Quantifying Inner Layer Interconnect Strain during Solder Reflow
Matching dielectric CTE above Tg to copper ductility prevents inner layer interconnect fracture during high temperature reflow thermal profile cycles.
Dielectric Resin Expansion
Thermal Displacement
Thermosetting polymer matrix materials undergo a volumetric increase when the operating temperature crosses the glass transition threshold. Dielectric resin expansion forces the copper traces within a multilayer circuit board to move from their original registration during high-heat soldering operations or extreme service environments. This mechanical shift places stress on internal via barrels and laminate bonds.
Structural integrity failures occur when the force of this movement exceeds the tensile strength of the interconnecting metallic elements.
Material Characteristic
Manufacturers evaluate this physical behavior through the coefficient of thermal expansion, which quantifies the dimensional change per unit of temperature. Testing involves heating a sample through its transition point while tracking the displacement using thermomechanical analysis equipment. Designers compensate for these movements by specifying materials with glass transition temperatures exceeding the peak processing temperatures of the assembly flow.
High rates of internal movement correlate with increased probability of barrel cracking and delamination at the interface between the metal and the dielectric.
Interconnect Risk
Copper plated through holes suffer fatigue during the life cycle of a product if the surrounding base material expands at a rate significantly higher than the metal. Rigid boards with high layer counts require low profile or low expansion resin systems to minimize the shear forces acting on plated interconnects. Failure to maintain stability during thermal cycling leads to intermittent electrical contact and eventual open circuits.
Controlled movement of the insulating matrix prevents excessive strain on the metallic structure.