Radio Frequency Interconnect Loss Driven by Substrate Dielectric Degradation Mechanisms
Substrate moisture absorption increases dielectric loss tangent, raising RF interconnect insertion loss and reducing long-term wireless link margins.
Glass Transition Temperature
Meaning
Thermal transition points in amorphous materials mark the change from a rigid, glassy state to a compliant, rubbery state. Glass transition temperature defines the limit where the polymer chains in a circuit board substrate gain enough mobility to move relative to one another. Below this point, the material is dimensionally stable and provides strong support for copper traces.
Above this temperature, the mechanical and electrical properties change rapidly.
Expansion Shift
Thermal expansion coefficients increase by a factor of three or more once the material exceeds this threshold. The glass transition temperature is the point where the rate of expansion in the thickness direction changes. This sudden increase puts mechanical stress on plated through holes and vias.
Processing Limit
Manufacturing steps like lead-free soldering subject the board to temperatures well above the transition point. High glass transition temperature resins are necessary to prevent delamination or barrel cracking during the assembly flow. Choosing a material with a high rating ensures the board remains flat and reliable through multiple reflow cycles.
Material Grade
Selection of a specific epoxy system depends on the expected thermal environment of the final product. Standard FR-4 materials typically offer a glass transition temperature between 130 and 140 degrees Celsius. Performance-grade resins for server or industrial applications reach 170 degrees or higher.
These values are verified using differential scanning calorimetry during the material qualification phase. Suppliers provide data sheets that specify the minimum value for each laminate type to guide the hardware designer.