The design considerations for power converters operating at room or at ultra-low temperatures are the same in terms of power density, reliability, and efficiency. In order to design power converters at cryogenic temperatures, the passive and active components should be carefully selected. Understanding the behavior of the components at cryogenic temperatures leads to power converter systems that have many superior advantages such as high power density (reduced size and volume), higher efficiency (reduced system losses), and even increased system reliability. This paper reviews the status of the recent research on the cryogenic temperature behavior of Si semiconductor devices, wide bandgap semiconductor devices, passive components, and power converter topologies.

Survey of Cryogenic Power Electronics for Hybrid Electric Aircraft Applications

Sn–Pb eutectic alloy is widely applied to the deep-space electronics that work under cryogenic environments, but little is known about its responses to the mechanical loading at cryogenic temperatures. In this work, a comprehensive investigation about the deformation and fracture behaviors as well as constitutive relations of the eutectic alloy at cryogenic temperatures was conducted through uniaxial tensile experiments over a full temperature range from 293 K to 77 K, in-situ cryogenic tensile experiments and fractography. With the declining temperature, the tensile strength and quasi-static toughness increase substantially while the elongation is maintained at 25%–30%; Sn–Pb eutectic alloy achieves the optimal combinations of strength, ductility and toughness at 123 K, which is due to a number of deformation twins activated in the Sn matrix together with the compatible deformation between the Pb-rich phases and the Sn matrix. Multiple 45° shear bands induced by the maximum shear stress contribute to reconcile the deformation difference between the two phases, which reaches its minimum at around 123 K. A ductile shear fracture on the 45° shear planes with respect to the tensile axis occurs under the maximum shear stress at temperatures ranging from 293 K to 123 K, while the maximum normal stress leads to a brittle fracture on the 90° planes at 77 K. Moreover, the Anand model fails in fitting the constitutive relations of Sn–Pb eutectic alloy when the temperature declines to 233 K or lower. Instead, the Hollomon equation has been successfully applied to fit the constitutive relations at these invalid temperatures.

Maximum shear stress-controlled uniaxial tensile deformation and fracture mechanisms and constitutive relations of Sn–Pb eutectic alloy at cryogenic temperatures

In this report for the Nasa NIAC Phase I study, we present a mission architecture and a robotic platform, the Shapeshifter, that allow multi-domain and redundant mobility on Saturn's moon Titan, and potentially other bodies with atmospheres. The Shapeshifter is a collection of simple and affordable robotic units, called Cobots, comparable to personal palm-size quadcopters. By attaching and detaching with each other, multiple Cobots can shape-shift into novel structures, capable of (a) rolling on the surface, to increase the traverse range, (b) flying in a flight array formation, and (c) swimming on or under liquid. A ground station complements the robotic platform, hosting science instrumentation and providing power to recharge the batteries of the Cobots. Our Phase I study had the objective of providing an initial assessment of the feasibility of the proposed robotic platform architecture, and in particular (a) to characterize the expected science return of a mission to the Sotra-Patera region on Titan; (b) to verify the mechanical and algorithmic feasibility of building a multi-agent platform capable of flying, docking, rolling and un-docking; (c) to evaluate the increased range and efficiency of rolling on Titan w.r.t to flying; (d) to define a case-study of a mission for the exploration of the cryovolcano Sotra-Patera on Titan, whose expected variety of geological features challenges conventional mobility platforms.

The Shapeshifter: a Morphing, Multi-Agent, Multi-Modal Robotic Platform for the Exploration of Titan (preprint version).

This paper presents reliability of surface mount technology (SMT) packaging assemblies compared to decades old pin grid array (PGA) assembled with hand soldering that were subjected to extreme cold thermal cycles representative of high-reliability applications for deep-space missions. The advanced SMT packages included commercial–off-the-shelf (COTS) column grid array (CGA), leaded thin small outline (TSOP), leadless quad flat no-lead (QFN), and passive technologies. Tin-lead eutectic solders were used for soldering; they were either in paste form for assembling SMT packages or solid wire for hand soldering plated-through hole (PTH) for PGA ceramic packages. It is a general perception that the PTH technologies are an order of magnitude more robust in thermal cycle reliability than SMTs. However, this may not be true under extreme cold environmental conditions. Design for reliability (DfR) may requires compromising the stand-off height of PGA assembly and the thickness of printed circuit board (PCB) to achieve the desired long-term reliability. This paper presents the key parameters affecting the reliability of SMT package assemblies including TSOP, QFN, CGA, and PGA assemblies as well as associated test results under standard and extreme cold harsh thermal shock/cycle conditions. The standard thermal cycle conditions for high-reliability applications included were in the range of (−55°C to 100°C) and (−55°C to 125°C) whereas extreme cold cycles were harsh (−135°C to 70°C) and (−120°C to 85°C), or milder (−110°C to 20°C). Test results presented include non-destructive optical/SEM and X-ray images and daisy-chain resistances as well as destructive failure analyses performed by X-sectioning. Finally, key findings for various packaging technologies under extreme cold thermal cycles are presented.

SMT/PTH Solder Joint Reliability under Extreme Cold Thermal Cycles

This paper studied the low temperature tensile mechanical properties of Sn63Pb37, Sn62Pb36Ag2, In50Pb50, and Pb88Sn10Ag2 solders. It found that Sn62Pb36Ag2 solder still maintained a high post-break elongation at -150 °C compared with Sn63Pb37, and its low-temperature mechanical properties were more excellent. The dispersed β-Sn phase in the solder played a key role in strengthening the toughness. Compared with In50Pb50 and Pb88Sn10Ag2, It also showed better process applicability and maturity.

Study on the Low Temperature Reliability of Leaded Solder

In addition to the challenges of cold operation for active devices, passive devices and packaging materials exhibit changes in electrical behavior as a function of temperature. Results on the temperature dependent behavior of capacitors and substrate materials as a function of temperature and frequency to −185°C will be discussed. These properties may further change if the materials exhibit degradation due to mechanical loading. Whether the electronics are intended to operate at cold temperatures or simply survive exposure to cold temperatures, the packaging technologies between the electronics and the outside world are required to survive multiple extreme low temperature cycles, which combine the challenges of cold temperature exposure and fatigue at low temperatures. To that end, the impact of extreme low temperatures on the mechanical behavior of various key electronic packaging materials will be discussed along with experimental results obtained from several representative assemblies that have been thermally cycled or shock cycled down to −120°C or lower.

Bianca Rhym

Papers

Electronic packaging and passive devices for low temperature space applications