Pad dimensions and land patterns for QFPs

Question

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.
Physical Package:

Recommended Footprint:

_{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?

1. Should the pin be centered on the pad?
(C1 = D - L) If so, what should Y1 be? L, L+tolerance, 2L?
2. 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?
3. 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.

Answer 1

If you're interested in solderability and manufacturability, then you should follow the recommended pad layout. I've never had a manufacturer's recommended pad layout give me grief, although I have run across a few vendors whose dimensioning requires a fair bit of headscratching and pencil-and-paper chicken-scratching in order to figure out offsets and spacing.

You are making some bad assumptions in your calculations though. No, pins are not often centered on the pads "lengthwise" but they are often centered widthwise. Y1 and C1 would most certainly be given by the manufacturer. The recommended land pattern will (in my experience) give more space for the pin on the "outside" of the pad and less underneath it. My guess is that that gives a good shape to the solder connection. You won't have anything in terms of heat dissipation unless there are a lot of grounds or you have a ground pad underneath. In the case of a lot of ground pins, you'll want to give them a lot of copper fairly quickly, but you'll want to connect the pad to the copper with thermals or you'll have soldering problems.

I wouldn't worry about minimizing board space, especially if you're not building a million of these. The half a millimeter you might save by shaving Y1 a little isn't worth it.

Answer 2

Funnily enough, I recently followed a manufacturer's recommended land pattern for a TQFP, and got bitten.

Microchip have an excellent repository of IC drawings and associated land patterns here: https://www.microchip.com/en-us/support/package-drawings

Amongst them is the "32-Lead Plastic Thin Quad Flatpack (3BB)" a 7x7x1.0mm TQFP with 0.8mm pitch. The land pattern recommends 8.50mm between pad centres, with each pad 0.55x1.60mm. This makes them 6.90mm apart, and 10.10mm from edge to edge.

This happens to be exactly the pattern in the KiCad standard library:

The edge to edge distance of the pins themselves is 9.00mm, and the length of the pin contact area is about 0.60mm. So regarding your questions about centring, this works out to be pretty much exactly centred.

Which is it came unstuck for me. After reflow I had a few bridges to tidy up but even after removing what I could see I still had a short. Removing the part revealed this:

Notice how the 3 or 4 pins at the top left have ridges towards the inside of the pad? There was so much solder building up behind the pin that I was getting an invisible short.

I fixed this instance by removing a substantial portion of the solder and reflowing. But in future I will be shortening the pads so they mostly extend to the outside of the pin, not the inside. This still provides plenty of solder contact area, but exposes it where I can see and touch it.

I think you still want a bit of pad on the inside to allow the part to self-centre, but I'd much prefer having to be more accurate with my placement than have to remove excess solder from under the part.

Here's what it looks like successfully soldered (note it's the same part, different location). Notice how much pad is visible on the outside of the pins, and therefore how much pad is hiding underneath the part.