How to Build a Footprint

Well, not really how to build one in a technical sense, but some thoughts on how to better ensure that you get it right. In theory, it shouldn’t be that difficult. You download the datasheet and build the land pattern based on the information in the datasheet. That usually works, but not always.

I had a through-hole battery holder that didn’t match up with any of the land patterns in my library, so I modified one that was close. That worked mostly okay, but there was one measurement in the data sheet that was a little ambiguous. I ended up with the mounting holes being off by a millimeter or so. Not too much, but enough to make the fit difficult.

I went in and shifted the leads over by the same amount, used it again, got another PCB fabbed and discovered that I had shifted the pins the wrong way! Then it hit me. In the first application, I had the battery holder on the bottom side of the PCB but I had looked at it through the mounting holes from the top side of the PCB. D’oh! One reason why I’m not a professional designer.

The other part was a little tiny SMT trim pot. Since there are pretty close to a million different little trim pots, the likelihood of me finding an exact match in my CAD library was precisely zero. I didn’t want to Gieger VR mistake close re-invent the little zig zag resistor symbol, so I just found a part that looked the same. Well, it was almost the same. The footprint I found is for a 4 x 4mm part and the part I ordered is 3 x 3mm. That’s a tiny trim pot. Somehow, when looking at the datasheet, I got the measurements wrong. Once the part came in the mail, it was quite obviously too small.

The pad pretty much ends right at the edge of the trim pot. We won’t be able to reflow that part. No solder paste would be touching the pads on the trim pot. I’ll see if our guys on the floor can figure out how to get the thing soldered on there. If they can’t, I’ll need to look for a larger part to put in it’s place.

Fortunately, I physically looked at the part and the PCB before assembly. Unfortunately, I got the measurements wrong. If at all possible, get some sample parts before you order your PCBs. Then you can print out a 1:1 image of your PCB and lay the parts out on it. That would have saved me in both of the above cases.

Duane Benson
Is it “datasheets” or “data sheets”?

http://blog.screamingcircuits.com/

About Duane

Duane is the Web Marketing Manager for Screaming Circuits, an EMS company based in Canby, Oregon. He blogs regularly on matters ranging from circuit board design and assembly to general industry observations.
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One Response to How to Build a Footprint

  1. Data sheets are the bane of most PCB layout guys.
    I had a classic error of one for a connector where it had been drawn at 2:1 then obviously auto dimensioned which produced the data sheet illustration twice the size of the real thing.
    Here’s one you can try for yourself of a Sharp LCD module :-
    http://pdf1.alldatasheet.com/datasheet-pdf/view/43093/SHARP/LM162KS1.html
    How many numbers do you have to add together to get the distance of pin 1 from the short edge?

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