Hi Eldon,
I am surprised you ave not seen failures like these earlier.
Also, did you perhaps switch to Pb-free solders?
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.
Sorry, but this is wrong. On heating the PCB expands more than the CC—no cracking.
But when the board comes back to ambient it seems to be
longer, and shows a gap.
On cooling, the PCB shrinks more, putting the CC, solder joints and all in tension—causes cracking and gaps.
After a board goes through 3 heat cycles at a CM,
wouldn't it be stable in CTE?
No, there is no 'stabilization' of CTE's.
And why would the other resistor not have the
same issue? Is CTE different in X and Y?
There are differences in CTE in x and y—by sometimes more than 2 ppm/C. The loading is also different if running parallel or perpendicular to the wave
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?
Obviously, something has changed—you need to find out what.
Werner
-----Original Message-----
From: Eldon Sanders <[log in to unmask]>
To: [log in to unmask]
Sent: Fri, Jul 2, 2010 7:31 pm
Subject: [TN] Cracked 2208 smt resistor end joint
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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