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When a PCB is tested in an in-circuit test fixture, a high density of probes can be in contact with the test card simultaneously. The widespread usage of 0.050" and 0.039" probes in high density areas leads to significant flexion of the unit under test (UUT). For example, a one inch square BGA can require 400 probes at 0.050" spacing. Consider 8oz probes, this component will support 200 lbs/in2 underneath its body.

In the case of a single-sided fixture, all the probes are applied underneath the card and a vacuum box secures the card from the top.

Single-sided fixture side view

Ideally, due to the forces on each side of the UUT, all probes should be counterbalanced by a pressure finger. If this were possible, the resulting force applied to the card would been nil. Unfortunately, this is rarely possible. The density and complexity of electronic circuits is increasing constantly. We therefore find ourselves constructing fixtures with a high density of probes that cannot be supported adequately, resulting in major flexion of the card under vacuum.

As we will see, major flexion of a UUT can result in breakage of certain components and enormous repair costs for damaged circuits. Moreover, some problems such as a solder fracture may not be detected during final test, or even worse, after delivery to the end client. It is therefore essential to use the best tools available to minimize this effect when designing our test fixtures.

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