ROGER HELD wrote:
> 
>      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

We often observe barrel cracking in small vias after thermal shock.  
This is a fairly common failure mechanism, and is observed particularly 
after thermal shock and not thermal stress (solder float).  I don't have 
any data to offer, but since you asked for opinions, I doubt the trapped 
gas is the problem.  I'd be more concerned about the copper ductility, 
and plating quality.

-- 
John Moylan

Delsen Testing Laboratories, Inc.
1024 Grand Central Ave.
Glendale,  CA  91201

Phone:	818 247 4106
FAX	818 247 4537
email:	[log in to unmask]
URL:	http://www.primenet.com/~delsen/

Delsen Testing Laboratories, Inc. is an independent testing
laboratory primarily engaged testing advanced materials.