How To Apply The Probe Spring Force And Working Stroke

As the former essay got an impact on our engineers testing applications, Here we have some advice for spring force and working stroke of test probe.

Before we start our topic, we need to know what is a test probe when you used a test probe, here you could read the first essay I wrote a few days ago. It will tell you the structure of a Test Probe.

Ok, you knew the probe definition, so we notice that there is a spring working inside of the probe that can give a probe flexible and compressed action. Yes, that’s spring’s main job.

And every spring will give out a force which can keep touch with the tested component and stop from damaging the PCB board. We will get the perfect performance when we use the right spring force with recommended stroke.

Usually, probe vendors will provide our customers advice as their own designed spring force and working stroke.

How is the right spring force at first?

Normally we could select the forces based on what you are testing.

For the clean board, we will recommend our dear customer to use 100grams to 200grams.

For the dirty board, we will advise engineers to use 200grams at least.

For the thin board, less than 100grams to 150 grams will be better options.

If you need puncture aggressively and worry less about the board damage problem or not easy to puncture the PCB boards, 200gram, even 300grams also can be placed in the test point.

All things can be changed based on the real test situation. Suitable test solution will last a long time and reduce the cost of ICT. This is not an absolute unchangeable decision.

We should focus what we want, if we need safety, we must have more eyes on damage issue. Never use the risky options; if we need the high FPY( First Passed Yield), and the board is thick, why not increasing high spring force pressure so that ensure a better contact. All test probes must be suitable for your PCB board test.

In the other hand, when we do use a wrong working travel also will increase the risk rate of board puncturing. So here we should listen to our probe vendors’ suggestions. 

In probe area, we always mentioned about full stroke( full travel) and recommended stroke( or working travel).

99% of probes suppliers will say, “Please apply the probes in the right recommended strokes. ”

Yes, it’s right, because they designed the test probes including the springs, they knew springs better than you, they tested the spring life over millions of cycles.

So what are full stroke and recommended stroke? ( see an example picture of 100mil test probe)

Almost ICT-probe designed spring forces are namely compressed at 2/3 full travel, So a 200gram probe means it is 200gram when compressed at 2/3 full travel. It won’t be matched for special designed test probes like double-end pin.

As we know the cylinder coil spring force constant is a straight line in the chart, but this does not work in probe spring, it is not a simple calculation. It can perform well between 0 to 2/3 travel, but it is going down when over compressed 2/3 travel. Like a person, you will get sick when you got overload work or jobs. So overload compression will cause a lifespan shortage. 

Nevertheless, overload compression could increase PCB puncturing issue.

Finally, Suitable is the best option, neither the highest FPY nor over protective test without FPY.