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June 2014 • SMT Magazine 71 (Figure 10), the failure location can't be exact- ly nailed down. For that, an MXI-like method would be required. Multipoint connections such as bus struc- tures provide pin-level fault diagnostics. What's special about boundary scan is its high testing speed and its flexibility when it comes to proto- type tests. Boundary scan is a structural proce- dure and, as such, independent from the chip's integrated functional logic. Finally, that means that each pin can be tested independently. So, this procedure can be ideally combined with stress testing, e.g., where thermal cycling in a climatic chamber tries to force bad solder joints to fail. But boundary scan also has its strengths in lab use. For rapid prototype verification de- signers often need to evaluate certain signals. For that purpose, graphical tools yield best re- sults. It not only features cross referencing be- tween layout and schematics. Boundary scan cells can be activated by simply clicking on the respective pin. The resulting logic signal states are displayed using customer specific colour schemes. Powerful in a Process Team The very existence of the discussed technol- ogies and system solutions isn't sufficient for a production with highest quality standards. The use of X-ray systems and Boundary Scan sys- tems in the production of BGA assemblies re- quires a thorough analysis of the entire produc- tion situation. Accurate knowledge of the ex- pected faults and their statistical distribution is of paramount importance. There are more than 100 parameters, which influence the definition of an optimised inspection and test strategy. So it is impossible, to name here the "ultimate" strategy. But one thing is certain: The combina- tion of AXOI and boundary scan for BGA as- semblies is capable of delivering a fault cover- THe TROuBLe WITH BgA SOLDeR JOInTS continues arTICLe figure 12: Components of the goepel picoTap designer Studio package.