When designing a bed-of-nails test fixture, it is important to select the correct test probes and receptacles. Since they are the interface between the Device Under Test (DUT) and the Test Instrumentation, your test results depend on this connection to properly function.
The selection process can be overwhelming when looking at the number of probes and receptacles that are available, this post breaks down requirements that can be used to help guide the probe and receptacle selection process.
While there are several different manufacturers each with many different series of test probes and receptacles, they all typically use a Grid Spacing to help narrow the selection.
The Grid Spacing refers to the minimum distance (also known as the pitch) between the centers of 2 test points. Typical grid spacings are
- 50mil (1.27mm)
- 75mil (1.91mm)
- 100mil (2.54mm)
The other main feature of a test probe is the method used to connect the receptacle to the test instrumentation. Typical methods to connect to a test receptacle include
- Wire Wrap
- Solder cup
Wired receptacles have a short, small gauge (~30AWG) wire attached to them that can be used to connect the receptacle to instrumentation.
Wire-wrap receptacles have a square post that can be used to wire wrap a small gauge wire around. These receptacles also work well with low-cost jumper wires that can be used to quickly wire up a test system.
Solder Cup Receptacles
Solder Cup receptacles have a small cup pressed into the bottom of the test probe that can be used to solder a wire to the receptacle. These can also be soldered to a PCB that can be used to interface the test probes to the test instrumentation.
Test Probe receptacles can be purchased from a distributor or directly from the manufacture. At FixturFab, we source almost all of our test probes and receptacles directly from Ingun. They have a great selection tool that can be found here.
We typically order receptacles from the following series
- KS-050 (50mil/1.27mm spacing)
- KS-075 (75mil/1.91mm spacing)
- KS-100 (100mil/2.54mm spacing)
For development fixtures that are only going to be used to test a low number of DUT’s, we typically use cheap probes and receptacles that are sourced through Aliexpress. For these receptacles, we typically search for the following part numbers:
We primarily use wire-wrap receptacles since they help us quickly connect development fixtures.
Test Probe Selection
Test probes also have many different manufacturers, but all have similar constraints. When choosing a test probe receptacle, the most important decision is what tip to use.
The number of tips that are available can be overwhelming. From past experiences, we found that we typically only use a couple of different tips.
We detail these below, but here you can find a comprehensive list of tip styles and what they’re best used for.
The crown tip style typically has 4 (or more) points that hit the test point at the same time. These are great for interfacing with Pad style test points and component pins.
The spear tip style has a single point that can is moderately aggressive for interfacing with Pad style test points.
The cup tip style uses a cup to self align and interface with component posts from through-hole connectors and other through-hole components.
The cone tip style uses a pointed cone to interface with unplated vias or unpopulated through-hole pads.
Test Probe Distributors
At FixturFab, we source the test probe and receptacle from the same manufacturer. For production test fixtures we use Ingun for all of our Test Probes. We have typically used the following test probes series
- E-050 (50mil/1.27mm spacing)
- E-075 (75mil/1.91mm spacing)
- E-100 (100mil/2.54mm spacing)
For development fixtures, we use P75-LM2’s, sourced from Aliexpress.