Grant :
I concur with the other comments that to truly identify the cause is extremely speculative without seeing the boards.
However, Stewart most likely hit the mark with the description of the film contact. This is often referred to as "ferrotyping". When in the mask is soft, ( usually the result of insufficient tack drying ) under vacuum the artwork causes the mask surface to flow slightly , leaving the texture of the film in place of the gloss imparted by the material, and drying.
Sometimes one can see similar results if there is Oxygen inhibition in the exposure step, this is usually seen as surface attack during develop , and results in a lower gloss surface with tell-tale marks and patterns from the developer itself.
Without knowing the combination of mask and conformal coat it is difficult to judge the effects , however in most cases it shouldn't be an issue.
What you might want to do is go back to your supplier and ask when the last time they profiled their soldermask ovens! That should give 'them something to think about.
Russell Smith
-----Original Message-----
From: Dougal Stewart [mailto:[log in to unmask]]
Sent: Tue, March 19, 2002 4:08 AM
To: [log in to unmask]
Subject: Re: [TN] Soldermask color
This may be as simple an explanation as the area of sheen being created by
the polyester silver halide or diazo film surface during the vacuum pull at
exposure, during the bare board fab process. Where there are high spots
caused by pads and tracks, the polyester is held off contact adjacent to the
feature, and the gloss will be apparent where the film comes back into
intimate contact. Normally, you would see this exaggerated more on one side
than the other as the normal exposure process uses glass on one side of the
frame and mylar for the other.
Different hues may be an optical effect caused by the above, or by different
thicknesses of the ink, again being thicker off the tracks and pads due to
slump.
How will it affect conformal coat ? - until you have an understanding of
what you have got, you will not be able to understand what effect you will
have.....
Dougal Stewart
email: [log in to unmask]
phone: +44 1896 822204
mob: +44 7984 629667
----- Original Message -----
From: "Grant Emandien" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, March 19, 2002 9:35 AM
Subject: [TN] Soldermask color
> Hi,
>
> We have received PCBs with solder mask exhibiting color variarion i.e.
light
> and darker hues on the same PCB. The color differences are more prevalent
> around vias but also evident on other areas. The light hue has a more matt
> finish while the darker hue exhibits a sheen, almost an oily appearance
but
> is not easily removed.
>
> Any comments on what it may be and possible effects (for conformal
coating)?
>
> TIA
> Grant
>
>
>
>
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