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TAP TIMES -4- OCTOBER 2022
High-power socket design (from 3) As the current increases, the heat in-
creases. For a fi xed contact pressure and
A rule of thumb is that a PC board with a 1 contact area the temperature increases
oz copper layer can handle approximately exponentially. For this probe, fi gure 3 be-
2.
35A/mm copper provides the heat trans- low shows the rise in temperature as the
fer path. In addition, the large center current increases.
GND pads in the PQFN provide a good
heatsink to the board.
Test Socket Design Challenges
The challenge for a test socket is to place
a compliant interface between the PC
board and the device that can handle the
current demands of the device under test.
The socket must also survive the thermal
issues that arise with high-current fl ow in
limited surface contact area and perform
reliably under the manufacturing demands
for high insertion count lifetime in a high- Note the rise in termperature as the current increases.
volumeproduction environment.
The temperature rise at 2 amps is 20°C.
The interface of two surfaces is the point Projecting the curve at 10amps continuous
of contact resistance, and this where the current through the probe the temperature
heat increases most from the current fl ow increase is greater than 150°C. Running
in the socket contact. this probe at 10 amps continuous is im-
possible.
For a single spring probe, the contact area
for a conic pointed tip and the contact Limit heat/current in the test
surface area of the PC board. The contact To keep the test socket intact, the test
resistance for a spring probe should re- program must limit the heat and the cur-
main fl at over current. Figure 2 below is a rent in the test.
measured contact resistance V-I curve.
The socket design must increase the ther-
mal mass of the contact technology. The
test board and socket must have a sense
line on the high-current pads on the part
to keep the voltage and current under
feedback control.
The second level of the high tempera-
ture problem for the contact is diminished
lifetime. Limiting the time that the high
current state is active will increase the life
of the pin and allow for testing at higher
currents. next page
