Transfer Molded DIP-IPM Package Improves Thermal Performance

May 1, 2010 12:00 PM
Eric Motto Application Engineer, Powerex Inc., Youngwood, PA.

The transfer molded DIP-IPM was first introduced in 1998 to address the rapidly growing demand for cost-effective motor control in consumer appliance applications. These devices soon became widely accepted due to their performance, reliability and cost advantages compared to conventional designs based in discrete devices. In the years that followed, continuous improvements in package thermal performance, power chip design, and HVIC (High Voltage Integrated Circuit) technology have enabled the development of a complete line of modules for motors rated from 100W to more than 15KW at line voltages of 100VAC to 480VAC.

In order to cost-effectively provide larger output power ratings, a new transfer molded package structure was developed. Fig. 1. shows the cross section of this new package compared to the previous large DIP-IPM. The DIP-IPMs are fabricated using a transfer molding process like a very large integrated circuit. First, bare power chips and the custom HVIC and LVIC die are assembled on a lead frame. Ultrasonic bonding of large diameter aluminum wires makes electrical connections between the power chips and lead frame. Small diameter gold wires are bonded to make the signal level connections between the IC die and lead frame. This part of the process is basically the same for both devices. Next, they are encapsulated. This is where the packages differ.

A cross section of the previous generation 3 Large DIP-IPM is shown in Fig. 1(a). In this device, a heat spreader made of copper is attached to the lead frame and electrical insulation is provided by a thin layer of the molding compound at the mounting surface. The copper heat spreader gives relatively good thermal performance, but the high thermal resistance of the molding compound limits this construction to devices with ratings of about 50A at elevated case temperatures.

The new generation 4 large DIP-IPM package cross section is shown in Fig. 1(b). This device uses a new low thermal impedance structure based on technology developed for the generation 4 super Mini DIP-IPM [1]. In this novel structure, a partially cured insulating resin sheet is adhered to the rear surface of lead frame after chip bonding. The other surface of the resin sheet is attached to an aluminium heat spreader. The lead frame with the resin and aluminium heat spreader attached is then transfer-molded using epoxy resin. The transfer molding process causes the resin sheet to cure simultaneously with the epoxy resin. The result is a stable high reliability joint with low thermal impedance. The thin insulating resin sheet stays in a fixed form during the process, so it does not need to have the fluidity of the epoxy resin over mold . Thus, it is possible to increase the amount of ceramic fill to improve the thermal conductivity.

In addition, it is possible to achieve a thinner insulating layer because it is not constrained by the limitations of the molding process. The extremely thin layer of high thermal conductivity resin yields a substantial reduction in thermal impedance compared to previous DIP-IPM designs. Fig. 2. shows a photograph of the new large DIP in its final form. The module features a compact 31mm × 79mm footprint.

NEW LARGE DIP-IPM

In addition to the six IGBTs and free wheeling diodes required for a three-phase motor drive, the new large DIP-IPM also contains HVIC and LVIC chips to provide gate drive and protection for the power devices. Fig. 3 shows a complete functional diagram of the DIP-IPM.

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