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What techniques are there for placing the components on a PCB at a desired height? e.g. so that potentiometers and switches can be both panel mounted and board mounted?
For example, are there adapter components designed to lift other components away from the plane of the board?
Reading the answers here I'd guess that nobody has invented such a thing yet, and that instead PCB designers have just had to come up with ways around the problem such as wired connections, actuator extensions, etc.
are there adapter components designed to lift other components away from the plane of the board?
There are plenty of them. Here's one type for instance that uses plug-in headers: -
But, they needn't be plug-in headers. They can be soldered at both ends such as this: -
Or taller ones like this: -
You can get many varieties such as this range from Samtec (amongst others): -
You can buy spacers that fit under a component such as an LED (a tube or something similar that the LED leads go through). Spacers are also available for other components, see companies such as Bivar and a number of Asian companies.
There are versions of the above that are made with a material that is designed to withstand wave soldering but dissolve in the cleaning process. Bivar has some such wash-away spacers and there are other suppliers as well.
However, my preferred method is to use a machined fixture or jig to hold the component and PCB in alignment while soldering is taking place. Sometimes, for hand soldering especially, you can use a 3D printed fixture or jig. This method can be more accurate than the use of spacers.
For example, to hold 3mm through-hole LEDs in specific alignment a certain distance above the PCB, or an LED display so that it is spaced a bit off the PCB to be closer to a window.
If the distance is too great or the leads too short, you can just use a sub-PCB and connect the two with connectors or flat cable or make the whole PCB using a rigid-flex construction, snap off the part that has to be at a different level and mount it with hardware etc. Photo from here. Such rigid-flex PCBs tend to be quite pricey. You can also get into different planes with the flex, but keep in mind the flex is a developable surface so it the rigid parts cannot be oriented arbitrarily without folds in the flex (as a connection made with a wire harness could be).
Custom placement like this usually has to be accomplished by hand.
After placement, the THT components are soldered with a solder wave machine, so there are no means to keep the components at a certain distance to the board during the automated solder process.
