Although flexible circuits are not new in today's technology roadmap, unmanned automatic testing of flexible circuits is challenging. These circuits can be very thin, have complex configurations, and in some cases are rigid bare plates and flexible combinations of hard and soft plates. Basically, there are three different ways to test these challenging circuits: hand, jig, and fly pin.
Manual testing involves only a digital voltmeter (DVM), by testing the continuity and discontinuity of individual networks. However, many semi-automatic, affordable DVM can only provide continuity of measurement, and IPC and MIL specifications of resistance resolution discontinuity testing can not be tested to the specification of isolation requirements.
Flexible circuits can, in many cases, be tested by using two dedicated and universal mesh clamps. These two units are needed in mass production. However, it is challenging to align the product with the actual fixture. Some, but not all, flexible circuits already have mounting holes that can be used to position the circuit onto the fixture, which makes jigs most suitable for mass production.
Unfortunately, the mounting holes can be electroplated and positioned, and repeatability may be affected. In other cases, if all flexible circuits are not fitted with holes, they will be "clamped" to a connector, which makes fixture positioning difficult. To overcome the challenge, "dykes" or "positioning boundaries" can be designed as fixtures so that flexible circuits may still be repeatedly positioned to clamp. Many of the current design of flexible circuit has very close and very small pad pad, in these very small pads on the wrong location may cause the "rat bite" or the pad itself completely destroyed, resulting in scrap.
Flying needle test
This approach may be the most advantageous option for testing small and medium volume orders. The first advantage is the cost of course. No need for fixtures and time spent on installation. It's a good choice for both hard and soft combination boards, proofing and small and medium volume production.