The low yields are on account of Porosity in Solder mask.This causes layer to layer short.
Solution:
1. Use pure thermal solder resist.
2.Use screen of 40 T/cm.
3.If u have a lot of $ stretch cloth at 45 deg.
4. Print on the PCB once in X axis and after semi cure print on Y axis.
Anil Kher

    -----Original Message-----
    From: Stephen Thomas <[log in to unmask]>
    To: [log in to unmask] <[log in to unmask]>
    Date: Monday, July 12, 1999 3:01 PM
    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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