Module without bond wires, solder and thermal paste

Power module packaging is driven by the ever-increasing demand for higher power densities, reliability improvements and further cost reductions.

The known reliability limitations of traditional solder joints and bond wires are holding back significant power density increases, made possible due to higher junction temperatures and the future use of wide band gap devices.

Silver sintering has already started to replace the solder joint between chip and DBC substrate, leaving one major reliability bottleneck - the bond wire interface on the chip surface.

For some years now, the elimination of bond wires in power modules has been under discussion in industry and academia. Most of the new packaging approaches have been based on soldered or welded bumps, as well as on embedded interconnection.

The packaging technology, named SKiN Technology, takes the Ag (silver) sinter joint and applies it to all remaining interconnections in a modern power module.

In addition to the double-sided sintering of power chips, the entire DBC is sintered to the heat sink. The resulting device has a very high power density and demonstrates remarkable thermal, electrical and reliability performance compared to traditional packaging technologies.

Silver sintering is an established technology that has started to replace the soldering of chips to DBC substrates in mass production. Due to its unprecedented reliability and thermal behavior, it makes power modules better suited to higher temperatures and demanding applications such as electric vehicles and wind turbines.

However, two issues remain unaddressed: how to replace wire bonding on the chip top side and how to connect the power module to the heat sink.

SKiN Technology resolves both these matters by using Ag sinter technology for all interfaces. The chip surfaces are sintered on the top side to a flex layer and the chip bottom to a DBC substrate, which in turn is sintered to a heat sink or base plate. Figure 1 shows a schematic drawing of this packaging. The special flex foil has a metal base power layer that is comparable to bond wire diameter in thickness and serves to connect the chip top surface.

Figure 1: A schematic drawing of SKiN device.

A thin metal layer on top represents the gate and sensor tracks that are connected to the power layer by vias. The two metal layers are insulated from each other by polyamide.

The top layer can also be used for SMD components such as temperature sensors and gate resistors.

The second sinter joint connects the back of the chip to a standard DBC substrate. All standard IGBT and diode chips can be used for this process, they need just an additional noble metal contact treatment on the chip top side.

The third joint connects the back of the DBC using large-area Ag sintering to an aluminium pin fin water-cooled heat sink. The power terminals are sintered to the DBC in the same process step, resulting in a power module in which all interconnections are made with Ag sinter joints.

The main advantages of SKiN and its performance improvements are as follows:

• Power density:

The use of an Ag sinter layer instead of thermal paste will increase the power density as a result of the reduced thermal resistance chip to coolant. The large-area metal connection on the chip top surface will further improve the heat spread of the die;

• Reliability:

The replacement of Al (aluminium) bond wires by sintered flex foil will increase the power cycling capability thanks to better CTE compatibility of the materials used and the large-area connection between chip surface and contact medium;

• Electrical properties:

The use of the sintered flex foil, instead of the Al bond wires, will increase the maximum surge current rating of the dies as a result of the increased cross-section and area of the chip surface contact. In addition, the module stray inductance is decreased due to reduced loop geometry and wide traces.

The prototype design used for this performance comparison is a 600 V, 400 A half-bridge power module with an aluminium pin fin heat sink.

The chipset of the prototype samples consist of 2 x 200 A, 600 V IGBT and 1 x 275 A, 600 V CAL freewheeling diode per switch. The power terminals are placed on the opposing short sides of the heat sink.

The auxiliary contacts from the IGBT to the gate driver are provided by the flex layer itself, which is extended across the long side of the DBC (see Figure 2).

Figure 2: SKiN half-bridge 600 V, 400 A module.

To benchmark the new packaging concept not only with traditional power module designs, identical devices have been built with standard Al bond wires for the die surface contact.

To obtain maximum performance, each chip is contacted with 12 bond wires (see Figure 3). The surface of the diode is contacted with three stitches per wire; the IGBT with four stitches.

Figure 3: Benchmark module with bond wires.

A significant difference between the flex layer and bond wire design is the contact area of the chip surface. While the bond wires are in contact with around 21% of the total metalised chip area only, the flex design exhibits a contact area of 50-85%, depending on the chip type.

The maximum power dissipation of the semiconductors is limited by the maximum junction temperature, the coolant temperature and the thermal resistance from chip to cooling medium.

Especially in motor vehicle applications where coolant temperatures above 85°C are needed, the temperature difference to the maximum allowed junction temperature becomes small which leads to reduced power densities and the need to reduce the thermal resistance to a minimum.

The new power module meets these requirements with a fitting solution: a high-density pin fin Al heat sink and an Ag sinter joint between the DBC substrate and the heat sink.

No thermal paste is used, which has a significant contribution in the thermal performance of standard packages.

The measurements were performed using a three-phase motor vehicle inverter setup (Figure 4) with a 50% glycol mixture and 70°C coolant temperature. The water inlet and outlet is on the left-hand side; distribution through the three modules is obtained using three parallel flow channels.

Figure 4: Inverter set-up used to measure thermal resistance.

For the test, all IGBTs are electrically connected in series and heated by an adjustable DC current. In this way it is possible to measure the power losses as well as the junction temperatures most accurately since it is not disturbed by switching transients.

The difference in thermal resistance (junction to water R_th(j-a)between the upper (TOP) and lower (BOT) IGBTs, as well as the variation between the half-bridges is less than 10% and is shown in Figure 5.

Figure 5: Rth(j-a) of the six IGBT switches.

The lower switch of a half-bridge has a slightly better thermal resistance than the upper switch. This is due to the larger DBC copper area beneath these chips, which leads to better thermal spread. This effect is well known in power module designs due to layout restrictions.

At a coolant temperature of 70°C, a flow rate of 10 l/min and a maximum junction temperature of 150°C, it is possible to draw 205 W/cm² chip area out of the system. Traditional high-power inverters with thermal paste between the power module and the water cooler reach just 100-150 W/cm² chip area.

The flow per half-bridge is only a third of the total flow rate. Of course it is also possible to arrange the design with a serial coolant flow through the three phases which will lead to an even higher power density.

The question remains as to how a traditional module would perform if it were mounted on the same high-density Al pin fin cooler used for the SKiN module. To investigate this, thermal simulations were performed where the Ag sinter joint between DBC and heat sink was replaced by a thermal paste layer of only 20 µm.

Such a thin thermal paste layer is only possible using pressure contact modules without baseplates, like the SKiiP or SKiM power module family. Modules with baseplate would require a much thicker thermal paste of 80-150 µm.

The simulation results confirmed the large impact that the thermal paste layer has. Even for a layer of just 20 µm, the total thermal resistance will increase by 23-30%, depending on the coolant flow rate.

The diode surge forward current (I _FSM) was measured using a standard half sine wave current surge of 10 ms duration at 25°C.

The surge current rating of the flex layer module is 27% higher than the bond wire module.

Due to the larger cross-section and shorter track length in the flex layer design, the surface contact fuses later than the bond wire module.

This behaviour is particularly important for active front-end or generator applications, since it compensates for the reduced chip area which has been made possible by the improved thermal behaviour of the integrated pin fin cooler.

Power cycling is the main qualification test to validate the lifetime required by the application mission profile owing to cycling loads. The most demanding applications are electric and hybrid vehicles, lifts and wind turbines.

The failure modes for power cycles are a combination of the typical bond wire lift-off and solder joint degradation in the layers below the chips. What cause of failure dominates, depends on numerous factors such as cycle time and chip size.

The replacement of the solder with a sinter layer has already eliminated one failure mode, leaving only the bond wire as the remaining reliability weak point.

Single-sided sintered power modules have already, in the past, demonstrated a two to three fold improvement in power cycling.

Power cycling tests were performed on both module types under identical conditions with ΔT_j= 110 K (40 to 150°C) and a complete cycle time of 14 seconds

The control strategy for the power cycling test was a fixed time adjustment, which is the harshest and most realistic test mode since it does not compensate for any type of degradation during the test.

The results for the SKiN power module by far exceed the target curve (red line), which is already 20 times higher than the industrial standard (green line). The modules passed more than 700 k cycles until failure.

In addition, short power cycles with a ΔT_jof 70 K (80 to 150°C) were started. Here the modules have already passed three million cycles. Tests will be continued to EOL.

The preliminary results demonstrate the reliability of the new double-sided sintered power module. The target was exceeded, resulting in a 70-fold improvement in performance over the industrial standard and a 10-fold improvement over the single sided sintered benchmark module.

It is important to mention that these results are from standard 600 V IGBTs with a chip thickness of only 70 µm and a standard aluminium top side metallisation. Only an additional thin noble metal surface finish is required. The SKiN packaging technology does not require any major changes in chip metallisation materials or layer thickness.

SKiN technology is a new packaging technology without bond wires, solder layers and thermal paste. All interconnections to chip top and bottom surface, DBC to heat sink and power terminals are made using Ag sinter joints.

The bond wires have been replaced by a special flex foil which increases the chip surface contact area by a factor of four.

To demonstrate the performance improvements of the overall system and in particular the flex foil, a comparison with a benchmark module featuring conventional Al bond wires was performed.

Due to the elimination of thermal interface materials and the integration of a high-performance pin fin heat sink, it is possible to double the power dissipation, compared with conventional designs. The elimination of the thermal paste layer alone leads to a 30% improvement in the total thermal resistance junction to water.

Owing to the modified geometry and increased chip contact area, a 27% increase in diode surge forward current capability has been achieved.

The power cycling performance demonstrates a 70-fold improvement over the industrial standard and a 10-fold improvement over the single-sided sintered benchmark module.

Further activities are underway to exploit the new possibilities of the dual-layer flex foil. These are, in particular, a further improvement in thermal resistance resulting from double-sided cooling and the integration of passive and active components for gate drive, current and temperature sensing.