To all Curtain Coat Skip Sufferers:
          
          I know of three ways to get around a skip problem, and neither is 
          without some risk.
          
          1.   You can increase your coat weight.  Going from 11 g/sqm to 
          13 g/sqm will get coverage over some skips.  There are limits to 
          what can be attained, and there is the material cost increase.
          
          2.   You can reduce your viscosity.  Going from 60 seconds to 50 
          seconds will dramatically increase the fill of small skips.  If 
          you only do that change, you will find the thickness of mask over 
          conductors and especially the corners of conductors will drop.  I 
          recommend increasing temperature in conjuction with reducing 
          viscosity.  This has the net effect of keeping solids the same 
          and minimizing the thickness loss over conductors.  For example, 
          increasing the Probimer 52M curtain temperature from 24 C to 27 C 
          and reducing the viscosity from 60 seconds to 50 seconds results 
          in no change in solids content (but you will see a different 
          ratio of coverage on conductor to coverage on laminate).
          
          3.   You can increase the temperature of the boards as they are 
          coated.  This has the same effect as raising the temperature and 
          lowering the viscosity.  The downside to this is that you may see 
          an increase in mottling and runback, with some thinning of mask 
          over pads.
          
          Any of these conditions will help unless you have channels that 
          are too narrow.  As a rule of thumb, any channel that is not at 
          least twice as wide as it is deep will get a skip.
          
          Good luck!
          
                                        Bruce Guttman
                                        HADCO Corporation
                                        [log in to unmask]
          


______________________________ Reply Separator _________________________________
Subject: SM SKIP IN CURTAINCOAT PROCESS
Author:  JMONAGHAN
Date:    7/24/95 7:31 AM


          
     Hello,
     Our company largest DS PCB manufacturer in Malaysia.We are 
     encountering
     the soldermask skip problem.I give below the parameters used.
          
     M/C Burkle Curtain coater
     Ink Peters GL2469sm
          
     Viscosity 60to65
     Beltspeed 78m/mim
     web weight 6.4to 6.8gms/600sqcm
          
     We use pattern plating process and plating thickness 40to 60 microns.
          
     The Sm skip is random and happens near closely spaced tracks(most of 
     the times).
          
     I Shall be highly grateful if anyone can give info to minimise the 
     sm skip problem.
          
     Thanks
     R.Srinivasan
          
     Likom PCB SDN BHD
     Malacca Malaysia
          
     E-mail :[log in to unmask]