February, 2019
www.us-tech.com
Page 73 The Magic of a One Millimeter Probe By Mike Ramsey, R&D Engineer, Plastronics
interesting changes occur to the impedance and signal frequencies that probes carry. These changes occur at 0.04 in. (1 mm) heights. This short path length forces impedance closer to characteristic 50W imped- ance, and signal frequencies that the probe can carry increase rapidly.
A
s interconnect paths get short- er, assuming there is a good electrical connection, some
technique is used to eliminate any extra resistance that may be gener- ated from the test setup. Average ini- tial contact resistance was about 38 mW. This is a static reading at full deflection. Contact resistance was also measured dynamically during probe pin cycling. After 150,000 cycles, the force
under test changed very little, but the average contact resistance dropped to 30.7 mW. No T033 pin under test exceeded the 100 mW threshold. The test results showed that the pins under test were making
good electrical connections. With a known good electrical con-
nection, the focus turns to the electrical performance of the spring probe. The first property to verify, based on test height, was impedance. Plastronics had an outside lab perform the imped- ance checks, since it requires special- ized equipment. The third party meas- ured the impedance at 54.9W in the field, which is very close to a character- istic impedance of 50W. The closer to the 50W target
impedance when signal or power comes to the probe, the smaller the
amount of signal or power reflected back down the path from which it came. This is important for energy conservation, as well as to reduce false positives from seeing a reflection. Another electrical property to
discuss is the probe’s ability to carry signals at high speeds. This is meas- ured by –1 and –3 dB insertion loss frequencies. These are a measure- ment of the signal speeds that the probe can carry before reaching select points of signal decay. Again, this is affected by the ground config-
Continued on page 77
T033 stamped spring probe. Until recently, there were very
few probes in the market to fit this niche. Plastronics funded a research project to produce the T033 series of probes. The T033 series has a mini- mum pitch of 0.02 in. (0.4 mm), and when it is fully actuated, has a test height of 0.04 in. (1 mm).
Developing Test Probes To ensure the best electrical
connection, the company focused on the normal force generated. The T033 is the latest addition to Plastronics’ H-Pin series of stamped spring probes. The H-Pin consists of two beryllium copper stamped metal parts and a stainless steel spring. The two metal parts clip together in such a way as to slightly deflect the spring that is between them. The assembled probe then has a
preload on it. For the T033, that pre- load is about 0.2 oz (5.2g). This means that as soon as the probe is deflected, even slightly, it exhibits this force. The probe’s force is provid- ed by the stainless steel spring. Stainless steel has a much higher stress relaxation curve at tempera- ture than the beryllium copper stampings. The stampings are deflected at very low stress, so that if they lose any stress from relaxation, it is kept to a minimum. The total force at full deflection
of the T033 stamped probe is 0.4 oz (11g). Based on the preload value, the full deflection distance of 0.001 in. (0.25 mm) and the force at full deflection, a linear curve of force vs. deflection can be created. That curve is force = 23.2 x distance + 5.2, where force is in grams and distance is in millimeters. A stable and flat force curve
enables a stable and flat contact resistance curve. To prove this, sev- eral test sockets housing the T033 H- Pin were machined and contact resistance was measured using a standard four-wire measurement. The four-wire measurement
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