Electronic test fixtures typically use spring loaded test points, often called pogo pins. Pogo pins come in a number of sizes and test point head styles for probing different circuit geometries. They are gold plated to prevent corrosion which would keep them from making good electrical contact to the Unit Under Test (UUT).
Commercially produced test fixture bases are made of a stable base material. Numerically controlled machinery drill out holes for the pogo pins from the drill files used in making the bare circuit boards. Guide pins going through the circuit board’s mounting or tooling holes align the board on the test fixture. The UUT is typically held in place with clamps or a vacuum system.
This is just what I needed, but I could not justify the high cost to have one commercially made for me. I don’t have an CNC drill, and my machining skills are not good enough to pull this off by hand. Then it occurred to me that I had the templates I needed in the form of blank circuit boards.
Pogo pins come in two parts. The main part is the pin itself with a main cylinder containing a spring. The test pin itself is free to slide up and down in the cylinder but is prevented from coming out. The test fixture base holds a sleeve that the pogo pin slips into. The fixture base is often 1” thick to give the sleeve rigid mechanical support. The wires in the test fixture connect to the sleeve as well. The pogo pin slips into the sleeve to get its precise position and electrical connection. A pin that wears out or breaks can be easily replaced.
I had a bag of pogo pins I picked up at an electronic flea market. Unfortunately they didn’t have sleeves. You can also buy pogo pins from Mouser Electronics. I decided to use two circuit boards with spacers instead of a thick plastic base. The wires solder directly to the pogo pins. This would mean replacing a pin will require unsoldering a pin. The number of pins in my test fixture was relatively small, and the quantities of boards tested annually is not that high, several hundred per year, so I decided to risk it. After 5 years I have not had a single pin fail, so it was a good bet.
The holes for the circuit board pads I wanted to probe were too small for the pogo pins so I drilled them out. Using the current holes as pilots ensured I would have the accuracy I needed. I drilled out two sets of boards. I then cut the traces leading from these pads to ensure I would not have any unexpected short circuits.
The two bare circuit boards are separated about 1” by spacers and hex nuts. The pogo pins slip through the newly drilled holes and are supported well enough to ensure the probe tips hit the proper places. Fortunately the pads were large enough to still allow soldering the pogo pins in place.