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Why is a surface finish one of the most important decisions you will have to make when purchasing a PCB? All PCB basically have copper tracks on them. If left unprotected these copper tracks will deoxidize and deteriorate. The end result is a PCB that is of very little use to you. So, a surface finish basically protects the copper tracks on the PCB from being oxidized.

The most common types of surface finishes for a PCB are; Hot-air solder leveling (HASL), Electroless Nickel Immersion Gold (ENIG), Immersion Silver and Immersion Tin. Lets see the benefits of using each of these types of finishes.

HASL
Here are two types of HASL.
■Tin/lead alloy HASL
■The lead free HASL

The process remains the same, no matter using a leaded alloy or a lead free alloy. The process consists of submerging the PCB into the alloy. The excess alloy is then removed from the board by air knives. These air knives blow air across the surface of the board to remove the excess alloy.

So what are some of the advantages and disadvantages of using HASL.

Advantages
■The PCB with a HASL finish is one of the most inexpensive boards.
■The PCB with a HASL finish remains solderable even after multiple wash and storage cycles.
Disadvantages
■Due to the uneven finishing of HASL surface, it is not suitable for more complex requirements and finer pitches.

Electroless Nickel Immersion Gold (ENIG)
Surface finishing with ENIG provides a smooth finish. Gold is an excellent choice for a surface finish as it doesn’t oxidize easily and it dissolves effortlessly onto the solder. The catch here is that gold cannot be applied directly on to copper because copper atoms diffuse into the gold. Nickel is used as a protective layer between copper and gold, as gold easily coats on nickel and it is easily plated on the copper tracks as well. That is what makes ENIG a great choice as a surface finish.

One of the commonly experienced drawbacks with ENIG surface finish is the ‘black pad’ effect. The nickel layer has been witnessed to break under stress.

Advantages
■Excellent solderability
■Smooth Surface
■Long shelf life
■Withstands multiple reflows
Disadvantages
■Higher cost
■Black pad effect

Immersion Silver
In the Immersion Silver process, the silver layer dissolves leaving a tin-silver alloy on the copper tracks. This proves to be a very reliable surface finish and has recently become a substitute for the HASL finish.

Advantages
■Excellent solderability
■Smooth Surface
Disadvantages
■Still in research phase

Immersion Tin
Very similar to the immersion silver technique, Immersion Tin however has some major health issues due to the carcinogen used in the tin solution.

Another problem faced with Immersion Tin is the whiskering found on tin. However the whiskering effect has been brought under control.

Advantages
■Excellent solderability
■Smooth Surface
■Low cost
Disadvantages
■Heath concerns

Source:http://www.seekic.com