Douglas DeVoto

Thermal performance and reliability characterization of bonded interface materials (BIMs)

Thermal interface materials (TIMs) are an important enabler for low thermal resistance and reliable electronics packaging for a wide array of applications. There is a trend towards bonded interface materials (BIMs) because of their potential for low thermal resistance (<;1 mm2-K/W). However, due to coefficient of thermal expansion mismatches between various layers of a package, thermomechanical stresses are induced in BIMs and the package can be prone to failures and integrity risks. Deteriorated interfaces can result in high thermal resistance in the package and degradation and/or failure of the electronics. The Defense Advanced Research Projects Agencys (DARPA) Thermal Management Technologies (TMT) Program has addressed this challenge, supporting the development of mechanically compliant, low resistivity nano-thermal interface (NTI) materials. Prior development of these materials resulted in samples that met DARPAs initial thermal performance and synthesis metrics. In this present work, we describe the testing procedure and report the results of thermal performance and reliability characterization of an initial sample set of three different NTI-BIMs tested at the National Renewable Energy Laboratory.

Reliability of Bonded Interfaces for Automotive Power Electronics

In automotive power electronics packages, conventional thermal interface materials such as greases, gels, and phase change materials pose bottlenecks to heat removal and are also associated with reliability concerns. There is an industry trend towards high thermal performance bonded interfaces. However, due to coefficient of thermal expansion mismatches between materials/layers and resultant thermomechanical stresses, adhesive and cohesive fractures could occur, posing a problem from a reliability standpoint. These defects manifest themselves in increased thermal resistance in the package.The objective of this research is to investigate and improve the thermal performance and reliability of emerging bonded interface materials for power electronics packaging applications. We present results for thermal performance and reliability of bonded interfaces based on thermoplastic (polyamide) adhesive, with embedded near-vertical aligned carbon fibers, as well as sintered silver material. The results for these two materials are compared to conventional lead-based (Sn63Pb37) bonded interfaces. These materials were bonded between 50.8-mm × 50.8-mm cross-sectional footprint silicon nitride substrates and copper base plate samples. Samples of the substrate/base plate bonded assembly underwent thermal cycling from −40°C to 150°C according to Joint Electron Devices Engineering Council standard Number 22-A104D for up to 2,000 cycles. The dwell time of the cycle was 10 minutes and the ramp rate was 5°C/minute. Damage was monitored every 100 cycles by acoustic microscopy as an indicator of an increase in thermal resistance of the interface layer. The acoustic microscopic images of the bonded interfaces after 2,000 thermal cycles showed that thermoplastics with embedded carbon fibers performed quite well with no defects, whereas interface delamination occurred in the case of sintered silver material. Both these materials showed a superior bond quality as compared to the lead-based solder interface even after 1,000 thermal cycles.Copyright

Reliability of Emerging Bonded Interface Materials for Large-Area Attachments

Conventional thermal interface materials (TIMs), such as greases, gels, and phase change materials, pose bottlenecks to heat removal and have long caused reliability issues in automotive power electronics packages. Bonded interface materials (BIMs) with superior thermal performance have the potential to be a replacement to the conventional TIMs. However, due to coefficient of thermal expansion mismatches between different components in a package and resultant thermomechanical stresses, fractures or delamination could occur, causing serious reliability concerns. These defects manifest themselves in increased thermal resistance in the package. In this paper, the results of reliability evaluation of emerging BIMs for large-area attachments in power electronics packaging are reported. Thermoplastic (polyamide) adhesive with embedded near-vertical-aligned carbon fibers, sintered silver, and conventional lead solder (Sn63Pb37) materials were bonded between 50.8 mm x 50.8 mm cross-sectional footprint silicon nitride substrates and copper base plate samples, and were subjected to accelerated thermal cycling until failure or 2500 cycles. Damage in the BIMs was monitored every 100 cycles by scanning acoustic microscopy. Thermoplastic with embedded carbon fibers performed the best with no defects, whereas sintered silver and lead solder failed at 2300 and 1400 thermal cycles, respectively. Besides thermal cycling, additional lead solder samples were subjected to thermal shock and thermal cycling with extended dwell periods. A finite element method (FEM)-based model was developed to simulate the behavior of lead solder under thermomechanical loading. Strain energy density per cycle results were calculated from the FEM simulations. A predictive lifetime model was formulated for lead solder by correlating strain energy density results extracted from modeling with cycles-to-failure obtained from experimental accelerated tests. A power-law-based approach was used to formulate the predictive lifetime model.

Gaining Traction: Thermal Management and Reliability of Automotive Electric Traction-Drive Systems

Increasing the number of electric-drive vehicles (EDVs) on America?s roads has been identified by the U.S. Department of Energy (DOE), the federal cross-agency electric vehicle (EV)-Everywhere Challenge, and the automotive industry as a strategy with near-term potential for dramatically decreasing the nation?s dependence on oil. Mass-market deployment will rely on meeting aggressive technical targets, including improved efficiency and reduced size, weight, and cost. Many of these advances will depend on the optimization of thermal management.

Degradation characterization of thermal interface greases

Thermal interface materials (TIMs) are used in power electronics packaging to minimize thermal resistance between the heat generating component and the heat sink. Thermal greases are one such class of TIMs. The conformability and thin bond line thickness (BLT) of these TIMs can potentially provide low thermal resistance throughout the operation lifetime of a component. However, their performance degrades over time due to pump-out and dry-out during thermal and power cycling. The reliability performance of greases through operational cycling needs to be quantified to develop new materials with superior properties. NREL, in collaboration with DuPont, has performed thermal and reliability characterization of several commercially-available thermal greases. Initial bulk and contact thermal resistance of grease samples were measured, and then the thermal degradation that occurred due to pump-out and dry-out during temperature cycling was monitored. The thermal resistances of five different grease materials were evaluated using NRELs steady-state thermal resistance tester based on the ASTM test method D5470. Greases were then applied, utilizing a 2.5 cm × 2.5 cm stencil, between invar and aluminum plates to compare the thermomechanical performance of the materials in a representative test fixture. Scanning Acoustic microscopy, thermal, and compositional analyses were performed periodically during thermal cycling from −40°C to 125°C. Completion of this characterization has allowed for a comprehensive evaluation of thermal greases both for their initial bulk and contact thermal performance, as well as their degradation mechanisms under accelerated thermal cycling conditions.

Thermal Performance and Reliability of Large-Area Bonded Interfaces in Power Electronics Packages

In automotive power electronics packages (e.g., insulated gate bipolar transistor [IGBT] packages), conventional polymeric thermal interface materials (TIMs) such as greases, gels, and phase-change materials pose a bottleneck to heat removal and are also associated with reliability concerns. High thermal performance bonded interfaces have become an industry trend. However, due to mismatches in the coefficient of thermal expansion between materials/layers and the resultant thermomechanical stresses, there could be voids and crack formations in these bonded interfaces as well as delaminations, which pose a problem from a reliability standpoint. These defects manifest themselves in increased thermal resistance in the package, which acts as a bottleneck to heat removal from the package. Hence, the objective of this research is to investigate and improve the thermal performance and reliability of novel bonded interface materials for power electronics packaging applications. Thermal performance and reliability of bonds/joints is presented for bonds based on a thermoplastic (polyamide) adhesive with embedded micron-sized carbon fibers, sintered silver (Ag), and conventional lead (Pb)-based solder materials. These materials form a bond between 50.8 mm × 50.8 mm footprint direct-bond-copper (DBC) substrate and copper (Cu) base plate samples. Samples undergo thermal cycling (−40°C to 150°C) for up to 2,000 cycles as an upper limit. Damage occurrence is monitored every 100 temperature cycles by several non-destructive techniques, including steady-state thermal resistance measurement, acoustic microscopy, and high-voltage potential testing. This yields a consistent story on the thermal performance and reliability of large-area joints under accelerated stress conditions.Copyright