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16 The PCB Design Magazine • August 2017 WHOSE FAULT IS THAT BAD BOARD? four had this problem. Going back on the route traveled by the electronic module, the follow- ing mistakes and miscues were found: A cus- tomer (also a designer) ordered the manufac- ture of a very expensive board to an EMS com- pany, providing all the components except for one—a 1,156-ball FPGA, which the distributor could not deliver together with the whole bill of materials (BOM). This was the first mistake: Assembling a board without a complete BOM. Having in mind that a third reflow process would be necessary for FPGA assembly, the technicians from the EMS company assembled one side of the board with lead-free solder paste, and the other with tin-lead. They assumed that during the second reflow process, the solder joints from the first side would not melt and the component would not drop in the oven. This was the second mistake: They mixed the sol- der alloys SAC with SnPb; the FPGA contained SAC alloy balls. In order to solder the new FPGA onto an 18-layer FR-4 board with components on both sides, a lead-free soldering thermal pro- file was used. The thermal process took place on an SMT rework and repair station using hot air for upside heating and infrared radiation for backside heating. Inherently, the heat affected the compo- nents in the vicinity of the FPGA package, sev- eral BGA sockets among them. According to the datasheet, these sockets had SAC balls, but they were previously soldered with an SnPb al- loy, too. At first glance, the solder paste melted and made joints with the socket balls (Figure 4). However, it was very curious to see that almost not a single ball remained on the socket's spring (Figure 5). It looks as if, instead, the ball was not properly attached to the socket's spring. Could it be a fake component (Figure 6)? This problem of distribution is another cause of bad boards, which seems to never stop. When a chain mistake occurs, the rule of 10 comes into play. The Latin expression "quod erat demonstrandum" is illustrative. Figure 4: The solder paste melted and formed the joint with the balls of the socket. Figure 5. Where are the balls? Figure 6: Is this new component good or fake?