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Flux-less Soldering

Background

Industry -Aerospace/Optoelectronics

Materials Used
Quartz window (with AR coating)
SST 304L housing

Requirement

To Indium solder and hermetically seal quartz glass window in SST housing with no flux for an ultrahigh vacuum application.

Problem:

Most solders oxidize in atmosphere and increase their surface tension making solder flow and fill behaviors more difficult and ultimately making a strong, hermetic seal challenging. Flux very effectively reduces the surface tension of the solder so that it can flow into the areas desired for bonding and increasing the overall likelihood of a successful bond. However, geometry that does not allow the flux to evaporate during processing can lead to flux entrapment that can severely impact hermetic performance. Due to the extreme coefficient of thermal expansion (CTE) mismatch between quartz (50×10^-7/C) and SST 304 (170×10^-7/C), high temperature brazing was not an option.

Solution:

Our solution was to develop a custom process that lowered the surface energy of molten solder without using flux. Our technique provides extremely low surface energy solder and increases the flow characteristics of the molten solution. Many customers come into processing with the understanding that flux will enhance their hermetic sealing area. What they have not seen is the issues caused by flux entrapment that can severely compromise hermetically sealed joint areas. IJ Research has developed a unique processing method were solder material can be brought to its liquidus and hermetically seal with no need for flux or any other aid that may hinder hermetic sealing.

Results:

The advantage of this processing method became clear once the customer who had an internal yield rate of just 30% transferred the process over to IJ Research were processing yields were increased to over 98%. Additionally X-Ray inspection performed on components for non-destructive evaluation shows that IJ Research’s soldering method was able to consistently obtain over 90% flow in the sealing area where the customer had previously been struggling to obtain over 50% flow. We were able to maximize yield and increase the assembly performance by applying our unique and highly specialized knowledge in establishing the most reliable and durable hermetic joints.