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72 SMT Magazine • September 2017 basic methods of repair as outlined in the IPC- 7721 board level repair guidelines. In one meth- od a two-part epoxy is used, while in the other a pre-adhered dry film adhesive is used. The two-part epoxy (IPC 7721 2.6-epoxy mixing) method for pad repair is outlined in the IPC-7721 Procedure 4.7.1 Surface Mount Pad Repair, Epoxy Method. It is the most reliable method for pad repair. This requires a higher degree of skill and patience by the technician. In this method, the site is prepped for the new pad by removing the existing pad or pad/trace combination with a sharp knife. After cleaning the area, the proper replacement pad or pad/ trace is soldered to the existing board using a lap solder joint. The two-part epoxy is mixed per the manufacturer's recommendations on a flat plate where the mixture can be seen (a glass plate is easiest). Using a wooden orange- wood stick, a thin layer of this two-part mix- ture is then placed underneath the replacement pad or pad/trace area and clamped using a non- stick surface clamp to hold the replacement pad or pad/trace to the PCB during the curing pro- cess. Curing per the epoxy manufacturer's di- rection, which is usually accelerated to some extent by elevated temperature conditions, will then complete the process. Prior to imple- menting the final step of overcoating with sol- der mask over the trace area and/or up around the periphery of the pads to gain some adhe- sive strength, the continuity between the var- ious conductors involved in the repair process should be confirmed. The dry film method for pad repair is out- lined in IPC-7721 Procedure 4.7.2 Surface Mount Pad Repair, Dry Film Method. It is the cleanest of the two approaches and requires a high degree of dexterity to perform. The pad and trace prepara- tion process as described previously is used in this method. However, when cutting out the replace- ment trace and film, great care needs to be taken in making sure that the replacement cut out dry film is adhered everywhere on the bottom-side of the foil. If a trace section needs to be lap sol- dered to that section on the board, the techni- cian needs to make sure that the overlapped sec- tion has the dry film on the board. Once it is af- fixed in position, the replacement materials will have the heated tip of a curing iron sized over the bonding area. While being adhered, the cur- ing temperature needs to be confirmed, as both the correct temperature and pressure needs to be applied to get the cure right. Cleaning, testing, and inspection are the same as in the case of the epoxy repair described above. Conclusion The above discussion is a snapshot in time with respect to the top BGA rework challenges. These challenges, in no order, were that of: ex- cessive BGA warping due to thinning packag- es, shielding of neighboring devices in an era of increasing component density, underfilled BGA device removal and replacement, as well as the repair of solder mask. In addition to these cur- rent challenges, others including reworking ev- er-finer devices pitches and integrating lower reflow temperature solders are rework trials that will be added to this list soon. SMT References 1. Gregory L. Tonkay, Robert H. Storer, Ron- ald M. Sallade, and David J. Leandri, "Critical Variables of Solder Paste Stencil Printing for Mi- cro-BGA and Fine-Pitch QFP," IEEE Transactions on Electronics Packaging Manufacturing, Vol- ume 27, No. 2, April 2004. 2. Garcia, Omar, et al, "Selective Reflow Re- work Process," IPC APEX EXPO Proceedings, 2015. 3. Wettermann, Robert and Gaynor, Adam, "Effectiveness of Different Materials as Heat Shields during Reflow/Rework," IPC APEX EXPO Proceedings, 2015. 4. Luetzlow, Norbert and Jahal, Kiuldip, "Quantifying Performance of Mechanical and Chemical Processes for Pre-treatment Prior to Solder Mask Application," IPC APEX EXPO Pro- ceedings, 2013. Editor's Note: This article was originally pub- lished in the proceedings of SMTA International. Bob Wettermann is the principal of BEST Inc., a contract rework and repair facility in Chicago. TOP 5 BGA REWORK CHALLENGES TO OVERCOME