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.

Expert Panel Responses

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 156.4 C. High-indium solder alloys may also be usable 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.

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.

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.

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.

100 In (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. 100 In 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 100 In 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 100 In can be found at www.indium.com

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.

One must not automatically assume that In100 is the ideal solder joint alloy for use in space or any other deep-freeze atmosphere. It may be IFF (if and only if) the product's service cycle is going to remain stable at a single very cold temperature for a long period of time, such as in cryogenics and exterior space use, but you need to understand that you NEVER get something for nothing when it comes to alloy properties! In100 may also be the LAST alloy you would think of using in deep space, because of the temperature VARIATION the product may see. Different alloys react differently to these variations in temperature.

Some, such as Sn63, do well in terms of the grain structure coarsening due to temperature cycling over time, which eventually leads to total failure but is expected. Others such as In100 cannot withstand more than a few hundred cycles to failure compared to Sn63's thousands of cycles to failure. Other alloys may provide a longer time to failure but have other detractions, such as being more prone to shock failures (too brittle) etc., etc. Therefore, one must consider the temperature cycling frequency and the cycling extremes the product will see, not just the coldest or hottest temperatures.

Of course, an alloy's melting points must be considered first, but the alloy properties by that I mean the properties of both the bulk solder joint and the interconnect alloy) and the established time to failure that the product is expected to see are far more important in determining expected product life than the simple hottest or coldest temperature peaks. Knowledge of these alloy properties requires a very good understanding of metallurgical principles and the correlating life cycle testing methods to validate which alloy selection and surface finish is optimal for the product's intended use.

Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics

Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.

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