Thermal Materials Improve Automotive Laminate Reliability
December 31, 1969 |Estimated reading time: 4 minutes
You only have to look as far as the local dealership showroom to know that sleek, small and fast are the trends in the automotive industry. This article examines how advanced laminate technologies combat the issues associated with these smaller and more powerful automotive electronics modules.
By George Sears
Driven by continual advances in the microprocessor market, electronic modules in automobiles are getting smaller and more powerful. As speed and output increase, so does heat generated. Although automotive electronics engineers have been using thermal pads or cushions successfully in the past, as devices get smaller but experience higher speeds and outputs, the industry must use thermal greases, adhesives and gels for heat dissipation. Further adding to the mix of available solutions is advanced laminate technology, which is suitable for dual-sided surface mount components, compatible with thru-hole components and can be used in the passenger compartment, under the hood and on or near an engine.
Things are heating up quickly, despite efforts to keep them cool. Ingenuity comes in the form of materials, not merely end products. While process and equipment modifications drive performance characteristics, material performance is the determining factor in meeting end-product goals (Figure 1).
Figure 1. Automotive electronics are exposed to a variety of harsh factors, such as higher-temperature environments.
All industries are affected by changing market forces; however, the automotive industry is pressured to deliver more gadgets, improved safety and additional sensors in a tightly defined area. Suppliers must develop products that withstand harsher environments at higher temperatures. Components must perform efficiently. Many devices are designed for use in the transmission or exhaust-gas-recirculation system and must operate in extreme temperatures and environments. In traditional ceramic technology, heat dissipation to the baseplate is a low-stress design. However, with smaller devices, the ability to use both surfaces of circuitry is ideal, if not essential.
Although advanced laminates have been available for several years, application in the automotive world is in its infancy. Automotive electronics are exposed to a variety of harsh factors ranging from higher-temperature environments and ambient weather conditions to road debris and automotive chemicals. The trend to design control modules for on-engine and on/in-transmission locations has increased the need for higher reliability. Steer-by-wire technology also requires more demanding performance criteria. The future development of fuel cell vehicles will require more advanced electronic designs and better-performing materials to handle higher-power devices and increased heat dissipation.
While the double-sided feature of advanced laminates offers flexibility and freedom of design, it leads to new demands regarding heat dissipation. Materials must offer higher thermal performance and lower stress on devices. Thermally conductive materials must act as the path between active devices mounted on the laminate circuit and the metal housing. These materials must be sandwiched between silicon ICs attached rigidly to the laminate and the aluminum baseplate or housing. Thermal interface materials (TIM) are preferred, if matched correctly with the electronic design. To maximize thermal dissipation, materials must be highly conductive while offering low modulus.
The use of thermal materials in conjunction with advanced laminates has been slow to gain acceptance because of the perception of high costs. Although the bottom line is important, the cost of using thermal materials is small because so little product is used. One common mistake is selecting an industrial-grade material for areas of extreme heat. Although industrial-grade materials are durable and suitable to handle these parameters, this is not always the case. Others opt for industrial-grade products even though they recognize they may be trading performance for perceived lower cost. The money saved on the material likely will result in decreased performance and longevity of the overall component.
An anti-lock brake system supplier recently needed a thermal interface material that would maintain thermal properties in a continuous, 200°C environment. An industrial-grade material would not realistically work in this scenario because of inferior heat dissipation or the failure to handle mechanical stress. High-temperature, silicone thermal greases tend to separate and dry out due to expansion and contraction. While thermal epoxy adhesives provide the mechanical properties necessary for the application, they likely would cause component failure due to their high modulus. Using a low-modulus adhesive is recommended in such situations.
Using gels as a TIM is relatively new in the auto industry. These were the norm because the industry did not demand the processor speed and high-heat output of other markets. Gels can offer higher heat dissipation and improved conformity.
In an application for an automotive electronics supplier, one gel* provided the thermal dissipation needed to allow for a compact, miniaturized module with increased functionality. The engine controller design with flip-chip-on-laminate technology required a specially formulated gel with high thermal conductivity, electrical insulation and a controlled, particle size, non-abrasive filler. An earlier design had used an industrial-grade grease. However, the thermal path was removed by grease pumping out of the interface. This lack of heat dissipation caused the die to overheat and led to high-ppm field failures after initial production start-up. An attempt to solve the design problem with thermal adhesives exposed another unexpected failure mode. Adhesives with large-particle-size oxide fillers of high hardness induced early electrical failures due to die cracking. The TIM was developed using a low-modulus polymer and small, soft-mineral filler that accelerated thermal-cycle durability.
Which advanced laminate material should you choose for the automotive environment? First, determine the amount of thermal dissipation, mechanical strength and stresses the device will undergo in end use. With an understanding of these characteristics, work with your supplier. The key to success is choosing a supplier with access to a variety of formulation chemistries, as well as a willingness to perform the testing necessary to select the best material for your needs.
* Gelease, LORD Corporation, Cary, N.C.
George Sears, manager of product development and technical service, the electronics group of LORD Corporation, may be contacted at george_sears@lord.com.