The #surfacefinish is a coating that covers the outer surface of the PCB after the copper has been etched to protect the exposed copper circuitry on the PCB and improve solder ability. Choosing the right surface treatment can help our circuit boards to avoid damage from the outside of the environment, extend the shelf life of the board, improve the durability of the board, and better achieve the needs of our final product. So do you know how to choose the right PCB surface treatment process? In this article, I will introduce you to nine common surface treatment processes to help you understand how to choose the correct PCB surface treatment process.
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PCB Surface Finish
The following 9 common surface treatment processes
&#;HASL (Hot Air Solder Leveling)
#HASL involves wetting the PCB with flux and then immersing the PCB in a tin/lead alloy furnace to form a thin protective layer on the surface of the PCB. Finally, the PCB is removed after the protective layer is formed, and the excess solder (lead or tin) is removed by blowing hot air across the surface of the board with an "air knife".
HASL
Advantages of HASL
&#;Low cost
&#;Can be reworked for repair
&#;Long shelf life
&#;Good corrosion resistance and test ability
&#;Exposing the PCB to temperatures up to 265°C allows for better identification of potential delamination problems in the assembly of components onto the board
Disadvantages of HASL
&#;Poor flatness or coplanarity of the HASL surface compared to other surface treatments such as submerged gold
&#;Not suitable for SMD and BGA with <20 mil
&#;Contains hazardous substances lead, not environmentally friendly and not RoHS compliant
&#;Exposing PCBs to high temperatures may expose them to thermal shock and affect other materials in the PCB
&#;May block PTH (Plated Through Hole)
&#;Poor wettability
&#;Not suitable for capacitive touch switches
&#;LF HASL (Lead-Free Hot Air Solder Leveling)
#LFHASL works similarly to HASL, but LF HASL immerses the PCB in molten lead-free solder (e.g. Sn/Ag/Cu (SAC), Sn/Cu/Co, Sn/Cu/Ni/Ge) and then blows off the residue on the PCB surface with a hot air knife.
LF HASL PCB
Advantages of LF HASL
&#;Environmentally friendly, complies with ROHS standard
&#;Durable
&#;Affordable price
&#;Suitable for optical inspection and electrical testing
&#;Reprocessable
&#;Very good bonding to copper with excellent solderability
Disadvantages of LF HASL
&#;Inconsistent coating thickness
&#;Solder bridge formation
&#;Not suitable for very dense, high pin count, closely spaced assemblies
&#;Not suitable for HDI (High-Density Interconnect) PCBs
&#;Can form tin whiskers, a fine conductive filament that can cause short circuits and various other problems
&#;OSP (Organic Solderable Preservative)
#OSP is a technique involving the immersion or spraying of water-based organic compounds that selectively bind to copper and provide an organometallic, non-conductive coating with a protective effect. Simply put, OSP is the formation of a thin, uniform protective layer on the copper surface of the PCB to prevent oxidation of the board during storage and assembly operations.
OSP PCB
Advantages of OSP
&#;Protects copper solder with resistance to oxidation, thermal shock, and moisture
&#;Low cost
&#;Lead-free, environmentally friendly
&#;Can be reworked for repair
&#;Very flat surface, ideal for fine-pitch component assembly
Disadvantages of OSP
&#;Thermal exposure can damage OSP films, which reduces the solderability of through-holes
&#;Not conducive to electrical testing
&#;Short shelf life, typically only 6 months
&#;ENIG (Electroless Nickel Immersion Gold)
#ENIG is a two-layer metallic coating of 2-8 µin of gold over 120-240 µin of nickel. The process of electroless gold deposition is to use the autocatalytic formation of nickel deposits as a barrier to copper surfaces and component soldering surfaces, followed by deposition by immersion to form a very thin protective layer of gold to avoid corrosion of nickel.
ENIG PCB
Advantages of ENIG
&#;Protects copper from passivation and oxidation
&#;Good wettability
&#;No hazardous substances, RoHS compliant
&#;Durable, long shelf life
&#;Very good solderability
&#;Flat surface
&#;Ideal for PTH (Plated Through Hole)
&#;Suitable for complex surface mount components (e.g. fine pitch BGAs and flip chips)
Disadvantages of ENIG
&#;Cannot be reworked for repair
&#;May suffer from black pad syndrome (phosphorus buildup between gold and nickel layers) resulting in PCB surface breakage and connection errors
&#;High Cost
&#;May cause signal loss for signal integrity applications
&#;Not suitable for touch contact boards
&#;The process is complex and difficult to control
&#;ENEPIG (Electroless Nickel-Electroless Palladium-Immersion Gold)
#ENEPIG is an upgrade of ENIG, which is a three-layer metal coating consisting of nickel, palladium, and gold. On the copper substrate is a chemically plated nickel layer, followed by a chemically plated palladium layer, and the final layer is an immersion gold layer. The thickness of the palladium is in the range of 3-6um, and its function is to prevent nickel from passing through the top gold layer, that is, to prevent nickel from being oxidized and corroded.
ENEPIG VS ENIG
Advantages of ENEPIG
&#;Provides a flat pad surface, allowing more areas to ground the wire
&#;Ideal alternative to soft-bonded gold
&#;Ideal for multiple reflow cycles
&#;No black pads
&#;Long shelf life
&#;Compatible with a variety of lead-free solders
&#;Supports alternative connections such as lead bonding, edge card connections, and push buttons
&#;Ideal for in-circuit testing and crimping connectors
&#;RoHS compliant
&#;Ideal for high-frequency applications with limited pitch
Disadvantages of ENEPIG
&#;Reduced solderability and poor wettability with thick palladium layers
&#;A complex process, not widely used
&#;High Cost
&#;Gold wire bonding may not be as reliable as soft gold compared to soft gold
&#;ISn (Immersion Tin)
#ISn is the deposition of the tin onto the copper surface through a chemical replacement reaction, forming a thin layer of tin (between approximately 1um-40um thick) that protects the underlying copper from oxidation.
Isn
Advantages of ISn
&#;Uniform plating, flat and smooth surface
&#;Multiple remelting and rework repairs are possible
&#;Ideal for HDI PCBs and fine pitch surface mount components
&#;Excellent protection of the underlying copper from oxidation
&#;Suitable for fine processing, lead-free assembly
&#;Suitable for backplanes and press-fit connectors
Disadvantages of ISn
&#;The material contains thiourea, which is carcinogenic
&#;Easily oxidizes and forms tin whiskers, which can lead to PCB short circuits
&#;Not suitable for PTH (Plated Through Hole)
&#;Short shelf life, only 3 to 6 months, not suitable for long term storage
&#;Difficult to test electrically,, requires special setup (soft probe landing)
&#;IAg(Immersion Silver)
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#Immersionsilver is a PCB immersed in a silver ion bath to obtain a thin layer of lead-free silver on the surface of the PCB through a chemical replacement reaction, which is used to protect copper traces from corrosion. Immersion silver is 5-12 µin thick and is mainly used for lead bonding, EMI shielding and membrane switches.
IAg PCB
Advantages of Immersion Silver
&#;Excellent surface flatness
&#;High stability (stable chemical properties)
&#;Suitable for fine-pitch components
&#;Low loss for signal integrity applications
&#;Can be reworked for repair
&#;Excellent solderability
Disadvantages of Immersion Silver
&#;High coefficient of friction/not suitable for flexible pins
&#;Stringent storage requirements
&#;Sensitive to contaminants in the air or on the PCB surface, must be soldered the same day the PCB is removed from storage
&#;Tends to lose gloss when mishandled
&#;Not suitable for micro-through holes with 1:1 aspect ratio
&#;May form silver whiskers (hair-like metal bumps on the surface of the board), leading to board breakage or short circuits
&#;Difficult to perform electrical testing
&#;Hard Gold
#Hardgold is first plated on the PCB copper surface with a layer of nickel, and then gold is plated on the nickel coating
The gold purity of hard gold is 99.6%, mostly used for PCB side contacts (usually called gold fingers) and keyboards, etc. The plating process of hard gold: acid cleaning &#; electroplating nickel &#; pre-plating gold &#; electroplating gold-nickel or gold-cobalt alloy
Hard Gold PCB
Advantages of hard gold
&#;Hard and wear resistant
&#;Lead-free, RoHS compliant
&#;Long shelf life
&#;Suitable for electrical testing
Disadvantages of hard gold
&#;High cost
&#;Requires additional conductive traces
&#;Poor solderability, not solderable above 17 µin thick
&#;Soft Gold
#Softgold is also composed of a layer of gold plated in a nickel coating. The purity of soft gold is 99.9%, while the hardness of pure gold is softer, so this surface treatment process is called soft gold. The plating process of soft gold: acid washing &#; electroplating nickel &#; electroplating pure gold
Soft Gold PCB
Advantages of soft gold
&#;Suitable for lead bonding
&#;RoHS compliant
Disadvantages of soft gold
&#;Poor solderability
&#;How to choose the right surface treatment?
Each surface treatment has its advantages and disadvantages, and we recommend that you compare the characteristics of various surface treatments before making your choice. The following are the main factors to consider when choosing a surface treatment
PCB Surface Finish Comparison
1) RoHS compliance
2) The expected yield of the project
3) The possibility of high frequency
4) Budget cost
5) The environment in which the end product will be used, is 6) the PCB susceptible to contamination and damage?
7) Fineness of component pitch, type of assembly
8) Reworkability
9) Shelf life
10) Impact and drop resistance
11) Whether lead build-up or wave soldering is required
12) Solderability
13) Thermal shock
14) PCB board surface aesthetics
Conclusion
In our experience, you usually only need to list the most important requirements for your PCB project and choose one of the surface treatment processes that meet the requirements to ensure that the manufactured boards will meet your requirements to the greatest extent possible. Of course, no matter which surface treatment process you choose, quality is our primary concern.
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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- 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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