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July 2010

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Subject:
From:
Eldon Sanders <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Eldon Sanders <[log in to unmask]>
Date:
Fri, 2 Jul 2010 18:31:15 -0500
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We are experiencing fractures in the end metallization or solder joint on one of 
two larger high voltage resistors, size 2208.  They are mounted on FR-4.    
Only one location is failing.  The other location is an inch away and oriented at 
90 degrees.  The failure rate is variable between assembly lots.  Some have 
no failures, and no observed cracks.  The most recent lot has a 2% failure 
rate, and another 2% that exhibit visible cracks in the ceramic but the solder 
joint is connecting the pad to the metallization and making electrical 
connection.  I believe the root cause is the poor choice of the FR-4 board 
material with such a large ceramic resistor.  However, about 80,000 boards 
have been built over 7 years without this failure mode.  So, I am searching for 
the root cause of this specific timeframe.  I have verified the process at the 
CM has not changed - the resistor is mounted on the 2nd pass SMT, and the 
board goes through wave soldering of TH components using a fixture.  The 
same date code resistor is used for both good assembly lots and bad.  There 
is some correlation to the date code of the bare fab.  On cracked boards 
there is a gap observable between the solder joint and the ceramic resistor 
body.  

My present theory is the crack occurs when the assembly is wave soldered 
causing excessive tension on the resistor due to CTE mis-match, and some of 
these crack.  But when the board comes back to ambient it seems to be 
longer, and shows a gap.  After a board goes through 3 heat cycles at a CM, 
wouldn't it be stable in CTE?  And why would the other resistor not have the 
same issue?  Is CTE different in X and Y?  Finally, would an error or change in 
the raw fab process cause a change in the CTE and cause this issue to occur 
after many years?

Thanks to all the incredible minds if TechNet.

Eldon Sanders 
Sr. Manufacturing Engineer 
Welch Allyn Monitoring 
8500 SW Creekside Place
Beaverton, OR 97008-7107
phone: (503) 530-7596         fax: (503) 526-4300
[log in to unmask]

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