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 voiding. The formation of voids is probably accelerated by the rough thick film structure containing impurities and defects. Thick film silver plated with Cu-Ni-Au displays a much more promising behaviour. Bond-pull values above 4.5 gram and 7% increase of electrical resistance is obtained after 6 months at 250ºC. The thin gold layer (< 0.01 mm) diffuses into the aluminium bond during the first hours of high temperature exposure and limits further AuAl growth. Aluminium and nickel forms intermetallics at a much slower rate, and thus a stable high temperature connection is obtained. The same type of behaviour is also observed for LTCC silver conductor plated with Ni and Au.

Aluminium wire-bonding to thin film gold on silicon or ceramic substrate also displays acceptable mechanical strength after HT ageing. For thin film gold on silicon 5 gram bond-bull strength is obtained after 6 months at 250ºC. Again, the thin gold layer diffuses and forms AuAl intermetallic during the first hours of HT exposure. The microscope images reveal no significant growth in the intermetallic layer during the long term HT ageing. In addition, the thin film gold is pure and very homogenous, and there are no signs of void formation in the intermetallic layer. For aluminium wire-bonding to aluminium thin film, good results were expected since this is a monometallic system. However, micro-cracks were formed in the periphery of the contact area during bonding. These cracks were growing during high temperature ageing and resulted in a variable mechanical strength with bond-pull values ranging from 1.7 to 6.3 gram.

The results presented in this work show that long term reliable aluminium wire-bonds for 250ºC operation is feasible both with thin film, thick film and LTCC substrate technology. For the screen-printed conductors, a plating system with nickel is necessary while standard thin film gold may be used directly. The choice of substrate technology will depend on the type of components involved, size requirements and compatibility with the higher level packaging.

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