What's the Difference Between HASL and HASL Lead-free ...

Jun. 10, 2024

What's the Difference Between HASL and HASL Lead-free ...

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.

For more information, please visit lead-free hasl.

Due to the good quality of ENIG, the price is higher too. Many customers choose the most common HASL process.

Many people know the HASL process, but they don&#;t know what the difference is between HASL and HASL-LF. Please refer to the difference as below:

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.

Introduction

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.

Overview of HASL Process

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:

PCBs are pre-heated to warm them up before soldering
Flux is sprayed or foamed over the board to remove oxides and improve solder wetting
Boards pass over a pumped wave or fountain of molten solder
Excess solder is air-knifed off as boards leave the solder wave
Hot air levels the remaining solder to a uniform thickness

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 Leaded HASL Coatings

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:

Melting point of 183°C allowing lower solder bath temperatures
Good wetting on copper traces
Ductile coating absorbs mechanical shocks and stresses
Resists corrosion and oxidation of copper conductors

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 Materials

HASL PCB Board

Lead-free HASL eliminates lead and uses greener tin-based solder alloys. The main options are:

1. Pure Tin (Sn100)

Melting point of 232°C requires higher solder bath temperatures
Prone to tin whisker growth causing electrical shorts
Harder coating prone to cracking under stress

2. Tin-Copper (SnCu)

Eutectic Sn0.7Cu0.05 alloy melts at 227°C
Improved strength, hardness and wetting over pure tin
3-4% copper reduces risk of tin whiskers

3. Tin-Silver-Copper (SAC305)

Eutectic Sn96.5Ag3Cu0.5 alloy with 217°C melting point
Silver content improves fatigue resistance and strength
Small copper addition prevents tin pest corrosion

4. Tin-Bismuth (SnBi)

Low melting alloys like Sn42Bi58 (138°C) or Sn57Bi43 (179°C)
Bismuth reduces melting point significantly
Lower process temperatures than pure tin
Brittle with poor drop/shock resistance

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 Coating Thickness

Lead-free HASL coatings are typically thinner than traditional leaded HASL:

Leaded HASL &#; Average thickness of 100-300 microinches (2.5 &#; 7.5 μm)
Lead-free HASL &#; Average thickness around 50-100 microinches (1.25 &#; 2.5 μm)

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.

Lead-Free HASL Process Differences

Switching from leaded to lead-free HASL requires optimizing the soldering process:

Higher preheat temperatures &#; About 150-160°C needed for lead-free solders compared to 130-140°C for leaded.
Higher solder pot temperatures &#; Lead-free solders need around 240-255°C bath versus 200-210°C for leaded. This imposes stricter requirements on solder pot metallurgy.
More aggressive fluxing &#; Extra flux aids wetting of lead-free solders onto copper traces.
Nitrogen blanketing &#; Nitrogen atmosphere prevents oxidation of lead-free solders.
Tight temperature control &#; Lead-free solders are less forgiving than lead, requiring tight control of preheat, flux, and bath temperatures.

With the right parameters, the lead-free HASL process can match leaded HASL for solderability and surface finishes.

Lead-Free HASL Process Options

There are three main equipment configurations used for lead-free HASL processes:

1. Pumped Molten Solder

Boards pass over continuously pumped wave of molten solder
Simple, low maintenance, and fast process
Limited flexibility in solder volume and dwell times

2. Foam/Spray Flux + Static Solder Pot

Foam flux applied first, boards passed through static solder bath
Allows greater control of flux amount, temperature, exposure
Simpler maintenance than solder wave pumps

3. Selective Deposition

Solder selectively sprayed only onto the required areas
Reduces solder consumption by 60-70%
Minimizes solder beads around edges and connectors
Slower process requiring precise application control

Each has its own pros and cons. The method is selected based on production environments, volumes, and flexibility needs.

Comparison of Properties

The properties of traditional leaded HASL versus lead-free HASL are compared below:

Contact us to discuss your requirements of multilayer pcb design tips. Our experienced sales team can help you identify the options that best suit your needs.

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The Advantages of Implementing High-Efficiency Solar Panels in Renewable Energy Solutions 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)CostLowerHigher

Pros and Cons of Lead-Free HASL

Some key advantages and disadvantages of switching to lead-free HASL are:

Benefits

Environmentally friendly &#; RoHS and lead-free compliant
Eliminates risks from lead contamination
Enhanced corrosion resistance
Improved leach resistance in humid conditions
Better creep resistance under thermal cycling
Reduced tin whisker problems with SnAgCu alloys

Drawbacks

Higher process temperatures require equipment changes
Narrower process windows require tight control
Material costs are higher
Harder deposit prone to damage from board flexure
Reduced shock/drop resistance
Brittle alloys like SnBi have poor mechanical properties

Reliability Considerations

For acceptable reliability some important factors to consider are:

Good metal finish on traces prior to HASL
Sufficiently thick copper weights on outer layers
Smooth base copper to maximize bonding
Void-free immersion in solder with adequate dwell times
Clean fluxes to prevent residue entrapment
Well-controlled solder bath temperatures and atmosphere
Graduated thermal profiles during preheat, immersion and cooling
Removal of solder beads at connectors to prevent tombstoning

With optimal processing, lead-free HASL can achieve the adhesion, solderability, and reliability needed for most applications.

Summary

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.

Frequently Asked Questions

What are the most common lead-free alloys used for HASL solder coatings?

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.

How does the thickness of lead-free HASL coatings compare to traditional leaded HASL?

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.

What PCB surface finishes work best under lead-free HASL coating?

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.

What solder pot temperature is needed for lead-free SnCu HASL process?

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.

What selective deposition methods are available for lead-free HASL processes?

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.

Why Is Lead Free HASL Finish A Hassle? - Epec's Blog

Hot Air Solder Leveling (HASL) has always been the main staple of PCB surface finishes. In the late s, 60/40 tin-lead reflow started to phase out of processes and was replaced with Hot Air Solder Leveling. HASL finish, the long running reliable surface finish, is still used today in military, aerospace, medical, and other applications.

The Evolution of Hot Air Solder Leveling

The long history of HASL representing an excellent solderable finish for printed circuit boards has kept it alive in the global PCB market, both domestic and offshore, even though it contains lead (Pb).

We recognized since the beginning of EU RoHS in mid- that HASL was here to stay. Longevity and reliability combined with the trust of customers meant finding a solution to keeping the process alive, which meant trying to remove the lead was going to be a necessity.

Given that PCBs are in everything from industrial applications to toys, the safety concern was obvious. Once the health risks from prolonged exposure to lead for children and adults were established, getting lead out of products became the focal point for the electronic manufacturer.

Originally the expectation was that lead would be entirely removed from everything we touched.

Fast forward to present day, and we've come to the realization that lead may never be 100% totally gone from all products. However, choosing a PCB surface finish that works best for you and your process while still keeping the environment safe is has become a much more practical solution.

What About Lead Free HASL Finish?

The Lead Free Version (LFH) of HASL PCB finish became the most frequently considered surface finish next to immersion gold early in the process of safer finish alternatives. So, why didn't lead free HASL become the new standard instead of simply becoming a hassle for manufacturers? Due to the complicated nature of applying a LFH finish, some circuit board manufacturing facilities may need to outsource this process.

PCB with HASL / Lead Free HASL Surface Finish

The chemistry involved in the makeup of LFH has changed over the years, as have the applications. Vertical and horizontal applications both initially had the same issue as HASL; a pooling, non-flat finish with a somewhat foggy appearance in areas of the PCB.

Pre-trial combinations of the LFH gave the finish a bad review. The combination of tin, silver, and copper alloy originally had poor results at the processing level, leaving behind a bumpy uneven coat that was dull and unattractive as well as having a poor performance in assembly. Removing the silver, changing the tin-copper, and tweaking the manufacturing process has allowed for a better, smoother surface coating than originally found. With this promising development in application, demand has increased, as well as brought more LFH in house and decreased time to customer product delivery.

Pb Free HASL Processing Challenges

LFH needed to be applied at a hotter temperature. On the first pass, the surface is left grainy and dull. Once the second pass has been added, both surface as well as appearance improve to a shiny, much flatter, much smoother, even coat. However, excess heat from two dips in the molten solution leaves copper on the hole walls, reducing the copper below the acceptable limits per IPC standards. This forced another process change once again adding a stain to the LFH finish.

After a lot of subsequent changes to chemistries and processes, lead free HASL is now a stable surface application to use on PCBs. Improved process eventually produced a consistent means of using lead free HASL surface finish, dramatically reducing the hassle experienced within the PCB manufacturing industry.

LFH Decrease In The PCB Industry

So after all the improvements, why does LFH still seem to be the least used surface finish in the industry today?

ENIG finish (Electroless Nickel Immersion Gold), OSP finish (Organic Solderability Preservative), and even immersion tin and silver all took the lead in surface finish popularity for manufacturing. While Pb Free HASL finish became more possible, it still remains a more complex process than other finishes. As technology continues to develop, real estate on surface becomes tighter and footprints decrease, making it harder for LFH to establish itself as the go to surface treatment.

Summary

Lead Free HASL has improved viability in terms of process, but it still has some groundwork to overcome when competing against other surface finishes. However, this once not-so-popular finish has consistently been gaining popularity among customers, prompting manufacturers to give it a greater consideration as a main staple up against immersion silver, OSP, and immersion tin.

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