HASL vs. Lead-Free HASL vs. ENIG: How to Choose a PCB Surface Finish

Bare copper oxidizes within days, and oxidized copper does not solder. The surface finish is the coating applied to exposed pads to keep them solderable until assembly. It is one of the few fabrication options that affects both the price of the bare board and the yield of the assembly.

This guide compares the three finishes available in our online quote: HASL, lead-free HASL, and ENIG. It is written for rigid FR-4 boards.

At a glance

HASL

HASL stands for hot air solder leveling. The board is dipped in molten tin-lead solder, and hot air knives blow the excess off the surface.

Strengths

• The lowest-cost finish
• Excellent solderability. The pad is already coated in solder
• Tolerant of handling and of more than one pass through the oven

Limits

• It contains lead, so it does not meet RoHS. If your product ships to a market or a customer that requires RoHS compliance, this finish is ruled out. If you mark RoHS as required on your request, leaded HASL is not an option
• The surface is not flat. Solder pools slightly on each pad, and the thickness varies from one pad to the next

That unevenness matters little for through-hole parts and large surface-mount parts. On fine-pitch parts it causes uneven paste deposits and bridging.

Lead-free HASL

The process is the same, with a lead-free alloy in place of tin-lead. It gives you the low cost and good solderability of HASL in a RoHS-compliant form.

Two differences are worth knowing:

• Higher process temperature. Lead-free alloys melt at a higher temperature, so the board sees more thermal stress during finishing. Standard FR-4 handles this on ordinary boards. For very thin boards, ask before choosing it
• Still uneven. The flatness limit is the same as with leaded HASL

For most through-hole and standard surface-mount designs that must be RoHS compliant, lead-free HASL is the sensible default.

ENIG

ENIG stands for electroless nickel immersion gold. A layer of nickel is plated chemically onto the copper, then a very thin layer of gold is deposited over the nickel. The gold keeps the nickel from oxidizing. During soldering, the gold dissolves into the joint and the solder bonds to the nickel.

Strengths

• A flat pad surface. This is the main reason to choose it
• Suitable for BGAs, QFNs, and fine-pitch parts
• RoHS compliant
• Long shelf life when stored sealed
• Suitable for exposed contact areas such as test points and keypad contacts

Limits

• It costs more than either HASL finish. The difference is most visible on small orders
• It is not the right choice for edge connectors that are plugged in and out many times. Those need hard gold plating, which is a thicker and different process

How to choose

Work through these questions in order:

1. Does the board carry BGAs, QFNs, or parts with a pitch of 0.5 mm or less? Choose ENIG.
2. Does the board have exposed contact pads or need a long storage life before assembly? Choose ENIG.
3. Does the product need to be RoHS compliant? Choose lead-free HASL.
4. None of the above? HASL is the lowest-cost option that will do the job.

If you are unsure about RoHS, choose lead-free HASL. You can compare both prices in the online quote.

Common questions

Can I mix finishes on one board? Not in a standard order. One finish covers every exposed pad.

Does the finish affect assembly? Yes. Tell your assembler which finish the boards have. A leaded HASL board is not a lead-free product, even when it is assembled with lead-free solder paste.

Is ENIG the same as gold fingers? No. ENIG gold is very thin and soft. Gold fingers on edge connectors use hard gold, plated much thicker to survive wear.

Does the solder mask color matter? Not for the finish. The finish only covers exposed copper.

Selecting it on your order

In the online quote, the surface finish is a single option. You can price the same board with each finish and see what ENIG adds for your board size and quantity before you decide.

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