Is it bad practice to place a bypass capacitor under its respective component?

Question

I am designing a double layer board and in my layout I am running into problems.

I want to keep the board as small as possible, but I should also be able to assemble it per hand. I made all my resistors and ceramic capacitors of 0805 of size.

My problem is that I am running out of space with these dimensions for possible routing on the top layer.

Since the bypass capacitors need to have a low impedance path to the component I was thinking of placing these bypass capacitors directly under an IC to save space. Wouldn't that potentially introduce a parasitic element and disrupt the working of the chip? I have no idea. Any share of expertise on the matter would be appreciated.

Answer 1

In many cases its the only practical way to design the board.

For example with a large BGA package the opposite side of the board is the closest place to put a capacitor to most of the possible power pin locations.

That said, 0805 is really quite large. 0402 should be hand solderable if you are in good physical shape (no hand tremors) and have good eyesight or a magnifying glass. 0603 should be hand solderable without magnification unless you have an uncorrectable vision problem.

Answer 2

(I hope you meant to place it vice versa to the placement surface of the IC.)

The voltage on the pins of the decoupling capacitor is the same as on the VCC/GND of your semiconductors. So the "risk" is not really there. Rather placing it close to the pins is fine - and having short but wider circuit wires towards it very fine! (Only when having very big copper surfaces then this will have bad impact on hand soldering processes as that's a thermal sink. Doing the pads in a mushroom shape can help prevent this.)

It also depends on what the vendor of your semiconductor has written in the data sheet. Sometimes they have "keep free" areas, e.g. close to highly sensitive ADC parts in the chip or in the area where oscillators are driven.

For simple logic I would say, its not a problem. In classical boards I have seen DIP sockets with capacitors built in, and also seen wired capacitors hand soldered directly on the backside solder points of the IC.