Thermal Radiographic Failure Analysis of Capacitor Encapsulant Fracturing
Multiple tantalum capacitors are SMT attached to PCBs. Failures or fractures were observed at contractor SMT assembly lines and not on RTX lines.
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Authored By:
Norman J. Armendariz, Ph.D.
RTX- Raytheon
AZ, USA
Summary
Fractures have been observed on polymer tantalum capacitors in a repeatable longitudinal direction along and within the top-region of the molded epoxy encapsulant of sub-contractor SMT (surface mount technology) assembled CCAs (circuit card assemblies).
In-situ radiographic experiments were performed on as-received capacitors using a transmissive x-ray fitted with a heating or hot-stage programmed to simulate SMT production reflow profiles that revealed material forming on top lead frame within the encapsulant below < 232oC.
Visual inspection afterwards confirmed longitudinal fractures were recreated with metal found in the fractures. EDS (energy dispersive spectroscopy) analysis found small amounts of Ag (silver) along with Sn (tin) in the metal.
Failure mechanism attributed to Ag from Ag-adhesives that reacted with the lead-frame Sn-plating, forming a Sn-Ag 3.8% eutectic alloy ~221oC, which expanded and fractured the mold epoxy encapsulant along the top lead-frame in a longitudinal manner.
Conclusions
- Capacitor encapsulant longitudinal fractures were replicated. Ag-Sn radiographically viewed in -situ forming on top lead-frames under simulated SMT reflow profiles.
- Both X-ray hot-stage reflow experiments showed material forming ~180-190o C. Actual temperature at top of device most likely higher since TC was on opposite side of PCB.
- Visual inspection afterwards and EDS confirmed Ag-Sn emerging from the fracture as observed in previous production failures.
Initially Published in the SMTA Proceedings
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