For electronic assembly integrated circuits (ICs) are mounted on printed circuit boards and encapsulated by an encapsulant (Glob-top) afterwards to protect them from mechanical damage, moisture and radiation which could negatively affect the device performance, reliability or lifetime. Well known encapsulants and die-bonding materials used at temperatures below 150°C are silicones, epoxies and polyimides. In the case of die-bonding metals like silver are intermixed to these materials to provide an electrically conducting contact. While die-bonding and encapsulation of ICs is a common technique for applications up to 150°C, these low temperature adhesives and encapsulants fail in high temperature applications for temperatures higher than 150°C.

We investigated the reliability of different adhesives and encapsulants for temperatures up to 250°C. In our study thirteen encapsulants and four die-bond materials were characterized. We performed temperature cycling tests between room temperature and 250°C and storage test at a constant temperature of 250°C.

The bonding strength of a test die in a ceramic housing has been evaluated by shear tests after 50, 100, 200, 500, 1000, 2000 and 3000 temperature cycles. The bonding strength has been also tested after a constant temperature storage of 250°C every 24 hours up to 600 hours and subsequently every 700 hours up to 3000 hours. One of the tested die-bonding materials withstands this procedure without destroying the chip or loosing its bond strength.

Chips with the best adhesive have been die-bonded to a ceramic housing and bonded with 25 μm aluminium wire afterwards. The resistance of the bond wires have been measured every 24 hours up to 200 hours and subsequently every 700 hours up to 3000 hours at a constant temperature of 250°C and after 50, 100, 1000, 2000 and 3000 temperature cycles. The data show that most of the encapsulants become brittle at these temperatures or exhibit a large mismatch in their thermal expansion to the thermal expansion of the IC or the bond wires, resulting in a high thermomechanical stress and therefore in a destruction of the electrical connection between the chip and the substrate. One type of material for encapsulation showed no brittle effects and no destruction of the bondwires over the full period of test. With this encapsulant one could enhance the reliability of the IC devices and improve their mechanical and physical properties at high temperatures.

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1. Reliability testing of wire-bond at High Temperature Storage