As consumers, we tend to think of our electronic devices in a rather static manner. If it works today, it should work tomorrow and in a year – so long as it is protected and used properly. However, a little engineering perspective informs us that this is not the case.
In reality, even “normal” usage of an electronic device pushes it slightly further to its end-of-life. The culprit? A physical phenomenon known in the industry as CTE mismatch.
To understand CTE mismatch, one must understand two basic physical truths: (1) all electrical currents generate heat; and (2) all materials deform with changes in temperature, according to their material properties.
The heat generated by electrical currents can largely be attributed to the first law of thermodynamics, which states that energy can neither be created nor destroyed and can only change forms. You can think of an electrical current as energy flowing through a material called a conductor: a material chosen for this task because of it’s high conductivity (or low resistivity). While scientists and engineers usually choose the materials with the highest conductivity for carrying electrical currents, a perfect conductor, with no resistance to electrical flow, has not been discovered. This means that even with the best conductor, some of the electrical current’s energy will be lost as it traverses the material. Since the energy cannot simply disappear, as indicated by the above first law, it is dissipated from the conductor as heat.
While not intuitively obvious, you are probably aware that materials expand has they are heated. This is why sidewalks and bridges feature expansion joints – otherwise, an elevated temperature could cause catastrophic failure. The way a specific material deforms with temperature is described by a material property called the coefficient of thermal expansion (CTE). The mathematical definition of CTE is not necessary to digest to understand this – the higher the CTE, the more a material expands with rising temperatures. Importantly, CTEs for different materials can vary drastically.
As we discussed in our first post, electronic systems use a variety of different materials. The ICs are usually made from silicon. Interconnections are made of solder, comprised of metals and intermetallics. Underfills are made from polymer based epoxies. Substrates and boards are combinations of metals and ceramic or organic compounds. All of these materials are mechanically assembled to stay together and function properly, which means that any physical deformation experienced by any one of the materials is going to affect its nearby components. Furthermore, all of the different materials have different CTEs, some varying by orders of magnitude.
Now, as our device is used and electrical currents flow, heat is generated thanks to the first law of thermodynamics. If the heat cannot be removed from the system, the components will absorb the heat and their temperatures will increase. Thus, thermal expansion will ensue. But, since the materials have different CTEs, they will expand differently. High CTE materials will experience compressive stress because they are coupled to low CTE materials. Low CTE materials will experience tensile stress because they are coupled to high CTE materials. This is the basis of CTE mismatch.
It is important to understand that CTE mismatch doesn’t cause immediate, catastrophic failure for the same reason that thermal expansion does not cause immediate catastrophic problems in day-to-day life – because thermal expansion is not so dramatic. Even large objects may only expand by millimeters under intense heat. However, electronic systems are powered on, powered off, allowed to idle, cranked into high gear, and expected to handle this thermal cycling for years of ordinary use. The combination of CTE mismatch and thermal cycling causes the aforementioned stresses to build and typically concentrate in the first-level interconnects. Eventually, the interconnects – and the device’s functionality – will degrade.
In our future blog posts, we will discuss the methods engineers use to combat CTE mismatch and make more robust, durable, and capable electronic devices.
