Authored By:
Bhanu Sood and Michael Pecht
Center for Advanced Life Cycle Engineering (CALCE)
University of Maryland, College Park, MD USA
Summary
Moisture can accelerate various failure mechanisms in printed circuit board assemblies. Moisture can be initially present in the epoxy glass prepreg, absorbed during the wet processes in printed circuit board manufacturing, or diffuse into the printed circuit board during storage. Moisture can reside in the resin, resin/glass interfaces, and micro-cracks or voids due to defects.
Higher reflow temperatures associated with lead-free processing increase the vapor pressure, which can lead to higher amounts of moisture uptake compared to eutectic tin-lead reflow processes. In addition to cohesive or adhesive failures within the printed circuit board that lead to cracking and delamination, moisture can also lead to the creation of low impedance paths due to metal migration, interfacial degradation resulting in conductive filament formation, and changes in dimensional stability. Studies have shown that moisture can also reduce the glass-transition temperature and increase the dielectric constant, leading to a reduction in circuit switching speeds and an increase in propagation delay times.
This paper provides an overview of printed circuit board fabrication, followed by a brief discussion of moisture diffusion processes, governing models, and dependent variables. We then present guidelines for printed circuit board handling and storage during various stages of production and fabrication so as to mitigate moisture-induced failures.
Conclusions
Printed circuit boards have come a long way since the time when they existed mainly as a platform for connecting components. Boards are now part of the design, as features and even components are built into layers. The electrical and thermal properties of printed circuit boards are becoming critical factors in system designs. Many board developers do not realize that moisture in boards can significantly alter board performance. Moisture can negatively affect the integrity and reliability of printed circuit boards.
The presence of moisture in a printed circuit board alters its quality, functionality, thermal performance, and thermo-mechanical properties, thereby affecting overall performance. Moisture content can vary widely depending on how boards are handled. Regardless of how much protection is used, some moisture will be absorbed.
Parameters will be changed, even if boards are baked to dry them before they are used; this can alter performance from the specification sheet listings. This paper summarizes moisture-related issues and provides guidelines to reduce the impact of moisture on the reliability of printed circuitry boards. The controls and guidelines provided in the paper can be implemented at different stages of PCB production.
Initially Published in the IPC Proceedings
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