Thermal shock occurs when a board heats up to a very high temperature in a short period of time.
Thermal shock reliability testing is used to ensure your board will not fail if a thermal shock occurs. Failure can occur in the board and the components.
There are some design choices that ensure the PCB in your product can be classified as compliant as IPC Class 2 or Class 3, or under MIL-STD standards.
Make sure your PCB can take the heat with thermal shock reliability testing.
Let anything heat up to a high temperature in a very short time period, and it will probably fail. The same is true for your electronics, which might need to survive a very hot environment. Even more important is slow and fast cycling between extreme temperatures, which places extreme stress on components and the structure of the PCB itself. If you can design your board to withstand thermal shocks and stresses, your board is likely to have a longer lifetime.
Ensuring your board can withstand large temperature changes requires understanding thermal shock reliability testing. By understanding the primary points of failure in a practical situation, your board has a chance of surviving a large thermal shock. Let’s look at the primary causes of failure and the most common failure points that arise during thermal shock reliability testing.
What Is Thermal Shock Reliability Testing?
Thermal shock reliability testing is a conceptually simple test: the board is brought to an extreme temperature in a short amount of time, and the tester determines whether the board fails due to this extreme temperature change. Obviously, the term “extreme temperature change” could mean anything, and what qualifies as a board failure depends on the design requirements and standards that govern your product. IPC has specified a general standard based on MIL-STD standards for thermal shock testing and reliability.
The IPC-TM-650 2.6.7 standards define thermal shock reliability testing requirements for multiple materials. Another standard thermal shock reliability testing method for defense articles is MIL-STD-202G, Method 107. Both standards specify examining structural reliability due to changes between extreme temperature values. The temperature test range in these materials is chosen in terms of the glass transition temperature (Tg) for the board’s substrate (IPC-TM-650 2.6.7) or over a standard range (MIL-STD-202G).
IPC-TM-650 2.6.7 Thermal Shock Testing
This test is based on similar methodologies as MIL-STD tests for thermal reliability. The upper temperature in the test is set to be below Tg of the laminate material. Specifically, IPC D coupons are specified to be subjected to thermal shocks ranging from -55 °C to the minimum of: Tg - 10 °C, reflow temperature – 25 °C, or 210 °C. This covers most moderate and high Tg laminates, and most typical operating conditions for commercial or industrial products.
MIL-STD-202G Thermal Shock Testing
This standard specifies standard tests using air-to-air methods or liquid-to-liquid methods. These methods are designed to control the rate of heat transfer into the device under test (DUT), and they each have different characteristics. These two sets of methods are summarized in the table below.
When the DUT is exposed to a high-temperature environment, it needs to continue operating in that environment so that the system can reach thermal equilibrium. Tests specify a dwell time, which is the time the DUT should remain in the environment to allow it to come to equilibrium. Because air-to-air heat transfer is low compared to liquid-to-liquid heat transfer, an air-to-air test method will have a longer dwell time. The dwell time also depends on the mass of the system being tested; a heavier PCB will have a longer dwell time in both tests.
The test range in MIL-STD-202G is divided into different categories. The lower-end testing temperature can be as low as -65 °C and as high as 200 °C (for liquid-to-liquid tests) 500 °C (for air-to-air tests). Air-to-air tests are more typical of real conditions, especially in aircraft or automobiles. However, they cannot simulate cases where temperature rise occurs very quickly, thus liquid-to-liquid tests are a better choice when there is the danger of a fast, large temperature rise during operation.
What Determines Thermal Reliability?
Whether we’re worried about thermal cycling or thermal shock, the important parameter that determines thermal reliability is the difference between the CTE values of various board materials. This includes the laminate, solder, and printed conductor materials. In addition, the ductility of conductors used influences whether a conductor will fracture during a thermal shock. A more ductile material can withstand a higher strain rate, thus it can sustain a faster temperature rise.
As the board temperature increases, stress is placed on different structures as the board materials expand at different rates. Due to differences in CTE values, thermal shock leads to the following forms of failure:
Solder fracture: Tensile and shear stress in solder balls tends to concentrate near the top and bottom of the ball, leading to fracture, especially at brittle interfaces between dissimilar metals (see below).
Delamination: When delamination occurs, the conductors and substrate become separated under extreme deformation.
False-color SEM images showing brittle solder fracture with crack propagation. [Source]
Here are some ways you can prevent failure due to thermal shock:
Closely match CTE values: Your conductors, solder, and substrate materials should have CTE values that are as similar as possible.
Use high-Tg substrate materials: When the Tg value of the substrate is larger, the CTE value for the substrate will remain low over a broader temperature range.
Use substrates with high thermal conductivity: If your board will operate in a hot environment where there is a danger of thermal shock, using a high thermal conductivity substrate (e.g., metal-core or ceramic) will allow heat to dissipate away quickly and possibly reach a lower equilibrium temperature.
Your PCB design and analysis software should give you everything you need to design your PCBs to withstand thermal shock and pass thermal shock reliability testing. The CAD tools in Allegro PCB Designer from Cadence integrate with a set of field solvers for thermal, electrical, and CFD simulations, giving you everything you need to examine thermal stress and reliability in your new system. Try this unique toolset when you need to design reliable PCBs for advanced applications.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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