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Process Development with Temperature Sensitive Components in Server Applications

Posted on Sep 01, 2006


As the electronics industry prepares for the possibility of Pb-free Printed Circuit Board Assembly (PCBA) processing without the

EU RoHS server Pb solder exemption, many studies continue to focus on attributes of assembly material chemistries, board finishes and processing techniques. These efforts generally target critical components such as ball grid array packages (BGAs) to ensure reliable solder joints that meet operational requirements at time zero and lifetime reliability targets. For consumer products, this approach may address the known failure mechanisms of the subject card, components, and assembly.

For high reliability products, there could be failure mechanisms in Temperature Sensitive Components that extend beyond critical components in BGA packages. These components include SMT aluminum capacitors, tantalum ceramic capacitors, crystals, oscillators, fuses and other components which have temperature limitations on the package body that restrict the peak reflow temperature and the time duration above 217°C. Exceeding these temperature and time limitations may not induce time zero fails, but may reduce the long term reliability of the component. These components have all been classified and specified for Pb-free processing by their suppliers. Users, including designers and assemblers, may consider these components as non-risk components capable of withstanding the Pb-free evaluation peak temperatures of up to 260°C as set forth for ICs in J-STD-020. Additionally, as part of temperature profiling efforts, most assemblers do not attach thermocouples to these components resulting in an absence of data collection to ascertain specification compliance. As a result, SMT attach profiling protocol does not generally include the specifications of these temperature sensitive components and subsequently, any induced damage may propagate over time. Depending on the lifetime reliability requirements of the product, product owners risk quality and reliability impacts caused by time dependent failures.

This paper examines the risks that temperature sensitive components pose to high reliability Pb-free server product PCBAs and discusses various issues including smaller process windows, profiling methods and accuracy, time zero quality, and expected reliability performance when using these components. Additionally, the intent of this work is to document the need to identify and monitor temperature sensitive components within industry standards, to extend awareness, and to enable designers / card assemblers to adopt optimum design features and processing techniques. The intended result is to help ensure that products can meet specified lifetime reliability requirements.

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