I'm working on a PCB design for an FPGA chip where I want to do power analysis and voltage fault injection. Because of that I want to minimize the decoupling capacitance of this chip, but of course it should still work. I don't know the exact amount of capacitance required and would like to easily experiment with different values and capacitors.
What would be a good way to quickly connect/disconnect (different) decoupling capacitors from a chip? I'd like a quick and easy solution without soldering again and again. I've thought about jumpers or SMD dip switches but was afraid that the inductance would be too high or the current capability too low for the capacitor to still be effective. Sockets for SMD capacitors would probably be perfect, but I've never seen something like that.
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1\$\begingroup\$ Your best bet really is just soldering them in and out. It's trivial to do, and won't cause you problems with inductance the way jumpers will. \$\endgroup\$– JREFeb 27, 2020 at 12:13
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3\$\begingroup\$ acquire hot tweezers, they are a handy tool for this sort if thing. \$\endgroup\$– vicatcuFeb 27, 2020 at 12:16
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1\$\begingroup\$ You can easily remove and replace SMD capacitors with a standard soldering iron. A little blob of solder along the length of the tip (longer than the length of the capacitor.) Touch the tip to the capacitor from the side so that you heat both joints at once. Melt the solder on both joints. Push sideways gently. The capacitor will come off and stick to the tip. Pluck it off with a pair of plain tweezers, or flick it into the nearest trashcan. \$\endgroup\$– JREFeb 27, 2020 at 12:22
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1\$\begingroup\$ I'm working on a blog post with pictures and descriptions of how to do it. I'll post a link to it when I'm done. \$\endgroup\$– JREFeb 27, 2020 at 12:23
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1\$\begingroup\$ Solder-bridge jumpers. \$\endgroup\$– rdtscFeb 27, 2020 at 13:32
2 Answers
The quality of decoupling depends not only on the number of capacitors, but also on their location. In your analysis the location and quality of connection will be of importance. Sockets may cause contact problems. Classical SMD jumpers increase distance.
However you can make custom capacitor device in your design software with 3 pads: one pad you solder one pin of cap to, another pad you solder another cap's pin, and third pad is very near the second pad so that you can easily remove the short with the "solder sucker" or any other removal tool. This will not be component soldering as is, you will just need to heat one of the cap's terminals and remove the solder using vacuum pump.
PCB Techniques I know for disconnecting paths:
- Jumpers (as you noted)
- 0R-SMD-components (need soldering)
- Using a scalpel for cutting paths (that doesnt have any advantage, it's just if you haven't anticipated that)
Sockets would be a nice way...
Maybe you should model a jumper/switch and see whether that inductance is relevant?
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3\$\begingroup\$ Inductance is usually a large consideration in decoupling capacitors. That's why they are placed very close to the consumer, and why there are discussions as to which capacitor to place closest to the consumer when using multiple decoupling capacitors of different values. It's also the reason for discussions about using vias (and where to place them) when the decoupling capacitor connects to ground on a different layer than the capacitor. \$\endgroup\$– JREFeb 27, 2020 at 12:18
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3\$\begingroup\$ To add to what JRE said, at the speeds of today's FPGAs, a decoupling cap with a jumper (even a short one) or a switch is almost useless. \$\endgroup\$– SteveShFeb 27, 2020 at 12:26