All materials will change their dimensions as a result of changes in temperature. Most materials will slightly expand as the temperature increases. Some will actually shrink. No matter the case, the value that dictates how a material will change its shape as a result of temperature is the coefficient of thermal expansion. This blog post describes this polymer property and works to create some context around the idea.
Other polymer properties worth checking out:
- Comparing Shore A And Shore D Hardness Values
- Comparing Lap Shear Strengths Of Polymer Adhesives
- High Vs. Low Modulus Of Elasticity
Coefficient Of Thermal Expansion
As materials are subjected to various temperatures, they may expand or contract as a result. The coefficient of thermal expansion (CTE) quantifies how a particular material will change its shape throughout changes in temperature. For any particular material sample, the amount that it will ultimately change its shape as a result of temperature is proportional to:
- The CTE of the material - the higher the CTE, the more the material will expand or contract
- The original size of the material sample - Larger samples will expand or contract more than smaller samples
- The degree of temperature change - the larger the temperature difference, the greate the ultimate dimensional difference
CTE is normally measured in PPM/°C. The following are some example CTE values for some specific materials
- Silicon: 2.6 - 3.0 PPM/°C
- Ceramic: 9.5-11.5
- Gold: 14.1 PPM/°C
- FR-4 PCB: 18 PPM/°C
- Filled Epoxy Resins (<glass transition temperature): 18-25 PPM/°C
- Lead Free Solder Alloy: 21.5 PPM/°C
Thermally Induced Stress
When two materials with different CTE values are bonded together and then are cycled through different temperatures, stress builds up at the bond interface as the two materials expand and contract at different rates. Overtime this stress can accumulate to the point of mechanical failure either by warping, fracturing or or even total bond failure. In sensitive applications it is vital that materials with similiar or near-similar CTE values are chosen. If choosing materials with near-similar CTE values is not an option, some polymer adhesive options do exist that help to protect against the accumulation of thermally induced stress.
How To Protect Against CTE Differences
If two materials of different CTE values need to be bonded together, the best strategy for reducing the risk of damaging the components due to thermally induced stress is to choose a low modulus or flexible polymer adhesive. Low modulus adhesives will absorb dimensional changes from either interface material, essentially eliminating stress along the bond line.
It may be tempting to choose an polymer adhesive that has a CTE midway between the two interface materials. This is not the best strategy for reducing thermally induced stress. Again, the most effective way to reduce mechanical stress between materials with different CTE values is to choose a low modulus or flexible adhesive to bond these components.