Solder bonding is a versatile lower temperature bonding process that is used in joining a range of metals, ceramics, glass and metal: ceramic composites. By definition, solders are joining filler metals that melt below 450°C. Solder bonding is typically used in the assembly of structures for its good thermal and/or electrical contact or for creating seals. The advantage of solder bonding stems from lower temperature exposure (less that 400°C), compared to brazing when joining thermally sensitive materials. Alternatively, compared to bonding with epoxy adhesives, solder bonding is a more conductive bond, but does require higher temperature exposure and the wetting of the molten metal to the bonding surfaces.
Because of it excellent thermal and electrical conductivity, solder bonding finds application in the manufacture of sputter targets, heat spreaders and cold plates and other related thermal management components. Solder bonding is also used to seal ceramic:metal and glass windows used in optical based sensors and in other fluid cooled enclosures. Figures 1 and 2 show several typical solder bonded parts.
Solder bonding (e.g. S-Bond®), despite being versatile and capable of joining most materials, one must consider several issues when active solder bonding…
- Thermal expansion mismatch
- Size and shape of bonded parts
- Interaction with post solder bond processing
- Galvanic corrosion coupling
In every application being evaluated for a solder bonding solution, the component and process design needs to consider the following issues.
- Minimize CTE mismatch of bonded materials to prevent distortion or fracture.
- Understand post bonding processes to prevent damage of bond interface.
- Know Service Temperature and Thermal cycling effects on bond interface.
- Understand effects of service environment on bond interface corrosion
Thermal expansion mismatch (CTE): solder bonding requires heating the component parts in an assembly to 120 – 400°C, depending on the solder filler metal being used. When similar materials are being joined there is no CTE mismatch so it is not a concern. However; many times solder bonding is being used to lower the CTE mismatch… but despite the lower bonding temperature, it is not alone a “silver bullet” universal solution. Even when heating to 250°C, melting for Sn-Ag based solders, upon cooling once the solder solidifies it can transfer a strain. Then the CTE derived stresses can distort metal assemblies, fracture a glass or ceramic components or fracture the bond. Thus, one needs to minimize CTE mismatch stresses by selecting assemble component materials that are as close as possible in CTE.
When matching CTE is not practical, then one should design the component parts with size and thickness in mind… larger bond areas will “accumulate” more stress and lead to more distortion and/or fracture. A solution for larger parts is to “tile” the component parts; by tiling (mosaic) the strain mismatch accumulation is interrupted and lower the accumulation of stress in the assembly.
Post solder bond processes such as post solder bond heating either with another solder process, welding or bake outs to dry or cure components. Coatings may also be required on a bonded assembly where the heat and or chemical exposure of the coating process, as in electro-plating (see the coating blog article), interacts negatively with the solder bond.
When post processing a solder bonded part, temperature exposures typically should be below 90% of the solidus temperature (temperatures where solder alloys begin to melt) to maintain the bond. The thermal cycle itself can be damaging to the bond, even if the temperature is below this limit, especially with assemblies that have dissimilar materials. The processes that can degrade solder bonds include, other solder steps, welding, bake out or curing, and coating. Therefore; one needs to understand their impact on the solder filler metals and the solder bond interface.
Service conditions can also limit the performance and life of solder bonds. Temperature in service generally needs to be restricted to be below 80% of the solidus temperature of the solder filler metal (although active solders such as S-Bond® can be used up to 90%) to maintain sufficient bond strength. Thermal cycles are more damaging than constant temperature exposure and can be more damaging when CTE mismatched materials. Joint design can mitigate some of these effects by…
- Selecting component materials to lower CTE mismatch
- Minimizing area of solder bond and consider tiling, if practical
- Using thicker cross sections, if possible, to limit distortion
- Orienting or mechanically supporting solder bonds/seals to lower bond stresses.
With proper design solder bonded assemblies can be superior to epoxy bonded joints and work very well and compete with brazed or welded joints