Steve ; I went around this last time using thickest silk screen I could get
printed over via's and track;
at the same time as all other white print .
Talk to your fab ; they can adjust the viscosity to a large degree ; saving
you all the add steps you do now .

paul

-----Original Message-----
From: Stephen Thomas [mailto:[log in to unmask]]
Sent: Monday, July 12, 1999 8:07 PM
To: [log in to unmask]
Subject: [TN] PCB Fabrication Question


I am attempting to screen print resistive carbon ink over a large area of a
PCB, for reasons I will not go into now. Under the carbon ink are tracks and
via's which must be insulated from the ink, this is currently being achieved
by plugging the via's with an epoxy and printing two layers of photo
imageable soldermask.

The reason I am posting this question to the sig is that the process
described above is producing low yields. Does anyone in sig land have any
innovative suggestions to resolve this problem.

TIA

Steve

P.S. Ideas currently under consideration

Use dry soldermask instead of photo imageable (out of favour, at least in
the UK)

Replace the soldermask with a polymide covercoat (as used on flexible PCB's)
- not sure that the carbon ink will adhere to its shiny surface.

Use a thicker carbon ink to reduce the risk of the carbon ink getting into
where it shouldn't.

Any ideas welcomed !!
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