when to and not to place components on bottom layer

[md4pcb]

hi,

I am new to PCB design and have followed Robert's courses on PCB design with Altium

In the course "Starting with Altium Designer", I see that Robert places components on the bottom layer. This is surprising to me as I don't see this on some of the PCB that I work with in my company.

I guess that those PCB boards don't need to, but I wonder if the board would be better or worse when they were designed with components on the bottom layer.

so I have some following-up questions to learn how to do this in the future.

is it common to place components on the bottom layer?

when we should and should not place components on the bottom layer?

when placing components on the bottom layer, what I should be aware of and should not do?

Do you have any references or material to learn more about this?

thanks,
md

[robertferanec]

Mostly the reason do not have components on the bottom is assembly cost - if you place all components on one side only, you only pay to assemble one side of the PCB. So if you need a very cheap board then consider all components on one side only (this only applies to cheap PCBs - for expensive PCBs is better to keep PCBs smaller, see below).

If you have more complex pcb or you need to be careful about board size, you will often have components on both sides for several reasons. Here are two:
- better for decoupling (unless we go to extremes, then watch this video: https://youtu.be/52fxuRGifLU )
- smaller PCB (often the area taken by components is large and if you place them all on one side, you need larger PCB)

A note: when placing all components on one side, you need to know what you are doing. That placement may not be optimal, especially for connections what should be kept as short as possible.

[md4pcb]

hi, Robert

thanks for your notes

From your comments, placing the components on both sides should be the first choice unless the cost is the main constraint ?

Regarding the decoupling, did you mean the decoupling capacitors? When placing them on both sides, they can be placed close to the source for the best effect? Do I get this correctly?

thanks
md

[qdrives]

The first choice is single side components.
One other downfall of components on both side is that the board need to go through the reflow oven twice, unless... you are using wave soldering too and the components on the bottom can go through the wave soldering process.
With components on only one side, wave soldering is not an issue. With components on both side it depends on the components (TH or SMD) and layout (wave soldering, selective wave, manual soldering or only reflow [Pin-in-Paste])
Components may need to glued to the board or use different melting point solder between top and bottom.

[robertferanec]

"From your comments, placing the components on both sides should be the first choice unless the cost is the main constraint ?"
- as qdrives explained, the first choice would be one side - simple, cheap, fast.

"Regarding the decoupling, did you mean the decoupling capacitors? When placing them on both sides, they can be placed close to the source for the best effect? Do I get this correctly?"
- yes