Improved PWB test methodologies

Abstract: Printed Wiring Board (PWB) and Printed Circuit Board Assembly (PCBA) testing aims to ensure an error free board after the etching and the assembly processes. After the etching process, several types of errors might occur such as opens and bridges, which are already, showstoppers in Direct Current (DC) applications. Mouse bites, spurs and others such as weak traces, which can be problematic in Radio Frequency (RF) and high-speed signals applications. Loading expensive component on defective boards can be economically catastrophic especially for high volume production. The rule of ten which has been reported by the production experts says that the defect costs ten times when detected in the next testing phase. Bare board also needs to be tested for the characteristic impedance correctness due to the process variations and the compounding raw material tolerances that can cause characteristic impedance mismatches. Although testing the characteristic impedance is not in interest in some application, sampling the characteristic impedance for a specific design is one way to test the manufacturing process stability for better tuning, otherwise PWBs might differ from each other even within the same batch. In addition to the possibility of defective PWB, the assembly process is never perfect to achieve 100 % of PCBA yield due to the possible errors in the process steps such as paste application, pick and place operations and soldering process which might lead to bridges, opens, wrong or miss oriented components. For low volume production, flying probes test technology is cost efficient as compared to bed-of-nails. The performance of the flying probes system depends on the test algorithm, the mechanical speed and the number of probes. To reduce the initial and maintenance costs of the probing technology and to accelerate the test time, Paper A introduces a new indirect method to test PWB continuity and isolation testing using a single probe for testing both continuity and isolation at the same time. RF signal is injected into the trace under test, instead of a DC current. The phase shift between the incident and the reflected signals is measured as it carries the information about the correctness of the trace when compared with a reference value of the same trace in the correct board. The method shown an important capability for detecting PWB defects such as 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 7 % to 68 %. Applying this approach to PCBA testing led to significant margins between correct and defective interconnect. The test cases in paper C shown 40 % and 33 %. Moreover, this margin has been proven to be important even for short microstrip line, which intended to connect two typical IC pins. This technique is strongly recommended to be applied to PCBA testing where probing is feasible. The approach can be applied to the complete layout testing or to boost a test strategy whose test solutions are not covering 100 % of the possible defects. By applying this test solution to bed-of-nails equipment, 50 % of the probes will be reduced, on the other hand, for a given design with NI isolated traces and NA adjacent pairs, employing this solution to flying probes system with two probes, leads to the reduction of the number of tests from (NI+NA) tests to NI tests as isolation and continuity are performed in one go. Flying probes system involves mechanical movements, which dominate the test time, reducing the number of the mechanical movements increases dramatically the test throughput. On the other hand, this method is believed to be extremely fast to test the correctness of the characteristic impedance which is prone to variations due to the instability of the PWB manufacturing process, in the same time one could employ the method to evaluate the process stability by checking after each batch of PWBs. Paper B and D provide insight into the impact of the PWB manufacturing variations on the characteristic impedance. Moreover single probe approach is believed to have a good potential for Sequential Build-Up (SBU) interconnects testing where connections between component pads and the upper layers are often impossible to test with the current test technologies.