Single Probe for Bare Board Continuity and Isolation Testing

Abstract

Bare board testing is vital before components loading. Defects after the PCB manufacturing process are possible such as opens, bridges, near-opens, near-bridges and characteristic impedance mismatches due to process variations and compounding raw material tolerances. Moreover, defects might cost about ten times more when detected in the next test phase, another motivation for unpopulated board test is loading expensive components on a set of defective boards might be economically catastrophic. Flying probe systems, which were developed in late 1980’s are commonly used and favorable to perform bare board isolation and continuity testing, especially when the volume is not big 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 test unpopulated and populated board with open sockets, using a single probe. Specifically, a coaxial probe injects one frequency signal into the PCB trace, the phase shift between the reected signal from the trace and the incident wave is detected and compared with the nominal value, which has been captured from a defect free board, which already underwent direct continuity and isolation testing. The conducted experiments have shown a good feasibility for practical use in the ATE (Automatic Test Equipment) for bare board and loaded board with open sockets. At the highest sensitivity of the phase shift detector, the prototyped tester is capable to distinguish between a defective and error free board with significant margins in case of defects such as opens, DC and RF bridges, 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 8 % to 65 %.