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Ask the Experts

April 14, 2025 - Updated
April 21, 2025 - Originally Posted

Low Operating Temperature Lead-Free Solder

We will be soldering copper wires inside a motor that operates in a cold environment at -200 °C and cycles up to 100 °C. Which lead-free solder alloy is designed to endure this temperature cycle range?

L/M.

Expert Panel Responses

An indium-containing solder would be best for this very low-temperature application. In100 (100% In) is however extremely soft. For this reason, an alloy of 97In3Ag (97% Indium, 3% Silver) would be suggested for the added strength that the Ag provides. This does lower the melting point from 157°C to 143°C, however it should still meet the need for the top of your temperature range.

Fritz Byle
Process Engineer
Astronautics

Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

For soldering copper wires inside motors exposed to extreme temperature cycling from -200 °C to +100 °C, standard lead-free solders like SAC305 or SnCu are not suitable, as they tend to become brittle under cryogenic conditions.

We recommend using indium-based solder alloys, such as:

Indalloy 1E (52In/48Sn) – eutectic alloy

Melting point: 118 °C
Reflow: 135–150 °C
Excellent ductility and cryogenic reliability

Indalloy 290 (97In/3Ag)

Melting point: 143 °C

Although we are not affiliated with Indium Corporation, they currently hold key patents and offer proven performance for cryogenic and aerospace-grade solders.
Other commercial brands to consider for high-reliability and specialty solder applications include:

AIM Solder (USA/Canada) – known for custom alloys and reliability testing support
Alpha Metals / MacDermid Alpha (USA/UK) – global supplier with strong R&D in automotive and high-temp solders
Senju Metal Industry (Japan) – top-tier Japanese supplier with indium alloys and cryogenic solder pastes
Tamura Corporation (Japan) – known for electronic-grade solders and specialty formulations
Koki Company Ltd. (Japan) – advanced no-clean and low-temp alloys
Stannol GmbH (Germany) – offers eco-friendly and specialty solders with traceability
Balver Zinn (Germany) – high-purity solders, including indium and silver-based options

Depending on your region and supply chain, these companies may offer equivalent indium-based alloys or alternative cryogenic-capable solutions. We recommend consulting their technical data sheets and requesting samples when possible.

Andres Rojas
Engineering Director / Master IPC Trainer (MIT)
AMMSA Solutions

More than 20 years of technical experience in the electronics industry in roles ranging from Process & Project Engineer to engineering manager and Technical Applications Engineer for Latin Americas. IPC Master Trainer, International speaker and consultant.

Usually solder is tested/cycled from -40 C to 150 C. I’d recommend you contact the solder manufacturer for information on a larger operating range.

Edithel Marietti
Senior Manufacturing Engineer
Northrop Grumman

Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.

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