Electronics Assembly Knowledge, Vision & Wisdom
Practices to Assure Consistent Hot Gas Rework
Practices to Assure Consistent Hot Gas Rework
Paper discusses practices observed at various CMs that can lead to inconsistency in rework and techniques that will reduce or eliminate the inconsistency.
Production Floor

Authored By:
Mitchell Ferrill, IBM Corporation, Endicott, NY, USA

Musheer Ahmed, Celestica Inc., Toronto, ON, Canada
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Hot gas rework is a critical process for the recovery of high priced electronic card assemblies which need defects repaired or components upgraded. It is a requirement that the rework process produce quality and reliability levels that are equivalent to original assemblies. Much time and attention is generally spent establishing a program, or recipe, for reworking a particular device location on a complex assembly.

This is normally accomplished by inserting thermocouples into the component solder joints in order to measure the temperature experienced during the process. Having the correct recipe is critical for achieving the proper metallurgical interaction, especially for mixed metallurgy (Pb free device attached to a card assembly with SnPb paste) and Pb free assembly. Most automated hot gas rework tools have a trigger (or tool control) thermocouple which is used to initiate the rework program. Once this thermocouple reaches a specific temperature a series of timed steps begin.

Care must be taken to locate this thermocouple in nearly the same location as that used to establish the recipe with the thermocoupled profile card. It is also critical that this trigger thermocouple be in intimate and consistent contact with the PCB (Printed Circuit Board) surface. If proper attention to these details is not taken, then the resultant peak temperatures actually reached on the product may be much different than expected. This can result in poor quality solder joints that pose a reliability exposure. This paper discusses some common practices observed at various CMs (Contract Manufacturers) that can lead to inconsistency in rework. It also discusses practices and techniques that will reduce and/or eliminate the potential inconsistency.
Hot gas rework will continue to be a critical process for electronic assembly. Due to the critical nature of the operation, rework must be more than a "press go" and walk away until finished process. Proper setup, control, and monitoring will assure that a high quality and reliable product results. This is especially important to assure that the proper metallurgical structure is formed.

Rework CMs need to have a focus on defining clear and precise process instructions, especially as it relates to positioning of the tool control (trigger) thermocouple. These instructions need to be clear enough to assure the thermocouple is positioned in the same location each and every time rework is performed.

There also needs to be a focus on establishing a reliable trigger thermocouple attach method, such that consistent and repeatable contact is maintained with the PCB. Reuse of thermocouple attach tape is not a good practice. Rework CMs should also take advantage of the trigger thermocouple temperature data output available from most hot gas rework tools. This data can be used as an indicator or process monitor to verify a successful rework has been completed.

This is done by taking some extra time during the initial rework profiling stage. Multiple profiling runs should be performed, to establish an expected peak trigger thermocouple temperature and tolerance. When the peak temperature falls within this window, this provides another indicator that the rework operation was successful.

Often visual inspection and electrical test are the only methods used to verify a successful rework. Visual inspection is difficult, usually requiring a mirror, and can only evaluate the very outer rows of the array device. Electrical test confirms metallurgical connection, but does not provide assess metallurgical structure. Following these recommended procedures and controls will assure the best opportunity for rework success
Initially Published in the SMTA Proceedings
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