Authored By:
Peter Lu, Robin Bridge, Max McKerlie
Curtiss Wright, Ottawa, ON, Canada
Summary
Commonly used integrated circuits have been increasingly moving from leaded packages that are reworkable with a soldering iron to leadless packages such as BGAs that require complex rework that cannot be visually inspected to confirm success. BGA packages have been decreasing in ball spacing (pitch) and increasing in I/O (ball count) making commonly accepted/documented time-dependent hot air local reflow profile development techniques no longer optimal. At the same time, the complex PCB interconnect structures (and resultant sensitivity to thermal cycles) needed to support these packages drive a need for tighter process control which is not attainable by using the classic time-dependent reflow profile. These factors combine to drive a need to improve the supporting processes around complex hot air rework.
Conclusions
We have shown that using a live feedback PCB temperature monitoring system to drive the parameters of a hot air reflow process, results are more consistent and there is better control over common practical variables that occur within the classic time-dependent methodology of hot air rework. The temperature values monitored are shown both analytically and experimentally to be sufficiently close and consistent to be relied upon to keep actual solder ball temperatures during the rework cycle within acceptable range for key criteria. From a practical viewpoint and proven within this paper experimentally it is clear that using a temperature-dependent profile during the BGA hot air rework process is less impacted by common production variables such as use of different machines, sub-optimal profiles generated with improperly configured samples, re-use of profile developed at a different PCB location, and TC1 Preheat trigger changing.
It has helped us reduced our total profile numbers from more than two thousand to around 50, and reduced the second rework rate significantly.
Initially Published in the SMTA Proceedings
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