A new 3D power module packaging without bond wires

The replacement of bond wires in power modules combined with a higher level of system integration has been discussed for several years in the industry and academia. Most of the new packaging approaches have been based on solder bump interconnection technologies ^{[1, 2, 3]} or on embedded technologies with sputtered or electroplated interconnection layers ^{[4, 5, 6]}.

The new proposed packaging concept is based on flip chip technology but instead of solder balls the power chips are attached to a flexible printed circuit board by ultrasonic welding. A key material development for this concept has been a flex board with a thick aluminum layer for the power connections. The chip is welded by an ultrasonic welding process to the aluminum metallization of the flex board forming a well known reliable Al welding joint. The flexible printed circuit board contains the power routing for the main current path on the bottom side as well as the auxiliary signals to control the IGBT`s on the top side layer.

Since there is an additional signal layer on top of the power devices a higher packaging integration can be achieved. On the upper layer driver ICs, sensors as well as passive components can be placed without the need of additional space.

1) SKiN^® Basic Idea

SKiN^® technology is based on a high volume Flipchip process. The standard power semiconductor devices are welded with the aluminum emitter and gate surface on aluminum punched dimple structures of a flex board. There is no need for a new power chip metallization. Standard aluminum top side metallization can be used. Semikron developed a flex board material with three layers. One layer is used for power devices and high current structures the inner layer is the isolation between the two layers and the third layer is designed for fine pitch logic structures.

Figure 1. Flex Circuit Packaging concept

The chips have an underfill for the need of the mechanical stability between flex board and silicon chips. The collector can be soldered todifferent substrates. On the logic side bare dies like driver IC`s or SMD components can be used.

2) SKiN^® Flex material

The flex board basic material can be produced in a high volume process. This process is a roll to roll application. The etching process and end finishing is done on sheets with a base dimension of 510 × 480 mm.

Figure 2. Etched power side structure

The thick aluminum layer on the power side can handle about 15A/mm² on a trace length of 10 mm with a resistance of 0,25 mΩ. In this configuration the flex is power full enough to design power modules up to 100 A without additional busbars. The Polyimide layer is in between the two electrical layers and has an insulation capability of 7,5 kV.

The logic side has a standard 35μm copper layer and is designed for fine line structures.

Figure 3. Etched logic side structure

The end finishes of these two metal layers are designed for different functionalities. There are some areas that are bondable and others that are solderable depending on the surface finish. These layers can be produced in high volume and it is similar to a standard PCB manufacturing process. The flex layers itself can be used for power and logic interfaces of the finished power module.

For the module designers there is a new degree of freedom in 3D structures. One of the results of these structures is that modules can be only 2 mm flat. The electrical design can be optimized for low inductivity or for ultra compact devices. The logic components like the drivers IC`s can be placed near to the power chips and all interconnects between power and logic side are integrated in the flex layer itself.

3) SKiN^® Bump structures

Semikron developed bump structures on which an ultrasonic “Flipchipbonder” can weld chips directly to these geometries.

Figure 4. Schematic of the bump structures

In Fig. 4 is shown that these bumps are directly punched through the flex material. One bump is specified to handle about 8A load current. The dimensions of these structures in height and distance from bump to bump are depending on the chip design. The bump geometry it self is designed to have enough robustness to handle the ultrasonic force from the Flipchip process. The complete dimple array of the final multi chip circuit can be stamped by a single precision punching tool. This process is fast and cost effective.

Figure 5. Typical bump structure for an IGBT chip

In figure 5 a typical bump structure for a 10 A IGBT chip is shown. There are two bumps for the emitter and one bump for the gate structure. The tolerance between the bump heights of this dimple array is less than 5μm.

4) SKiN^® Thermal mechanical modeling

FEM modeling results show that the dimple array in SKiN technology offers a flexible structure that help to reduce the thermal mechanical stress, of the different materials.

The aim of this modeling work was to understand which kind of bump geometries and which kind of underfill material is the best choice for this new packaging design. The complete structure was analyzed under thermal conditions of -40°C to 125 °C. Coupled boundary conditions have been applied on the models, which forces all nodes on each single side to have the same displacement to the direction normal to there symmetry planes.

Figure 6. FEM Analysis of Dimple Display

Furthermore, the material characterization of CTE and Tg values of different underfill materials gave us a better  understanding to reduce the stress in the package.

In Figure 7 there is the result of the relationship between the underfill material and the stress behavior in this SKiN package over a temperature range of -40°C to 125 °C.

Figure 7. Stress relation diagram

5) SKiN^® Flipchip welding process

The aim of this process was that the power chips are welded directly on the bump structure of the flex board material. (Figure 8).

Figure 8. Flipchip on Bump Structure

A new ultrasonic flip chip bonder has been designed for this application. The machine has a working area of 200 mm x 250 mm and the Z -axis is 5 mm.

The chips are flipped directly from the wafer to the ultrasonic bond head.

Figure 9. Flipchipbonder

Advantages of this ultrasonic flip chip process:

• Monometallic connection with lowest possible electrical contact resistance and inductivity in combination with high reliability.
• Repeatability of the bonder axes ± 3 μm
• Precisely controlled force- and ultrasonic output
• Display of progression of bump deformation over time

Productivity of ultrasonic flip chip process:

• Cycle time: 1,0 - 1,2 s
• Process time: 100 msec / chip

Figure 10. Cross section of welded joint

6) SKiN^® Integration of low power components on logic Level

With the use of a flexible PCB a higher level of packaging integration can be achieved. On top of the power dies is a whole new layer available for other circuit components like passives, gate driver ICs and micro controllers. Since the top layer has a standard 35μm copper thickness fine pitch components as well as standard SMD packages and bare dies with thin wire bonds can be used. In standard copper lead frame IPM modules it is only possible to integrate the driver ICs on the same level next to the power dies and use wire bonds for the connection. With the flex PCB structure it is possible to place the low power components virtually on top of the power dies allowing for a smaller footprint and a further reduction of parasitic elements.

Due to the further miniaturization and the use of different SMD and bare die packages it is desired to use a no clean process for the attachment of the top layer components. Conductive adhesives  have been proven to be a reliable attachment process for the low power components. Another advantage of the conductive adhesive is that it provides a stable high temperature joint, even if the finished component is soldered by a PCB reflow process.

Figure 11. Bonded Driver IC and SMD Components on Flex

Fig. 11 shows the logic side of a demonstrator device with a bare driver IC and other passive components that are placed on the top layer of the flexible PCB after ultrasonic flip chip attachment of the power dies.

7) SKiN^® Flex board interface

Another advantage of the flex PCB is the easy and variable interface of the power terminals  and control signals of the final power module. In a standard power module the power and control interface is usually made by solder terminals, spring contacts or a lead frame. In any case it is necessary to contact the die top side by bond wires that are usually routed through the bottom contact substrate that again needs to be connected to the external interfaces.

The flex PCB can combine all these functions and interfaces in a single component with a single process step, replacing not only the bond wires but also the power and control interface components. The flex board top side carries the low power auxiliary signals that can be soldered or welded to a PCB or even make contact with a flex plug in connector. The flex board bottom side carries the high voltage high power traces that can again be soldered or welded to a PCB, copper busbar or even directly to the motor winding. Since the flex board is flexible it can be bended in any direction and a true 3D package integration independent from the cooling surface can be achieved.

8) SKiN^® IPM Module

To demonstrate the advantages of the new 3D packaging technology like high integration level and replacement of bond wires and lead frame with a flex PCB a low power IPM (Intelligent Power Module) topology has been chosen. 

Figure 12. Cross section of 600V IPM

Two IPM package with different topologies and voltage ratings are currently under development.

Both modules are SMD components. The flex board is extended and folded out of the package bottom side, so that it can be soldered directly to a main inverter PCB. The low power signals are on one side of the package and the high power signals on the opposite side forming a dual inline structure, no additional lead frame is necessary. For a reliable reflow solder contact to the main PCB all flex traces have AgPd plating. Fig 13 shows a package out line and a cross section of the 600V IPM package. The flex layer is bended so that the low power top as well as the high power bottom side makes contact to the inverter PCB.

Figure 13a. Cross Section of 600V IPM

Figure 13b. Engineering Sample

Fig. 14 shows the switching waveform of a 600V module at 10A and different DC voltages. Due to the low inductive package design and the close coupling from driver IC to the power semiconductors very low over voltages and clean switching  waveforms can be observed.

Figure 14. Switching Waveform of 600V IPM

9) Summary

A new packaging technology has been presented that combines a flip chip ultrasonic welding process with a flexible two layer board. The specially developed isolated flex board has a bottom layer made of thick aluminum and a copper layer on the top. The unique feature of the assembly is the welding joint from the power die to a punched dimple structure on the aluminum layer.

The copper layer is used for low power circuits like gate driver and passive components allowing a real three dimensional package integration.

First products that are tested with this new packaging concept are 600V and 1200V IPM modules in 3 phase AC as well as CIB topology.

This packaging concept allows new approaches of electrical and mechanical module designs and is the ideal solution for highly integrated applications.

Reference List:
[1] J. Bai, G. Q. Lu, X. Liu: Flip chip on flex integrated power electronics modules for high density power integration, IEEE Transactions on Advanced Packaging, vol. 26, no. 1, 2003
[2] J. Catala, J. Bai, X. Liu, S. Wen, G. Q. Lu:Three dimensional packaging for power semiconductor devices and modules,  IEEE Transactions on Advanced Packaging, vol. 28, no. 3, 2005
[3] M. Mermet-Guyennet: New structure of power integrated modules, CIPS Proc., 2006
[4] R. Fillon, E. Delgado, P. McConnelee, R. Beaupre: A high performance polymer thin film power electronics packaging technology, Advancing Microelectronics, 09. 2003
[5] K. Hase, G. Lefranc, M. Zellner, T. Licht: A solder bumping interconnect technology for high power devices, IEEE PESC Proc., 2004
[6] SIPLT TM Siemens planar Interconnect Technology innovative replacement of wire bonding. (Cooperation technology)

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