In Electronic and Electrical assemblies, why is the Glass Transition Temperature and CTE important when selecting a Polyurethane or Epoxy System?
In demanding electrical/electronic applications, thermosetting epoxies and polyurethanes may be used to bond components or hermetically seal the assemblies through a potting or casting process. In operation, these assemblies will experience some form of thermal cycling, whether it be through heat generation, or through an environmental temperature change.
Due to these thermal changes, a polymers T(g) (glass transition temperature) and CTE (co-efficient of thermal expansion) should be factored into the decision tree when selecting an appropriate polymer system.
As presented in the TMA graph in figure 1, the T(g) is the temperature at which the polymer transitions from the glassy, rigid state into the flexible, rubbery state. A marked change in CTE also takes place at this transition temperature. A typical filled epoxy system may have a CTE in the range of 30-60 ppm below its T(g) and a CTE in the range of 120-180 ppm above its T(g).
Why is it important to know these physical properties when selecting a polymer for potting or bonding electronic assemblies?
Components in an electronic assembly may be pressure sensitive or delicate in nature and they may have CTE’s much lower than 30 ppm. When these assemblies thermal cycle in operation any mismatch in CTE, or change in CTE, can cause damage to the assembly.
This damage may show up as stress cracks in or around components, or a loss of adhesion between components. In the end, this damage may compromise the hermetic seal and or affect the reliability of the device.
This challenge has been addressed successfully in the industry by one of two ways. The first is to provide a polymer system with a high T(g) and low CTE. An example of this would be an epoxy system with a high loading of inorganic filler. This system would have a T(g) higher than the upper most operating temperature of the device, and a filler loading as high as possible to provide the lowest possible CTE and lowest shrink. The goal is to provide a system that closely mirrors the dimensional changes of the components in the assembly during a thermal cycle. This helps minimize the development of any stresses within the assembly. An approach that has been successfully implemented in many industry applications. A second way to address this challenge is to provide a polymer system with a very low T(g). An example of this would be a low hardness shore A polyurethane system. This system would have a T(g) below the lowest operating temperature of the device. This would provide a polymer system that remains flexible and rubbery throughout the entire operating temperature range of the device. As a result, this low stress polymer will not damage delicate components during thermal cycling. The low modulus environment cushions the components and allows them to move freely without damage. This approach too, has found industry success in the bonding and potting of electrical/electronic assemblies.
By understanding the fundamental properties of T(g) and CTE, we can overcome the challenges and provide two polymers with very dissimilar properties that will perform in a demanding thermal cycling environment.
