Power Module Substrate Options Available to Lower Semiconductor Junction Temperatures for Increased Reliability


Jerry Moudilos, Field Applications Engineer at Vincotech

Parameters often change during the design cycle which can lead to redesigns and PCB board re-spins. This can create delays in the product development program as different power modules need to be selected and a PCB re-spin is often required to accommodate these new modules. This paper examines changing a power module's substrate as a possible option for lowering its semiconductors junction temperatures without changing the fit, form, or function of the power module.

 

Designing for Electrical Performance and Temperature Requirements

Often during the design cycle, the output power of the design can change, leading to higher than estimated output currents for the power module. Parasitic inductances due to the product’s layout and assembly can also lead to higher than expected losses.

In many cases, the design has enough margin for the module to accommodate these higher current requirements and higher losses at the expense of higher power dissipation and higher semiconductor junction temperatures. These higher semiconductor junction temperatures decrease the reliability of the power module and a redesign is often necessary to meet the product's operational and reliability requirements. This redesign often requires a larger power module with higher current rated semiconductors or a power module with semiconductors which have lower conduction losses and/or switching losses. This new power module also requires a PCB re-spin and re-characterization of the module's electrical performance.

Another option available to the designer is to improve the thermal design of their product.  Ambient temperature is a fixed parameter in most applications and the designer must either increase airflow or coolant flow for forced air or liquid cooling systems. For convection cooling systems, the heatsink surface area must be increased which will increase the weight and size of the product which is often not desirable or possible.

Another possibility that does not require a PCB re-spin is to decrease the power module's thermal impedance from its semiconductors junctions to the module's case. This can be accomplished in a power module by simply changing its substrate material. Since this substrate modification does not require the power module's layout, bond wire lengths/routing, or semiconductors to change, its electrical performance will be similar to the power module with the less thermally conductive substrate. This helps minimize the designer's re-characterization effort which is often required when major components such as power modules are changed due to re-designs.

Vincotech offers many power modules with aluminum oxide (Al2O3) substrates and with the more thermally conductive aluminum nitride (AlN) and silicon nitride (Si3N4) substrates. Many of these AlN and Si3N4 substrate modules were designed specifically for customers who were satisfied with the Al2O3 substrate module's electrical performance but were not able to meet their desired thermal and junction temperature requirements.  

Substrate changes are relatively quick to implement because minimal engineering work is required. The lead time for these AlN and Si3N4 substrate modules is determined by the availability of the AlN and Si3N4 substrate DCBs and is typically about 4 - 8 weeks.

 

Case Study for Power Modules with Different Substrates

In this paper, we will examine the six-pack IGBT4 V23990-P829-F10-PM power module which is constructed with an Al2O3 substrate and the six-pack IGBT4 V23990-P829-F-PM power module which has an AlN substrate.   Both these power modules have similar semiconductors, bond wire length/routing, and layout.  These modules also have similar housings and footprints which are shown in Figure 1:

 

Housing for the Vincotech V23990-P829-F-PM and V23990-P829-F10-PM Power Modules

Figure 1. Housing for the Vincotech V23990-P829-F-PM and V23990-P829-F10-PM Power Modules.

 

These power modules were required to operate with the following operating conditions:

  • Vdc = 900V
  • Vout = 480V
  • Iout = 20A
  • fswitching = 20kHz
  • pf = 0.8
  • Tcase = 80ºC
  • Tjmax = 120°C

The semiconductor junction temperatures for both the V23990-P829-F10-PM and the V23990-P829-F10-PM power modules were obtained from Vincotech's ISE simulator software and are shown in Figure 2:

 

Figure 2. Vincotech V23990-P829-F-PM and V23990-P829-F10-PM Power Module Semiconductor Junction Temperature Comparison

 

The curves in Figure 2 clearly show that the V23990-P829-F10-PM power module with its Al2O3 substrate DCB would not be able to meet the design's junction temperature requirement while the V23990-P829-F-PM power module with its AlN substrate DCB will have ~16°C margin. In this case, the V23990-P829-F-PM power module can be dropped into the application's V23990-P29-F10-PM footprint with no changes to the PCB layout or the product's thermal design.

The V23990-P829-F-PM power module also offers the option to increase the product's output current since its IGBT junction temperatures are 104.6o C. If required, this module's output current can be raised to 30A while maintaining the 120o C maximum junction temperature limit.

Although AlN substrates have lower thermal impedances than Al2O3 substrates, they do have a few disadvantages. Power modules with AlN substrates are more expensive than modules using Al2O3 substrates.  In this example, the price of the AlN V23990-P829-F-PM power module is approximately 27% more than the price of the Al2O3 V23990-P829-F10-PM power module. Also, AlN substrates are more brittle than Al2O3 substrates. Similar tools can be used to assemble AlN and Al2O3 modules to their heatsink or coldplate, however, a pre-tightening process is required for the AlN substrate modules to minimize stresses to the substrate during assembly. 

Other substrates such as Si3N4 are commercially available and offer better thermal impedances than Al2O3 substrates at lower costs with more mechanical robustness than AlN substrates. Si3N4 substrates do have higher thermal impedances than AlN substrates but in many applications, they offer a viable trade-off between cost and performance.

 

Case Study Results

Transitioning from a paper design to a final manufacturable design often results in higher than expected semiconductor temperatures due to design requirement changes, parasitic inductances and over-optimistic assumptions and estimates. The additional losses which result from these conditions manifest as higher than expected semiconductor junction temperatures which will lower the expected lifetime of the product.

Lowing the product’s power rating or re-designing the product’s cooling system is an option however, these options are costly and time-consuming. Changing a power module's substrate material can be an easy option to lower its semiconductors' junction temperatures or to increase its power output for a given maximum junction temperature. Power modules with an AlN or a Si3N4 substrate will have similar mechanical and electrical characteristics as their Al2O3 variants which allow these modules to be a drop-in replacement in the customer’s existing layout. The AlN power module will more than its Al2O3 equivalent power module and will require additional care when it is assembled to its heatsink assembly.

Other substrate materials such as Si3N4 are available and offer a comprise solution which has less thermal resistance than Al2O3 substrates at a lower cost than AlN substrates. Existing modules with Al2O3 substrates can be quickly redesigned with AlN or Si3N4 substrates at low risk since no design changes are required other than a change in the module’s Bill on Materials.

 

References

More information: Vincotech    Source: Bodo's Power Systems, March 2019