Green molding compounds for high temperature automotive applications

Temperature is the main accelerating factor for most of the failure mechanisms affecting semiconductor devices. The limitations imposed by temperature are particularly severe on power devices used in an automotive environment. The number of electronic components in cars and trucks is continually growing. The development of new automotive controls is possible due to the integration between automotive components (engine, transmission, brakes) and electronic controls. Unfortunately, these electronic controls are often in direct contact with these components in a hostile high temperature environment. A high temperature reliability goal, often cited, is 20,000 hours at 200degreesC (the average "key-on" lifetime for a truck). The limitations imposed by temperature are particularly severe in the components assembled in plastic packages, due to the thermomechanical and chemical properties of the materials used as molding compounds. In particular, antimony trioxide (Sb2O3) and brominated (Br) flame retardants used in molding compounds are well known to degrade the gold wire/aluminum bonding pad interface at high temperatures, due to the formation of Kirkendall voids, leading to increased electrical resistance and loss of strength. Recent development of molding compounds using brominated flame retardants and a transition metal oxide flame retardant has lead to greatly improved high temperature reliability. Wire bonds in a device molded with this new type of molding compound show no strength degradation and no evidence of Kirkendall voids after 3,000 hours at 180 and 2.5 amps current. Unfortunately, at 200degreesC, this new type of molding compound does show a sharp increase in resistivity at only 700 hrs. Further development has lead to a "Green" molding compound containing neither Sb or Br, but rather a mixture of transition metal oxides as flame retardants. This Green molding compound shows good high temperature reliability at 200degreesC and 2.5 amps current. It can be concluded that degradation of gold/aluminum wire bonds is a major constraint to reach high temperature operation of automotive power devices and that the selection of a proper molding compound is fundamental to achieve this target.