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92 SMT007 MAGAZINE I JANUARY 2019 experimentation via life cycle testing will show what is right for your specific application and end-use operating environment. A well-chosen material set will, in most cases, withstand up to six heat cycles (three rework cycles) for the majority of Class-2 or Class-3 designs. The materials included in the design of the PCB (including the components and boards) have an impact on the maximum number of rework or heat cycles that an electronics assembly can withstand. As a reminder, an entire assembly can be compromised if only a single component is damaged. Make sure to look up the specifications for all of the compo- nents to investigate the temperature limit specified by the component manufacturer. It's important to note here that passive compo- nents may have a lower time-over-temperature exposure limit than larger active components. This is known in the component specification world as the process sensitivity level (PSL). Furthermore, check what these values mean for other elements on a PCB, which are further defined in the IPC J-STD-075: Classification of Non-IC Electronic Components for Assembly Processes. During a reflow process, the board is stressed as well as the solder joints and all of the components. The total number of heat and cooling cycles should be part of the decision to limit the number of rework cycles. In general, the typical number of rework attempts (multiply by two to get to the number of heat cycles) for Class-2 and Class-3 PCBs is three. Make sure to enumerate all of the heat cycles the board has already gone through. For a typi - cal double-sided PCB, this could include both the primary side component placement and reflow as well as the secondary side place- ment and reflow. Develop a profile for each of the processes using a thermal profiler to prove that you have not exceeded any of the component temper- ature/time limits. If we simply remove and replace one of the devices, then the count of reflow cycles already numbers four. For BGAs and other active devices, the usual number of heat excursions is equal to the number of times the die is exposed to the liquidus temper - ature of the soldering alloy used. However, wave soldering, baking, and conformal coat- ing curing processes may heat and relax the PCB. If reballing of the device is required, then another two heat cycles—one for the removal of the BGA balls and the other for the re-attachment reflow—must be added to the total. There are some ball removal processes that do not raise the die temperature above liquidus. In addition to the technical limitations placed on the maximum number of rework cycles, there are also economic decisions that may drive the cost versus benefit of a rework process. Many times, the alternatives evalu- ated include the removal and replacement of the device or scrapping the board and replac- ing with a new assembly. At times, the cost and availability of these options push the deci- sion in a certain direction. PCB rework yields, overhead costs, and opportunity costs in using labor (which could be producing higher margin PCBs) enter the decision-making process. Thus, the question that needs to be answered is, "When is too much too much?" SMT007 Further Reading 1. IPC—Association Connecting Electronics Industries. "IPC-7711/21: Rework, Modification, and Repair of Elec- tronic Assemblies." November 1, 2011. 2. Coyle, R., Meilunas, M., Popowich, R., Anselm, M., Read, P., Oswald, M., & Fleming, D. "Interconnection Reli- ability of Interposer and Reballing Options for BGA Back- ward Compatibility." SMTA International proceedings, October 14, 2012. 3. Ma, H., Xie, W., Subbarayan, G., & Lieu, K.C. "Effects of Multiple Rework on the Reliability of Lead-free Ball Grid Array Assemblies." IEEE 61st Electronic Components and Technology Conference, 2011. Bob Wettermann is the principal of BEST Inc., a contract rework and repair facility in Chicago. To read past columns or contact Wettermann, click here.

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