Steve,

I wondered how long it would be before you replied, you always seem to be
around to offer your 2 cents.

Anyway, my customer is only noticing the problem on a couple boards. The
parts in question are 20 mil QFPs. However, there are other 20 mil parts on
the boards that do not have the problem. There are also other boards with
the same QFP that are not having a problem. Solder paste being used is Alpha
LR735. The customer is currently looking into the screenprinter and
stencils. The old IR oven was really cooking the heck out of their boards.
They have a big shiny bga on their boards that reflects the IR light making
it nearly impossible to solder. In order to compensate the smaller, darker
components were reaching 183 immediately and staying above for as much as 3
minutes just to get the BGA above 183 for a minimal period of time. I think
this "cook the heck out of em" recipe created a need to print gobs of paste.
Now that the convection oven is in place they do not have to sacrifice their
profiles.

If you disagree perhaps you will comment further.

Brian


>Hello Brian!
>
>     I guess I'd give a shot at your questions seeing how there's no math
>involved (BOY! Am I ever embarrassed!) But the first thing I'd ask is if their
>solderpaste deposition is what it needs to be based on the pad sizes that are
>used for the devices. If I may ask, how fine is this pitch? Is there
>soldermask dams between the pads?
>
>     As far as the parts shifting from "power loading", I've not been so lucky
>to experience that, except for a time or two when I've had to hand load some
>fairly populated proto's and the paste has dried quite a bit...you can deal
>with the touch-up on just a few boards when that happens. It would seem to me
>that as long as the paste isn't dry, and the part is placed accurately, and
>deep enough into the paste (not just set on top of it) it really shouldn't
>move even if there's a slight jolt here and there going down the conveyers...I
>might hedge a little in my answer if the part is ceramic, or if it's a PLCC,
>but QFP's usually stay where they're put...
>
>Going from I/R to convection really should make things easier. It should be
>the other way around, that is; he was experiencing bridges before with I/R,
>but go away with convection...at least that's what I remember when we switched
>ovens at one company I worked at. I've only had a chance to go through that
>once, every place else I've worked used convection ovens exclusively.
>
>I'd almost bet that it has something to do with the solderpaste deposition
>and/or footprint of the part..
>
>-Steve "I'm no math whiz" Gregory-
>
>

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