Packaging electronics designed to manage high voltage and high current requires specific design and manufacturing techniques. Most of the differences in packaging technologies are related to increased temperature and power handling capability. Standard power packages are currently being used to handle the most common power transistor devices. Component packages such as the TO-220 can be observed on countless assemblies. It is common to see these components coupled to a heat sink to help manage the high temperatures of operation. This package is excellent for smaller devices that process 20 amps or less. However, larger power devices may require some custom packaging to make them work efficiently. The EMPF Power Packaging Laboratory was developed to process high power and high reliability devices and offers these tips when considering packaging power devices:

The Right Stuff
Consider using metallic die attach materials in lieu of organic polymer die attach materials. The traditional polymer materials may experience some degradation at the higher temperatures generated by larger power devices. High lead materials used for die attach have shown to be able to handle the increased amount of strain at high temperature. Even eutectic tin-lead die attach materials are effective at relaxing the excess stress caused by power electronic temperature extremes. Gold-tin die attach materials should also be avoided for high-power applications because they may not absorb the stress of the die expansion at temperature. To help dissipate the heat generated by the high-power die operation, consider the use of aluminum silicon carbide (AlSiC) between any heat spreader and the die. AlSiC has excellent thermal conductivity properties and also matches well with the thermal expansion characteristics of the die.

Avoid the Void
The amount of voiding present in any thermally conductive material, such as die attach, should be kept to an absolute minimum. The presence of voids creates thermal losses that can lead to localized hot spots on the die. To minimize voiding in die attach materials, the EMPF utilizes die placement equipment that has scrubbing and ultrasonic capability. These operations promote wetting and help dissipate voiding in the die attach. Also, forming gas or nitrogen are the preferred bonding environments as they can allow for fluxless reflow and help eliminate the formation of new voids.

Bond Big
Aluminum wire bonds are the preferred method of interconnect between the die and leadframe pads. The wire bonds can range in size from 5-20 mils in diameter. Typical high-power aluminum bonding wire will be greater than 10 mils. The wedge bonders used to create these wire bonds must have capillaries capable of handling large wire and must have the ultrasonic power capable of creating reliable intermetallics through ultrasonic welding at room temperature.  Gold wire is typically avoided in these applications because of the negative effects of diffusion and intermetallic formation at higher temperatures. If gold wire is bonded to aluminum pads in a power package, there is a risk of the following occurring:

•     Kirkendall voiding at the bond connection increases the electrical resistance
•     Purple Plague intermetallic formation decreases the mechanical strength of the joint
•     Horsting voiding increases the resistivity
•     White Plague at high temperature decreases the mechanical reliability of the bond

There is Safety in Numbers
Multiple wires at the same connection can be used for high-current applications to reduce the aluminum wire temperatures and increase long term reliability. However, increasing the number of wire bonds in the circuit will also increase the inductance of the circuit. The benefit of using multiple wires must be carefully weighed against the detriment of increased inductance. High-current applications may warrant the need for power ribbon. The larger aluminum power ribbons can carry more current than standard aluminum wire and therefore fewer number of wire bonds can be used for the same connection.

Successful electronics power packaging involves careful planning of the design, materials, equipment, and processes. Die attach, voiding, and wire bonding are three of the most critical parameters that must be addressed when working with power packages. Many conventional electronic practices, processes, and equipment are not applicable for packaging high reliability power devices.