Selecting Tooling Shields for Thermal Protection during Selective Soldering Cycles
Selecting selective soldering tooling shields requires matching composite thermal resistivity against component clearance gaps to guarantee zero thermal damage.
ESD Resistivity
Material Conductivity
Surface electrical impedance measures the opposition to current flow across the outer layers of printed circuit board laminates and assembly coatings. Esd resistivity acts as a gauge for how effectively a substrate dissipates static charge rather than allowing voltage buildup. Manufacturers define this property in ohms per square to categorize materials into insulative, dissipative, or conductive groupings.
High resistance blocks current pathing, whereas low resistance facilitates rapid energy discharge during grounding procedures. Precise control of these values prevents dielectric breakdown and hardware failure.
Validation Method
Testers apply a constant voltage between two electrodes placed at a fixed distance on the component surface to derive the logarithmic resistance value. An operator utilizes a megohmmeter to record the current flow through the dielectric material or surface finish. Calibration occurs using reference standards that verify the accuracy of the probe contact pressure and environmental conditions like humidity or temperature.
Stable measurements prove that the antistatic coating application is uniform across the entire panel surface. Deviation from specified threshold ranges suggests contamination on the board finish or failure in the secondary coating process.
Production Boundary
Final assembly lines require strict adherence to these electrical parameters to minimize damage during robotic placement or manual handling stages. Excessively low resistivity allows unintended current paths to form between trace geometries if the board surface remains damp or contaminated. Conversely, values exceeding the upper limit prevent the orderly removal of static charge from the module during contact with transport fixtures.
Engineers select materials based on their ability to maintain consistent resistivity levels under the thermal stress of solder reflow cycles. Proper grounding of all metallic fixtures further mitigates the risk of catastrophic discharge event.