Eldon,
How is the pad where the solder is cracking designed? Is there any kind of thermal relief to help with the flexing during the CTE event?
Toby, CID
Sent from my iPhone
On Jul 2, 2010, at 7:31 PM, Eldon Sanders <[log in to unmask]> wrote:
> 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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