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I am trying to align a PCBA very accurately (within +/- 0.5mm positional tolerance) on a plastic enclosure. I'm thinking about using two 2mm diameter dowel pin that is press-fitted into the plastic enclosure and then having the two clearance holes on the PCBA align with the press-fitted dowels on the enclosure. Is this a good plan? Any other suggestions of ways to make this work?

Thanks!

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  • \$\begingroup\$ This really depends on what type of assembly process you're using... \$\endgroup\$ – Ron Beyer Feb 5 at 16:54
  • \$\begingroup\$ This is all manually assembled if that's what you're asking. Enclosure is either machined or injection molded \$\endgroup\$ – Philip C Feb 5 at 17:08
  • \$\begingroup\$ I was just thinking about the thermal expansion, but I expect it's immaterial. What thermal range are you anticipating for the end result, and how thick is the PCB? \$\endgroup\$ – jonathanjo Feb 5 at 17:14
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    \$\begingroup\$ Aligning to what, exactly? The size of the injection molding can change somewhat depending on process conditions, for example. If it's something like an LCD display aligned to a PCB, molded features such as pins close to the display and in the same plane should be adequate. \$\endgroup\$ – Spehro Pefhany Feb 5 at 17:17
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    \$\begingroup\$ One option is to use flathead screws to mount the PCB. The angled head will center the screw where it passes through the PCB, eliminating the need for a tightly controlled diameter. 0.5mm is actually a pretty reasonable tolerance imo. \$\endgroup\$ – Drew Feb 5 at 17:18
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If the plastic enclosure is one of the typical injection molded ones available in the market they have wall thicknesses that are in the 0.1" (2.54mm) range. This thickness is not enough to firmly support dowel pins.

A better strategy is to look for an injection molded case/box that has molded in mounting points (a.k.a mounting boss). The best ones will have pressed in brass thread inserts. These mount points can be used to accurately locate the PC board if the holes in the PC board are designed to match the box and are sized correctly.

The mounting points can also be used with additional standoffs if the board needs to be elevated.

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A dowel pin will be more than accurate enough for this. Just make sure that your holes are toleranced right on both the PCB and the enclosure, they should be press fit in the enclosure and +0.02-0.1mm hole size on the PCB. When you were talking about very accurate assembly I worried about the plastic enclosure, which would likely not be stiff enough for repeatability of 0.001" (0.025mm). But 0. 5mm is big enough that even screw standoffs might be good enough using close fit hole sizes.

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