The control of the metallurgy of new brazing alloys, together with their wetting characteristics and the process temperature were found to be essential in optimizing the electrical performance of the final assembly of a power semiconductor device. Considerable effort was devoted to 3 main areas, namely, reduced temperature brazing on coated molybdenum plates using existing aluminium brazes, low temperature brazing with newly developed aluminium based alloys acid low temperature bonding with newly developed nonaluminium based alloys. Positive results were obtained in each case.
Coated molybdenum plates have been developed to the point of implementation in standard production. Selected coatings were rhodium, palladium and molybdenum disilicide, which permit wetting by the aluminuim-silicon braze alloy at a lower temperature than is possible with the standard process. All 3 coatings gave very good results but rhodium was finally chosen for economic reasons.
Aluminium based alloys and nonaluminium based alloys were systematically explored. A silver-aluminium-geranium alloy was developed and tested in current devices with encouraging results. However for use in industrial processes the alloy should be available as preformed discs and the production of such disc has yet to be proved.
Nonaluminium based alloys have also been devised and evaluated and suitable processes have been developed for application in a manufacturing environment. In particular a diffusion soldering process has been developed which combines features of conventional soldering and diffusion bonding processes. It relies on the reaction between a thin layer of molten solder with metal on the component to form one or more intermetallic phases that are solid at the joining temperature. The joint will not remelt thereafter unless it is heated to the higher temperature at which one of the intermetallic phases melts. 2 alloy systems were selected for detailed assessment, the silver-tin and silver-indium sy stems. For production purposes the silver-tin system was chosen. It has a bonding temperature of 250 C and a remelt temperature in excess of 500 C.
The development of new materials and structures for the backing plates in high power semiconductor devices covered 3 main areas, namely, tungsten alloy compensating plates of improved expansion match, composite compensating plates of controlled expansion and direct bonded high copper heat sinks.
The development of tungsten alloy and composite materials has led to the feasibility of innovative structures being demonstrated with the practical realization of inoar-tungsten/copper plates. 50 mm diameter thyoistor units were bonded to tungsten-nickel-copper alloy plates with inoar interleaving. These survived 1000temperature cycles between liquid nitrogen temperature and 200 C.
Direct bonded high copper heat sinks were addressed at a feasibility level, but their development was soon recognized as too demanding and costly to be practical for commercial use.
The possible innovations identified range from the feasibility level to near future exploitation possibilities and to introduction in actual manufacturing processes.
THE PRESENT TECHNOLOGY FOR THE FABRICATION OF SILICON-HEAT-SINK ASSEMBLIES IN HIGH POWER SEMICONDUCTOR DEVICES INVOLVES THE BRAZING OF VERY LARGE AREA SILICON WAFERS TO RIGID MOLYBDENUM DISCS.
CRITICAL FEATURES ARE THAT THE BRAZE MUST INTERACT SUFFICIENTLY TO FORM A JOINT STRONG ENOUGH TO WITHSTAND THE HIGH STRESSES ASSOCIATED WITH THE THERMAL CYCLING AND YET THE PENETRATION OF THE BRAZE MUST BE LIMITED TO AVOID APPROACHING THE ACTIVE REGION OF THE DEVICE. THIS RESEARCH PROJECT IS AIMING AT THE IMPROVEMENT IN TECHNOLOGY OF SILICON-HEAT-SINK FABRICATION THROUGH EXPLOITATION OF THE FOLLOWING OPTIONS:
- DEVELOPMENT OF AL-BASED ALLOYS FOR BRAZING TO MO AND W;
- DEVELOPMENT OF TECHNIQUES FOR REDUCING THE BRAZING TEMPERATURE;- DEVELOPMENT OF NEW NON AL-BASED ALLOYS FOR LOW TEMPERATURE SOLDERING;- DEVELOPMENT OF TUNGSTEN ALLOY COMPENSATING PLATES OF IMPROVED EXPANSION MATCH AND OF COMPOSITE HEAT-SINK ASSEMBLY;
- DEVELOPMENT OF DIRECT-BONDED HIGH COPPER HEAT-SINKS.
Funding SchemeCSC - Cost-sharing contracts