Al2O3 or AIN ceramic, to which the die is soldered。 The DBC is often mechanically attached to

a heat sink without using another intervening base plate。 This approach was described earlier as

used by Semikron in its SKiiP power module。 The design has a low profile and reasonably good

heat path due to the thinness of the DBC。 It has good thermal cycling reliability because the CTE

of the DBC is dominated by the low CTE of the ceramic layer and because the mechanical

attachments of the DBC to the heat sink can accommodate differential movement during cycling。

Packages using a copper base plate to stiffen the substrate layer and make the package suitable

for attachment to a heat sink have to contend with the solder attachment to the base plate。

Semikron eliminated the base plate by using DBC with a plastic frame to evenly distribute

pressure across the DBC when pressed directly against the heat sink。 Built into this plastic frame

are the leads that conduct current into and out of the package。 Because this package eliminates

materials that have high differences in CTEs, it would seem an ideal candidate to modify for

high temperature use。 Between the DBC substrate and the heat sink there is a layer of thermal

interface material, which in the high temperature concept would most likely be graphite that can

withstand 300°C。 All plastics could be replaced with higher temperature plastics, a number of

which are available。

7。 3。 2 Die Attachment and Material Selection

Besides the plastic in the frame, the solder used in the package must be replaced with a solder

able to withstand temperature over 300°C。 With this first design concept, the goal is to keep the package as conventional as possible, so the commonly used gold-silicon eutectic solder (Indium 

product) has been chosen。 This ensures that the package can be readily assembled with minimal

change to the processes established for standard temperature power semiconductor packages。

Next the materials and geometry of the DBC layer have to be chosen。 Commercial DBC offers a

choice between two ceramics Al2O3 (alumina) and AIN。 There are tradeoffs in selecting between

the two。 The thermal conductivity of alumina is almost an order of magnitude less than that of

AIN。 The CTE of Alumina is roughly 6 to 7 ppm compared to AIN with a CIE of4。 5 ppm/K。

The CTB difference is important; alumina is better suited for use in DBC because the CTE

difference between alumina and copper is smaller than with AIN。 This makes alumina based

DBC less prone to thermal shock failure than AIN based DBC。 In is design, however, the

stresses in the AJN DBC were not enough to cause failure, so AIN was chosen for the ceramic

due to the better match of the CTE with the device, thereby lowering the stresses in the

semiconductor die and the solder layer attaching the die to the substrate。 The following thermal

stress analysis will elaborate。

7。 3。3 Thermal Analysis

Figure 7 shows the 3D package with the overall themz1 profile。 The pressure frame and the

external structure are omitted for clarity。 By allowing the junction temperature of the die to reach 300 °C, the thermal package can be made very small-only about 4 inches on a side。 The model predicts these temperatures by assuming a 0。 5% loss in a 60 kw inverter bridge, which is 33% of the 1。 5% losses assumed in a silicon IGBT bridge。 The losses are assumed to occur 2/3 in the transistor and 1/3 in the diode。 Also assumed is an effective surface heat transfer coefficient of the heat sink fins of 25 W/m2/K。 This heat transfer coefficient is probably conservative (low), but is still believed to fall within a typical range of the heat transfer coefficients obtained when using forced convection with a fan。