The process requirements in the PCB production are very important, which directly determines the quality and positioning of a board. Such as HASL, gold plating pcb , ENIG.
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Due to the good quality of ENIG, the price is higher too. Many customers choose the most common HASL process.
1. On the surface, Lead-tin is brighter and lead-free tin (SAC) is dim.
2. Lead in HASL is harmful to humans, but lead-free not. The lead eutectic temperature is lower than lead-free, which depend on Lead-free alloy composition. For example, The eutectic of SNAGCU is 217 degrees, the pcb soldering temperature is eutectic temperature plus 30 to 50 degrees. The lead eutectic is 183 degrees. The mechanical strength and brightness with lead is better than lead-free.
3. The Lead content in HASL-LF is not more than 0.5, while HASL has a lead of 37.
4. The lead will increase the activity of the tin wire during the welding process, so Lead wire is easier to use than lead-free wire. However, lead is toxic and not good for the human body. What is more, Lead-free tin will have a higher melting point than lead-tin, so the solder joint is much stronger.
In printed circuit board (PCB) manufacturing, a protective and conductive coating is applied on the copper traces to facilitate soldering of components. The two most common solder coatings are Hot Air Solder Leveling (HASL) using tin-lead solder alloys and Lead-free HASL using tin-based lead-free alloys. This article provides a detailed comparison between traditional leaded HASL and modern lead-free HASL processes.
The Hot Air Solder Leveling (HASL) process deposits a thin solder coating on the PCB conductors to protect them from oxidation and provide a solderable surface. The key steps are:
This coats the exposed copper traces with a smooth solder layer ready for component assembly. Lead-tin alloys like Sn63/Pb37 used to be common for HASL but environmental concerns over lead has led to a transition to lead-free solders.
Traditional HASL used tin-lead solder alloys containing 30-40% lead to coat PCBs. The most common composition was 63% tin (Sn) and 37% lead (Pb) or Sn63/Pb37:
However, lead is a toxic heavy metal banned under RoHS. This prompted a shift to lead-free HASL processes using tin-based solders.
Lead-free HASL eliminates lead and uses greener tin-based solder alloys. The main options are:
1. Pure Tin (Sn100)
2. Tin-Copper (SnCu)
3. Tin-Silver-Copper (SAC305)
4. Tin-Bismuth (SnBi)
Among these, SnCu and SAC alloys are most popular for lead-free HASL. The specific alloy is selected based on soldering process needs and costs.
Lead-free HASL coatings are typically thinner than traditional leaded HASL:
Thinner coatings are used to conserve higher priced lead-free solders and account for their higher hardness. Thick leaded deposits tend to crack and warp under board flexure.
Switching from leaded to lead-free HASL requires optimizing the soldering process:
With the right parameters, the lead-free HASL process can match leaded HASL for solderability and surface finishes.
There are three main equipment configurations used for lead-free HASL processes:
1. Pumped Molten Solder
2. Foam/Spray Flux + Static Solder Pot
3. Selective Deposition
Each has its own pros and cons. The method is selected based on production environments, volumes, and flexibility needs.
The properties of traditional leaded HASL versus lead-free HASL are compared below:
For more information, please visit lead-free hasl.
PropertyLeaded HASLLead-Free HASLSolder AlloyTin-Lead (SnPb)Tin-Copper (SnCu), SACLead Content30-40%<0.1%Melting Temperature183°C217-232°CSolder Bath Temperature200-210°C240-255°CCoating Thickness100-300 μin (2.5-7.5 μm)50-100 μin (1.25-2.5 μm)WettabilityGoodModerate to GoodSolderabilityExcellentGood to ExcellentDuctility/HardnessDuctile/SoftHarder/More BrittleCorrosion ResistanceModerateGoodLeach ResistanceModerateBetterCreep ResistanceModerateBetterFatigue ResistanceModerateGoodDrop/Shock ResistanceGoodModerateWhiskeringNoMinor risk (mitigated)CostLowerHigherSome key advantages and disadvantages of switching to lead-free HASL are:
For acceptable reliability some important factors to consider are:
With optimal processing, lead-free HASL can achieve the adhesion, solderability, and reliability needed for most applications.
The electronics industry has widely adopted lead-free HASL surface finishes to meet environmental regulations and market demands. While the switch from conventional SnPb involves process changes and cost impacts, lead-free coatings offer important advantages. With proper control of materials, fluxes, temperatures and equipment parameters, high quality and reliable lead-free HASL coatings can be achieved. The process continues to evolve with newer solder alloys and selective deposition methods for further improvements.
The popular lead-free HASL alloys are pure Tin (Sn100), Tin-Copper (SnCu), Tin-Silver-Copper (SAC305/405), and Tin-Bismuth (SnBi). SnCu and SAC305 are most common allowing better wetting, strength and whisker resistance than pure Tin.
Lead-free HASL deposits are typically 50-100 microinches thick versus 100-300 microinches for leaded HASL. Thinner coatings conserve costlier lead-free solders and prevent cracking issues seen with thick lead-free layers.
Organic Solderability Preservatives (OSPs) and Immersion Silver provide excellent solderability under lead-free or no-lead HASL. Other options like ENIG, Immersion Tin, and Immersion Gold also work well.
To suit the higher 227-240°C melting point of SnCu alloys, the solder pot temperature needs to be maintained between 245-255°C, compared to 200-210°C used for leaded SnPb solders.
Jet printing selectively sprays lead-free solder using drop-on-demand inkjet heads only onto required areas. Another method uses laser-cut stencils allowing solder paste application just on exposed traces. Both dramatically reduce solder usage.
Posted:05:20 PM December 06,
writer: NextPCB
Hot air solder leveling and lead-free HASL (Hot Air Solder Leveling) are the two prominent surface finishes in the PCB Manufacturing and Assembly Process. However, there is a noticeable difference; Lead-Free HASL does not use tin and lead solder.
Yet, when choosing the right surface finish for your circuit boards, it is vital to choose a cost-effective and RoHS-compliant surface finish. Suppose you want to use HASL as your surface finish choice and need help deciding which surface finish to use. In that case, this guide examines the pros and cons differences between HASL and lead-free HASL.
Surface Finish is metalizing the outer layer of a PCB being soldered. The function of PCB surface finish is to offer protection against corrosion on the copper circuitry. Also, it fosters easy soldering of circuit board components.
There are varied kinds of surface finishes employed in the PCB Industry. They are: Hot Air Solder Leveling (HASL), Electroless Nickel/Immersion Gold (ENIG), Immersion Silver (ImAg), Organic Solderability Preservative (OSP), and Immersion Tin (ImSn)
Hot air solder leveling (HASL) is the least expensive PCB surface finish. Many PCB Manufacturers commonly use this surface finish as it is economical and widely available. When applying Hot Air Surface Leveling on circuit boards, the board is dipped in molten solder and then scrapped off with a hot air knife.
PCB Manufacturers that employ through-hole technology or larger surface mount technology designs, hot air surface leveling excels in these designs. For smaller boards, it is not ideal. Also, The solder used is typically Tin-Lead solder, making it RoHS non-compliant.
Lead-free hot air solder leveling is a type of surface finish that employs lead-free solder alloys. Instead of the conventional Tin-Lead eutectic alloys used by HASL, Lead-free HASL employs other solder alloys that do not contain lead.
When applying Lead-free HASL on the circuit board, molten solder is first used on the circuit board to achieve wetting. Then, air knives (usually set at a temperature above the lead-free solder melting point) are used to scrap off excess solder.
After the solder has solidified, the circuit board is passed through the washer, where solder flux residues are removed.
Despite being considered an environmental alternative to the standard HASL, lead-free HASL is least employed in surface finishes. The reason is that other innovative surface finishes are available in the PCB market. They include OSP and ENIG.
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Many Customers understand the HASL process. Still, they need to learn the difference between HASL and Lead-free HASL. However, the differences between them are:
Choosing the proper surface finishing during PCB production is essential. This careful consideration is because PCBs are found in every electronic component worldwide, ranging from colossal industrial engines to children's toys.
Hence, PCB production is taken seriously to avoid exposing toxic materials such as lead to children and adults, which could adversely affect their health. Hence, since EU RoHS's inception in , HASL has taken on a new approach by not utilizing lead in its surface finishes.
Hot Air Surface Leveling (HASL) application in surface finishing is a common trend in the PCB industry. This guide has provided differences between HASL and lead-free HASL as they provide respective physical and mechanical strengths in their application.
Also, while HASL offers cost-effective benefits to PCB surface finishes, it is not RoHS compliant. As a result, should ensure high safety upon application of these finishes.
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