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Reliability of Reworked Lead-free Assemblies

Question: Our current soldering process for a product proceeds like the following: two heat excursions, thru reflow, two thru a wave and one thru rework. How many more rework cycles can we do before compromising the reliability of the assemblies?

Answer: As the products become more sophisticated the ability to repair them becomes more dependent upon the particular laminate and the type of solder being used. The intent is based upon IPC-4101 and whether or not leaded solder or lead-free solder is being used.

You state “…Our present process: 2 heat excursions, thru reflow, 2 thru wave, one thru rework. How many more rework cycles can we do before compromising
the reliablity of the assemblies?”

In the past we used to recommend 5 thermal cycles and a removal and replacement was consider one cycle. I know this was incorrect, but that is the way we used to do it. This allowed us to fabricate the board with a fused tin/lead electroplate, (1 reflow cycle), wave solder (second reflow cycle) and three repairs for a total of five heat cycles. This was all based upon the work done by Roger Wild from IBM.

With the denser boards of today and the surface mount process, there are more thermal excursions due to components on both sides of the board, hence we have to be careful as to how and when we heat the board again to conduct rework. These thermal cycles is all acerbated by the small aspect ratio on the surface mount boards. The “Z” axis expansion can cause cracking in the barrels that won’t be found until the product is in the field, at which time it will be difficult to find and fix, plus not pleasing the customers.

I would recommend looking at each repair cycle location and the impact on the product reliability from exposing that area with localized external heat application. Some work should be done by building scrap boards and then get the appropriate number of microsections made up, to see if there was any damage to the internal layers and the microvias so a proper decision can be made as to whether that process can be used for the rework operation. I would also strongly recommend the use of thermal hot plates to elevate the product temperature and reduce the amount of thermal shock the board will see when exposed to the rework cycle of component removal and replacement process.