Hi Roger,
According to our suppier the boards have 4-5uM of nickel and 0.1uM Au, which fits very nice within the immersion gold specification that was recently discussed on Technet.
Our solder paste contains 2% silver (62/36/2), with a RMA flux.
Problems occur during the second soldering step (we use nitrogen inerted convection reflow) on about 50 % of the boards. We see a serious degree of dewetting (gold still visible) especially at the fine-pitch components.
Strange thing is that this happens on boards of two suppliers (both new for us), where with other suppliers we usually have excellent results. It's difficult to imagine that both our new suppliers provide bad boards, but on the other hand I can't imagine what's wrong with the assembly process since we normally solder gold boards (with similar specs) without any difficulty.
I'll try to get the pads analysed somewhere. Thanks for your help !
Daan Terstegge
Unclassified mail
>>> Roger Massey-G14195 <[log in to unmask]> 09/10 3:07 pm >>>
Daan,
Can you define exactly what the platings are, in terms of thickness
and purity if any, ie Ni is 4-6uM with 0.5uM Au etc!! and what the
solders etc are Is the problem effect the whole batch, or just some of
the boards?
Regarding your specific questions:-
Prebake of boards will enhance any diffusion of Cu through the Ni
and into the Au, I cant see why this would effect solderabilty, but
may effect reliability The degree of diffusion etc will be related to
time and temp of prebake and the thickness and quality of the Ni
plating.
Effect of previous soldering operations would all effect the
diffusion as above
I assume that as long as the solder paste is relatively active it
should work fine on Ni-Au as HASL
Have you done any surface analysis of the pads? sounds like they
are contaminated with something, either from the plating, storage
handling, or even within your own facility and process.
I think we need more info, and you should get some basic surface
analysis of the pads themselves.
Roger
Motorola AIEG
______________________________ Reply Separator _________________________________
Subject: [TN] soldering to immersion gold boards
Author: "TechNet E-Mail Forum." <[log in to unmask]> at #email
Date: 10/09/99 14:17
Hi Technet,
I'm having some problems lately with soldering of pcb's with an electroless
nickel/ immersion gold conductor finishing, which brings me (after some
discussions with our board supplier) to the following questions :
Is solderability influenced by pre-baking the boards ? Does the first soldering
step have an effect on solderability for the second soldering step (double sided
reflow) ? How about the time between all these thermal excursions ?
Is it possible that a solder paste that performs excellent with HASL-boards (and
normally also with immersion gold boards) is a bad choice for soldering to
immersion gold ?
Greetings,
Daan Terstegge
Unclassified mail
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