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Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


HASL vs. ENIG

genny

#2193

HASL vs. ENIG | 14 December, 2000

Hello all, Our company has always used HASL finish on our products. Recently however, we have a had a problem on a couple of boards that I have been told cannot be avoided with HASL, and we should switch to ENIG(electroless nickel immersion gold). I have searched this forum and learned all kinds of interesting and worrysome things about ENIG and wanted some more info. We have a PCB with a ground plane side that is almost all tinned(very little soldermask) This is due to needing to mount a heatsink directly to the pcb, and needing a good thermal contact. During HASL, some solder gets trapped in vias or pools on the surface and during reflow this solder can sometimes run out and form peaks that will push through the thermal gasket used and short to the grounded heatsink. I agree that ENIG will not have this pooling or excess solder problem, but I am worried about the reliability of it, ie. black pad syndrome or nickel oxidation of the pads. Also the problem of weak solder joints if the right temp profile isn't used and a proper intermetallic bond isn't formed. I have been told ICT will catch black pads, but am not sure if I believe it, or if it will catch the weak bonds. Can you tell me if ICT testing will always pick up black pads, or if components may appear to connect at first but after time or temperature variations may turn into cold solder joints? Thank you for your responses.

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#2194

Re: HASL vs. ENIG | 14 December, 2000

Wow, someone read the fine SMTnet Archives!!!! Scary, get their name!!! [OK, I�ll stop ... Several points are: * Most of the stuff in the fine SMTnet Archives is all the bad things that we can make-up to respond to the questions people ask. There are many happy campers using ENIG. * Planarity of ENIG probably would make it a good matching surface for your thermal gasket / heat sink. * If you change to ENIG and get black pads, you�ll have an observable problem before the product leaves the plant. Two issues in precluding that are: 1 Finding and pummeling the bird brain that can-up with the ICT solution!!! Lordy, even if ICT can catch black pads, who in earthly heaven wants to be repairing things that far down the line? Using the ICT as a screening point will assure you of having bad boards in every possible level of completion. Ugh!!! 2 Determining how to find a supplier with a perfect process, monitor your in-bound materials, go forth and never think about black pad!!!

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#2195

Re: HASL vs. ENIG | 14 December, 2000

OK, now let me ask the really dopey question ... so if the irregularity of the HASL surface is punching holes in the thermal gasket and causing shorts to the heat sink, why are you putting solder on that surface? I wonder how the thermal conductivity characteristics of an ENIG surface compares with a copper / copper oxide surace. Bare copper has to be cheaper, eh?

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genny

#2196

Re: HASL vs. ENIG | 15 December, 2000

It is only after the reflow heating process that the peaks may show up, as the solder build up in some vias(due to HASL'ing) might drip down during reflow. I don't think I've ever heard of it being desirable to have a bare copper surface that will oxidise. I feel there should be some finish on the board. I do know that soldermask has a fairly poor thermal conductivity relative to any metal interface, which is why we didn't apply it. The quantities that we eventually plan to manufacture of this product mean that we need to choose a good reliable and economical manufacturing process. We'd prefer that it be a process that is fairly easy to source as well. Some of the more exotic finishes are not readily available from a lot of board fab houses. Thanks for your replies.

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