Thermal Design Challenges of Highly Integrated Power Circuits
Preface
At present, with the continuous development and progress of science and technology and manufacturing processes, the development of semiconductor technology is changing with each passing day. For power semiconductor devices, their manufacturing process has also evolved from planar process to trench process, and the power density is getting higher and higher. At present, power semiconductor devices are not only single switching devices such as IGBT or MOSFET device types, but also hybrid power device types such as intelligent power modules IPM. In the IPM module, both power devices and drivers and control circuits IC are integrated. Such power semiconductor devices have a higher degree of integration. The packaging structure of this hybrid integrated power semiconductor device is somewhat different from that of traditional single power semiconductor devices. Therefore, its heat dissipation design and heat propagation method are also different from traditional power semiconductor devices, which will bring greater thermal design challenges to users. This article takes Infineon's CIPOS™ Nano IPM module IMM100 series as an example to illustrate the heat propagation model of Infineon's innovative PQFN packaged devices, and combines the comparative analysis of heat dissipation results under different heat dissipation conditions to give PQFN package heat dissipation suggestions in applications and device steel mesh design and reflow soldering temperature reference curves, hoping to provide users with application references for such devices.
PQFN Package Thermal Propagation Model
The CIPOS™ Nano IPM module IMM100 series adopts Infineon's innovative PQFN package design, integrating the three-phase inverter circuit, drive circuit and controller MCU in a single package. Its package size is 12mm×12mm and the thickness is only 0.9mm. Figure 1 is a cross-sectional view of the PQFN packaged IPM module. From Figure 1, it can be seen that the drain of the three-phase inverter MOSFET is directly used as the pin of the PQFN package, which has a very small thermal resistance, so that the heat generated by the power MOSFET can be quickly transferred to the copper skin of the PCB pad.
Figure 1. PQFN package IPM plan view
Figure 2. Bottom view of PQFN packaged IPM module
Figure 2 is a bottom view of the PQFN packaged IPM module. V+, Vs1, Vs2 and Vs3 are the main heat dissipation paths for the power MOSFET, and V- is the main heat dissipation path for the IPM integrated drive and control chip.
Figure 3. Schematic diagram of heat propagation when PQFN packaged IPM module is soldered on PCB
Figure 4. PQFN package IPM module thermal resistance model
Figure 3 shows a schematic diagram of heat transfer when the PQFN packaged IPM module is soldered on the PCB board. From the figure, it can be seen that the main heat transfer path generated by the IPM module is through the PCB board and copper foil, and only a small part of the heat is transferred from the top of the IPM module to the air. The reason is that due to the special packaging structure of the IPM module, the power MOSFET chip is fixed on the metal frame, and the bottom of the metal frame is directly soldered to the surface mount pad of the PCB as the pin of the IPM module. Therefore, the thermal resistance Rth (j-CB) from the chip to the PCB pad is relatively small. Correspondingly, the plastic sealing material from the chip to the top of the IPM has a relatively large thermal resistance, so the thermal resistance Rth (j-CT)+ from the chip to the shell is larger than Rth (j-CB) .
Figure 4 is the thermal resistance model of the PQFN packaged IPM module. According to the previous analysis, the heat dissipated by the IPM module is mainly transmitted through the PCB board and copper sheet, so the heat dissipation power PD ,B is much larger than PD ,T , and the effect of the heat dissipation power PD ,T on the temperature rise of the IPM is negligible compared to PD ,B .
That is, PD,B >> PD ,T , PD ,T XRth (j-CT) ≈ 0
Therefore, according to Rth (jc) =(Tj - Tc ) / PD ,
It can be concluded that T j ≈T C
In other words, it can be roughly considered that the case temperature of the IPM is approximately equal to the junction temperature. Based on this conclusion, the approximate IPM junction temperature can be easily estimated in practical applications to determine whether the IPM is working in the safe operating area. It should be noted that the junction temperature estimated based on this conclusion is only an approximate value, not the exact junction temperature in the strict sense.
PQFN package has four different heat dissipation forms
Comparative test analysis
According to the characteristics of PQFN packaging and actual application scenarios, four different application scenarios were used in the laboratory for thermal performance comparison tests. The first application scenario is to use a conventional FR-4 material PCB board (1.6mm thickness, double-layer board), and the IPM module relies on the PCB for heat dissipation without any external heat dissipation measures; the second application scenario is to use a 9X9X5mm small aluminum profile heat sink pasted on the top of the IPM module to assist in heat dissipation based on the first application scenario; the third application scenario is to use an additional fan for forced air cooling based on the first application scenario, and the fan is 12VDC/0.11A; the fourth application scenario is to use an aluminum substrate instead of a FR-4 material PCB board, the thickness of the aluminum substrate is 1.6mm, and the copper sheet is 1oz. Figure 5 is a schematic diagram of the actual circuit board for four different application scenarios.
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Figure 5. Schematic diagram of physical circuit boards for four different application scenarios
Based on the above four different application scenarios, the output phase current and shell temperature of IMM101T-046M were tested in the laboratory. During the test, the inverter carrier frequency was 10kHz and the DC bus voltage was 300V. The relationship curve between the IPM output phase current and the relative shell temperature was drawn according to the measured data as shown in Figure 6.
From the test data, it can be seen that when the PCB board of the same FR4 material is soldered to the IPM module of this PQFN package, adding an additional heat sink or cooling fan on the top will also be of great help in reducing the IPM shell temperature. Although the previous article stated that the heat generated by the PQFN package is mainly conducted from the PCB board and the copper skin, since the thickness of the IPM module of the PQFN package is only 0.9mm, the thickness of the plastic sealing material between the top surface of the IPM and the silicon wafer is relatively thin. According to the previous conclusion, it can be approximately considered that the IPM shell temperature and the junction temperature are the same. Therefore, when the heat sink is used to cool the top of the IPM, it will also have a significant effect on reducing the temperature of the silicon wafer. When the cooling fan is used for cooling, while reducing the shell temperature, it will also reduce the temperature of the copper skin near the IPM module, so that the heat generated by the IPM is more quickly conducted from the pad to the copper skin, further reducing the temperature of the silicon wafer.
Comparing the test data of application scenarios 1 and 4, it can be seen that without any additional heat dissipation measures, under roughly the same IPM temperature rise conditions, the output current capacity of the IPM is approximately doubled when an aluminum substrate is used. In some applications where the structure is relatively compact, the application power density of the IPM can be increased.
Figure 6. Relationship between IPM output phase current and relative shell temperature rise
FIG7 is an infrared temperature graph when tested in four different application scenarios, obtained at the same 300V DC bus voltage and 10kHz carrier frequency.
Figure 7. Infrared temperature graphs during testing in four different application scenarios
PQFN package steel mesh design
and reflow soldering temperature profile recommendations
PQFN devices are surface mount packages. Because they mainly rely on surface mount pins to dissipate heat through PCB and copper foil, the area of some main heat dissipation pins of PQFN is relatively large, and the pad area on the PCB is also relatively large accordingly. This will inevitably cause voids in large-area pads during reflow soldering. Too large a proportion of voids will increase the thermal resistance between the device pins and the pads, reducing the thermal conductivity. In actual batch soldering, it is generally required that the soldering void rate is less than 25% to ensure the thermal resistance performance requirements. Taking some optimization measures when designing PCB pads can reduce the soldering void rate and improve the soldering quality from a design perspective.
The first measure is to divide the steel mesh of the large pad into small pieces;
The second measure is to place 0.3mm diameter vias at the cross gaps of the steel mesh divided into small pieces;
Measure three is to recommend the use of a steel mesh with a thickness of 0.127 mm. It is not recommended to use a steel mesh with a thickness less than 0.1 mm or greater than 0.15 mm.
Figure 8 is a schematic diagram of a large pad steel mesh being divided into small pieces. By taking the above design improvement measures, the actual PCB production and PQFN device welding were performed, and the welding void rate was relatively low after X-ray photography. The experimental test void rate was about 15%. Figure 9 is a recommended PCB library component design diagram. Figure 10 shows the recommended reflow soldering temperature curve. Based on this recommended soldering temperature curve, users can adjust the soldering equipment parameters in combination with their own solder paste and soldering process to obtain a lower soldering void rate and improve soldering quality.
Figure 8. Large pad steel mesh is divided into small pieces
Figure 9. Recommended PCB library components for IMM101T series devices
Figure 10. Recommended reflow soldering temperature profile
Summarize
Through the above test results and analysis, it can be seen that this innovative PQFN packaged IPM module is somewhat different from general devices in actual applications. Since it uses PCB and copper foil as the main heat dissipation method and has a very small package size, this PQFN packaged IPM module can be used in applications with a smaller structure, such as hair dryers, air conditioner indoor fans, ceiling fans, etc. If additional heat dissipation measures are adopted, such as sticking a heat sink on the top or using a cooling fan, the current output capacity of the module can be increased, expanding the application power range of the PQFN packaged IPM module. When an aluminum substrate is used instead of a FR-4 material PCB board, the current output capacity of the IPM module can be increased by about one time.
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