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PCB007-Aug2021

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82 PCB007 MAGAZINE I AUGUST 2021 dation of the electrical characteristics. Histori- cally this has been easy. Fixture technology and fixture testers were all the rave through the '80s, '90s, and into the new millennium. Using headed pin technolo- gy through featureless music wire allowed the successful test of most designs. Even with the higher density, the CAM systems were able to split fixtures to A and B tests to encapsu- late the extremely dense PCBs. Unfortunate- ly, this has some drawbacks. e extra fixtures and increased drilling and assembly time adds cost and time to the equation. Whether ET is captured in the quoting process by the manu- facturer will be the decision of the individual manufacturer, but these costs are real in both time and materials/labor. As today's designs get more complex, eco- nomics come in to play as to whether fixtures are the answer. Extremely dense designs are now requiring quad-density or greater to still utilize fixture technology, or they are being converted to dedicated spring probe fixtures that are very costly. ese variables are usually not considered during the quoting process (re- member DFT) and can cause loss of margin by the time the electrical deliverables are met. Now, today's flying probe machines can test extremely dense designs without using fix- tures. e cost is much less but there are trade- offs from the fixture test. Flying probes can- not provide the full parametric test that fixture testers can provide. is can be an issue when testing some military, aerospace, and high re- liability medical products. ese require the "simultaneous" test of the PCB for opens and more importantly, shorts. So, in some of these cases a fixture or multiple fixtures will be re- quired. is increases cost and can cause de- lays in the manufacturing window due to man- ufacture of the fixture(s). Where is this going? Design for test. Speak- ing for the ET arena, PCB designers are en- couraged to take into consideration how their electrical deliverables can be met. Sure, fly- ing probes can tackle most designs and re- quirements of today, including test of buried passives, HiPot, inductance, and even capac- itance. However, in high-reliability products that require full parametric tests, fixtures will be required. Here is where consideration will be most beneficial. Consult with your manu- facturer on electrical test density capabilities to make sure your design can be tested effec- tively. If your design is beyond the capability, consider adding a pad or feature to allow the effective probing of the network. Sometimes adding a small 0.010-0.015" pad, spaced appro- priately for ET capability, may turn your un- testable design into a routine build. is can be increasingly effective for wire-bond technolo- gy where direct probing of the wire-bond pris- tine area is prohibited. Doing so may stress you out a bit finding real estate to add these features for test, but usually this would not increase the price of the actual PCB build. However, it does make a significant difference in the testability of the PCB design. While DFM is an absolute necessity in the market of today, DFT needs the attention more today than ever. An overall great design implementing DFM and DFT will make the manufacturing cycle less expensive and keep the time frame for deliverables in check. Don't forget ET. PCB007 Todd Kolmodin is VP of quality for Gardien Services USA and an expert in electrical test and reliability issues. To read past columns or contact Kolmodin, click here. As today's designs get more complex, economics come in to play as to whether fixtures are the answer.

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