In a message dated 07/25/98 9:09:04 AM Pacific Daylight Time,
[log in to unmask] writes:

> Good Morning!
>  I hope all of you had a good weekend and are back for an exciting week.
>  I had a strange problem of solder short a few days ago. The shorts were
>  between 2 pins of an SOIC towards the plastic body, and not at the board
>  level. Can the soldering experts of TechNet help me understand this
>  strange phenomenon ?
>
>  Regards
>  Krishna
Hi Krishna,

    I hope you had a good weekend too. Well, I'm no "expert", but I get
by...hehehe. Sounds to me like you've got what's called "wicking" going on.
You see this a lot with vapor phase reflow more than you do with traditional
means of reflow (I/R, Convection). The fact that you see it more with vapor
phase gives you a little indication of what could be causing it in your
case...and I'm assuming that you're NOT using vapor phase.

But what causes this to happen in vapor phase is that heating rates are pretty
darn quick in a vapor phase oven, and it can be kinda' tricky sometimes to
make sure the pads on the PCB and the leads of the device are at the proper
temperature, at the proper moment for the solder to wet to them the way we
want...ideally, you want the leads and the pads to be at the same temperature
at the point the solder goes liquidous. When the leads on the devices are at
liquidous before the pads when the solder melts, the solder wets first to the
leads and tends to flow up the leads of the device.

So, with that being said, sounds to me like you may have a reflow profile
that's not giving the board and parts a chance to preheat and come up to
temperature smoothly together which could be causing your problem. Could be
you're running too fast a belt speed, or the set point temperatures in the
first zones of your oven are too high.

Has there been a temperature plot done on the reflow profile? That would be
the first place I'd start. What kind of oven are you using and what are the
reflow profile settings? Give us a little description of your board, the
solder paste your using, and I'm sure many of us can give you settings for a
starting profile that should be pretty close that you can easily tweek in.

I hope this helps,

-Steve Gregory-

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