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Coefficient of Thermal Expansion Mismatch

“Materials expand because an increase in temperature leads to greater thermal vibration of the atoms in a material, and hence to an increase in the average separation distance of adjacent atoms. The linear coefficient of thermal expansion 'a' (Greek letter alpha) describes by how much a material will expand for each degree of temperature increase” (Ronan, 2018)

 Within the electronics industry when you have two materials of a different thermal expansion rate you can find the stress that this causes between the two materials can lead to fractures and cracks in the medium that you use to join these two materials, in our case the solder joint. Thermal expansion will affect the reliability of pcb designs that are used in harsh environments. Ceramic BGAs are at a higher risk of CTE fractures due to a large thermal mismatch between the ceramic material and the PCB laminate. Whereas plastic BGA packages better match the FR4 or polyimide substrates. BGAs with smaller sphere sizes are more susceptible to CTE fracturing, larger sphere sizes provide a better stand off and more mechanical compliance. The larger solder mass in the intermetallic connection also provides better thermal conductivity and ability to absorb the CTE issues. 

The recommendation is that thermomechanical stresses due to different coefficients of thermal expansion in the materials used in electronic assembly should be factored in during the design phase. The issue with this is that this is not always possible due to the expansion rates not always being known. Stress fractures can occur not just at the assembly stage, but when the assemblies are in their end use environment. The stress the PCB and components can be placed under can cause CTE to take place and fractures to show over a period of time due to the temperature and environment of the end use. 

Utilising the very latest laser reballing techniques we can also add solder spheres to the underside of QFNs, LGAs and any Bottom only Terminated Component. This has the same effect of assisting with CTE issues on these parts as the component will be placed with a larger solder mass and better thermal conductivity between the component and the PCB. 

To learn more about how we can help you alleviate TCE issues please contact our one of our technical team.

 



 

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