Abstract
There is an increasing demand for sensors and electronics that can be operated in demanding environments with high temperature (HT), pressure and vibrations. Applications include sensors and actuators for down-hole well logging and control, and distributed control systems for automotive and aerospace industry.Reliable interconnects are essential for making microelectronic systems with long life time in harsh environments. Intermetallic growth accelerates as the temperature increases, and the material system must be carefully selected to avoid mechanically and electrically weak connections. Standard chip metallization is aluminium, and aluminium wire-bonding is recommended to obtain a mono-metallic system at the chip level. The challenge is to find a suitable substrate metallization compatible with aluminium wire-bonds at high temperatures.SINTEF has evaluated several different substrate metallizations with respect to long term wire-bond reliability. Thick film gold and silver thick film plated with copper-nickel-gold, gold and aluminium thin film and LTCC silver conductor plated with nickel-gold have been among the candidates. Test substrates with the different metallizations were fabricated and wedge-wedge aluminium wire-bonding was performed with 25 mm AlSi or AlMg wire. The substrates were subjected to long term ageing at temperatures in the range 200 to 250 deg C. The duration of the test was minimum 6 months and some of the test substrates have been running for 2.5 years. Bond pull-strength and/or electrical resistance were measured during ageing. After ageing, selected wire-bonds were cross-sectioned and inspected using scanning electron microscopy.The results obtained show that gold thick film on ceramic substrate is not suitable for aluminium wire-bonding in HT environment. The bond-pull strength is reduced to 1 gram after 6 months ageing at 250ºC. This is related to a degradation of the wire-bond by AuAl intermetallic growth and corresponding Kirkendall void