Technetters,

I am new to this group, but I have scanned the archives and there seems 
to a lot of expertise available, so I thought I bring up an issue we are 
battling with  that I was hoping getting some suggested remedies for.

We have an issue where 0402 Capacitors are turning during reflow. They 
are basically clocking beyond the 50% pad coverage rule. We can have as 
much as 10% of the caps on one board do this. These are lead free caps 
with matte tin termination. The vendor is of course claiming that it is 
compatible with eutectic solder and a leaded process. We are using 
Sn63Pb37 water soluble solder with an RTS profile.

The boards in question have been built for a while and this is a new 
problem. The placement is perfect. The problem does not appear when we 
use this same cap on our lead free assemblies.

Within a batch of, say 50 boards, the issue seem to vary with each reel 
different. We have not narrowed it down to a specific date code yet, but 
that experiment is next. These are all from the same vendor so we are 
also planning to try a different vendors capacitors.

One idea I had was that there is some sort of out gassing that move 
these caps, but we do not see any blow holes. I am not observing any 
other obvious defects and the issue is exclusive to these caps.. The 
easy answer is of course to go to a lead free assembly, but that is not 
really an option at this point.

Could oxidized caps on the caps cause this. Would using a different 
solder with an more aggressive flux be an idea? I have also thought of  
reducing the aperture in the stencil to reduce the solder. 

Any input would be highly appreciated.

-- 
Leif Erik Laerum
Quality Assurance Manager
Texas Memory Systems
[log in to unmask]
Tel: (713) 266-3200 x468
www.texmemsys.com


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