Reflection Phase Shift for PWB and PCBA Production Testing

Abstract

Testing of printed wiring boards (PWBs) and printed circuit board assemblies (PCBAs) is part of electronics production that has a great impact on profitability. High throughput and low cost testing is always needed for high quality and reliability. Bare board testing, that is, testing before loading components, is crucial, and can detect such defects as opens, bridges, near-opens, near-bridges, and characteristic impedance mismatches due to process variations and compounding raw material tolerances. If not detected at the bare board stage, the cost of defects can increase 10-fold. Another motivation for an unpopulated board test is that loading expensive components on a set of defective boards could be economically catastrophic. Flying probe systems, which were developed in late 1980s, are commonly used and favorable to perform bare board isolation and continuity testing, especially when the volume is not great enough to justify bed-of-nails purchase. Flying probe system performance for a given bare board depends on the test algorithm, the mechanical speed, and the number of probes. To reduce the cost on expensive test probes and probe maintenance and to accelerate the test time, this paper presents a new and cost-efficient approach to testing both populated and unpopulated boards with open sockets, using a single probe. Specifically, a coaxial probe injects one frequency signal into the PWB trace, and the phase shift between the reflected signal from the trace and the incident wave is detected and compared with the nominal value. This nominal value is determined by testing a defect-free board that already passed direct continuity and isolation testing. By applying this test solution to bed-of-nails equipment, we reduce the amount of probes by 50%. By employing this solution to flying probe systems with two probes, for a given design with NI isolated traces and NA adjacent pairs, we reduce the number of tests from (NI + NA) tests to NI tests as isolation and continuity are performed in one go. Flying probe systems involve mechanical movements that dominate the test time. By reducing the number of mechanical movements, we will dramatically increase test throughput. The experiments demonstrate feasibility for practical use in automatic test equipment (ATE) for PWB and PCBA testing. At the highest sensitivity of the phase shift detector, the prototyped tester is capable of distinguishing between a defective and error-free board with significant margins in case of defects such as opens, DC and RF bridges, and exceeded and different width lines. The margin in the measurement between a defective and a correct board, which depends on the type of the defect, is about 7% to 68%. In the case of loaded board testing, the approach is capable of detecting opens with important margins (our test cases showed 40% and 33%), which makes it a strong candidate approach to be applied officially to PCBA testing where probing is feasible. The approach can be applied to the complete layout or to boost the test strategy where the applied test solutions do not cover 100% of the possible defects.