geopolitical issues and everything going on, the supply chain logistic has become so difficult that you have your boards getting manufactured in one part of the world and then the assembly happening in another part of the world and then maybe the OEM is in a completely different part of the world and understanding when these manufacturing and the life cycle of these manufacturings occur, not understanding the shelf life may put you in a very difficult situation because we have a customer where they got the board manufacturer manufactured a few months ago, and they are almost reaching to a shelf life by the time they're going to do assembly. So they're sort of, they have to go through a lot of testing that, hey, can I even use this board before I do the assembly?
So some of these issues also keep coming back. So the signal integrity along with the reliability sort of go hand in hand. Coming back to hard gold. Yes, it's a very viable option. A lot of people just dumb absolutely high-thick gold onto these copper pads and realize a very good signal integrity. There is a minor issue with reliability because if you have a very thick gold and if you try to solder those components of those pad, there is an issue of gold embrittlement. So gold gets dissolved in the solder, but if there is too much gold, it creates the solder embrittlement, and there are reliability issues and failures along the way. Having said that, a lot of people, if they are not soldering, they just dumb lot of very high thick gold and realize a good signal integrity.
The only concern I have, not I, but even the industry has and lot of manufacturing supply chain has is, you are talking about hard gold is almost a couple of microns of gold. Like if you think about your PCB of 18 x 24, and you have 50%, 60% coverage of copper and you are putting one or two micron of gold, the gold pricing is almost what, last time I checked, and that is our raw material is close to $60 a gram. It costs multifold compared to the actual PCB without gold. It costs significantly higher, right? Absolutely higher. So that is where LiloTree comes into picture, that we have our product that we have specifically designed from a signal integrated perspective, reliability perspective, and cost effectiveness perspective.
First, the product is called Nickel-less ENIG Premium. So basically what we have is we put a barrier layer treatment on top of copper. That is organic treatment that we do to passivate copper. So copper does not diffuse through the top layer. And then we only push 50 nanometers instead of multi micron and hard gold, only 15 nanometers, which is only two micro inches, which is equivalent to the ENIG gold that people are aware of, that ENIG is the standard cause for 1.5 to 2 micro inches of gold and that is the amount of gold we put in on top of this barrier layer treated copper surface. So now the gold is very minimal, only 35 to 50 nanometers and a barrier layer also doesn't cost much because it's organic based solution. So with a very cost effective solution, the signal will most likely pass through copper instead of even gold.
It may actually get to into gold when you have 100+ Gigahertz frequency, but even if it gets into gold, so signal gets into gold or copper, you are still getting the best signal integrity performance, but the cost is actually in most cases cheaper than ENIG. So we have actually brought down the cost even cheaper than ENIG, the best signal integrity you can think of because it's absolutely overlapping copper. So even 100+ Gigahertz will have signal integrity performance as good as copper insertion loss, which is completely non-lossy surface finish. But the most important part is, again with the gold outer layer, you have shelf light more than 12 months. And this barrier layer treatment that I mentioned actually participates in the solder joint formation and forms very, very robust solder joint. So you will not have a solder joint failure.
Usually it used to be a weak link that usually the failure happens at the solder joint with nickel-less ENIG Premium, that has become a strong link. Failure will happen somewhere else, but never in solder joint. And you have the solder material and the PC materials are so strong. Overall reliability has increased tremendously because the joints have increased. I mean, the strength of the joint has increased tremendously as well. So that is why you have the reliability aspect, you have the signal integrity aspect, and then the cost aspect all covered with our product nickel-less ENIG Premium. And that's why it has been sort of becoming very popular in the market with every aspect covered of the manufacturing.
Zach Peterson:
Yeah. You mentioned the popularity in the market, and that was something I wanted to follow up on. What does the adoption look like? What has the market response been.
Dr. Kunal Shah:
The market is good. The adoption is sort of, again, it's organic, but it's sort of coming from the need perspective. So it's like, there are a lot of RF and microwave based manufacturers are reaching out to us and adopting our product one by one. We do have adoption in US, but quite a lot of adoption internationally, from likes of Europe, Australia, Asia, that is Taiwan and China. So, again, organically, we are sort of getting picked up based on the need because all those folks are doing RF and microwave and high-frequency manufacturing, and based on the need, they reach out and they start adopting. A lot of time what happens is, we work with a lot of these OEMs. They do a lot of testing, they conduct in-house testing, make sure it works. And then they sort of reach out to their PCB manufacturer that, hey, I want, the specified nickel-less ENIG Premium in their design. And then the PCB manufacturer reach out to us and adopt our product. So it's purely an organic based sort of a need based adoption than anything else. So which is always satisfying for the fact that, hey, we are supplying something that is needed in the market and then basically growing from there.
Zach Peterson:
So you mentioned the usage in like microwave RF. Is that the ideal usage setting or is there maybe another class of design where it's really applicable or do you see this growing to be used in every assembly? I'm sure you'd love to have it used in every assembly.
Dr. Kunal Shah:
Absolutely. But there is a reason also. One is my wishlist and second is a technical reason that I'll explain. But yes. So RF microwave is sort of early pickers because that is out of the need. But a lot of times now, the 5G high end, so a lot of times, you may not see much difference in terms of insertion loss at 2.5 GHz. But as you go into 5G, high bends in 2 mm waves, around 25, 27.5, around 25 to 35 GHz, you see a significant difference of what type of surface finish used. So now the industry's going towards these technologies, the people have started realizing, oh, I need to be aware of what surface finish I use. So that's where some of these newer customers for us have started sort of reaching out to us and now started adopting that way.
But also if you know that all the automotive industry run around 77, 78 GHz. So that is also a big market in terms of high frequency. And then as we know, with aerospace aviation, the RF microwave, anything around close to 100, 100+ GHz is sort of ideal market, but sort of coming back to your question is, oh, do I like all assembly should be using. Answer is yes, because again, one is my preference and wishlist personally, but technically speaking, one is, ENIG is the most widely used and don't get me wrong, we also sell our own version of ENIG and we are very proud of it. But typical ENIG has its own issue with the black pads between nickel and gold. There are a lot of corrosion issues that always has been prevalent in the market. Then the second thing is also the intermetallic is formed between nickel and tin, which is always weaker and brittle compared to a copper-tin intermetallic, even without using our barrier layer.
Now, we are adding our reliability even more with copper-tin intermetallics with our barrier layer to even stronger intermetallics. But with the standard ENIG, the solder is always brittle, right? And even the process of using nickel is also very cumbersome. It almost requires a babysitting of nickel bath. There are a lot of issues with the nickel plating and you may have an overplating and the skip plating in the PCB pads and they have to ex out and throw out those boards. And these are very expensive boards. And just because there is an overplating or underplating or rather skip plating, you have to ex out. Now, one of the thing you guys, as a designer community, you guys might be doing HDI where the line spacing is into five micron, 10 micron, 15 micron.
If you have overplating, you are actually touching from one pad to one another and creating a bridge. So it's actually a very detrimental to an electrical performance. You cannot have these lines touching to each other. And if you do nickel plating, there is a chance of overplating and these things happen at a micron scale, and it goes into plating. So there is nothing you can control. It just happens and comes out and you just have to ex out because there is an overplating happening. So as we go into these newer generation technologies, the failure is not an option. And these failures happen at a micron scale level. So why do you have to go through all this trouble to plate nickel, where there is an alternative out there which is cheaper in cost and provides you better performance and better reliability. And that's why we suggest that the nickel-less ENIG Premium process is such that it can actually eliminate the need for nickel in the long run or even in the short run with most of these high reliability and HDIN high-frequency application.
Zach Peterson:
So you mentioned something just a moment ago about the intermetallics between copper and in, I believe, being stronger with the presence of a barrier layer.
Dr. Kunal Shah:
Yes.
Zach Peterson:
I'm not a material scientist at the level anywhere close to where you are. I mean, like I said, I've done metal oxides, but not metals. So I was wondering why the presence of a barrier layer would have any effect on it, because it is an organic barrier layer as I recall you saying, right?
Dr. Kunal Shah:
Right absolutely. So typically I'll explain to you what is a conventional case and then what barrier layer does and how it improves it. So typically, usually what happens when you have the solder or reflow process, how the intermetallics are formed is basically copper diffuses into tin and then as that diffusion another material, which is tin, that coexisting between copper and tin is of course is called intermetallics. But usually in the scenario, like if you do multiple reflow, the copper keeps on diffusing. Also as the life of these devices like if you test it at two years, five years, 10 years, this copper always keeps on diffusing. There is no stoppage of this diffusion and basically this intermetallics becomes thicker and thicker.
It starts off with a sub micron into multi micron thick intermetallics. Now, usually whenever you have the strength, you evaluate it. Intermetallics is always weak because it's a dissimilar material come together. Those phases have to actually gel much better compared to a homogeneous material, which is a single material. Like if you're talking about tin material by itself, or SAC305, it's much stronger than the intermetallics because there are dissimilar materials together right. Now, thicker the intermetallics, more chances of failure of less strong material than a thinner intermetallics, right? So the idea of the barrier layer, it's actually at the treatment level, it actually forms this organic solution forms, organometallic compounds, and not only at the surface, but it actually within the copper moderate, it seeps through diffuses through almost 400, 500 nanometers of copper surface and completely passivates the top layer of this copper surface. Now what happened when the soldering process gets on, this diffusion of the copper is actually prevented by this organic metallic compound. So it is prevented to a point it's not completely stopped but instead of diffusing multimicron, it diffuses only one or two microns. So usually this diffusion sort of delaying process because of this barrier layer treatment makes this intermettalics thickness almost half or one-third of the thickness compared to a traditional intermetallics thickness. So by reducing this thickness, the robustness has gone multifold.
Zach Peterson:
So, well, it sounds like the main mechanism, just to maybe summarize all this.
Dr. Kunal Shah:
Yeah.
Zach Peterson:
The main mechanism is that the barrier layer allows this formation of these organometallics or rather organic based intermetallics, then just have a higher or a lower diffusion coefficient. Exactly. So it's more difficult.
Dr. Kunal Shah:
Yeah.
Zach Peterson:
So it's more difficult for this, or it just takes longer, I guess, for this,
Dr. Kunal Shah:
It takes longer for it to diffuse, exactly. And that makes the intermetallics thickness very, very small. And overall the reliability increases because every time people measure, okay, what is the intermetallic thickness at day zero? And what is intermetallics thickness five years from now, 10 years from now, and you will see multifold increase in this thicknesses, but what barrier is able to do is, it's actually reduces drastically. So what you see after 10 years, in traditional ones, or what you see in one year in traditional ones, you may see it after 10 years or 15 years. So that reduction in the diffusion of this copper is what is the key with the barrier layer treatment.
Zach Peterson:
I see. Okay. So this is interesting because it seems to me that given enough time and the growth of these these intermetallic layers, eventually it's possible, I guess you could say that one of these products would, might fail due to the growth of that intermetallic layer in a standard plating process. With yours, you're just essentially extending that time.
Dr. Kunal Shah:
Absolutely. And that extension, it could be orders of magnitude delaying. So what you would see it in several years, like 10 years, 15 years, or rather what you see it in one year getting really worse, this will take several years, 10, 15, 30 years, 40 years. And because the reduction is drastically reduced and that, like what I see it, if I give you a comparison, we and our customer did a long-term study in terms of what happens to these intermetallics. So after 1000 hours of aging, which is almost some of these are sort of done such a harshly to simulate the 10 year, lifetime, 15 year lifetime, right? So after 1000 hours aging at 150 degrees Celsius, the traditional copper-tin intermetallic grew by 10 micron, 12 micron, and the barrier layer treated intermetallic grew by two or three micron. So almost one-third of the size of these traditional intermetallics. So you can see it's an order of magnitude, slower growth of these these intermettalics, and that makes the overall solder joint robustness significantly higher.
Zach Peterson:
Yeah. That was the next place I was going was what exactly happens in the solder joint. So it sounds like you're just delaying the natural embrittlement of that solder joint.
Dr. Kunal Shah:
Absolutely. So basically, you are trying to reduce the phase, which is the intermetallics as small as possible because that is the weakest link. You'll always have a dissimilar material that provides the failure. Like if you look at an example of a welding, always the failure happen at the welding site instead of actual metal component. So wherever there is a dissimilar material joining and forming the joint, that's always the weakest. So our goal is to keep it as small as possible so that it doesn't get exposed as much with the stressful environment and does not create a failure in that phase itself.
Zach Peterson:
I see. Okay. So, I mean, that's great to understand, and I'm kind of wondering now why didn't anyone else think of this sooner? Was there just not enough of a market for the products that might need this to even motivate people to do these kinds of investigations?
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Dr. Kunal Shah:
I think one of the things is if you look at the electronics failure, that industry, like if you track the electronics failure in the industry and from a scientific perspective, number of paper published in the electronic industry, the major majority of failures come from solder joint failures, right? So that is being sort of something that people have been looking into it for decades to how to improve reliability of solder joint from a different perspective or can I have a different solder chemistry? Can I do this? Can I do this? And people have tried to sort of address it from a different perspective to see how can I improve the solder joint reliability. And I think we came upon just because our material science background and we realized, alright, it can be brought better.
We sort of having some inkling that, hey, how can we reduce this size of corporate diffusion and improve the solder joint reliability and this is the idea came about, and then we executed it and sort of perfected it. And now it's in the market, but we are not the first one who are trying to make solder joint reliability better. A lot of people have done it in different ways. I believe one of the things is it's easy to use and easy to realize in a mass market based product and not specific to the application of the area. So you can use this process for all PCBs and realize, so one of the things we do it is we can actually also improve the reliability of ENIG and we call it as ENIG Premium where a slightly different chemistry of barrier layer is applied on top of nickel to improve the nickel-tin intermetallics, right? So we do there also to improve sort of a standard ENIG to an ENIG Premium and realize the better reliability. So this is the idea of and this is our way of sort of contribution in terms of how we can improve the reliability of overall electronic assembly. And then sort of adding into it is the nickel-less ENIG Premium not only improving the reliability but also realize the nickel-free option and realize the signal integrity performance, which is being optimum for these products.
Zach Peterson:
I see what you're saying here. It's, everyone was looking at just the solder and not where the solder is actually going, which is the plating material,
Dr. Kunal Shah:
The plating material. Exactly. So a lot of people is like, oh, it's a solder. If you look at the solder companies, and if you talk to them, they'll always say, Hey, this solder material we have come up with or this popular solder material has the best solder joint reliability because of the chemistry has this, this and that, and makes it much more stronger joints and stuff like that. So it's always a solder manufacturer's responsibility, how to make it better. I think, we are the only ones from a surface finish perspective that how can we sort of create a good symbiosis with good sorted material and with good surface plating to create this bridge or make this intermettalics much more stronger.
Zach Peterson:
Yeah, it's interesting because, if you're talking about solder joint failure and trying to improve reliability through solder chemistries, of course, it's going to fall on the solder manufacturers to focus on that. And so I think for them, they're stuck working within the parameters that are given to them, at least at the industrial level, which is what are the surface platings out there? And just by approaching it from a different direction of looking at the plating itself, you can then take that next step towards increasing reliability.
Dr. Kunal Shah:
Absolutely. That's very well put. Yeah.
Zach Peterson:
Great. So, I mean, this is really interesting, but you mentioned one other thing earlier, which was multiple reflow issues with some of the traditional surface platings that are used, like an RF. So for example, immersion silver. So when you say multiple reflow, how do you overcome that issue? I mean, is this just a design challenge to where you need to now, if you are going to use, let's say immersion, silver, you need to design everything so that you don't have to go through multiple reflow. Is there something else that can be done on the back end to improve the reliability of that assembly once it passes through reflow? Or is this really just a matter of you got to go for an alternative plating material.
Dr. Kunal Shah:
I think you may have to just, so it depends again, so a lot of time as you know as a designer, you have double sided boards, right? So once the leaf flow is happening on one side, the other side, the surface finish is getting compromised. Like for example, immersion silver, seeing a temperature of 265 degrees Celsius for a lead-free profile will get tarnished or OSP will get compromised because in organic layer, at 100 degrees, 120 degree Celsius, this material starts to get compromised. So now this is 265 degrees Celsius. Some of these OSP materials will get compromised. It may not see the life of OSP after two reflow cycles or three reflow cycles. So they're completely compromised per se. So, I mean, again, you could actually look at the design to try to eliminate, so you can actually reduce the number of reduce reflow cycle.
But if the complexity of board is such that a designer cannot get away with these options, then you have to look for an alternative because yes, you are using these surface finishes for a signal integrity perspective. But if it's not manufactureable or if it creates a lot of manufacturing issues in terms of solder ability, because if it's compromised, the incoming solder, it will not wet well, and you may have pockets. And a lot of time you may not see some of those things. It may just appear fine from outside under the optical microscope of some sort. However, there may be pockets of areas which are not wet well, so you already have a compromised joint in the field, compromise solder joint and that gets into the field because it's a finished product now.
So a lot of these things have to be thought through beforehand. And then, as you said, you have to look at the alternative option where you have to look at which alternative will let me do three reflow cycle, four reflow cycle. We have tested six reflow cycle. And it doesn't get compromised because it's a gold-based surface. So, we do not see any issue with that. And also the underneath barrier layer also passivates copper. So even in any excursion of this reflow cycle, it does not get compromised, six plus reflow cycle. So, even harsher environments and temperature, the barrier layer has made the copper underneath way more robust in terms of corrosion resistance. And then you have gold layer on top of it.
So it's sort of a multi-layer of support in terms of what environment can throw at and still it can hold up just fine. And the solder wet well completely and form a very strong solder joint. So yeah, I mean the OSP and immersion silver is something that needs to be looked at in terms of reliability. There is another alternative out there, which is very seldomly used, but people have started talking about, which is called immersion silver immersion gold. So they put immersion gold on top of immersion silver. Not very many manufacturers have that plating option, but it's still out there in principle. However, it is also another thing is then you have eight microinches of silver and then additional eight microinches of gold. So again, you are adding more and more of this precious metal on top of it, try to solve one problem and add another problem of cost and complexity and everything on top of it. In comparison, our process is absolutely simple.
Any PCB manufacturer can use it because it is barrier layer treatment for 30 minutes at 50 degree Celsius. The failure is not an option because it's a very forgiving process. And then you just put for five minutes for 15 nanometers or two microinches of gold, and the process is done versus any alternative process, you have to go through multiple processes. There is an EPIG and EPAG. I'll just add more names, which are nickel-free option, which is electro-less palladium, immersion gold or autocatalytic gold. These processes are very cumbersome, like you have to go through some of the catalyst to put in palladium on top of copper, then go through the palladium process. And again, the palladium bath, it can be unstable and you have to babysit it and whatnot.
And then you have to go through immersion gold or autocatalytic gold and everything. So it's a very, very cumbersome process and very expensive, like palladium as you can see, these days, the pricing, with the geopolitical issues, it's almost 1.5 times the cost of gold. So it's even five times more costly than gold. And you are still putting 200, 300 nanometers of palladium and another 200-300 nanometers of gold. You're just adding this precious metal to realize this performance, but it just makes very, very expensive. So everybody has to sort of look at the performance, the reliability and then the cost. So, yeah.
Zach Peterson:
Well, I think it's great that you are working towards expanding adoption of a product that tries to balance all of those areas. Because I think sometimes as designers, we get focused in one of those areas, like cost or reliability and kind of the rest of it goes out the window. Yeah. So it's great to hear that there's an option out there that helps balance that. If someone were to explore incorporating your product into their assembly, how do they go about doing that?
Dr. Kunal Shah:
So, multiple ways they can do it, they can actually contact us directly, be it OEM or assembly or the PCB manufacturers, we do have our prototype lab where we can actually apply our technology. They can evaluate, test it out however they would like to, and then sort of qualify the process. Second option is the PCB manufacturer from OEM or assemblies feedback. Once they specify on their design, they can actually contact us and we can ship the solutions and they can set up in their plating lines. So again, this process does not require any capital expenditure. They can use the existing plating line, swap out the solution, putting these new solution and start using it right away. So that is second option. Third option, lot of even these early adopters have also done is a lot of times that, oh, I'm not seeing that RPC manufacturing is adopting it yet.
They want to adopt it, but they're still taking time, but we want this product right away because of all the benefits that's been described. So we have a plating line in house as well. So they actually send us the production board with a bare copper. We applied our technology and then ship it back to the either assembly or the OEM folks. And then they ship it to assembly house. And then they create an entire final assembly. So multiple ways they can realize the benefit of this technology by either having PCB manufacturer adopting or they can ship the production board to us. We can apply the technology and ship it back. So either ways, we can have this technology ready for them.
Zach Peterson:
That's great to hear. So we're starting to run up against our time window, but what I would like to do is direct any listeners to the show notes. They can find a link to your company website in the show notes. And I think that would probably be a good way to learn about the product and then also get in contact with you guys.
Dr. Kunal Shah:
Absolutely. And anybody has any further question because there's so much information out there that may not be there on the website, that they can always reach out to us and we can provide additional information, maybe testing results and lot many more information that is available to us.
Zach Peterson:
That sounds great. That sounds great. Well, once again, we encourage anyone that's interested to go check out the show notes and make sure that you go check out LiloTree.com. I believe that's the company email web address.
Dr. Kunal Shah:
Yes.
Zach Peterson:
So go check out LiloTree.com to learn more about this product. And of course you can find some information there for incorporating it into your assembly. Kunal, thank you so much for joining me. Like I said earlier, I've seen you out there. I haven't had a chance to talk to you yet, but this has been a great chat, and it's been a pleasure having you on.
Dr. Kunal Shah:
Absolutely Zach. It's been great discussing with you. Thank you. Thank for having us.
Zach Peterson:
Absolutely. Thank you. And to everyone that's listening out there, make sure that you subscribe to the channel. You'll get updated on all the new videos and of course, new podcast episodes as they come out. And I think last but not least, don't stop learning, stay on track and we will see you next time, everybody.
The PCB Surface Finish you select may be the most important material decision you make for your electronic assembly. The Surface Finish you select will influence the cost, manufacturability, quality, and reliability of the final product.
Just a few years ago, nearly three fourths of all the electronics were produced with SnPb or Hot Air Solder Level (HASL) as the finish. Today, other surface finishes have emerged, including: ENIG, ENEPIG, Soft and Hard gold, Silver OSP, and White Tin, and, SnPb solder now makes up just over 10% of the finishes in use.
More complicated board specifications are becoming common printed circuit board manufacturing requirements in the continuous drive to advance technology, such as: etching buried components, blind vias, drilling ever smaller holes, laser drilled blind vias, soldermask dams as low as 1.0 mil, and thicker high count multi-layer boards. These changes pose considerable assembly and integration challenges. Not to mention the increased demands to meet the restrictions in place to safeguard against the overuse of lead products and materials. Therefore, a number of Surface Finishes have grown in popularity and currently share the marketplace for PCB Manufacturing, each with attributes that make it attractive for certain applications.
When considering which surface finish to use, the following questions must be considered (in addition to general cost considerations):
All of these will influence your selection of the optimal PCB Surface Finish for your project.
Let’s explore a few of the more common surface finishes and their general use applications:
HASL Finish is the predominant ‘leaded’ surface finish used in the industry.
HASL stands for Hot Air Solder Leveling. The circuit boards must be immersed in a tin/lead alloy for this finish. ‘Air knives’ then remove the excess solder by blowing hot air across the surface of the board.
HASL Finish has many advantages when using the printed circuit assembly (PCA) process. It is one of the least expensive surface finishes available and the surface finish remains solderable through multiple reflow, wash, and storage cycles. For an In-circuit test (ICT), a HASL Finish automatically covers the test pads and vias with solder. However, the flatness, or coplanarity, of the surface is poor when compared to available alternatives. This considerably bumpy finish is not only an aesthetic issue, but can also be problematic when sending your boards to assembly. However, it’s corrosion resistance and testability are excellent.
Lead-free HASL Finish is a great alternative to the leaded HASL Finish. Not only is the coating planarity of most lead-free HASL Finishes reportedly better than leaded HASL Finish, concerns with copper dissolution and heat damage to the circuit boards have largely been overcome with different solder alloys such as SnCuNi, SnAgCuNi or SnCuCo While lead free HASL Finish may not be the best coating for projects with small spacing between components due to its tendency to bridge across the gap during heating, it is currently being used on products with component pitch as low as 0.5 mm.
The most significant advantage of using a HASL Surface Finish, whether leaded or lead free, is its excellent solderability.
These coatings have been used with great success on many boards despite their higher per unit cost.
The best features of using an ENIG Surface Finish is its flat surface and excellent solderability. The Electroless Nickel is an auto-catalytic process that deposits Nickel on a Palladium catalyzed Copper surface.
Immersion Gold is a replacement chemistry. In other words, it attaches to the Nickel by replacing atoms of Nickel with atoms of Gold. The recommended Gold thickness is 2-4 µin. The purpose of the immersion Gold layer is to protect the Nickel surface and maintain its solderability.
While the Nickel serves as a barrier layer to Copper. eventually, it too will diffuse to the surface of the Gold and cause the same solderability issue, it just happens at a slower rate than Copper).
Typical ENIG specifications are defined by IPC- 4552 Specification for Electroless Nickel/Immersion Gold. The Nickel thickness must be in the range of 3-6 µm, which is sufficient to prevent penetrability through to the base layer of Copper.
An ENIG Finish provides many advantages, including:
The Gold readily dissolves into solder and does not tarnish or oxidize. While the Nickel strengthens the PTH, increases thermal cycle life, and acts as a barrier that prevents Copper dissolution during wave solder and rework.
One risk to be aware of when using an ENIG Finish is its tendency to create “black pads.” While the actual cause of this phenomenon is still open to debate, the leading hypothesis is that it is mostly likely a contamination of the Nickel that then migrates into the Gold, turning it black. This tends to be particular problem when the gold plating process is not well controlled.
One simple solution to this potential issue, is using a similar alternative surface finish: ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold). ENEPIG Finish solves this issue by depositing electroless palladium over the nickel layer, which prevents any contamination from migrating to the Gold. Of course, for those on a tight budget it is important to note that ENEPIG is more expensive than the already costly ENIG Finish.
Immersion Silver is one of the more recent additions to the list of surface finish options. It has been used mainly in Asia and is continuing to grow in popularity in both North America and Europe.
Immersion Silver is a preferred surface finish for those concerned with solderability and being able to easily probe directly to the finish during ICT. During the soldering process, the silver layer dissolves into the solder joint, leaving a (6-12 µin) Tin/Lead/Silver alloy on the Copper, making very reliable solder joints for BGA packages. Another benefit to using Immersion Silver is the color contrast that enables easier inspection.
This surface finish received a boost in popularity after the Underwriters Laboratory performed temperature/humidity/bias testing with favorable results, in which no electromechanical migration took place.
However, when scaled up to higher volumes for commercial electronic production, the Immersion Silver showed a number of weaknesses. These includes: a tendency to cause micro-voids, tarnishing any exposed silver almost to the point of turning black, and “creep corrosion” when used in an environment high in air-born sulfur and humidity.
In recent years, however, the micro-voids issue has since been eliminated thanks to improved plating processes. Additionally, the tarnishing problem does not necessarily cause board failures. It is usually only an issue of perceived poor quality by customers based purely on aesthetics.
Immersion Silver is a good surface finish if one is confident that the product will not be exposed to sulfur during shipping or use of the product. It is a favorable surface finish for most other attributes.
Organic Solder Preservative, commonly referred to as OSP Finish, is the leader in low cost surface finishes. It is designed to produce a thin, uniform, protective layer on the copper surface of the PCB that shields the circuitry from oxidization during storage and assembly operations. While OSP Finish has been around for quite some time, it has only recently gained popularity as customers increase their search for Lead-free and fine pitch options.
OSP is a superior PCB Finsih over traditional HASL, particularly for PCB assembly, in regards to co-planarity and solderability. However, it does require a significant process change with the type of flux and number of heat cycles necessary. Also, careful handling is very important given the degrading affects the acid from fingerprints have on the OSP, thus potentially leaving the copper susceptible to oxidation.
OSP is an organic coating that is deposited with a wet in-line panel process. It is one of the most common finishes and is an excellent selection for less complex assembly projects. Unfortunately, this finish falls short when wave soldering is required for double-sided boards. This is because the surface mount thermal exposures can break down the film and allow oxidation of the Copper in the barrels, thus reducing the solderability of the through-hole vias.
This finish also encounters some difficulty during circuit testing. Since it is a non-conductive coating, probing through the coating is not recommended.
OSP Finish is ideal for fine pitch assembly since the smooth surface allows the stencil to press firmly against the surface of the copper pads. It is a great choice for high volume orders at a low price.
Another finish that is ideal for those looking for a flat surface finish is Immersion Tin. However, one significant problem with Immersion Tin is the fact that it is made up of the carcinogenic ingredient Thiourea.
Immersion Tin also has a tendency to cause whiskers and intermetallic formations. Whiskers are particularly problematic when working with fine line/spaces and part insertion, increasing the possibility of electrical shorts. Copper and Tin intermetallic formations often occur during deposition and continue to grow. This significantly shortens the shelf life of the stored parts.
This finish has also been listed as being particularly difficult for wave soldering after assembly. w exposed to elevated temperature, the thin tin layer can often almost completely be converted to SnCu intermetallic, leaving very little tin for soldering. This issue with solderability increases after the first reflow cycle or long term storage of the PCBs.
Choosing the right PCB Surface Finish is essential for predicting the cost, manufacturing process, quality, and reliability of any printed circuit board. Each surface finish has important strengths and weaknesses to consider while looking forward to PCB assembly. One way to ensure that you select the optimum finish for your boards can be to determine what problems are most important to solve and making sure that they are satisfied. For example, soldering circuit boards will require the right surface finish opposed to other PCB assembly methods.
For more information, download our Surface Finishes Chart, to help you pick the right surface finish for your PCB needs.
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An Expert’s Guide To PCB Surface Finishes
HASL Or ENIG? A Comparison Guide For The Surface Finishes
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