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March 10, 2014

Very Low Temp PCBs

We have a requirement for PCBs to be used at temperatures as low as -200 degree C. Are there certain solder types designed for very low temperatures? What would you recommend for a PCB surface finish?

J.V.

Experts Comments

In general, you want to look at indium-based solder alloys. Pure indium is used for seals in cryogenic applications and would definitely serve. It melts at 156.4C. High-indium solder alloys may also be useable in this range.

With regard to surface finish, you will of course not want a solder-based finish, and you will want to ensure that the selected finish will perform well with Indium solders. The solder manufacturer may have specific recommendations.

You will, I believe, also want to look carefully at the laminate materials you select. The PWB itself will become very brittle at the low temperatures. An alternative laminate, perhaps a perflourinated material, may perform better than an epoxy in this regard.
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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.
Application-specific. If the product will just sit there and not be subjected to any changes in temperature or mechanical loads, then most if not all existing materials should be fine. The key exception is pure tin, which can turn into tin pest at those temperatures. This would include tin plating and potentially lead-free solder. SnPb or high Pb solders should be fine as well as HASL, silver, and ENIG platings.
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Dr. Craig D. Hillman
CEO & Managing Partner
DfR Solutions
Dr. Hillman's specialties include best practices in Design for Reliability, strategies for transitioning to Pb-free, supplier qualification, passive component technology and printed board failure mechanisms.
I would think that dealing with such cold temperatures the solder joint will be the main concern.

As for surface finish I would recommend staying away from an ENIG finish. Due to the brittleness of the nickel. Depending on the heat cycles the product would see at assembly I would recommend using regular solder.

If the boards are required to have a lead free surface finish I would recommend an Immersion Tin or Silver or going with the Nihon Superior SN100 lead free solder HAL finish.
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Paul Reid
Program Coordinator
PWB Interconnect Solutions
Paul Reid has over 35 years experience in bare board fabrication, quality and reliability. Working for PWB Interconnect Solutions, which does thermal cycle evaluations (IST) of representative coupons, Paul provides failure and root cause analysis of how PWBs fail. His area of expertise includes how circuit board's copper interconnections and material fails in assembly, rework and in the field, as a result of thermal cycling.
100In (melts at 157 deg C) has historically been used for space applications and for cryogenic sealing where temperatures are close to absolute zero. So this would be the metal of choice. 100In can be manufactured as solder paste with a flux specific to In.  

For the PCB surface finish, ENIG is preferred. Given end reliability requirements & potential wire-bonding, thick Au (1-3 microns) metallizations may be necessary.

Unlike Sn-based solders, there is no problem of Au embrittlement with 100In even for thick Au as the rate of dissolution of Au in In is 13 times slower than that of Au in Sn.

More info on the physical properties of 100In can be found at www.indium.com
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Karthik Vijay
Technical Manager - Europe
Indium Corp.
Currently with Indium Corporation and responsible for technology programs and technical support for customers in Europe. Over 15 yrs experience in SMT, Power, Thermal & Semiconductor Applications. Masters Degree in Industrial Engg, State University of New York-Binghamton.
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