A Pogo Pin Based Electronic Test Fixture

Production testing of electronic equipment can take a lot time if you need to make a lot of connections to the circuit board.  This can be even more time consuming if the connection points don’t have connectors, but get wires soldered into the circuit board in the final assembly. 

I found myself needing to come up with a test fixture for a product I produce in moderate quantities in my electronics and ham radio side business, Unified Microsystems (www.unifiedmicro.com).  

The board has 5 input signals, 10 outputs, and 3 power connections.  Making connections to each unit and running the tests by moving a scope probe between the outputs was taking far too much time. I needed something better.

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, 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. 

This assembly is mounted to a piece of plywood along with additional test circuitry. Long screws provide alignment for the circuit board being tested. Hex nuts are adjusted so the board can be pushed down about ¼” once they reach the tips of the pogo pins.  This ensures good electrical contact but protects the pogo pins from being damaged by too much force.

Normally a test fixture would have a clamp or some other method of holding the UUT in place. It turns out that I can easily run the test with one hand, so I simply hold it in place with my left hand.  A clamp could have been added with a little more assembly complexity.  

The circuit board being tested is a device that takes four binary inputs and decodes them into 10 relay driving outputs.  It is used with some ham radio transceivers to drive a remote antenna switch to automatically select the proper antenna.  Some commercial customers use it when they need optically isolate binary signals and select one of 10 relays or other electronic devices. 

The UUT’s outputs drive 12V outputs, but the inputs require 5V TTL input levels. A simple 7805 regulator provides the 5V.  Four switches provide the inputs.  An LED array is powered from 12V to simulate the relays and provide visual indication the correct output is active.

Close up showing the mounting of the pogo pins.

It takes about 15 seconds to put the UUT on the fixture, and run through the binary combinations while watching the LEDs. I could have put a small micro on board to automatically generate inputs and monitor the outputs, but I didn’t think the extra work would justify the savings. A different UUT might require additional steps like adjusting trimmers.

An electronic test fixture using pogo pins can be build quickly and inexpensively by using bare circuit boards as the base. The breakeven for the time spent building it compared to the time saved in testing was probably around 100 boards. It has paid for itself many times over.

The test fixture in use.

© 2009 - 2016 Gary C. Sutcliffe



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