The present invention relates to an epoxy encapsulating material that has unique flexibility characteristics.
Electronic components often must operate under severe or harsh environments such as in automotive, marine and aerospace applications. In order to protect these sensitive components they often are coated with a protective encapsulating material.
It is known to use various epoxies as flexible encapsulating materials for electronic components. Encapsulating materials can be exposed to continuous temperatures of up to about 140.degree. C. and intermittent temperatures of up to about 150.degree. C. Such exposure is referred to herein as "thermal aging". However, the known epoxies such as EPOLITE FH-1432 commercially available from H.B. Fuller Co. and XN-2248 Resin/XY-2233-1 Hardener commercially available from Nippon Pelnox Corporation exhibit substantial increases in hardness and modulus with thermal aging. For example, according to measurements obtained by the present inventors, EPOLITE FH-1432 has a flexural modulus of 1111.6 measured at -40.degree. C. after thermal aging of 504 hours at 140.degree. C. and a percent hardness change of 30.77 percent after thermal aging of 504 hours at 140.degree. C. and XN-2248 Resin/XY-2233-1 Hardener has a flexural modulus of 850.2 measured at -40.degree. C. after thermal aging of 504 hours at 140.degree. C. and a percent hardness change of 32.30 percent after thermal aging of 504 hours at 140.degree. C.
Failures occur because of stress induced on an electronic component over time due to changes that occur in the encapsulating material's mechanical properties or viscoelastic properties. These changes occur during thermal cycling and will induce extreme pressure on electronic components. Below -20.degree. C., the conventional chemistry for flexible epoxies causes them to become very rigid and have very high modulus. Also, the hardness of these materials increases dramatically when exposed to high temperatures of 130.degree. C. or more for any length of time. This is due to the additional crosslinking of the epoxy resin and curative. This combination of limited glass transition temperature, very high modulus below the Tg and thermal age hardening causes poor thermal cycle performance and dramatically increases stress on electronic components. The varying stress fatigues the solder or the component itself and eventually causes an open circuit or a short. Consequently, it would be very advantageous to have an epoxy encapsulating material that continues to exhibit flexibility after thermal aging.