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Posted on 23 July 2019

SmartPIM and SmartPACK

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Self-acting PressFIT Assembly

PressFIT contact to the PCB and mounting to the heat sink within seconds in a single step. Robust housing for easy yet secure mounting and good thermal properties. Top contact quality and optimum space utilization are only some of the points that the new IGBT modules provide.

By Marc Buschkühle and Thilo Stolze, Infineon Technologies

 

The simplification of handling of IGBT modules has always been the objective in the development of new concepts. For this reason, the often-copied Econo family was born at the beginning of the 1990's. In this way it became possible for the first time, to implement a power section of an inverter by mounting just one IGBT module. Prior to this, three 34mm IGBT modules had to be mounted and contacted with the driver PCB and load connections. A further step toward simplifying the mounting process and increasing reliability was the introduction of the PressFIT technology. In this technology, the contact with the printed circuit board is established not by soldering but by simply pressing the module pins into the board.

Inverter designs, especially for approx. 2.2-55 kW, today are focused on components that are easier, faster and – in particular – safer in contacting and mounting. The new housing concept establishes contact between the module and the heat sink and PCB by using a single assembly step. Only one screw is needed in this regard for SmartPIM1 (rectifier, chopper and inverter) and SmartPACK1 (SixPACK).

During the mounting process, the PressFIT contact is pressed into the printed circuit board, the PCB is stabilized and the module is fixed on the heat sink in the same step. The entire mounting concept does not require any additional tools. Only a screwdriver is needed to fasten the screw.

Regarding reliability, the modules offer all the advantages of Press-FIT technology and further benefits:
• Self-acting PressFIT Assembly
• Duplex Frame Concept for rugged mounting
• Ceramic cracks during mounting impossible
• Best in class FIT-rates for interconnections
• Mounting possible on both sides of standard FR4 PCBs
• Reliable cold welded connection of module pins and PCB
• Thermal interface defined by design not by mounting procedure
• Family concept for inverters from approx. 2,2 kW up to 55 kW

The New "Smart" Design

The use of modules with PressFIT technology has gained more importance in inverter designs. The combination of the press fitting with the mounting of the heat sink is now the continued logical step in the simplification of the process.

During assembly of Smart modules cold welding, known from Press- FIT contacts, is done for connection the pins to the PCB. In the same step, the module and the PCB are mechanically fixed to the heat sink.

Figure 1 illustrates the mounting principle. A screw is directed through the bracket, the PCB and the module into a thread in the heat sink. By tightening the screw, the counter holder presses the module pins into the holes on the PCB. The specific geometry at the head of the PressFIT pins becomes plastically deformed just like with other PressFIT modules. At the same time, the IGBT module is pressed onto the heat sink and fastened by the bracket for the screw.

SmartPIM1 self-acting assembly

After the screw has been tightened, the entire mounting process for the module is completed within a few short seconds.

Due to the combination of the module mounting with the contacting, the working steps required during mounting are reduced and production costs can be decreased.The module construction in combination with the counter holder is displayed in Figure 2. The main module parts were marked and labeled.

SmartPIM1 module construction

Although single-step mounting is the fastest and also the preferred method, but it is only one of the possible ways of mounting modules. Modules in the Smart family can be used in much more flexible ways. The most common method of contacting by soldering as used in traditional inverter concepts, despite all new methods, is just as possible as press fitting.

Depending on the production philosophy of the user, the module is soldered similarly to a standard soldering module or press-fitted like a simple PressFIT module using a fixture. A rough overview can be seen in Figure 2. As a result, the IGBT modules of the Smart family are flexible and easy-to-use in all inverter and production concepts.

Duplex Frame Concept for Rugged Mounting

A point of criticism that is often experienced in practical use in reference to the handling and processing of modules without a base plate is their susceptibility to ceramic or chip breakages that can cause insulation failure.

In the design of the Easy modules a number of years ago, mounting using clips was successfully introduced for exactly this reason.

Ceramic breakage is practically excluded with this method. The reason for this is the decoupling of the screwing force from the ceramics (DCB) by the fastening clips.

In the case of the Smart1, this decoupling of forces is likewise solved on the module side by the duplex frame concept. Processing of the IGBT modules is very uncomplicated as a consequence.

A glance at the design of the frame displayed in Figure 4 shows that the module comprises an inner housing and an outer frame. The inner part of the module is connected with the outer frame by decoupling bars. This applies likewise to the sleeve in the middle that covers the screw.

During the mounting process, the module is pressed towards the heat sink, while the force is transferred from the outer to the inner housing for optimization of the thermal transition. The PressFIT pins distributed throughout the entire area of the module also press onto the DCB and heat sink.

Owing to the equal distributed pressure during and after mounting, the thickness of the thermal conductance paste on the module base can be minimized, thereby reducing the thermal resistance. Owing to the equal distribution of force during mounting, the thickness of the thermal conductance paste on the module base is minimized, thereby reducing the thermal resistance. Figure 3 shows a typical distribution of the thermal paste.

Optimized grease thickness up to transparency

After the press-in process, as shown in Figure 4, the force of the screw is absorbed by the outer frame and the inner rivet. Only a defined share of pressure is transferred to the inner core of the module.

Inner module core is always secured by force decoupling outer frame

Due to this, the module is very robust what was proven in numerous tests, where the DCB could not be damaged, also up to loads of more than 15KN according to a torque of more than 20Nm. Although when the plastic frame was damaged at the end, the insulation of the module is still alive.

Ceramic breakage, such as experienced for modules without clips or structural load protection, is in this way eliminated for SmartPIM1 and SmartPACK1.

The duplex frame concept also offers yet other features: The PCB is strongly fixed in between the counterholder with PCB, the outer frame and the heat sink. All forces and mechanical tension that externally affect the PCB are automatically kept separate from the inner module by the outer frame and its fixation around the inner core. Under normal conditions mechanical stresses, such as vibrations, are non-critical for the IGBT module.

Highest Reliability by PressFIT-Technology

Along with the mounting, the quality of the electrical connection to the PCB is a vital criterion.

The use of the modules in rough industrial environments demands a high level of reliability of the electronic components. This demand is met by replacing solder connections by a connection method of superior quality.

The electrical and thermal connection of the IGBT modules in the Smart family with the PCB is established by the PressFIT contacts.

The principle is the familiar cold-welding connection that is specially used for permanent contacts to which stricter reliability requirements apply. Figure 5 illustrates different cuts and REM images that provide an insight into the connection method. The two wings of the PressFIT geometry are malleably deformed during the press-fitting and pressed against the interior hole surface. The force needed for the cold-welding results in this way and tolerances are balanced. There is no necessity for particular finishing of the contact surfaces on PCB-side.

PressFIT Cold-welded and gas tight contacts

To qualify the PressFIT connection, some of the wide range of tests were carried out significantly beyond the specifications stated in the applicable standards (IEC- 60352-5/IEC 60749/IEC 60068-2 Part 43/5). The tests included the following: TST (Thermal Shock Test), H3TRB (high humidity, high temperature, reverse bias), power cycling, corrosive gas, salt mist and vibration.

All tests were performed using standard PCBs (with minimum and maximum hole diameters). The boundary conditions were tightened in individual tests in order to ensure suitability for power modules. For example, the temperatures in some cases were considerably higher than the usual highest temperature of 105 °C specified for standard printed circuit boards. Another aspect is the five times greater concentration for the corrosive gas tests compared with the standard (standard concentration: 10 ppm; test concentration: 50 ppm). The gas-tightness of the connection in this case was also proven under more stringent conditions.

No measurable contact deterioration could be detected in any of the tests. This signifies secure carrying capability for large and small currents throughout the entire lifetime of the module.

In addition, significantly lower voltages and currents, as experienced in shunt contacts for example, remain stable throughout the module lifetime.

By consequence, the PressFIT technology is optimally suited for use in power semiconductor modules, particularly in regard to future stricter reliability requirements and demands for greater functionality.

Power density

Along with improvements in quality, a further general trend can be discerned in power electronics toward minimization of the construction area. This aspect was likewise considered in the design concept for Smart1. An uncomplicated layout and optimum heat dissipation are of course essential.

On the one hand, the possibility of being able to freely choose where to position the contacts in the pin pattern further simplifies PCB layout design, while on the other hand, the individual chips are distributed throughout the entire module to achieve better cooling.

The clear separation between the individual module connections for power input, motor output and DC makes it unnecessary for conductor paths to cross on the PCB. One example can be seen in Figure 6. Additional connections between PCB layers are not needed as a result of parallel use of contact holes as vias in the PCB, and the layout is also made simpler.

Simplified PCB-Layout with power flow through design

To further optimize the available space, the distance between the module and PCB is suitable for optional positioning of SMD components such as gate resistors or gate emitter capacitors.

A collar around the tightening screw in the middle of the module is used to extend the available clearance and creepage distances. Therefore the distances comply with at least the requirements stated in IEC 60664.

An optimum thermal connection to the heat sink is a key factor for an IGBT module. In addition to the favorable and equally distributed pressing characteristics of the module already described, care was taken to obtain the best utilization of the whole module area of the SmartPIM1 in the internal layout design. To do so, the resulting heat is directed across the largest possible area to the heat sink and the heat sink is utilized to the best possible effect.

When comparing data sheet values the definition of the measurement method must be considered.

Great differences exist in some cases. For characterization of the thermal transfer resistances to the heat sink, the most critical ("worst") case is always considered at Infineon. This is in contrast to other manufacturers who only consider the Rth value that can be reached in best cases when heating just one chip.

In order to obtain a fast and simple comparison with other IGBT modules, a good rule of thumb is to add an additional 20% to the value of the thermal resistance of the other modules.

To consider the effect of thermal coupling of individual chips in real inverter operation, the worst case in supplying current to all chips at the same time is – as previously stated – determined by Infineon. In this way, the user is always on the safe side in thermal resistance terms when using SmartPIM1 and SmartPACK1. Figure 7 shows a comparison of the methods using a simulation.

Infineon considers application relevant thermal coupling effects [a] in contrast to competition [b]

Smart, a complete family

The IGBT modules SmartPIM1 and SmartPACK1 are the first members of new product family.

With the larger members, Smart2 and Smart3 a whole family of Smart modules will evolve.

The individual module sizes are geared toward the different frame sizes in today's inverter concepts typical to the market. For example, the Smart2 modules will cover the range occupied today by the Econo2 modules.

The Smart3 modules will reach the performance range of the Econo3.

The basic concept of one-step mounting, the module height of 12 mm and the PressFIT are of course the same as found in the Smart. Figure 8 provides an overview of current classes of the individual 1200 V modules.

Smart family portfolios

Therefore, with the Smart family it is possible for the first time to offer an integrated solution for inverters up to 55 kW.

Conclusion

The SmartPIM1 and SmartPACK1 represent another great step taken on the constant path to the simplification and improvement of IGBT modules. A robust, versatile module solution for the future has been developed with the secure PressFIT technology and a housing design allowing secure and particularly fast mounting. Contact to the PCB and attachment to the heat sink can be established simply by screwing. Resulting forces are decoupled from the screwing forces and are transferred to the interior of the module only to an extent that is required for the beneficial thermal contact. The contact to the PCB is gas tight owing to the cold-welding associated with PressFIT connections and by consequence the most stable connection method currently available for IGBT modules. The first stage in the development of the Smart family is complete with the SmartPIM1 and SmartPACK1.

 

 

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