Rick
There are several explanations but there are also a lot of questions to
be asked.
1. When it balls up, is this on top of the gold or is the gold dissolved
and the ball sits on top of whatever is underneath?
2. What is underneath the gold (any other platings etc.)?
3. What exactly do you mean by "flash", how thick, composition, how applied?
4. How is the underlying surface prepared to receive the gold flash?
5. How old are the circuits and how have they been stored?
6. Is the vendor an established one who knows what he is doing or is he
a cheap bucket shop?
In all probability, two (not exclusive) of the most likely explanations
is that either a) he is using an old style of immersion gold which
co-deposits unsolderable organics and phosphates or b) the gold is too
porous. My favourite gold porosity test is to connect the plating to be
tested to the + side of a ~4 V DC power supply and the -ve to some gold
wire. Immerse them in a 5% hydrochloric acid solution with the wire a
few mm from the test sample. Leave for 24 hours. If the solution changes
colour, the gold wire has a dark powdery coating, all the gold has
fallen to the bottom of the beaker or there are visible signs of
corrosion on the test sample (or any combination of these), then you
know that you have a porous gold :-( Many years ago, I had some
electrodes made by plating 5 um of pure, dense gold on the copper of a
PCB substrate. This was then hammered by blasting the surface with 5 mm
ball bearings, to close the pores and then another 5 um was plated on
top. Even with that thick coating and the mechanical pore-closing, about
half the pieces made failed that test. As a result, we changed to solid
gold.
My first guess would be that you have an unsolderable surface under the
gold, possibly due to oxidation. If you have nickel under the gold, the
chance of this are high. "No-clean" fluxes often don't have enough oomph
to cut through heavy oxidation and, if it is nickel, you must have a
good halide activator to do so, which many such fluxes simply don't have.
Brian
Rick Thompson wrote:
> Has anyone had any experience using Lead-free solder paste on a Flash gold
> finish? We're currently having some solder wetting issues with lead-free on
> flash gold where some of the pads on the boards exhibit complete lack of any
> wetting. It almost looks like what you get when you reflow solder paste on
> a ceramic substrate, where the solder just balls up. Is this an inherent
> problem with flash gold or do I have some kind of abnormal processing issue
> at the board vendor?
>
> Thanks in advance.
>
> Rick Thompson
>
> Sr. SMT Process Engineer
> SMTEK International, Inc.
> +1 (805) 532-2800
> [log in to unmask]
>
>
> Rick Thompson
>
> Sr. SMT Process Engineer
> SMTEK International, Inc.
> +1 (805) 532-2800
> [log in to unmask]
>
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