Hi Steve,
Three things are contributing to skew;
1) pads are too wide
2) A bit too much solder
3) Overlap to extension ratio is >>1

For example we found during a study a decade + ago that component movement
and resulting defects like a tombstone or drawbridge were a minimum when
the length of a chip resting on the pad equaled the extension of the pad
beyond the end of the termination. Another factor is that you have this
condition optimized when overlap(O) = extension (E) = nominal termination
band length (T). Defects sort of disappeared when stencil thickness and
corresponding volume of solder was reduced over the years but leapt back
into the fray with really small chips like 0603/0402/0201 came into common
use.

The simple solution is reduce the stencil aperture to where you want the
parts to stay (plus a little for luck) and have at it. You probably have
enough solder already just in the HASL board finish for a reliable solder
joint so stencilled solder paste is will be supplying the flux.

I don't know if that paper is still on the AVX web site but the data was
generated when IPC-SM-782 was first being formulated and that started in 1984.

Nice photo.



John Maxwell


At 05:33 PM 2/24/2004 -0500, you wrote:
>Hi All!
>
>Got a little issue here that I need to dig in deeper to, but I thought I'd
>post a picture on my web page and ask for your valued opinions. Was building a
>board today that we've built before, never had any problems with it. But today
>certain 0805 resistors started skewing  on their pads during reflow, and I
>don't know why. Take a look at:
>http://www.stevezeva.homestead.com and look at "skewed components". It's not
>really a defect because they're 100% on the pads, and the wetting and fillets
>are fine too. They just look funky...and this just started happening. Not so
>much as a pimple on the boards before when we built them.

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