If this were a hi-rel application, I would be very worried. An
improperly cured coating is potentially dangerous and I would want to
know more about what would appear to be an astoichiometric chemistry and
the free components.
The white "residue" is probably a very fine alumina added to the mask to
give it the desired rheological properties (and to reduce the cost). It
has a refractive index close to that of the mask resin, so is normally
invisible. When the mask surface is eroded by solubilisation in water or
by evaporation of an unreacted component, a small quantity will be laid
bare, becoming visible. If this is the case, the alumina itself is
harmless, but unreacted chemicals will always be present.
It would seem your board supplier is trying to weasel out of his
responsibility for producing faulty goods.
Without being certain, I thought that MIL-P-55110 went out of being
several years ago. Unfortunately, I was unable to check it as Assist is
offline just now. I suggest you check with
http://astimage.daps.dla.mil/quicksearch/ to find the current situation.
In any case, it was a specification for bare printed circuits, not a
qualification procedure for suppliers.
IMHO
Brian
> Bogert wrote:
>
> 3/30/002
>
> Folks, we have an OEM who purchased FR-4 printed wiring boards from a
> MIL-P-55110 qualified board manufacturer. The OEM wave soldered
> the PWAs using HF1189A water washable, organic flux and cleaned the
> assemblies using DI water in an in-line cleaning process. Subsequent
> Omegameter 600 ionic cleanliness testing passed. However, almost all
> of the solder side of the PWAs exhibited white residue. Basically it
> looks like a water mark. While you can remove the residue
> using abrasion, such as by a pencil erasure, it comes back, even after
> several subsequent baking operatioins and additional in-line
> cleaning. The OEM sent out samples for outside chemical evaluation
> and found minute traces of carbon. The OEM traced the problem to
> incomplete curing of the LPI solder mask. The white residue only
> appears on the solder side of the PWA. There is no evidence of the
> residue on the component side of the assembly.
>
> The OEM proposes that a sample assembly be sent out for SIR testing,
> and that if the test passes, they will request us to accept the
> condition as is. My concern is that over time in a humid environment,
> we may experience leakage current paths that cound cause operasting
> failures. The OEM has 20 PWAs that are part of the lot. However,
> only 6 experience this condition. It appears that I should reject all
> PWAs that show evidence of the white residue. I am concerned with the
> presence of carbon, even in trace amounts. The OEM noted that almost
> everything will show traces of carbon, so not to worry.
>
> Bottom line is, is there any technical concern if the white residue is
> present? Any input on this topic wo\uld be appreciated.
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