ROGER,
        WHAT YOU ARE SEEING IS A THE RESULT OF AN UNSUPPORTED PLATED 
     THROUGH HOLE.  WITH OUT A SOLDER PLUG OR A COMPONENT LEAD IN THE HOLE 
     TO CONSTRAIN  THE LAYERS THE VIAS WILL TEND TO SHEAR IN THE CENTER OF 
     THE BOARD.  QUESTIONS I HAVE FOR YOU ARE; ARE THERE PADS ON EVERY 
     LAYER FOR THE VIA?  THIS WILL HELP ENSURE GOOD PLATING FROM THE 
     FABRICATOR.  WHY IS THE VIA TENTED? IS THE SOLDER MASK RELIEF NOT 
     LARGE ENOUGH TO CLEAR IT?  I WORKED FOR A COMPANY THAT BUILT DOWN HOLE 
     SENSORS FOR THE OIL FIELD INDUSTRY.  WE SAW TEMP CHANGES FROM -20C 
     (DECK OF AN ARTIC DRILL RIG) TO 150C (DOWN HOLE).  WE USED PADS ON 
     EVERY LAYER, SOLDER PLUGGED, AND FINALY A SMALL LEAD SOLDERED IN THE 
     VIA HOLE.  WE EVENTUALY DID AWAY WITH VIA HOLES (WE USED ONLY THROUGH 
     HOLE COMPONENTS).  HOPE THIS HELPS.
     
     ROB BUTTERWORTH
     SR PWB DESIGNER
     ADC VIDEO SYSTEMS
     MERIDEN, CT 06450
     [log in to unmask]


______________________________ Reply Separator _________________________________
Subject: FAB: Barrel Cracking due to Air Entrapment
Author:  [log in to unmask] at internet-mail
Date:    6/6/96 12:49 PM


     To TechNet,
     
     I have had a problem with a sample PCB evaluation and I hope someone 
     can help me with some supporting data.
     
     The problem is that the evaluation board had a shear crack in the 
     barrel of a 12mil small via hole after 100 cycles of thermal shock. 
     What we discovered is that the hole had been plugged on both sides 
     with soldermask (not my idea).  We feel like the expansion of the 
     trapped air eventually caused the barrel to shear.  What I need is 
     some confirming data to show that the theory is correct.
     
     In the past, we had some boards which were plugged on both sides with 
     mask (but not thermal shocked).  The resultant after reflow was that 
     one side of the plug popped (outgassing) and that the barrel did not 
     crack.  Was the difference in resultants due to the cycling at lower 
     temperature (-65C ~ +125C) vs. short term exposure at high temp (270C 
     for ~7 secs.)?
     
     Any opinions or sources of data would be greatly appreciated.  Thanks 
     in advance for all your responses.
     
     Roger Held
     Hitachi Computer Products (America), Inc. 
     [log in to unmask]
     (405) 360-5500 x142