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22 SMT Magazine • November 2014 functional test. The detailed inspection insures that the component was placed and reflowed on the circuit board and that there is no ob- servable physical damage to the board. Boards may be subjected to detailed visual inspection under magnification or subjected to X-ray anal- ysis. The circuit board is functionally tested. This testing ensures the circuit board and the reworked part are functioning to specification and there are no opens or bridges as a result of the rework process. Once testing has been com- pleted, the unit is ready for the final step in the rework process. Finally, the last step in the process is to dis- pense epoxy underfill. Typically, the operator will use the epoxy defined in the manufacturing process. Generally, underfill is dispensed along a corner or in a line along the edge of the surface mount device (Figure 12). The device is heated to 125°C to 160°C in accordance with the man- ufacturer's recommendations and taking into account the circuit board properties. Capillary action will absorb the material and distribute it underneath the part filling the space between the component and the circuit board. The board is maintained at a constant temperature until the epoxy has cured. Typical cure time to achieve the optimal strength is approximately five minutes. However, the cure time can be lon - ger depending on the epoxy properties. considerations After reviewing this article, readers will un- derstand the challenges and processes needed for successful repair of surface mount devices and the associated devices present on a PCB as- sembly, particularly in today's mobile devices. Attendees will understand the challenges inher- ent in each stage of the process and understand the need for process control to manage the risk of rework. A robust and repeatable rework pro- cess is essential for high rework yields. Proper preparation and application of solder and flux will add to the robustness and repeatability of the process. Reclamation of expensive compo- nents can be worthwhile if safely removed. This article pays particular attention to the challenges associated with underfill epoxy and the lack of a robust, repeatable process and the need for a better solution for pad cleaning and preparation. Additionally, the industry lacks a re- liable approach to removing epoxy from printed circuit boards which needs to be addressed. smt Paul Wood is the advanced product applications manager at oK international. rework cHaLLenges For smartPHones anD tabLets continues feaTure Figure 11: Mobile device fixture. Figure 12: Dispensing epoxy underfill.