Single Probe for Bare Board Continuity and Isolation Testing

摘要

Bare board testing is vital before components loading. Defects after the PCB manufacturing processare possible such as opens, bridges, near-opens, near-bridges and characteristic impedance mismatchesdue to process variations and compounding raw material tolerances. Moreover, defects might cost aboutten times more when detected in the next test phase, another motivation for unpopulated board test isloading expensive components on a set of defective boards might be economically catastrophic. Flyingprobe systems, which were developed in late 1980's are commonly used and favorable to perform bareboard isolation and continuity testing, especially when the volume is not big enough to justify bed ofnails 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 probemaintenance and to accelerate the test time, this paper presents a new and cost ecient approach to testunpopulated and populated board with open sockets, using a single probe. Specically, a coaxial probeinjects one frequency signal into the PCB trace, the phase shift between the reected signal from thetrace and the incident wave is detected and compared with the nominal value, which has been capturedfrom a defect free board, which already underwent direct continuity and isolation testing. The conductedexperiments 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 signicantmargins in case of defects such as opens, DC and RF bridges, exceeded and dierent width lines. Themargin in the measurement between a defective and a correct board, which depends on the type of thedefect is about 8 % to 65 %.