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Edit: after a lot of trial and error I discovered that JB Weld works amazingly well for this. You want the original two-part epoxy that comes in two completely separate tubes (not the newer stuff that comes in a single double-plunger tube) since it has a ~4 hour set time. If you mix the two parts and then load it into a syringe and hook it up to an air compressor driven solderpaste dispenser (called a "glue dropper") you can plug holes really really fast and the results are great. Dries rock-hard. Keep in mind that JB Weld is ferrous (dried epoxy will stick to magnets, but that doesn't matter for my application. Also JB Weld is electrically conductive, so if you're plugging more than one hold you can't use this for hot-pluggable devices since an incorrect insertion would make electrical contact on the pair of plugged holes, by way of the JB Weld. Otherwise it's ideal. You'll have to throw away the syringe and tip but they only cost a dollar or so.

Everybody's familiar with the ubiquitous 2.54mm box header (image below).

One common way of preventing reversed or misaligned insertion is to get the male header with a shroud and the female header with a corresponding key (second image below). However this isn't always mechanically practical. It also requires buying the headers in exactly the size you need instead of cutting them down from larger sizes.

An alternative way to prevent flipped/offset insertion is to omit one of the male pins (they can be pulled out of the header with pliers pretty easily prior to soldering; only pressure holds them in place) and plugging the corresponding hole of the female header. However the plugging has to be done with a glue or epoxy that dries VERY hard, otherwise you can mis-insert the connector by pushing a bit too hard and deforming the glue. I've been looking at electronics epoxies and there seem to be a dizzying array of them!

Question:

Can anybody recommend an epoxy that dries very hard, hard enough to prevent a pin being driven through it? Preferably a one-part, air-dry (not bake-dry), pneumatic-needle-dispensable epoxy if possible.

A pneumatic-needle dispenser (like the kind used for solder paste) would make it really easy to key a whole bunch of headers at a time; just stick the needle in the correct female header position, press the footswitch, move on. Unfortunately I've found a lot of these headers use plastic that can't withstand reflow ovens, so an epoxy that doesn't require oven drying would be preferred (although even the heat-dried epoxies use much lower temperatures than reflow).

Thanks

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  • \$\begingroup\$ Sounds like a job for superman. I mean glue. Seriously. \$\endgroup\$ – Passerby Oct 11 '16 at 4:41
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    \$\begingroup\$ Also, for production, you normally have a single size header for pick and place, not cut to size ones. So that's not really a problem. \$\endgroup\$ – Passerby Oct 11 '16 at 4:42
  • \$\begingroup\$ Another method would be to use the pulled-out pin from the male connector for the plug then cut flush and glue it in. \$\endgroup\$ – ratchet freak Oct 11 '16 at 11:26
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    \$\begingroup\$ Note that the female header does not fit the box header properly; since the box header is designed for IDC connectors they are much wider inside than the female header. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 11 '16 at 12:48
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    \$\begingroup\$ Superglue is too runny, it will leak out of the bottom of the connector where the pins enter the plastic housing. You'll end up with the hole being coated instead of plugged and the connector will be glued to the PCB. No bueno! \$\endgroup\$ – user4718 Oct 12 '16 at 0:21
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Industrial solution to the keying problem is not an epoxy, but a special product called "Male Keying Plug" made of thermoplastic.

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    \$\begingroup\$ Thanks, but the pins seems too expensive, 16c at DigiKey in qty.1 What about a pack of toothpicks, plug in, and break out? \$\endgroup\$ – Ale..chenski Oct 12 '16 at 1:01
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    \$\begingroup\$ @Ali_Chen that's actually a pretty clever idea for prototypes! I don't think I'd want to ship a toothpick-laden device to my customers, though... :) \$\endgroup\$ – user4718 Nov 2 '16 at 20:07
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Take a pin and cut away the plastic matrix, leaving all metal. Dip the tip into epoxy (just a little bit - you don't want excess getting into the used sockets). Now insert the pin into the header as far as it will go, and snip off the excess. The little bit of epoxy will bond the pin to the socket contacts. Alternatively, tin the pin, insert into the header, then apply a soldering iron to solder the pin to the contacts. Neither the epoxy nor the solder treatment requires the production of a very strong bond - just enough to keep the pin from falling out, and the retention force of the contacts will do a pretty good job of than even without help.

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