PCB design for noobs: can I bend an electrolytic capacitor's legs?

Hello.

I'm designing a custom PCB for a project. The project will use a common 16x2 LCD screen, which will be removable and attached to a single-row of female pin headers soldered to the PCB, similar to an Arduino. This will leave some space under the LCD, which I'm populating with the other components I need. One (actually two) of these components are decoupling capacitors (a 10uF electrolytic and a 0.1uF ceramic) for a RF module located under the LCD screen.

Now, here's the problem: while the ceramic capacitor is short enough to fit under the LCD screen I'm afraid the electrolytic capacitor isn't. Since I'm hoping to keep them as close to the RF module as possible, as it is needed for effective decoupling on the RF module power supply line, can I (very carefully) bend the electrolytic capacitor's legs 90 degrees and solder it parallel to the PCB? My other option would be to put this capacitor next to the LCD screen, which would be around 1cm (0.4 in) away from where it is now. I'm not an expert on decoupling but that seems like a lot and wouldn't it defeat the purpose of using it in the first place?

Yes, you can 'lay it down'.

missdrew:
Yes, you can 'lay it down'.

I see, thank you! :smiley:

Electrolytics have too much inductance to do decoupling at RF at all - the 100nF ceramic is doing all the high
frequency work, and that cap does need to be close to the chip(s). For UHF/microwave work you might
see 100pF/1nF/10nF in cascade for decoupling, with the 100pF being closest to the chip's pin (less than
1 mm away is typical). At microwave frequencies a MLCC ceramic cap in the 30pF--300pF range can be
pretty close to an ideal hard short. An electrolytic capacitor is seen as an inductor at these frequencies,
mainly due to the leads and the size of the thing.

Google "self resonant frequency capacitor" to find out more about why decoupling caps are cascaded
in this manner. You'll also find calculators for inductance per cm of pcb trace too, which is why the
cap needs to be close to the chip.

Note that at UHF and beyond the caps need to be SMT to work well, even 2mm of lead length for
a through-hole ceramic cap would compromize its performance.

I would expect an RF module to always have its high-frequency decoupling built in as SMT components
anyway. The distance from the module to the host PCB is just too long for effective decoupling to
work on the host pcb.

However if the RF module is using some on-off keying it will draw current in pulses that are slow (and large) enough to be decoupled by an aluminium cap.

yes you can, but I ask "why"? could'nt you use something like this: https://www.digikey.com/en/products/detail/nichicon/UZR1H100MCL1GB/2551032

or use a tantalum, like this? https://www.digikey.com/en/products/detail/rohm-semiconductor/TCSM1C106M8R/12349998

This topic was automatically closed 120 days after the last reply. New replies are no longer allowed.