Evaluating Accuracy of Thermocouple Attach Methods
The goal of this study was to identify a non-destructive method for thermocouple attachment that provides a small offset to the "actual temperature under a BGA."
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Authored By:
Dr. S. Manian Ramkumar and Tim Grove
Center for Electronics Manufacturing and Assembly Rochester Institute of Technology
Rochester, NY
Brian O'Leary KIC
Summary
The oven recipe, which consists of the reflow oven zone temperature settings and the speed of the conveyor, will determine a specific time (temperature profile for a given PCB assembly. In order to achieve a good quality PCB assembly, the time) temperature profile should be within the product and process specifications. This is determined by the solder paste, components and substrate tolerances.
As a result, the accuracy of the profile becomes a critical element in the quality of the electronics assembly. The methods by which thermocouples (TCs) are attached to the PCB assembly, to record the profile as the PCB travels through the oven, significantly impact the measuring accuracy of the profile.
Many electronics assemblers do not have the luxury of sacrificing production PCBs and BGAs for the purpose of measuring their profiles. Yet they need to make sure that these assemblies are processed in spec. Area array packages have solder balls hidden under the package, making it particularly difficult to achieve the correct thermal profile. Improper melting of solder balls will lead to poor solder joint formation and will damage the BGAs or the entire assembly.
These components also tend to be expensive and, hence, represent a particular challenge for assemblers. The goal of this study was to identify a non-destructive method for TC attachment that provides a small offset to the "actual temperature under a BGA."
Conclusions
Using aluminum tape to attach a TC directly onto the top of the BGA provides a good approximation of the temperature readings under a BGA. Furthermore, this offset can be calculated with a reasonable level of confidence by using a formula developed in this research and displayed in this article. For a relatively small BGA and thin PCB, that offset is less than 2 C. Thicker boards and larger BGAs produce larger offsets, which can be approximated by the referenced formula and associated graph.
Initially Published in the IPC Proceedings
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