Automobile scale module series (7): SEMIKRON SKiN technology

SKiN, a chip interconnect technology

It is not inevitable that the autumn air will be miserable, but the mild cold is just a pleasant thing. In the last quarter of 2023, the weather is getting cooler, but I feel good. I really enjoy the coolness when I walk on the road after get off work every day, but everyone should keep warm and enjoy the new climate.

Earlier we talked about Cu-Clip and copper bonding interconnection technology and eMPack automotive modules. Among them, we all mentioned a chip interconnection technology from Semikron Danfoss - SKiN technology. Today we will talk about it. Those things~

chip interconnect technology

Chip interconnection technology includes two categories, bonded wires and unbonded wires. Among them, those with binding wires are familiar to us, and they are also the most technically mature category, such as the common 62mm, Easy, EconoDual, etc., which are basically in the form of binding wires. Of course, there are also ribbon bindings. line form. The loop stray inductance of this type of module generally ranges from 9nH to more than 20nH. In fact, there are certain limitations for high-speed switching devices such as SiC .

Because during the hard switching operation, the presence of stray inductance in the loop will generate a peak voltage (ΔV=Ls*di/dt). The larger the stray inductance, the larger the peak voltage will be. It reduces package reliability due to increased likelihood of partial discharge and insulation failure. In addition, in order to increase the current capability of the module, multiple chips need to be connected in parallel . An asymmetric layout will result in dynamic uneven current flow between parallel chips. This will create a transient temperature imbalance between them and may reduce the reliability of a given device.

The other is unbound wires, which are more flexible for effectively reducing stray inductance and optimizing layout. The Cu-Clip we talked about before and the SKiN we are going to talk about today both fall into this category. Of course, in addition to these two, there are many other forms, including Siemens' planar interconnect technology (SiPLIT), which deposits copper on a high insulating film for interconnection on the top of the chip. It has been characterized that the parasitic inductance can be reduced About 50%; General Electric's (GE) power overlay interconnect technology (POL), which uses a flexible substrate made of polyimide and copper to connect on top of the chip. In addition, including the development of 2.5D and 3D packaging forms, the module structure adopts multi-layer DCB and vertical power loop design to further optimize loop stray inductance.

SKiN Interconnect Technology

SKiN technology has been in use since 2011 and involves sintering the chip to a DCB substrate, the top side of the chip to a flexible circuit board , and the substrate to a pin-fin heat sink. This technology reduces the size and weight of the module, as well as extremely low stray inductance (can be as low as 1.4nH), while also having high electrical performance and module reliability.

The picture below is the module interface diagram using SKiN technology

The key technology lies in a polyimide flexible circuit board with patterned metal tracks on both sides. The metal at the bottom is called the power side. It is a thick metal layer. It is mainly used to carry load current . The thickness depends on the specific metal material and the current that needs to be carried. Generally, 100um is more suitable.

The top metal is called the logic side and only requires a relatively thin metal layer, such as 30um copper, because it carries gate, auxiliary or sense signals.

The material and thickness of polyimide itself depend on the specific application conditions, such as the required operating temperature and voltage level . Generally, its thickness is tens of um.

In order to connect the gate of the device from the power side to the logic side, open connections are made on the flex board.

The picture below is an early 400A/600V IGBT half-bridge module from Semikron Danfoss using SKiN technology. 

Schematic diagram of flexible film

The power side and the top part of the chip are sintered with printed silver paste, and the auxiliary contacts are connected through metal traces on the flexible film. To prevent the flexible film from being damaged by sharp edges during subsequent sintering, a thin layer of organic material is filled around the chip. The picture below is a picture of the DCB substrate ready to be connected to the flexible film (the chip has been sintered onto it),

The substrate and the flexible film are then sintered and connected, and then the corresponding electrical tests can be performed.

The next step is to sinter the DC and AC terminals , the heat sink, and the power section above.

A plastic frame is then added to facilitate the installation of modules in the subsequent system.

The above whole process is as shown below

All of this is done by silver sintering, including the chip to the DCB substrate, the DCB substrate to the heat sink, the top of the chip to the flexible material, and the entire module has no solder and bonding wires. And the stray inductance of the loop should be very low. The loop diagram is as follows:

summary

Today, we want to learn more about what SEMIKRON's SKiN technology is like, and at the same time, what is the structure of the flexible circuit board that we also talked about when we talked about eMPack in the previous article.

The main reason is that I said before that I wanted to talk about SKiN, so today we will briefly talk about it.

I hope you like today’s content!

references

Thomas Stockmeier, "SKiN: Double side sintering technology for new packages"