In a message dated 02/17/2000 3:00:22 PM Central Standard Time,
[log in to unmask] writes:

<< Hey Everybody-

     We are using 96.5/3.5 tin/silver solder paste on our top side and I have
 been noticing that on some QFPs the paste forms a heel fillet, a negligible
 toe fillet, and does not wick over the top of the lead "foot".  I am
 checking to see of the components have a palladium finish (I am pretty sure
 they do).  If it turns out they do, is there an issue with tin/silver paste
 and palladium coated leads?  We are peaking at 230-235 C (ten zone
 convection Conceptronics) so the boards are getting plenty hot.  Why does
 the paste seem to bulge up at the toe fillet, but not wick onto the lead
 surface?

 -Ryan Jennens
 TelGen Corporation >>

Hi Ryan!

You said some QFP's...I'm gonna assume that there are others that are okay....
Is the toe of the leads on the ones that don't have toe fillets actually
plated? I ask that because a lot of the time there will be bare copper from
when the cut and form was done to the leads of the QFP.

I remember reading in a SMT Magazine article one time that what will
determine if you get a toe fillet or not, was whether the cut and form was
done "Burr-up" or "Burr-down". If I remember correctly, that refers to the
direction the cutting tool travels when cutting the leads. When the leads are
cut, some of the plating kinda' smears along the cut ends of the leads, and
allows the toes to solder.

Again if I remember correctly, leads that have been cut "Burr-up" will more
likely have a toe fillet than the "Burr-down" cut leads...

By the way, the only palladium plated parts I've seen are from Texas
Instruments...

-Steve Gregory-

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