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I'm working on a PCB that has through-hole components on both sides of the board. The "top" side of the board is mounted flush to a Delrin plastic block (the only top-side component is a gas sensor that is fed air samples through hose fittings in the plastic block).

The flush mounting means that I have to add grooves to the plastic block to accommodate the soldered pins of the bottom-side components. Assuming a standard 0.062" thickness FR4 board, how deep do I need to make the grooves in the plastic block? The only thing I could find is this NASA workmanship standard that states 0.5mm to 2.29mm, but I'm not sure if that will always hold true.

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    \$\begingroup\$ Depends on how flush you're cutting them. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Feb 14, 2015 at 20:54
  • \$\begingroup\$ How many and what kind of thru-hole components do you have? Modern designs tend to use very few, mostly connectors, and the pin heights are usually fairly well specified by the manufacturers. \$\endgroup\$
    – Dave Tweed
    Commented Feb 14, 2015 at 21:07
  • \$\begingroup\$ I have a SIP power supply module and a connector that are a known protrusion, but the rest is a few resistors and ceramic capacitors. I'm not familiar with how flush those are typically cut after bending/insertion. \$\endgroup\$
    – Joe Baker
    Commented Feb 14, 2015 at 21:11
  • \$\begingroup\$ How tall is the gas sensor? \$\endgroup\$
    – Warren Young
    Commented Feb 14, 2015 at 21:28
  • \$\begingroup\$ The sensor itself is much taller at 17mm -- the primary purpose of the plastic block is a gas sample chamber for the sensor, so that's already opened up. It mounts into the block with an o-ring on a lip around the edge of the sensor for this purpose. \$\endgroup\$
    – Joe Baker
    Commented Feb 14, 2015 at 21:31

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Do you even need grooves? We make several products using through-hole components that are intended to mount using VHB double-sided foam tape. The boards are 0.062" thick double-sided with PTH and we use a table-top vertical belt sander to bring the component leads almost flush with the solder mask. In other words, the solder mask isn't touched by the sand paper but the leads are all sanded flat and sitting just proud of the solder mask.

This works well for small boards.

For what it's worth, there are commercial machines available that use a rotary saw blade to do the same thing. The board is held horizontal in a mounting / clamping system on the base and the saw motor is vertical on a sliding X-Y mechanism. The saw blade simply cuts all of the leads almost flush with the board surface.

This system is suited for boards of all sizes but especially for those boards larger than can be handled easily to be sanded with the belt sander.

Also note that these techniques are suitable only for PC boards with plated-through holes.

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  • \$\begingroup\$ And single-sided, of course. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Commented Feb 14, 2015 at 21:11
  • \$\begingroup\$ @IgnacioVazquez-Abrams: Not necessarily. Only one side of the board needs planing in the OP's application. You can load the bottom-side components, wave-solder them, plane the top side of the board, then proceed with the remaining assembly steps. All this increases cost, of course, but it may be necessary. \$\endgroup\$
    – Warren Young
    Commented Feb 14, 2015 at 21:24
  • \$\begingroup\$ Given that I'm only looking at a handful of simple components (guessing 17 pins total on that side of the board), what do costs look like for planing the board as opposed to the extra machining on the mounting block? \$\endgroup\$
    – Joe Baker
    Commented Feb 14, 2015 at 21:55
  • \$\begingroup\$ @JoeBaker: Are we to take it that this is a low-volume production, since you speak of machining plastic? If this were molded plastic, the extra machining steps in making the mold would pretty much disappear at volume. \$\endgroup\$
    – Warren Young
    Commented Feb 14, 2015 at 21:56
  • \$\begingroup\$ It's machined Delrin and yes, very low volume. I just added the material to the question. \$\endgroup\$
    – Joe Baker
    Commented Feb 14, 2015 at 22:00

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