Lead-free Reflow and Influence of Moisture
This paper examines concerns relating to lead-free reflow, PCB degradation, and the influence of moisture absorption.
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Authored By:
Kerin O'Toole, Bob Esser
Seth Binfield and Craig Hillman
DfR Solutions, College Park, MD
Joe Beers
Gold Circuits
Summary
The cracking and delamination of printed circuit boards (PCB) during exposure to elevated thermal exposure, such as reflow and rework, have always been a concern for the electronics industry. However, with the increasing spread of Pb-free assembly into industries with lower volume and higher complexity, the occurrence of these events is increasing in frequency.
Several telecom and enterprise original equipment manufacturers (OEMs) have reported that the robustness of their PCBs is their number one concern during the transition from SnPb to Pb-free product. Cracking and delamination within PCBs can be cohesive or adhesive in nature and can occur within the weave, along the weave, or at the copper/epoxy interface (see Figure 1). The particular role of moisture absorption and other PCB material properties, such as out of plane expansion on this phenomenon is still being debated.
Conclusions
Measurable change of capacitance was recorded after each reflow. Discrimination between different test structures and MSL exposures strongly suggests approach captures material degradation, as opposed to an increase in resistance at contact pads due to oxidation. However, contact resistance should be quantified in a next round of testing through ESR measurements. Strong difference in shield-over-shield capacitance between test structures B and C, due to the presence of non-functional pads, is very interesting and should be further characterized.
Initially Published in the IPC Proceedings
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