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.
Lead-Free Reflow Warpage
Thermal Deformation
High soldering temperatures required for tin-silver-copper alloys can cause printed circuit boards to deform during the assembly process. This thermal phenomenon is termed lead-free reflow warpage, and it can cause defects such as open joints or solder bridging. As the board is heated above two hundred degrees Celsius, the differing coefficients of thermal expansion of the laminate material and copper layers generate internal stresses.
These stresses distort the board planarity.
Substrate Stress
Asymmetrical copper distribution across the different layers of a circuit board exacerbates this bending behavior. To minimize lead-free reflow warpage, designers must balance the copper density on both sides of the substrate. This balanced design distributes the thermal forces more evenly.
Measurement Technique
Optical profiling systems use shadow moire techniques to map the flat surface of a board as it goes through a simulated oven run. These measurements quantify the dynamic lead-free reflow warpage to ensure that the deformation remains within IPC standards. This profiling occurs before mass production starts.
Process Optimization
Adjusting the oven profile can mitigate some of the thermal shock experienced by the assembly. By reducing the cooling rate and using proper supports, the effects of lead-free reflow warpage are kept within acceptable tolerances. This optimization improves first-pass yield.