This may be the AVX paper that John Maxwell spoke of:
www.avxcorp.com/docs/techinfo/soldpad.pdf



Dave Fish



----- Original Message -----
From: "John Maxwell" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, February 24, 2004 5:54 PM
Subject: Re: [TN] Skewed Components


> 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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