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100 SMT Magazine • June 2016 Generally, the recommended bake conditions for SMT packages are 8–24 hours at 105–125°C, but consult with IPC J-STD-033 for greater details and specificity. Don't forget that a partial bake is some- times worse than no bake at all because initially the ingressed moisture is being pushed towards the critical interface at the center of the package. Review of IPC-1601 The IPC-1601 standard describes the MSD protection and moisture ingress mitigation of the PCB itself. It prescribes exposure times and bake cycles. Even though it's localized, if there is mois - ture globally in the board, you could damage it at or near the site where you are applying the solder reflow temperatures. Typically, boards will show measling or delamination (Figure 2) locally near the rework heated area if there has been moisture absorption. Internal defects such as partial cracks and delaminations cannot be detected with stan - dard test and inspection tools, but they will reduce the MTBF and increase the risk of early life failures. The key concern in baking PCBs will be impact on the solderability of its surface finish. Some sur- face finishes are much more robust than others. For example, organic solderability preservative (OSP) and immersion silver (ImAg) or tin (ImmSn) sur- face finishes may not do as well as electroless nickel immersion gold (ENIG) or hot air solder leveling (HASL) surface finishes. There are various IPC docu- ments that provide guidelines for baking PCBs of different surface finishes. As a default, use a 150°C 24 hour bake-out cycle with the above caveats. Controls There are various controls that should be in place as part of PCB rework having to do with MSD controls. Make sure the trays, reels or boxes of parts clearly identify the parts and boards, the floor life, MSD level and make sure that they are time-stamped. Also make sure to maintain the as- sociation between the log sheet and the individ- ual tray, reels and parts. Keep track of cumulative exposure time associated of both replacement components and boards. Be aware that the floor life clock is not reset by reflow. Make sure that the remaining floor life of MSDs is tracked for assembled on boards for double-side reflow and rework. When the factory ambient conditions exceed 30°C, 60% RH, the floor life indicated on the MS label is no longer applicable. In this case, the floor life must be de-rated. These are some of the basic elements that need to be in place for an effective verifiable MSD program. Conclusion Depending on the construction of the PCBs and whether or not the parts on the PCB con - tain MSD devices, boards may need to be baked prior to rework. Care must be taken to make sure the elevated temperatures can be taken by both the PCB and the components on the board fall within the temperature withstand guidelines of both. If the board or the parts are baked, the sub- sequent handling and protection of MSD devices needs to be assured. The simplest is to consider the board another MSD device in which the in- vestigation of the moisture sensitivity level and appropriate protection outlined in IPC-1601 will tell you how to handle the assembly in rework. If there is no danger to the board or parts in the bake-out process, let the axiom "when in doubt bake it out (at the lowest temperature)" apply for modern SMT assemblies. SMT Bob Wettermann is the principal of BEST Inc., a contract rework and repair facility in Chicago. Figure 2: Resultant delamination near rework area on PCB. TO BAKE OR NOT TO BAKE (IN REWORK)—THAT IS THE QUESTION