Substrate Viscoelastic Properties and Thermal Expansion Mismatch in MEMS Packaging
Substrate viscoelastic relaxation and thermal expansion mismatch induce time-dependent, hysteretic offset drift in MEMS requiring mechanical anchor isolation.
Micro-Plastic Solder Creep
Time Dependent Material Deformation
Permanent strain accumulates in solder joints when they are subjected to mechanical stress at elevated temperatures. Micro-plastic solder creep occurs even when the applied load is far below the ultimate tensile strength of the alloy. The slow flow of metal eventually leads to the cracking and failure of electrical connections.
Homologous Temperature Factor
Solder is particularly susceptible because its melting point is relatively low compared to the operating temperatures of electronics. When the environment reaches half of the absolute melting temperature of the metal, micro-plastic solder creep becomes a dominant failure mode. Thermal expansion differences between the circuit board and the components drive this continuous deformation.
Grain Structure Evolution
Atoms within the alloy migrate along grain boundaries to relieve internal pressure. Over time, micro-plastic solder creep causes the grain structure to coarsen and weakens the mechanical integrity of the joint. The process is accelerated by the power cycles of the equipment which create fluctuating thermal stresses.
Reliability Assessment
Design engineers use accelerated aging tests to predict the lifespan of a product. Understanding micro-plastic solder creep allows for the selection of alloys and potting compounds that minimize strain. Proper thermal management remains the most effective way to slow the rate of material flow.