High temperatures and thermal cycling are very bad for electronic devices. To prolong their life and ensure reliability, designers have three main techniques at their disposal:
- Integrate passive cooling features
- Incorporate active cooling
- Adopt thick film substrates
Each approach has advantages and disadvantages. However, many designers find that thick film substrates are often the most effective method of thermal management. This blog post will review all three approaches with a focus on the characteristics of thick film substrates. First though, a look at why thermal management has become a critical aspect of electronics design.
The Importance of Optimal Thermal Management
Some electronic packages are placed into high temperature environments. Some vehicle underhood and most exhaust applications are of this type. Other packages generate significant heat internally as a result of the power handled. Rising component densities only exacerbate this problem. The most challenging applications combine all three. This is when compact high-power devices are mounted in high temperature locations.
A second issue is the duration of the elevated temperature. Sustained high heat affects the operation of transistors and degrades capacitors. It can also deform conventional epoxy-fiberglass circuit board material (FR4.) Thermal cycling is however a bigger problem, particularly if there’s a mismatch in the expansion coefficients of bonded materials. Here cycling will create stresses that lead to cracking and failure. (And high peak temperatures can cause the other problems identified previously.)
Avoiding problems and premature failure in-service means addressing these thermal management challenges during the design phase. Here are the three main techniques.
Passive Cooling Features
This refers to using conductive materials, heat sinks and radiating structures to move heat away from the source. FR4 circuit boards can be manufactured with metal planes on the internal layers, thermal planes and vias (interconnects between layers.) These take heat to a finned heat sink where it radiates away. Their major limitation stems from the properties of FR4 itself.
FR4 has low thermal conductivity and a relatively high coefficient of thermal expansion. Thus it doesn’t do a good job of moving heat away from the source but will expand appreciably as it gets hot. Inevitably, this leads to stresses and cracking, especially in and around vias.
A further limitation stems from the relatively low glass transition temperature (Tg) of FR4 – nominally around 130°C. This means that when heat isn’t removed promptly there is a real risk of distortion.
In some applications heat-generating electronics are cooled by blowing ambient air over the surface. This requires a significant temperature differential between ambient and the circuits, and also adds size and cost to the device. Furthermore, fans are not appropriate when the electronic device must be fully enclosed for protection against water ingress.
In some specialized cases electronics may be placed in an air conditioned cabinet. This is not practical for applications like those in transportation.
Thick Film Substrates
The use of ceramics, generally alumina, is the substrate of choice for Wells thick film printing applications that require thermal management. Electrical circuits are screen printed onto the surface before being dried and baked. Unlike FR4 circuits which use copper or silver polymer, platinum-silver inks are used as these adhere better to the ceramic and have similar coefficients of thermal expansion.
The term, “thick film” refers to the thickness of the lines or traces printed. At 10–20 μm (0.0004-0.0008”) thick they are only thick in comparison to another form of electronics, the “thin film circuit.”
Thick film substrates provide superior thermal management for four main reasons:
- Thermal conductivity is many times higher than FR4, so heat flows away from the source more quickly which minimizes hot spot formation.
- The coefficient of thermal expansion is significantly lower than that of FR4, meaning it takes more energy to create the same amount of expansion. Thus ceramic substrates experience far less stress than do those of FR4.
- While up to 11 layers of circuits may be printed onto a ceramic substrate vias are used only infrequently. This eliminates the most common source of cracking.
- Ceramics won’t soften or burn, unlike FR4. This makes them the only practical choice in very high temperature applications.
Ceramic substrates are produced in many different grades, so properties vary. (The same is true of FR4.) However, for those interested in making comparisons the following numbers give an indication of the superiority of ceramic over the alternative. (Note also that FR4 offers different performance through its thickness versus over the surface.)
Coefficient of thermal conductivity:
Typical ceramic: 24 W/m.K
FR4 (over the surface): 0.9 W/m.K
FR4 (through the thickness): 0.3 W/m.K
Coefficient of thermal expansion:
Typical ceramic: 6.7 ppm/K
FR4 (over the surface): 13 ppm/K
FR4 (through the thickness): 70 ppm/K
Drawbacks of Thick Film Substrates
Given the significantly superior thermal management characteristics it’s reasonable to ask why ceramics haven’t replaced FR4 everywhere.
The main reason is economic. Thick substrates are more expensive to manufacture. This results from the high cost of the inks used and the complexity of the manufacturing process and equipment needed. However, in some applications these costs are offset by eliminating fans and/or large heat sinks and using less copper.
A second potential issue is the size of ceramic substrates in comparison to FR4 equivalents. These are usually bulkier than FR4 and have a lower component density. Together these mean thick film substrates are often larger than an FR4-based product.
Superior Product Performance
Electronic devices have always been challenged by high temperatures, but the problem is growing more acute as power levels and component densities rise. Thick film ceramic substrates offer superior thermal management capabilities, which results in higher device reliability and longer life. Wells Engineered Products recommends the use of thick film substrates in applications where heat would otherwise impact performance and durability.