One of the key management issues in testing semiconductor parameters is the contact resistance of the probes.

Typically, a parametric test system inputs current or voltage into a device under test (Dut) and measures the device's response to that input signal. The path of these signals is: from the tester through the cable bundle to the test head, then through the test head to the probe card, then through the probe to the solder joints on the chip, to the DUT, and finally along the original path back to the test instrument.

If the results are unsatisfactory, the problem may be caused by the measurement instrument or software, or it may be caused by something else.wafer testing Usually, the measuring instrument introduces some noise or measurement error. The error is more likely to be caused by other parts of the system, one of which may be the contact resistance, which can be affected by probe parameters such as the probe material, the diameter and shape of the tip, the solder material, the contact pressure, and the flatness of the probe table. In addition, wear and contamination of the probe tip can have a significant negative impact on test results.

The integrity of the test signal requires high quality probe contact, which is directly related to contact resistance (Cres). Contact resistance is becoming increasingly important as signal voltage decreases, contact pressure decreases, and new material devices such as gallium arsenide absorb more current.

Contact resistance is the interlayer resistance of the probe tip in contact with the solder joint. It is usually not possible to give a specific metric because actual contact resistance is difficult to measure. Generally, signal path resistance is used instead of contact resistance and in many cases it is more relevant.wafer probe Probe card users often need to specify a nominal value for the path resistance when detecting false solder and open circuits. Signal path resistance is the total resistance from the solder joint to the tester, i.e., the sum of contact resistance, probe resistance, solder resistance, alignment resistance, and spring-loaded pin interconnect resistance. However, contact resistance is an important component of signal path resistance.

In the actual need to use, the probe contact a resistance to a large extent to develop depends on the material of the different solder joints, the number of times of cleaning, and through the condition of the probe, and it is with the nominal value of the difference between the time more. The contact resistance of tungsten rhenium alloys (97%-3%) is slightly higher than that of tungsten, and the fatigue resistance is similar. However, because the internal structure of the lattice of tungsten rhenium alloys is more compact than that of tungsten, the planar design of the tip of the probe system is more sufficiently smooth. Therefore, the possibility of contamination of the tip of these research probes is less risky, easier to clean, and its resistance during contact is more secure and stable than tungsten. Therefore, tungsten rhenium alloy is a more effective choice.

Contact pressure is defined as the pressure (measured in mils or microns) exerted by the tip of the probe (measured in grams by the enterprise) on different areas of the student contact. A contact pressure that is too high can damage the solder joint. A contact pressure that is too low may result in the inability to can pass through the research oxide layer and therefore we produce unreliable test analysis results.

The top pressure is mainly controlled by the drive of the probe stage, and additional Z-direction movement (vertical travel) causes it to rise in a straight line. In addition, probe material, probe diameter, beam length and taper length all play an important role in determining tip pressure.probe holder Figure 1 explains the relationship between contact pressure and contact resistance. Essentially, as the probe begins to contact and gradually penetrates deeper into the surface layer of solder joint oxides and contaminants, the contact resistance decreases and current begins to flow rapidly. These effects increase as the probe contacts the subsurface layer of the solder joint metal. Although the contact resistance gradually decreases as the probe pressure increases, the nominal contact resistance of both metals is eventually reached.

When it comes to probe contact resistance, we also need to consider balanced contact pressure (BCF), overspeed, probe-to-probe flatness, probe contamination, and cleaning.