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May 2014 • SMT Magazine 61 PCBa TEST: ENaBLING THE RIGHT maNaGEmENT PRaCTICE continues feATuRe figure 2: graphic line representation showing the process from paste printing through shipping. vision of zero defects being created in the indus- try, even best-in-class manufacturing sites typi- cally have 100~500 DPMO (defects per million opportunities) for lower complexity boards and ~2000 DPMO for higher complexity boards. There are three main contributors for prod- uct yield: design, materials, and manufacturing processes. As shown in Figure 1, there are many defects being created and these variables are dif- ficult to control in manufacturing. A review of test and inspection strategies Most SMT processes employ various imag- ing and test methods to produce defect-free PCBAs—these include pre- and post-reflow au- tomated optical inspection (AOI), 2D or 3D au- tomated X-ray inspection (AXI), in-circuit test (ICT, MDA, flying probers) and finally, func- tional test. Some of these test processes could be inline or offline sample tests. Most test and production managers would balance their test strategies, quality and output. Table 1 gives a quick review of these test technologies. While inspection technologies like AOI and AXI are now mature and have im- proved the process yield, defects do still flow through to the test stages. These are further complicated by the challenge of powering the board in manufacturing test, which captures a range on defects which may not be visible oth- erwise. With this broad spectrum of test strategies available, some of the critical questions often in the mind of test managers are: How should my test engineer define the test strategy? The definition should be based on a mix of ROI calculations, coverage of test, diagnostics effectiveness, and critically, DFT implementa- tion. As the goal of test is to find defects, di- agnostics effectiveness plays a crucial role in deciding what makes an optimal test strategy. Diagnostics effectiveness is all about how fast and accurately the root cause of the failure can be determined. The higher the diagnostic ef- fectiveness, the lower are the rework costs and process change costs. Of course these need to be balanced with the cost of each test stage. How- ever, it is important in the test management process to ensure the test strategy allows for the best diagnostics effectiveness.