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90 SMT007 MAGAZINE I JANUARY 2019 One of the typical questions process engi- neers pose regarding the PCB rework process is, "How many heat cycles are too much?" Asked in another way, the question is, "How would one define a limit on the number of times a PCB can be reworked while still being reasonably assured that the reliability has not been impacted based on its operational envi- ronment?" The answer will depend on a multitude of factors, but it is largely influenced by the board's function in the end-use operating environment. For example, a plane control module that is protecting passengers while in flight (Class 3) has a different set of maximum conditions compared to a giveaway toy that came from the local fast food eatery (Class 1). In reviewing this topic, consider that rework cycles consume part of the useful thermome- chanical life of the PCB. There are several factors to consider when calculating the maximum number of allow- able rework cycles. Some of the most relevant factors include PCB design, materials (includ- ing the components involved in the design of the PCB), and the number of heat cycles or heating and cooling cycles the board has expe- rienced. The multitude of factors and there being no one number is also supported in the rework and repair guidelines document, IPC-7711/21, which states as follows, "This document does not limit the maximum number of rework, modification, or repair actions to a printed circuit assembly." PCB design has an impact on how the many cycles should be allowed for rework. There are hearty laminates that can take multiple heat cycles. Higher layer- count PCBs with high aspect ratios through vias are more prone to failure after only a few soldering cycles if the materials of construc- tion are not robust. In general, larger pads hold up under more heat cycles; however, with smaller pads, there is a greater chance of board delamination after only a few cycles. Testing and Knocking Down the Bone Pile by Bob Wettermann, BEST INC. How Much is Too Much? Figure 1: How many times should I attempt to rework a PCB?

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