The Issue of Coefficient of Thermal Expansion (CTE) Mismatch
Yes, S-Bond can join a wide variety of materials, including aluminum, copper, stainless steel, refractory metals and ceramic to metal brazing with aluminum oxide, aluminum nitride, silicon carbide and other oxide, nitrides and carbides… however, with this wide variety of materials joining capability, we have a lot of inquiries about aluminum soldering to stainless steel or aluminum oxide, graphite bonding to aluminum, titanium to silicon carbide, etc.
Our answer is “Yes we can join them, BUT….” The big BUT… it depends on the materials and the joined assembly size and geometry. Our response is based solely in the CTE mismatch of the materials being joined. Materials expand at different rates depending on the composition (atomic elements), structure (atomic arrangement) and thermal properties. A material’s volume will change based on the relationship and when derived to any linear dimension, the relationship of the increase of length per unit length per °C (or °F) is established that leads to the linear expansion relation.
A table of common metals, ceramics and glass is seen below showing that materials vary widely. Many errors or “miscalculations” occur from aluminum soldering to any other metal or ceramics. With a linear CTE of 23 x 10-6 / °C, aluminum is one of the most expanding metals when heated. Alternatively, SiC, quartz and tungsten have almost zero or not much expansion at all when heated.
The most common design error made in aluminum soldering is to solder or braze bond large aluminum components to any other metal or ceramic. Many times aluminum soldering uses low temperature curing adhesives since soldering or brazing aluminum must be heated from 200 – 550°C. When the solder or braze is bonded, then the aluminum will contract to its room temperature dimension. An example, when S-Bond 220 joining aluminum to steel, if the component parts require heating to 250 °C using the CTE values in the table below, a 12” plate of Al will grow by almost 0.060” while the 12” plate of steel will only grow by about ½ that amount with a CTE of 10.4 ppm for steel vs. the 23 ppm. Thus the steel plate only grows at 250°C by about 0.030”… so upon cooling, the aluminum will try to return to length, by 0.060” where the steel will only return, upon being bonded at 250°C by 0.030” setting up a strain difference and leading to either bending of the plates, as seen in the figures below… or by the accumulation of stress from the strain mismatch, the stresses may be sufficient to begin the fracture of the joint at the edges of the S-Bond solder joint where the stresses were higher than the tensile strength of the S-Bond.
Note that if the steel plate were replaced by a ceramic plate, that the strain difference upon cooling, if the design permits fracture, will deflect the ceramic plate enough to fracture the plate.
Note that S-Bond bonding is a soldering process and compared to other brazing processes that have to heat assemblies to over 700°C, S-Bond solder assembly is a lower temperature, hence lower thermal expansion sensitive process than brazing.
So, S-Bond can bond most materials…BUT, when one is solder bonding dissimilar materials even when bonding at 250°C (480°F) one must still properly accommodate CTE mismatch into their assembly designs by some of the following techniques.
1) Using better matched CTE materials (e.g. ceramic to Kovar®).
2) Using multi-layers to over a distance accommodate CTE.
3) Bond smaller areas/components or make a mosaic breaking the larger CTE materials into smaller pieces.
4) Stiffening a design to resist bowing (may still fracture joint).
5) Use lower temperature joining processes, such as exothermic materials that only heat the joint areas, a recent commercially developed nanofoil has been developed and can reheat and solder joints via a patented NanoBond® process.
Feel free to consult with me about your particular application, we are prepared to discuss bonding options using our S-Bond dissimilar materials bonding techniques.
