PCB Surface Finish Lead-Free HASL vs ENIG - PCBONLINE

PCB surface finish is a metal or organic coating on PCB pads that protects the copper and boosts solderability. Lead-free HASL (hot air solder leveling) is the most common PCB surface finish, and if you don't specify which surface finish you want, we assume you want lead-free HASL for your boards.

For more information, please visit lead-free hasl.

However, lead-free HASL is not allowed for flexible PCBs and rigid PCBs with a thickness smaller than 0.5mm. You have to replace it with ENIG (electroless nickel immersion gold) or OSP (organic solderability preservatives). This article reveals HASL and ENIG and compares them.

Part 1: HASL and Lead-Free HASL

HASL means using tin/lead solder to spray the PCB pads and then using hot air to blow the PCB surface so that the pad surface is level. The thickness of HASL is 1mil to 2mil (25.4μm to 50.8μm, or 0.025mm to 0.05mm).

HASL is a cheap PCB surface finish. And its solderability is good.

However, HASL doesn't meet the lead-free requirements in most of the world, and in most cases, we use lead-free HASL.

The properties of lead-free HASL are pure tin. The thickness of lead-free HASL is 0.1mil to 1mil (2.54μm to 25.4μm, or 0.0025mm to 0.025mm).

Lead-free HASL is the default surface finish from the PCB manufacturer PCBONLINE. Compared with HASL, PCB pads with lead-free HASL look dim, and the soldering temperatures are higher.

HASL

Lead-free HASL

Properties

Tin and lead (37% of lead)

Tin (lead less than 0.5%)

PCB pad finish

Glossy

Dim

Solderability

Good

A little worse than HASL

Reflow soldering temperatures

210℃ to 245℃

240℃ to 270℃

Wave soldering temperatures

About 250℃

About 260℃

Though lead-free HASL is common, it can't be applied to rigid PCBs with a thickness smaller than 0.5mm and flexible PCBs, neither HASL. These boards can't bear the spraying force. If you spray tin on these boards, they bend, and only the middle of the boards is sprayed with tin. Sometimes, rigid PCBs with a thickness smaller than 0.5mm may even break if you apply HASL or lead-free HASL to them.

In such cases, you can choose the other PCB surface finish -- ENIG. Or you can also change lead-free HASL to OSP. But OSP is easy to oxide. If you don't solder the PCBs at once, OSP oxides. And the manufacturer has to seal and package the OSP PCBs at once.

Part 2: What is ENIG

ENIG, also known as immersion gold, or chemical Ni/Au, is one of the best PCB surface finishes for almost all PCBs. In any case, if HASL or lead-free HASL can be used, you can replace it with ENIG. But inversely, HASL or lead-free HASL can't replace ENIG.

ENIG means plating a nickel alloy coating on the PCB pads and then a gold coating with chemical reactions. Besides PCB pads, ENIG can also be applied to other areas, such as the PCB edges or wires for thermal dissipation purposes. The thickness of ENIG is 1μm, 2μm, and 3μm. (From the equation 1mil=25.4μm, you can understand how thin the PCB finish ENIG is compared to HASL.)

ENIG PCB pads have a gold color and look shiny. Besides, the solderability of ENIG is the best among all the PCB surface finishes. And because gold has a dense crystal structure and is hard oxide, the protection of ENIG to copper of the PCB pads is also excellent.

Part 3: ENIG vs HASL vs Lead-Free HASL

ENIG PCBs are high-standard PCBs and they are usually used for high-end applications. As we mentioned above, ENIG can take the place of HASL and lead-free HASL in any case if your budget allows, but HASL and lead-free HASL can't replace ENIG.

Below is a comparison between EENIG and HASL.

ENIG

HASL

Lead-free HASL

Meeting lead-free requirements or not

Yes

No

Yes

Price

High

Low

Middle

Thickness

1μm, 2μm, and 3μm

25.4μm to 50.8μm

2.54μm to 25.4μm

Finish method

Chemical reactions

Spraying

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.

Spraying

Properties

Nickel and gold

Tin and lead

Tin

Applications

Can be used for almost all PCBs

Simple FR4 PCBs with a thickness larger than 0.5mm

Common rigid PCBs with a thickness larger than 0.5mm

Part 4: One-Stop PCB Manufacturer Hassle-Free

PCBONLINE is a one-stop advanced PCB manufacturer founded in 1999, with two large advanced PCB manufacturing bases and one assembly factory. If you have needs for PCB and PCBA fabrication, PCBONLINE can be your reliable PCB/PCBA supplier from prototypes to bulk production.

In PCB surface finishes, PCBONLINE has these advantages:

• PCBONLINE can fabricate surface finishes including lead-free HASL, ENIG, hard gold plating, immersion silver, OSP, carbon ink, immersion tin, and palladium plating.
• We can achieve selective PCB surface finishes on your PCBs without a limit.
• Our engineers do free DFM (design for manufacturing) and you can stay hassle-free for your project.
• We manufacture flexible PCBs, rigid-flex PCBs, high-frequency PCBs, FR4 PCBs, HDI PCBs, AlN PCBs, alumina PCBs, aluminum PCBs, copper-base PCBs, and glass PCBs.
• We not only fabricate and assemble PCBs but also source electronic components for PCB assembly and assemble the PCBA to be box-built.
• One-on-one design and engineering support throughout your project.
• Free sample and functional jig testing for bulk production.

If you need PCB design, fabrication, assembly, and box build assembly, please feel free to contact PCBONLINE by email at info@pcbonline.com.

Conclusion

This article gives an introduction to HASL and ENIG separately and compares them. ENIG is universal for PCBs, especially for high-reliability PCBs. HASL doesn't meet the lead-free requirements but lead-free HASL does. If you don't know whether HASL can be used for your PCB or not, ENIG must be okay. You can also contact PCBONLINE for suggestions.

© This article is an original work of the PCBONLINE team. Please indicate the author PCBONLINE if you reprint. If the article is reproduced without permission or indicating the author's source, PCBONLINE reserves the right to investigate the infringement.

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

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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