Hi there Kim!

    From what I've always been told by vendors, it's about .004" total...that
is .004" from the lowest lead to the highest lead. That's probably the biggest
thing I don't like about fine pitch QFP's. Because the problem is that no
matter what you do, even if you get fresh parts from the vendor, there may be
parts in the trays that meet spec but still will give you problems in the
process if they're at the high end of the tolerance.

    Thankfully, most of the time in my experience most everything I've had to
place has been pretty good. The times that I've experienced a lot of problems
have not been caused by the vendors, but by stockroom or materials personnel.
For some reason, it seems that they always have to open up the matrix trays
and count every single QFP, and there's a lot of those who do that not using
the care that they should when stacking the trays back together.

    If you're getting A LOT of rejects from your machine for coplanarity
problems, and you know that the QFP's haven't been handled much, there
definitely could be a problem with them...or, maybe you've got the tolerances
in the data on your machine set too tight. Can I ask what type of system that
you're using to place the parts? How is it checking coplanarity? Laser?
Vision? I know the option we have on our machines for checking coplanarity
uses a laser. We call it a laser-lead locator. If I set the tolerance too
tight, it'll reject every QFP it looks at...so I open things up to the point
that won't give me defects if I can...it all depends on how fine a pitch I'm
placing, and how much paste I'm laying down....

                                                                          C-ya
L8'ter..

                                                                        -Steve
Gregory-

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