I’m designing a bed of nails test jig. I want my test probes to fit snugly in the circuit board, but want to allow for variation in the through hole plating of the hole.

In Altium, when you specify a hole size, is that the drill hole diameter the hole size after or before through hole plating? i.e. does the PCB manufacturer make an allowance for the plating and actually drill a slightly larger hole?

I am going to use the RC50-SC and RC100-SC receptacles to hold my test probes. What would be an appropriate hole size to specify in Altium in order to get a snug fit?



Cheers Phil

  • \$\begingroup\$ This is a question you should be asking your PCB fab. \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 28 '15 at 2:01
  • \$\begingroup\$ I always include a "read.me" text file with my job files. The file lists, among other things, the drill tools and number of holes for each tool, and I add the words "Finished hole size" to the table heading. Alberta Printed Circuits (and probably some other board shops) states that their standard hole sizes are Finished (after plating) size, but it would pay to check with the board shop if the sizes are critical. \$\endgroup\$ – Peter Bennett Aug 28 '15 at 2:48
  • \$\begingroup\$ I talked to our PCB house and they were no help. Good tip on including a read me file. \$\endgroup\$ – philby Aug 31 '15 at 2:55

You would be best advised to not specify plated through holes for your pogo pin receptacles. Carefully toleranced and drilled holes of the manufacturer specified size are required for mounting this type of receptacles. There are a number of solid reasons behind this:

  1. Pogo pins and their receptacles need to be located with very good precision so that the probe tips line up perfectly with the device being tested. The sloppy tolerance of plated holes will not suffice for maintaining close pin placement.

  2. The pogo pins need to be mounted almost as close to perfectly vertical as possible to maintain a minimum of spring pin binding as the device under test is clamped down over the pogo pin fixture. Normal circuit board thicknesses are not adequate to holding the pins in the optimum positions. Properly built and reliable fixtures often use a substrate material that is 0.375 to 0.500 inches thick.

  3. Pogo pin receptacles are not normally soldered into their mounting holes. They are meant to be press fitted into their precision hole and pushed down to a depth of a collar around near the upper edge. This allows for the receptacle to be pushed out and replaced should repair become necessary.

  4. The types of receptacles that you linked to are meant to have descrete wires attached to the solder cup end of the component. Other types will come with a 25 mil square post welded or cripped to the bottom end of the unit. These are often used with a wire wrap technique to attach the test probe wires.

  5. Lastly the typical receptacle is designed with special features formed into its sides to permit the inserted pogo pin to get firm contact in the socket area while also not being a total interference fit. This permits pins to be removed and replaced should they get damaged during use. Trying to solder the barrel of a receptacle of this type into a plated hole can render the side wall features to become non flexible. Two things can happen - it can be hard to insert new pins - and the rigidity of the receptacle can lead to premature pin and/or socket failure.

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  • \$\begingroup\$ A couple of alternative approaches (that I use) for points 2 and 3 (which others may find useful). I tend to use two-part spring probes, one part solderable receptacle, and one part replaceable spring probe. I then construct the jig from standard 1.6mm PCB, but have two PCBs held apart with 10mm spacers to hold the receptacles in place. The receptacles are then soldered to both PCBs (top and bottom) before the replaceable part is fitted. This allows for easy maintenance, and high precision location without the need for a thicker (and generally more expensive) substrate. \$\endgroup\$ – stefandz Aug 28 '15 at 10:49
  • \$\begingroup\$ I talked to our PCB house and they were no help. I'm looking at using a 2.4mm thick PCB to hold the probes. It is only a small board that I'm testing, so I shouldn't get too much flex in a 2.4mm thick PCB. Thanks @Michael for the detailed answer. \$\endgroup\$ – philby Aug 31 '15 at 2:54

Normally the hole size you specify in your EDA program will be the finished hole size within some tolerance.

However, there are a (very) few manufacturers who interpret this as the tool size, so the hole may be somewhat different in size (-0.1mm in the case of one supplier) so it's best to check with your manufacturer.

For round holes you definitely don't want the minimum hole size to intersect with the maximum pin diameter.

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