When a component must survive low Earth orbit (LEO), material selection is the specification decision everything else depends on, and so is the manufacturing partner behind it.

Space satellites need solar panels to generate power, converting sunlight into electricity. These space solar arrays, made up of individual solar cells mounted on panels, convert sunlight at better than 30% efficiency, handle radiation well and act as the baseline for many of today’s aerospace missions.

Despite the proven capability of these space solar arrays, the environment in which they operate can be brutal. Extreme temperature swings, constant radiation and no atmospheric pressure require engineers to consider these effects on the entire solar array. TPC companies are experts in manufacturing busbars, the conductive strip bonded to the solar cell, to withstand these harsh conditions.

In LEO applications, matching the busbar to the solar cell is not optional – major space missions depend on them, and the difference between a ten-year mission and a two-year one often comes down to this single specification decision. Original Equipment Manufacturers (OEMs) rely on advanced manufacturers like TPC to provide the guidance and components to create high-reliability missions.

Companies like Elcon and E-Fab act as advisors and co-designers for highly sensitive space applications. By providing customers with a proven approach for installing and interconnecting solar cells in LEO, OEMs can have greater confidence that their busbars are matched for reliability.

Why Matching Busbars Matters for LEO Cycling

TPC specializes in fabricating custom busbars that provide superior power distribution, rigidity and support for various space applications. It offers a range of materials and consults customers on how to build the component to project specifications that meet the application’s requirements.

A busbar is a crucial component in allowing for efficient current management while maintaining a compact design. The component is designed to provide strength to applications intended for use in unforgiving environments, and few environments test that durability more severely than in the thermosphere, where LEO operates.

A satellite traveling at 400 kilometers above the Earth orbits every 90 minutes. With each orbit, the solar array goes through one full thermal cycle, from about +120 °C in sunlight to −150 °C in shadow.

This happens roughly 5,500 times a year. That means over a 10-year mission, the busbar sees more than 55,000 cycles. For comparison, a 15-year Geostationary Earth Orbit (GEO) satellite, which travels above the Earth at about 35,000 kilometers, accumulates fewer than 1,800 orbits in total. One year of LEO exceeds three times the entire lifetime budget of a GEO array.

These LEO cycles impact the busbar bond, as every cycle produces differential strain at the bond between the busbar and the cell. The strain amplitude is proportional to the Coefficient of Thermal Expansion (CTE) mismatch; in other words, the greater the difference in how much each material expands or contracts with temperature change, the greater the stress placed on the joint between them.

As the material strain from CTE accumulates damage with cycle count, it is critical to choose a busbar material with a reliability that has a direct, quantifiable impact on mission life.

Selecting Materials

Informed by production experience across multiple LEO programs, TPC advises its customers on selecting the best materials for busbar connectivity, and the preferred material depends heavily on its application.

Based on TPC research and analysis, the following are the most commonly recommended materials, each with pros and cons:

Kovar

This material is the current industry standard for connecting busbars to solar cells. It’s a metal alloy with a CTE close enough to the cell beneath it that the bond survives thousands of temperature cycles without cracking. While highly resistant, Kovar¹ does not conduct electricity well. To combat this, it can be first coated in silver to support current-carrying and then in gold for solderability and to protect it from the harsh space environment.

Copper

This material has a strong resistivity and works well as an electrical conductor. It expands and contracts at roughly three times the rate of the cell beneath it, which can put a large amount of stress on the bond when the satellite moves between sunlight and shadow. Copper² works in harness wiring and in geometries with built-in compliance, such as S-loops, but is not optimal for a rigid busbar soldered directly to a solar cell under LEO cycling.

Silver

This is the most electrically conductive pure metal available and bonds readily to solar cells during manufacturing. That said, it has the same mismatch problem as copper. In LEO, atomic oxygen erodes unprotected silver³ continuously; the oxide layer is mechanically unstable under thermal stress, exposing fresh material. Early program data from Eureca⁴ showed spacecraft with unprotected silver degraded within one to three years.

Molybdenum

Molybdenum⁵ is reliable in core technical requirements: its expansion rate is well-matched to the germanium cell substrate, which serves as the foundational building blocks for high-performance space and terrestrial solar cells. It is non-magnetic and it resists atomic oxygen degradation effectively. The obstacle is manufacturing: molybdenum and silver will not bond directly to one another, requiring an intermediate layer of platinum or nickel to make the joint sufficient. This adds both cost and process complexity, which has limited broad adoption.

Invar

The material provides an exceptionally low thermal expansion rate, which is useful in some precision applications. However, its expansion rate falls too far below the germanium cell substrate to make a reliable bond. It is also ferromagnetic, or strongly attracted to magnets and can become permanently magnetized, which creates interference risks on magnetically sensitive spacecraft. Invar appears frequently in patent literature as a candidate for interconnect material but has rarely been adopted in actual solar array production.

Making Partnership Part of the Mission

The physics governing busbar material selection are well understood and represent a narrow margin for error. A material system where the supply chain takes shortcuts has the potential to compound into failures years later, in an environment where there is no opportunity to intervene.

In acknowledging the need for stringent quality standards and optimized material selection, a trusted advisor adds imperative value. The right partner brings material knowledge, direct experience navigating suppliers, specifying process requirements and expert-informed inquiries that prevent problems from reaching the assembly.

At 5,500 thermal cycles per year, for the full life of a mission, the difference between a controlled supply chain and an uncontrolled one shows up in the data. TPC has built its reputation on exactly that kind of supply chain discipline, bringing the material expertise and process rigor that LEO missions demand.

 

Frequently Asked Questions

Q: What makes LEO such a harsh environment for solar arrays?

A: Satellites in LEO operate in the thermosphere, where they face extreme temperature swings, constant radiation and atomic oxygen erosion, all without the protection of a full atmosphere. Unlike ground-based applications, there is no opportunity to inspect or repair components once a mission is underway.

Q: Why do busbars need to be matched to the specific solar cell they’re bonded to?

A: Every solar cell material has its own rate of thermal expansion, and the busbar must move with it through thousands of temperature cycles without cracking the bond. A mismatch, even a small one, accumulates damage over time in a way that a short ground test will never reveal.

Q: Why does material selection matter more for LEO than other orbits?

A: LEO satellites complete roughly 5,500 thermal cycles per year, far more than a GEO satellite accumulates in its entire lifetime. That relentless cycling puts far greater stress on the busbar bond, leaving almost no margin for a material mismatch.

Q: What role does a manufacturing partner play beyond supplying components?

A: The physics of material selection leave little room for error, and supply chain shortcuts can take years to surface as failures in orbit. An experienced partner brings process knowledge and supplier oversight that prevents those problems from ever reaching the assembly.

 

Works Cited

A full citation on TPC’s LEO busbar flight qualified stack recommendation is available on request.

¹ Glenn, G.S. and Rupp, M.L. (The Boeing Company), “In-Plane Solar Cell Interconnect with Integrated Diode Tab,” European Patent EP1128445B1, filed February 22, 2001, granted July 28, 2010. Silver-plated and silver-clad Kovar interconnects tested across multiple stress-relief configurations. LEO thermal cycle data: 20,000 to 40,000 cycles over a typical spacecraft lifetime; 1,800 cycles for a 15-year GEO mission. https://patents.google.com/patent/EP1128445B1

² Hanley, G.M. (Hughes Aircraft Company), “Solar Cell Assembly,” U.S. Patent 6,034,322 A, filed July 1, 1997, issued March 7, 2000. “Interconnects for electrically joining circuitry on the cell to an adjoining cell are either pure silver or silver plated kovar, molybdenum or Invar. These latter materials provide a better thermal expansion match to the cell material.” Copper is not listed as an interconnect substrate, implicitly confirming its unsuitability for direct bonding to III-V cells due to CTE mismatch. https://patents.google.com/patent/US6034322A/en

Note: Copper’s CTE of approximately 17 ppm/°C and resistivity of approximately 1.7 µΩ·cm are standard published values available in any materials reference.

³Koontz, S., King, G., Dunnet, A., Kirkendahl, T., Linton, R., and Vaughn, J. (NASA Johnson Space Center / Los Alamos National Laboratory), “Intelsat Solar Array Coupon Atomic Oxygen Flight Experiment,” 1995. NASA Technical Reports Server (NTRS). Tested Intelsat VI silver foil interconnects in LEO (STS-41 flight experiment and ground simulation). Silver interconnects were the key AO materials concern. Available data on silver AO degradation showed “high variance.” https://ntrs.nasa.gov/citations/19950037627

⁴ “Post-Flight Investigation of the ASGA Solar Cell Experiment on EURECA,” 1994.
https://www.osti.gov/biblio/191191

⁵Zhu, J.-J., et al., “Study on Silver-Plated Molybdenum Interconnected Materials for LEO Solar Cell Array,” IOP Conference Series: Materials Science and Engineering, vol. 229, 012010, 2017. Proposes molybdenum as a direct alternative to Kovar for LEO solar array interconnects. Highlights superior atomic oxygen resistance and non-magnetic behavior. Discusses Pt and Ni interlayer requirements due to Mo-Ag immiscibility. https://iopscience.iop.org/article/10.1088/1757-899X/229/1/012010