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

Applying the probe spring force and working stroke correctly is essential to ensure reliable and accurate testing of circuit boards. Here’s a step-by-step guide on how to apply the probe spring force and working stroke effectively:

Understanding Spring Force and Working Stroke

• Spring Force: The force exerted by the probe's internal spring when it is compressed. It ensures good electrical contact between the probe and the test point.

• Working Stroke: The distance the probe can travel when it is pressed down. It is the range within which the probe can move and still provide accurate readings.

Steps to Apply Probe Spring Force and Working Stroke

Select the Appropriate Probe:

• Choose a probe that meets the requirements of your testing setup, considering factors like material, spring force, and stroke length.

• Ensure the probe is compatible with the test fixture and the specific type of circuit board.

Determine Required Spring Force:

• The required spring force depends on the type of test and the characteristics of the test points (e.g., plated through holes, surface mount pads).

• Ensure the force is sufficient to penetrate any oxidation or contamination on the test point but not so high that it damages the board.

Set Up the Test Fixture:

• Install the probes into the test fixture, ensuring they are aligned correctly with the test points on the circuit board.

• Verify that the probes can move freely within their holders and are not obstructed.

Adjust the Working Stroke:

• Adjust the height of the test fixture or the probes to ensure the probes can reach the test points with the correct working stroke.

• The working stroke should typically be between 2/3 to 3/4 of the probe's full stroke to ensure optimal performance and longevity.

Calibrate the Test System:

• Conduct a preliminary test to verify that the probes are making good contact with the test points.

• Check for consistent readings across multiple test points to ensure uniform spring force and working stroke application.

Monitor Probe Performance:

• Regularly check the probes for signs of wear or damage, which can affect spring force and stroke performance.

• Replace worn or damaged probes promptly to maintain testing accuracy.

Test Environment Considerations:

• Ensure the test environment is clean and free of debris that could obstruct the probes or affect spring force.

• Maintain consistent environmental conditions (e.g., temperature, humidity) to ensure stable spring force and stroke performance.

Practical Tips

Documentation: Keep detailed records of the spring force and working stroke settings for each test setup. This documentation can help in troubleshooting and replicating tests.

Supplier Specifications: Refer to the specifications provided by the probe manufacturer for guidelines on optimal spring force and stroke settings.

Regular Maintenance: Implement a regular maintenance schedule to inspect and calibrate the test probes and fixture, ensuring consistent performance over time.

By carefully selecting the appropriate probes, adjusting the spring force and working stroke correctly, and maintaining the test equipment, you can achieve reliable and accurate testing of circuit boards in a no-halogen process.

Every spring will give out a force that can keep in touch with the tested component and stop it from damaging the PCB board. We will get the perfect performance when we use the right spring force with the 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 200 grams at least.

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

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

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

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

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

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

99% of probe 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 strokes and recommended strokes? ( see an example picture of a 100mil test probe)

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

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 goes down when over-compressed 2/3 travel. Like a person, you will get sick when you get overloaded with work or jobs. So overload compression will cause a lifespan shortage. 

Nevertheless, overload compression could increase PCB puncturing issues.

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