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

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.

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

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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:

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.

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