Posted on 01 September 2019

Spring contacts - reliable alternative to critical solder contacts

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More and more new electronic circuits and chip technologies are now being used in solar inverters to increase the efficiency rate of inverters and, consequently, overall system efficiency. The consequence for power electronic modules: they have to be efficient, flexible, and environmentally compatible, all at the same time.

By Daniel Seng, Product Manager, SEMIKRON Elektronik


The photovoltaic sector is currently experiencing an uninterrupted period of growth. In 2006, sales of solar thermal installations in Europe went up by more than 35%, taking the total solar thermal capacity to around 1,900 Megawatt (MW). According to the German Solar Thermal Industry Association (BSW-Solar e.V.), the German solar thermal market and the total turnover generated by the sale of solar thermal installations saw a 58% increase on the previous year’s figures last year, totalling 2 billion euros.

To help ensure that solar thermal inverters are highly efficient, MiniSKiiP power modules have a very low thermal impedance. This helps improve power dissipation and ultimately means that the power circuitry is utilised more effectively. A solar inverter featuring MiniSKiiP came out top in tests carried out by Germany’s consumer watchdog Stiftung Warentest and achieves an efficiency rate of up to 95.6 % - and still uses standard silicon IGBT and diode chips.

To help boost the efficiency of solar inverters, MiniSKiiP power electronics modules boast a very low thermal impedance

The MiniSKiiP module boasts a high degree of integration on a minimum of space. A complete Converter Inverter Brake circuit configuration (CIB) with a chip rating of 100A and blocking voltage of 1200V is integrated into a compact MiniSKiiP 3 case (59 mm x 82 mm) (Figure 2). MiniSKiiP modules also feature an unusually flexible interface. The power and auxiliary terminals can be positioned nearly anywhere on the surface of the module. This flexibility is also possible inside the module, due to the many different circuits in the same case.

Cross section of a mounted MiniSkiiP module with spring contact technology

Efficiency and environmental compatibility are not only important once the inverter is in operation. These factors are vital even in the assembly stage. With MiniSKiiP modules, the power switches, PCB and heat sink are connected in one step. The power circuitry is not soldered onto the PCB, as in other modules, but is connected using SEMIKRON’s patent-protected pressure-contact technology, which is based on spring connections between the driver board and the power module instead of rigid solder connections. As a result, demanding and timeconsuming soldering processes can be done away with in the production of the inverter. Thermally or mechanically induced stress that can occur when the inverter is in operation is reduced thanks to the moving contacts, guaranteeing long-lasting and safe electrical connection. This can also be seen in long-term applications where the module has been used in decentralised drives, mounted directly onto the motor. Today, more than 300 million of these springs are in use and have proven their reliability – in full compliance with the RoHS directive, of course.

Solder pin versus spring contact

Nowadays, other companies are also trying their hand at using solder- free connection technology for modules in this power class. In press-fit technology, for example, the load connectors are fitted into existing holes on the PCB, meaning that soldering is avoided entirely here. One shortcoming of this technology is the fact that soldering or bonding is still needed to connect the terminals to the DCB. With the spring contact technology used in MiniSKiiP, by way of contrast, this would not be necessary (Figure 3). And the less soldering needed in the production of power modules, the less the impact on the environment.

Temperature cycling of spring contact and solder pin

To further simplify the customers’ processes, we now offer our customers the option of ordering MiniSKiiP modules with a pre-printed thermal paste layer (Figure 5). The thermal paste layer is applied in a highly automated printing process. The customer can then do away with this sensitive and time-consuming step in his final assembly process – a clear time and cost advantage. In addition, the thermal paste layer is spread optimally and has the ideal thickness, resulting in better thermal performance and increasing the lifetime and durability of the power electronic module.

MiniSKiiP module with thermal paste layer

The fact that the customer can choose between two different covers for the MiniSKiiP module also offers the customer greater flexibility in terms on PCB layout. Depending on the final application requirements, a flat cover is available for very compact designs or a cover with space for individual SMD components on its underside. As MiniSKiiP modules featuring spring contacts require no through contacts on the PCB, SMD components can be easily placed onto the PCB. This allows for ultra low inductive PCB designs, which can boost the overall efficiency of the complete converter.

Power rating and topologies of the MiniSKiiP power module

MiniSKiiP modules are designed for 600V and 1200V chip off-state voltages and predominantly feature Trench IGBT technology in combination with SEMIKRON CAL diode.

In the 1200V range, the latest Trench IGBT4 technology is used in combination with the CAL I 4 diode. These chips may be used for a junction temperature of up to 175°C.

Due to this chip combination and the 25°C increase in junction temperature, better inverter performance is possible than in previous technologies with the same chip rating. Power dissipation is also reduced by around 20% in comparison to the predecessor generation, making the complete inverter much more effective overall.

In addition to the CIB configuration, standard modules are also available in standard inverter topology, non-controlled rectifiers with brake chopper, as well as half-controlled rectifiers with brake chopper.

For solar inverters, purpose-designed modules in two-phase semiconductor topology with 600V and 1200V IGBT’s are available. With many different possible spring arrangements, an extremely flexible layout is possible. This means that the chips can be optimally arranged in the module and inverter efficiency consequently further increased owing to very low-loss switching. These modules can also be equipped with SiC diodes instead of CAL diodes. This can once again boost the overall efficiency of the complete solar inverter. In short, with the right combination of innovative circuit topology and innovative case you have the optimum module for solar applications, designed to minimize power losses.

With the dimensioning and simulation tool SEMISEL from SEMIKRON one can select, configure and dimension modules. SEMISEL can be found at and offers an extensive online help function, should help be required. This free tool is the fast and reliable way of selecting the right power circuitry for a wide range of applications. The customer can enter a specific heat sink characteristic, for example, and will be given realistic temperatures for the complete power electronic system. Different customerspecific load conditions can also be entered and the resulting losses and temperatures viewed.


Spring contacts, reliable alternative to solder contacts

The success of the power module MiniSKiiP lies in the solder-free connection of the power and auxiliary terminals to the PCB and DCB. Electrical contact is established through mechanical springs which allows for an extremely easy module assembly or disassembly on the customer´s side. With the spring contacts a longer lifetime is guaranteed especially under thermal and mechanical stress. Today, more than 300 million spring contacts are reliably in use today.



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