While semiconductor die get so much of the attention due to their ever-shrinking feature size and ever-increasing substrate size, the ability to effectively package them and thus use them in a circuit is also critical. For this reason, considerable effort is devoted to developing and perfecting practical, technically advanced, thermally suitable cost-effective packages for components ranging from switching power devices to multi-gigahertz RF devices.

Regardless of frequency, package parasitic inductance is a detrimental issue, as it slows down slewing needed for switching crispness of digital devices and responsiveness of analog ones (of course, reality is that digital switching performance is still constrained by analog principles.).

Now, a researcher team at the US Department of Energy’s National Renewable Energy Laboratory (NREL; recently renamed as the National Laboratory of the Rockies) has developed a silicon-carbide half-bridge module that uses organic direct-bonded copper in a novel layout design to enable a high degree of magnetic-flux cancellation, Figure 1.                 

Figure 1 (left) 3D CAD drawing of new half-bridge inverter module; (right) Early prototype of polyimide-based half-bridge module. Source: NREL

Their Ultra-Low Inductance Smart (ULIS) package is a 1200 V, 400 A half bridge silicon carbide (SiC) power module that can be pushed beyond 200-kHz switching frequency at maximum power. The low-cost ULIS also allows the converter to become easier to manufacture, addressing issues related to both bulkiness and costs.  

Preliminary results show that it has approximately seven to nine times lower loop inductances and higher switching speeds at similar voltages/current levels, and five times the energy density of earlier designs — while occupying a smaller footprint, Figure 2.

Figure 2 The complete ULIS package is very different than conventional packages and offers far lower loop inductance compared to exiting approaches. Source: NREL

In addition to being powerful and lightweight, the module continuously tracks its own condition and can anticipate component failures before they happen.

In traditional designs, the power modules conduct electricity and dissipate excess heat by bonding copper sheets directly to a ceramic base—an effective, but rigid, solution. ULIS bonds copper to a flexible Dupont Temprion polymer create a thinner, lighter, more configurable design.

Unlike typical power modules which assemble semiconductor devices inside a brick-like package, ULIS winds its circuits around a flat, octagonal design, Figure 3. The disk-like shape allows more devices to be housed in a smaller area, making the overall package smaller and lighter.

Figure 3 This “exploded” drawing of the complete half-bridge power module shows the arrangement of the electrical and structural elements. Source: NREL

At the same time, its novel current routing allows for maximum cancellation of magnetic flux, contributing to the power module’s clean, low-loss electrical output, meaning ultrahigh efficiency.

While conventional power modules rely on bulky and inflexible materials, ULIS takes a new approach. Traditional designs call for power modules to conduct electricity and dissipate excess heat by bonding copper sheets directly to a ceramic base—an effective but rigid solution. ULIS bonds copper to the flexible, electrically insulating Temprion to create a thinner and lighter module.

The stacked module layout greatly improves energy density and reduces parasitic inductance (based on simulation data).  Typical half-bridge module inductance is 2.2 to 5.5 nanohenries, compared to 20 to 25 nH for existing designs. Further, reliability is enhanced as the compliance of Temprion reduces the strain caused by the differences in the coefficient of thermal expansion (CTE) between mated materials.

Since the material bonds easily to copper using just pressure and heat, and because its parts can be machined using widely available equipment, the team maintains that the ULIS can be fabricated quickly and inexpensively, with manifesting costs in the hundreds of dollars rather than thousands, Figure 4.

Figure 4 The ULIS can be machined using widely available equipment, thus significantly reducing the manufacturing costs for the power module. Source: NREL

Another innovation allows  the ULIS to function wirelessly as an isolated unit that can be controlled and monitored without external cables. A patent is pending for this low-latency wireless communication protocol.

The ULIS design is a good example of the challenges and dead-end paths that innovation can take on its path to a successful conclusion. According to the team’s report, one of the original layouts looked like a flower with a semiconductor at the tip of each petal. Another idea was to create a hollow cylinder with components wired to the inside.

Every idea the team came up with was either too expensive or too difficult to fabricate—until they stopped thinking in three dimensions and flattened the design into nearly two dimensions, which made it possible to build the module balancing complexity with cost and performance.

The details of the work are in their readable and detailed IEEE APEC paper “Organic Direct Bonded Copper-Based Rapid Prototyping for Silicon Carbide Power Module Packaging” but it is behind a paywall. However, there is a nice “poster” summary of their work posted at the NLR site here.

I wonder is this innovation will catch on and be adopted, but I certainly don’t know. What I do know is that some innovations are slow to catch on, and many do not because of real-world problems related to scaling up, volume production unforeseen technical issues, testability…it’s a long list of what can get in the way.

If you don’t think so, just look at batteries: every month, we see news of dramatic advances that will supposedly revolutionize their performance, yet these breakthroughs don’t seem to get traction. Sometimes it is due to technical or implementation problems, but often it is because the actual improvement they provide does not outweigh the disruption they create in getting there.

Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.

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