I would like to add a few general comments.  I have been involved with 
     electroless copper for a number of years, using most of the methods 
     available for measurement.  SPC of the thickness is frustrating to say 
     the least.  The process is so dynamic & also relies on more than just 
     the copper bath to influence the thickness (although many of these 
     variables have only nuance effects).
     
     The measurements vary so much that my real bottom line is:
     "I want to know if the bath/line has so much variation from one load 
     to the next that something has just had a major screwup."
     
     That is, I am looking for those wide swings in thickness from one rack 
     to another that indicate REAL problems (variation of 30-40 millionths 
     or more).  i.e. Did a replenishment pump die since the last load was 
     processed?  Did someone bump a calibration knob on one of the pumps?  
     Was the wrong add made?  etc.
     
     These are the things that you are REALLY interested in.  The thickness 
     of the Dep copper within 10 or 15 millionths of the target isn't 
     really going to make much difference to your processing unless you are 
     already on the verge of etching it out of the holes in your plating 
     line, creating PTH voids.
     
     Matt Byrne
     Hadco, Owego


______________________________ Reply Separator _________________________________
Subject: FAB:  follow-up on electroless thickness measurements
Author:  [log in to unmask] at SMTPLINK-HADCO
Date:    4/1/96 06:20 PM


     
Thanks to all who responded to my question about measuring thickness of 
electroless copper.
     
Here are some comments.
     
1.  Coupons must be all the same size, routed, not sheared.  Shearing may 
not make a big difference, but routing is best.
     
2.  Deposition varies with material:  epoxy-glass, polyimide-glass, etc.  
My thought is that, with different materials, you may need different SPC 
chart for each material.  Most of what our plant processes, almost all, 
is epoxy-glass, so we can evade this problem.
     
3.  Deposition may vary from lot to lot of the same material.  When 
setting up an SPC chart, may want to get a mix of lots to use for 
coupons.  That way, variability in those lots will be included in the SPC 
chart even when you don't know what the variability is.
     
4.  Bake the coupon before weighing.  Will the copper-clad coupon have 
enough water on the surface to make a difference?  If the coupon is baked 
(and weighed) before deposition, will it not absorb water when it is put 
through the wet process?
     
     
Thanks again to all who contributed.
     
Louis Hart
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