Meaning
A physical difference in the rate of dimensional expansion or contraction between joined materials during temperature changes causes mechanical stress in electronic assemblies. High differential expansion, known as cte mismatch, occurs when silicon dies with low expansion rates are soldered to laminate substrates with high expansion rates. This difference leads to shear stress at the solder joints during thermal cycling.
Interface Stress
Temperature fluctuations generate localized forces at the boundaries between copper, solder, and FR4 laminate. The magnitude of these forces depends on the temperature range and the mechanical stiffness of the components. Solid joints resist this force until the shear strain exceeds the yield point of the metal.
Failure Mode
Repetitive expansion differences eventually crack the solder interconnections or cause pad cratering in the circuit board. Delamination of internal copper traces can also occur when the vertical expansion of epoxy resin pulls the layers apart. Early thermal modeling identifies these high-stress joints prior to assembly.
Material Selection
Designers balance these thermal expansion differences by selecting epoxy resins with inorganic fillers or using low-expansion underfills. These specialized materials constrain the lateral movement of the assembly during operation. Matching the expansion rates of the joined layers extends the service life of the product.
