Introduction to Thermal Cycling Life Testing
Materials expand or contract with temperature change. Water expands as it freezes, whereas steel contracts as it cools.
This motion can limit the life of your system.
Materials and mechanical engineers include the expected motion into their designs, well the better engineers do.
Even centuries ago, craftsmen used expansion slots or features when attaching wooden table tops to their frames.
The motion due to temperature change will occur and has the potential to create immense strain within your product.
Types of damage caused by thermal cycling
There are a couple of ways a change in temperature can damage your product or system.
Understanding how you material and design risk incurring damage leading to product failure permits you to design and interpret the life testing results appropriately.
Material phase transition
If the change is severe enough that it crosses a material transition point, it may melt, for example.
To avoid this type of damage select materials that will retain the essential material properties for the functioning of the design.
If the rate of change is too fast (thermal shock) some materials cannot accommodate the change in dimension that occurs on the cold side as the warmer side attempts to catch up, thus may fracture.
As an example, consider what happens when you splash a bucket of hot water on a frozen windshield, it shatters.
If the entire windshield is slowly warmed to the same temperature as the hot water, it will not shatter.
An easy way to crack ceramic capacitors is to touch one edge with a 400°C soldering iron tip.
Heating the circuit board pad and component by touching only the pad allows the capacitor to heat slow enough to avoid the huge thermal stress relieved by cracking the ceramic.
Accumulated cyclic damage
Let’s assume the temperature range for the thermal cycling does experience thermal shock and does not cross material transition points (freezing or melting, for example), thus the bulk material’s motion is relatively elastic, meaning the material retains its properties such as tensile strength, hardness, etc.
If you bend a wire coat hanger wire nothing much appears to happen other than the change in shape.
Yet even a small amount of deflection comes at a price.
The material structure, the bonds that create and hold the shape and properties, changes or accommodates the change in shape.
The material may break chemical bonds, scission polymer changes, alter the local accumulation of elements, create microvoids or cracks, or even accelerate material diffusion across boundaries.
Even well-designed springs incur a small amount of damage with each deformation.
Eventually, the accumulation of small changes leads to a failure.
Embrittlement and hardening are just two of types of changes that may occur within the material as it experiences even small thermal changes.
Coefficient of thermal expansion mismatch
It is rare that we create a product from a single material.
Gold bars are an example of single material product. The functionality is limited compared to an iPhone, for example, yet a solid gold bar will not experience damage due to a mismatch of different coefficient of thermal expansion (CTE) materials.
CTE is a measure of the relative motion due to a change in temperature.
It allows us to estimate the dimension change of a bulk material given a specific change in temperature.
The mismatch parts come into play when two materials are connected in some fashion and have different responses to a temperature change. If they have different CTE values, they will move relative to each other across the attachment points.
This relative motion may cause significant strain, thus often causes damage as the involved materials and attachment accommodate the induced strain.
Consider a table top
Consider the table top attached to a frame and legs.
The wood will expand with temperature (even more so with moisture) and if the table in simply pegged or nailed to the supporting frame, the dimension change across the large table top (wood expands more across the grain than with the grain, by the way, thus for some materials the CTE is different for different orientations) and the relatively smaller frame will more much less.
This relative motion will place a significant strain on the peg or nail making the attachment.
With a very poor design or over time, the relative motion will cause the attaching method to fail.
A well-designed attachment will accommodate the relative motion.
One technique is to make the attachment using a slot, allowing a sliding motion to occur thus avoiding the accumulation of damaging strain.
Railroad tracks and concrete sidewalks include expansion gaps to permit the iron rail or concrete slabs to expand into the gaps without causing buckling, for example.
In both cases, the relative expansion of the metal and concrete is much creating than the surrounding earth.
The design accommodates the difference.
Consider a circuit board
Consider a circuit board with a large integrated circuit (IC).
The silicon-based IC has a CTE of approximately 2 ppm/°C, whereas the CTE of the printed circuit board (PCB) is possibly 21 ppm/°C.
This order of magnitude difference in CTE is the mismatch. As the assembly heats up the PCB will move much more than the relatively unresponsive IC thus creating a strain across the solder joints attaching the IC to the PCB.
Solder responds by deforming when under strain thus retaining the connection.
Solder materials then respond to the strain by relaxing, and to oversimplify a bit, the relaxation process causes damage to the solder.
As the system cycles thermally and continues to relax it accumulates damage and eventually will form a crack which eventually creates an open in the circuit.
Understand the failure mechanism caused by thermal cycling
This very brief introduction to thermal cycling and the ways it can damage your product implies that you must understand the specific failure mechanisms before designing your thermal cycling based life test.
The thermal cycling profile will replicate the same path to failure as seen in normal use.
If you accelerate the life testing by cycling more often than will occur in use, you may inadvertently accelerate different mechanisms than desired.
For example, a nichrome wire based toaster, if cycled quickly by making toast one cycle immediately after the last, will not allow the wire to cool, thus minimizing the stress across the wire attachment.
The result will not correspond to the life behavior during normal home use.
Testing a subsystem when the attachment to the enclosure causes the most CTE mismatch accumulation of strain should include the enclosure.
Testing an IC attachment method, well, should include the solder joint and PCB. When flexing a tube, rod, cable, take care to understand how the damage for that specific material occurs.
Flexing too far or too fast will likely destroy your ability to equate a test cycle to a use cycle in a meaningful manner.
The first, and I would suggest, most important step in crafting a thermal cycling life test is to understand how the damage occurs.
Understanding the failure mechanism permits you to design a test which you can confidently interpret and estimate the expected life during use.
If you currently just toss a few units into a thermal cycling profile, how are you interpreting the results?
Do you have a firm understanding of how the damage occurs in testing and in use, plus understand the relationship between the two?
If not, it’s time to find out and get the most value from your testing.
Do you have an interesting failure mechanism related to thermal cycling? Leave a short description in the comments and help your community learn about the many facets of thermal cycling and its impact on system life.
Mechanical Systems Reliability Testing (article)
Basic Approaches to Life Testing (article)
Failure Modes and Mechanisms (article)