I'm putting together a footprint for a 100-pin, 14x14mm TQFP, and I'm finding conflicting designs. The pitch and width of the pads are all basically the same, but the length and centering of the pads horizontally varies a good deal.
The following images are from the Microchip packaging specifications document (see page 282-283), so that we can have names to use for the dimensions.
There's a table with numbers for each dimension, but the exact details aren't really important here.
Where should the pin go on the pad lengthwise?
- Should the pin be centered on the pad? (C1 = D - L) If so, what should Y1 be? L, L+tolerance, 2L?
- Should the inner edge of the pad line up with the inner edge of the pin? (C1 - Y1 = D - 2L) If so, how far should the pad stick out in front of the pin?
- Should the pad and pin have some other dimension?
Note that the question is basically moot if Y1=L. I'm assuming that I'll want a little extra pad to hit with the soldering iron.
It might be relevant that L1 is allowed to vary by ±25%, which feels like a bigger variation if you read 'between 0.45 and 0.75mm'. It might not be relevant.
I'm interested in solderability, avoiding invisible solder bridges under the chip, routing traces underneath (and outside of) the chip. Of course, I don't want to use absurdly long pads for heat dissipation and board space reasons.