Thermally conductive adhesives are uniquely qualified to meet the increasingly diverse requirements of advanced electronics systems. From their traditional use as fastening materials, adhesives find wide application in bonding and encapsulation in nearly every application segment, including military/aerospace, medical, automotive, and industrial, among others. In their traditional application in computer and communications systems, thermally conductive adhesives have long played a vital role at the chip level for die attach, at the PCB level for heat sink bonding, and at both the chip and board levels for all types of thermal management methods.
As the semiconductor industry pushes toward smaller, higher-density devices, product manufacturers face greater challenges in assembling die, package, and other components into products able to cope with increasing heat loads. Further complicating this critical manufacturing step, each product not only presents growing thermal requirements, but also must meet unique manufacturing and lifecycle requirements dictated by the needs of the application. With advances in epoxies, silicones, and other materials, however, manufacturers can find adhesives able to meet nearly any combination of requirements for thermal, environmental, and structural stability.
Advances in semiconductor process technologies continue to yield faster, high-density devices that often increase heat loads in target designs. The ability of thermally conductive adhesives to help dissipate heat has motivated their use as the preferred fastening approach in applications requiring careful thermal management (Figure 1). Besides their use in areas such as power electronics and solar energy, thermally conductive adhesives play a vital role in the rapid growth of the LED lighting industry. Since light output falls as temperatures rise, the thermal conductive properties of adhesives are particularly important in this arena. By using these adhesives, LED providers are able to deliver highly efficient LED lighting products. Furthermore, because these adhesives remain resistant to typical environmental stresses such as weathering and corrosion, these products can help deliver the extended operating life expected in the marketplace.
In supporting highly diverse application segments, adhesives can deliver a correspondingly broad array of performance characteristics required for manufacturing suitability, structure support, and protection. For example, during manufacturing, adhesives need to flow freely to prevent formation of voids, yet provide minimal curing times to help speed the overall manufacturing process (Figure 2). At the same time, adhesives need to adhere firmly to surfaces, providing a strong bond between heat sinks and components for PCBs, or between a die and package for integrated circuits.
In fact, the interface between die and package faces can experience some of the most significant stresses in an electronic assembly. When two dissimilar materials are bonded, differences in the coefficient of thermal expansion (CTE) in the materials can result in stress and strain that might eventually weaken or even fracture the bond between them. Even normal device operation can cause thermal cycling that can add stress to this bonding interface. By providing adhesives with the right CTE, engineers can reduce thermal expansion mismatches between die and package, thereby reducing stress while providing necessary structural support for the assembly.
In the face of growing demand for more effective thermal management, engineers are looking for adhesives correspondingly more effective in conducting heat away from sensitive devices and entire assemblies. Adhesive suppliers typically specify a material’s thermal properties in terms of bulk thermal conductivity, which describes the material’s ability to transfer heat through itself. In electronic design, however, thermal management remains focused less on material specifics than the broader issue of dissipating or transferring heat from a die through a package, or from a component through a heat sink.
For an engineer, the internal heat transfer capability described by the bulk thermal conductivity specification addresses only part of the real objective. In a typical product assembly, a more practical characteristic is the material’s thermal resistance. Thermal resistance describes the ease with which heat can transfer across the interface between the die (or component) surface and bonding material, across the bonding material, and finally across the interface between the bonding material and package (or heat sink).