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PCB-Nov2014

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56 The PCB Magazine • November 2014 by Todd Kolmodin Gardien serviCes usa Flying Probes Expanded C o l u m n testiNg todd Flying probes have been around for a long time now, but there are mysteries for some about how these machines really can perform the same as the "grandfathered" bed-of-nails machines. Although the "grid" has been known for years to provide the required electrical test, it has failed somewhat—due to pitch, density and cost—to provide the economical solution for both time and budgets. The flying probe has taken a new stance in the ET world, allowing operators to reduce cost and still provide expe- dient throughput in today's challenging ET/FA arenas. This month let us explore the parameters of the flying probe vs. the bed of nails. Using either format we are in the end concerned with the electrical functionality of the individual PCB. In the electrical test theatre we are driven by the customer requirements, fabrication drawings or industry standards. This is a very important point. Fabrication drawings, PO documents or special requirements need to be stipulated in advance so that the PCB manufacturer can en- sure that all requirements for the given PCB are met. This alleviates any unnecessary delays in the delivery of the product. There needs to be a full, open communication channel between the OEM and the PCB manufacturer regarding all requirements for the manufacture of the PCB. This is just not limited to the physical manu- facturer but the final electrical attributes given copper lengths of any given net. This is crucial when a specific class is called for manufacture and physical attributes of given nets exceed thresholds for continuity for the given class. This leads to delays due to nets on the product outside of specification parameters. I digress, so let us move to the flying probe vs. grid. Bed of Nails These workhorses are still widely used to- day. They provide full simultaneous continuity and isolation (shorts) testing in one single pass. Albeit expensive, they have been the go-to test for a great many years. The negative now is the expense of adding fixture to today's designs. Pitch, density and continuity of design revi- sions make fixtures a rather large expense in the design cycle of a given PCB. But the bed of nails machines can perform tests extremely quickly. Many OEMs still call out "simultaneous test" in their specifications that require bed of nails test. This term means that the continuity and isolation test is done immediately. With a bed

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