PCB Surface Preparation Before Solder Mask on Non- ...

A circuit board is made of copper. Usually, final finishes are applied after the solder mask process. In some cases, for special applications, the final finish may be applied before solder mask. In this case, we have solder mask on ENIG or galvanic nickel-gold. It is also possible to have tin or tin-lead under solder mask; this was an old technology that no longer plays a role today.

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The methods for surface preparation of PCBs with copper are very well known. All these methods of pretreatment, chemically or mechanically, increase the surface roughness and ensure good solder mask adhesion. Many technical papers provide details on this topic. Yet, there is almost no literature available on the topic of surface preparation of ENIG or galvanic nickel-gold.

The main criterion for copper preparation is to achieve the desired level of surface roughness. But the gold surface is even, glossy, and very thin. I don&#;t recommend increasing the surface roughness of the gold, as the gold surface itself will be destroyed. As a consequence, mechanical methods&#;such as brushing, pumicing, or jet scrubbing&#;are not applicable to gold.

In addition, chemical microetches, such as mixtures of acids with oxidants, are not recommended. The gold surface generally resists these blends, but due to the fact that the gold thickness is just 0.03&#;0.07 µm after ENIG, the result is that the gold surface is porous. Acids and oxidants can attack the less noble metals beneath the gold with a negative impact on solderability and reliability. Even after galvanic gold plating with a higher thickness, the surface is porous. In addition, the edges of the tracks are generally not plated, allowing another point of entry for the oxidants.

Nickel-gold Surface (ENIG, ENIIPIG, ENIPIG, Galvanic Ni-Au)

PCBs with a nickel-gold finish can be processed directly over solder mask coating. But to do so, the rinse process following the nickel-gold plating step is good. The rinse water should not exceed 10 µS conductivity. The hold time between ENIG plating and solder mask coating must be held to less than one hour, as well.

To read this entire article, which appeared in the February issue of PCB007 Magazine, click here.

So my usual goto finish for PCB&#;s (particularly for 4 layers) is ENIG which is electroless nickel plating with a thin layer of gold. This finish is very flat and looks bling especially with a matte black solder mask which would make Gucci smile.

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Now the collective Google wisdom is that when you are using smaller than say 0.8mm pitched parts ENIG is a must over HASL (which is hot air solder leveling) which is as bumpy as the arse end of the moon in comparaison. I use a lot of 0.5mm pitched parts including STM32 micros etc hence the use of ENIG.

The only downside of ENIG is the cost - JLCPCB charges $16 for ENIG on small runs which is not overly expensive but when you are doing a $2 double layer test board (you get 5) an extra $16 is huge.

So time for some penny pinching and test out how smart the internet is about ENIG vs HASL.

I have designed and received a few new Gadgeteer modules and mainboards to test a few theories out and to slip these R & D marvels past the CFO&#;s inspection i ordered all of them with HASL to see how they fair.

This display adapter uses a 0.4mm pitched socket which is pretty tiny and the Google collective says you cant use HASL and must go with ENIG. Here is said 0.4mm pitch part in HASL from JLCPCB - looks ok to me, not as bling as gold but since there are no exposed pads after soldering its a moot point - for reference those via&#;s are 0.3mm holes

So the real question is what happens when we try to solder the socket?

I am now struggling to think why I need ENIG at all and will just save my $16 per order for further frivolous spending

PS - just to blow my own trumpet the above socket was soldered by hand without using a stencil

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