Power Electronics

Sinter Technology Enhances Power-Module Robustness

State-of-the-art packaging technologies such as sintering can boost reliability of power modules in high-power applications.

High-power applications such as automotive, wind, solar, and standard industrial drives require power modules which fulfill the demand for high reliability as well as thermal and electrical ruggedness. Packaging development engineers in the New Technologies Department at Semikron in Nuremberg, Germany were challenged to develop, optimize, and employ a new packaging technology that could meet the demands of power modules used in extreme environments. The result was sintering technology.

Silver sinter technology has been used to connect chips to substrates since 1994. Even back then, the properties of sintered silver bonding layers — and the benefits they boast in terms of reliability — were analyzed and reported within the contexts of numerous international congresses. At that time, however, it turned out that this type of bonding technology was not quite ready for use in large-scale industrial electronics.

These sinter-chip/substrate connections are constructed solely from special silver particles, which produce, in certain circumstances, sinter-bridge formations that create a reliable connection between the two bond parts. Fig. 1 shows the silver particles before and after sintering. In relation to this, it is important to know that each and every one of these particles is surrounded by a special coating material.

Producing a bond is simple: just place the amount of particles needed for the desired layer thickness between the two bond parts and apply a given temperature and pressure to the bond for a given time. The result is a stable sinter connection. This basic process is, however, only sufficient for the first-technology evaluations.

The past years have been devoted to the industrialization of sinter technology. An independent sinter paste has been developed, which today is the basis of the sinter paste approved and implemented by Semikron. Additionally, sinter engineering tools have been developed to manufacture multi-chip direct copper bonds (DCBs) in formats of 5 x 7 inches. The sinter press was designed to handle pressure loads depending on the process action. The production staff responsible for assembly is well-trained and the process in place continuously improved.

Contact strength achieved by the sinter layer between chips and substrates is extraordinarily high. Sintered layers display high load-cycling capability in reliability tests. A further advantage of sinter technology is that no solder-stop layers have to be washed out.

The achieved accuracy of chip position relative to the substrates is as much as 50 µm. In contrast, a positional accuracy of just 400 µm is achieved with solder technology, a fact that can be considerably encumbering in the subsequent image-processing procedures.

The Table shows the material parameters for the silver-diffusion sinter layer compared with a standard solder layer. The high temperature stability of sinter technology indicates that the connecting layers do not age. Fig. 2 shows the power-cycling capability of sintered chips vs. soldered chips. If you consider the thickness of the sintered layers, the sintered layer is 4.5 times thinner than a standard soldered layer yet provides four times the thermal conductivity, resulting in excellent thermal properties in the sintered connection.

The layers also demonstrate far-higher cycling capability than soldered layers due to the fact that the melting point of the silver used in the sintered connection is also four times higher than that of the Pb-free solders commonly used today. The long-term experience has paid off — an alternative packaging technology, completely solder-free, has been established.

APPLIED SINTER TECHNOLOGY

Sinter technology was used in the SKiM® IGBT module family for 22- to 150-kW traction drive converters in electric and hybrid vehicles (Fig. 3). SKiM ® technology offers a five-fold increase in thermal-cycling capability versus modules with a base plate and soldered terminals.

Instead of soldering the DCB — the ceramic substrate required for isolation — to the copper base plate, the connection to the heat sink is assured by way of pressure-contact technology for all thermal and electrical contacts (Fig.1). Pressure points positioned directly beside every chip guarantee that the DCB is connected evenly. The fact that no base plate is used ensures superior thermal-cycling capability and low thermal resistance. Fig. 1 shows a cross-section of the module case, the pressure contact system, and the spring contacts for the gate connections. Solder connections are omitted entirely, making the SKiM family the first-ever 100% solder-free module series on the market.

In the past 15 years, the maximum permissible chip temperatures have risen steadily. Today, state-of-the-art silicon components such as IGBT 4/CAL 4 diodes, can be operated at a maximum chip temperature of 175°C.

In the future, the use of silicon carbide will create even greater challenges in terms of sufficient thermal-cycling ability in the connecting layers, for silicon-carbide components can be operated at temperatures as high as 300°C. Sinter technology developed by Semikron, however, is ideally suited for such high temperature ranges, thanks to a connecting-layer melting point of 961°C — around 740°C higher than the solder connections commonly used today. The high temperature stability of sinter technology means connecting layers do not age, as verified in reliability tests performed today.

Over the years, the application areas for power-semiconductor modules have changed dramatically. In the past, semiconductor modules were used in easily accessible and stationary control cabinets with defined cooling technology/systems.

Today, in contrast, power modules are destined for mobile applications, i.e. vehicles with cooling conditions of up to 110°C. The challenge faced today is how to ensure that the power-semiconductor component can generate its maximum permissible current, IC(max) under such temperature cooling conditions. Sintering technology allows the relationship between these two parameters, as well as the IC current that can be controlled at increased chip temperatures, with no compromise to reliability.

SINTERING'S FUTURE

Sinter technology developed by Semikron is a key technology that allows for the production of components that are more powerful and reliable with a longer life-time. The same principles applicable to the SKiM module family for electric and hybrid vehicles — base plate-free module, pressure-contact system, and sinter technology — were applied in the development of the fourth generation of the intelligent power module, SKiiP, for applications such as wind and solar power, elevator systems, trolley buses, metro, and underground vehicles.

The benefits of sinter technology (Fig. 4) continue in the SKiiP fourth generation: five times the thermal cycling capability, unbreakable joint between the die and DBC, and twice the power-cycling capability.

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