Authored By:
Jeffrey Marcus Jennings
Harris Corporation
Melbourne, FL, USA
Summary
Control of moisture exposure and proper application of drying methods during the surface mount assembly process are essential to prevent moisture-related damage from soldering operations. For many component types (nonhermetic surface mount devices (SMD), non-integrated circuit (IC) electrical components, etc.), the handling, storage, and drying process are clearly specified (see IPC/JEDEC J-STD-020, -033 & -075). Unfortunately, the guidance these specifications provide for electrical connectors is less definitive despite some of their constituent materials potentially exhibiting similar moisture related soldering process incompatibilities. In addition, these pecifications exclude manual soldering operations, referred to as "Point to Point" and wave soldering, which are more commonly used during connector installation. The potential for moisture-related damage is particularly concerning for radio frequency (RF) connector polymeric dielectrics which often are selected based on their electrical performance characteristics rather than their processing robustness.
In the current study, the tendencies of various commonly used RF connector polymeric materials to absorb and desorb moisture under various humidity and temperature conditions are characterized with the impact of sample size effects on these properties being considered. Differences in the dimensional stability of these materials during soldering temperatures in a moisture saturated versus dried condition are compared utilizing thermomechanical analysis (TMA). The materials are then exposed to various soldering process conditions, such as convective reflow, and solder pot dip to qualitatively identify any potential process incompatibilities. These results are then used to classify the relative "process" and "moisture sensitivity levels" for the various materials evaluated and provide recommendations for any special moisture-related handling, storage, and drying conditions for solderable connectors containing these materials.
Conclusions
Moisture effects on the polymeric materials used in solderable RF connector dielectrics have been studied. For the thermoplastic materials evaluated, the moisture absorption and desorption processes occur via Fickian diffusion. The time to saturated state depended on the material type, exposure condition and size of sample whereas the saturated % mass change depended on material type and exposure condition. Analysis of these materials in cylindrical geometries (solid and hollow) typical of connector dielectrics (within the size range evaluated) is simplified by using the ratio of their volume to surface area as an effective length in determining diffusion coefficients. The polymers evaluated fall into three categories of absorptive saturation behavior: highly absorptive; slightly absorptive; and non-absorptive. Exposure to soldering processes yielded two types of deleterious responses in the materials tested. Highly absorptive materials in a saturated state exhibit severe blistering while excessive softening materials deformed significantly when exposed to the soldering temperatures independent of moisture saturation
Initially Published in the SMTA Proceedings
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