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68 SMT Magazine • June 2014 THe TROuBLe WITH BgA SOLDeR JOInTS continues arTICLe the distance between two pads and the minimal insulation clearances may be violated. Then the classical round pad layout must be chosen, which has to be considered when parameteris- ing the X-ray test. A sole evaluation of the solder joint accord- ing to its ball form is often insufficient for dis- tinguishing between good and bad. In the case of single-sided assemblies, the transition be- tween pad and solder ball can be evaluated us- ing a high resolution 2.5D X-ray system. A vis- ible necking-down would indicate a "resting" ball: the head in pillow situation. If, however, the assembly is populated on both sides or has even more than two soldering planes, this ap- proach is somewhat problematic. The 2.5D X- ray image then shows strong superimpositions of the BGA with components from the other side. Figure 7 shows a board section with three soldering planes (TOP = capacitors, BOTTOM 1 = BGA_1, BOTTOM 2 = BGA_2). Here, only a 3D X-ray system will provide a remedy. GOEPEL's ScopeLine MX [5] is a semi-auto- mated X-ray system for offline analysis. Better Functionality without nails As an addition to X-ray inspection of com- plex BGA assemblies, boundary scan is the method of first choice. As opposed to classic in- circuit test physical nails are "moved" into the chips and become virtual nails (Figure 9). This design-integrated test electronics is se- rially controlled via a test bus. The virtual nails are in fact boundary scan cells, arranged as a shift register (boundary scan register). The syn- chronous handling of the cells makes the elec- trical test of BGA solder joints a simple task. However, in the case of directed connections figure 8: Scopeline MX-1000 for semi-automated bGa analyses (MXI). figure 9: Transition to design-integrated test electronics.