The EMPF was recently contacted to help a customer who was experiencing poor test yields on some very complex boards. After the initial telephone conversations, samples of the boards were sent to the EMPF for evaluation and repair. The EMPF was also requested to determine the root cause of the failures and to provide recommendations to the subcontractor for improving the board quality and increase production yields.

The boards received for evaluation were multiplayer, approximately 14" X 18", mixed technology (double sided surface mount with through hole components) including multiple BGA's. The bare boards were also .092" thick and Solder Mask Over Bare Copper (SMOBC). Initially, the board assemblies were inspected for the obvious type solder and manufacturing defects using 4X magnification. The results of this inspection indicated that the solder flow and wetting of the solder met the requirements of the IPC Specification. Overall the workmanship and quality of the boards appeared to be good.

The next step of the evaluation was to X-ray the BGA's and BGA sockets that were soldered to the board. In addition, an ERSA scope was used to inspect the outer rows of solder joints under these packages. The results of this inspection identified poor wetting and some possible cracks in the solder joints around the balls of the BGA sockets. It was obvious that the balls on the BGA sockets did not collapse and wetting of the solder paste did not take place during the reflow process. Digital photographs of the solder joints were taken and were included in the final report supplied to the customer.

The customer was contacted and made aware of this finding. What they requested was for ACI to remove the BGA sockets from the board and attach the BGA's directly to the Printed Circuit Board in those locations. There were 12 of these sockets that had to be replaced on each of the boards. To remove the sockets, the PDR 40 BGA Rework Station was used with a reflow profile that was developed particularly for that device and that board. Each device was removed one at a time using this profile and the board was allowed to cool between each thermal cycle. This minimized overheating and stressing the surrounding components and the board itself. After the sockets were all removed, the excess solder on the pads was removed and the pads were prepared for the reattachment of the BGA's.

When it was time to resolder the 12 BGA's on this board, much thought was given by the engineers to determine what this process should be. The options were to either place and reflow the BGA's using the rework station that was used to remove them, or to place all the BGA's and mass reflow them in the convection reflow oven. After evaluating the other components on the board, the latter process was selected. Thermocouples were placed on the board and a reflow profile was generated. The advantage of this process was that it eliminated the board warpage that might be caused by the intense localized heating associated with the rework process. By placing all the parts and reflowing the board, it also reduced the number of heat cycles to which the board and components were being exposed. The boards were reflowed using this process and the BGA's were X-rayed and inspected to verify the successful reflow of the BGA's.