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I'm working on an open source filter design where the PCB can be reused for many future designs. It is based around a Sallen-key filter since a few changes to the capacitors and resistors can change it from a low-pass to a band-pass or a high-pass. When looking at layout, I've realized it would be very beneficial to make the capacitor and resistor pads fit both larger (like tantalum caps) to smaller 0402 packages.

Is there anything I should be careful of while designing this 'universal pad'? Is it possible someone has built something like this already?

Anindo Ghosh
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OhmArchitect
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  • Parasitic capacitance is the first thing that comes to mind. Larger pads -> larger parasitic capacitance. What is the highest frequency you plan to use? will few additional pico Farads matter? What size of capacitors? 0805? 1210? – Lior Bilia Aug 17 '13 at 17:39

3 Answers3

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If somebody might want to assemble this board with automated equipment (pick and place), you don't want to do this. When you place a small part on pads that are too big for it, the excess solder will move the part around and cause defects, such as "tombstones".

If your circuits will only be assembled by hand-soldering, its certainly possible to make pads that will fit more than one size part. For example, a footprint that can fit 0402 or 0603, or a footprint that can fit 0603 or 0805. But trying to span the broad range from 0402 up to tantalum capacitors is going to make it more difficult to solder your board.

It would probably be better, unless you need to work with very high frequencies, to just design with maybe 0805 footprints. Hand-soldering you could probably put anything from 0603's to 1012's and A or B size on an 0805 footprint. And it's possible to get even very low valued capacitors in larger footprints, whereas if you choose a smaller footprint there will be some maximum C value available in that size.

The Photon
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  • Great answer, I have a large stock of 0402 and 0603 parts a buddy from work gave me. He's also going to lend me his soldering microscope and assures me that 0402 parts are possible to solder with it. I'm pretty sure the values will range from 10uf to 1nf(ish), so I will most likely have to go with tantalum caps for the higher values. – OhmArchitect Aug 17 '13 at 18:18
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Assumption 01: the board is intended for hand assembly, tabletop experiments and hacking. Assumption 02: the frequencies are low (say, under 1MHz)

In that case, I would make the passive pads 3216 (tantalum size A) or 3528 (tantalum size B). If you are planning to have pads for a large tantalum cap, then having small 0402 ceramic capacitors doesn't give an advantage, because the large pads for the tantalum cap are there anyways. Size A or B can comfortably accept 0805 and 1206 passives.

P.S.
3216 comes from metric 3.2 x 1.6mm.
1206 comes from imperial 0.12 x 0.06 inch
They are pretty much identical, if you convert from inches to mm.

Nick Alexeev
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I did this a few year ago for a customizable audio active crossover, what i did is set the variable values on small daughter board.

Djkrugger
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    How was the signal integrity? Luckily for this design I don't intend to deal with that much current! However, high current designs can be a lot of fun! – OhmArchitect Aug 17 '13 at 20:07