Adhesive compounds play a critical role in the fabrication of assemblies for electronic, optical, and mechanical systems. In securing multiple components into a single structure, bonding agents such as epoxies and silicones help stabilize the assembly despite the various sources of stress that can arise in the target application. Among these stress factors, differences in thermal expansion of the components of an assembly represent one of the most insidious and potentially damaging sources of stress.
Manufacturers face continued challenges in finding an optimal approach for dealing with differences in thermal expansion. Among the alternatives, adhesives and potting compounds have emerged as the most effective. As a noted industry expert explains:
“There are several possible solutions to the expansion mismatch problem. One is to use a resilient adhesive that deforms with the substrate during temperature change. The penalty in this case is possible creep of the adhesives, and highly deformable adhesives usually have low cohesive strength. Another approach is to adjust the thermal expansion coefficient of the adhesive to a value that is nearer to that of the substrate. This is generally accomplished by selection of a different adhesive or by formulating the adhesive with specific fillers to “tailor” the thermal expansion. A third possible solution is to coat one or both substrates with a primer. This substance can provide either resiliency at the interface or an intermediate thermal expansion coefficient that will help reduce the overall stress in the joint.”1
With the emergence of specialized adhesives, manufacturers can bond diverse materials into assemblies able to withstand thermal expansion effects.
Dealing with Thermal Expansion
Thermal expansion occurs in most materials due to an increase in the energy of molecular interactions associated with an increase in temperature. For any material, it’s the coefficient of thermal expansion (CTE) that expresses this change per degree of temperature change — typically specified as a linear quantity in units of in./in./°C. In assessing product reliability, thermal expansion remains a key concern to manufacturers, particularly when assemblies use materials with dramatically different CTEs.
Because materials with different CTEs expand or contract at different rates, a bond between those materials can experience significant stress. In fact, manufacturers routinely deal with this challenge in bonding different assemblies due to the wide variation in CTEs of materials typically used in most applications. For example, at 20 °C, CTE is 2.56 × 10-6 in./in./°C for silicon, 7.4 × 10-6 in./in./°C for cermet, 10.8 × 10-6 in./in./°C for steel, and 23.0 × 10-6 in./in./°C for aluminum.
In a typical assembly, forming reliable bonds between different materials can be challenging. Manufacturers must contend with issues such as the optimal design of surfaces for bonding, adhesive dispensing method, curing technique, and more. In addition, application requirements will typically drive manufacturers to target different degrees of flexibility in their assembly structures. Using moderate-to-high CTE adhesives, for example, manufacturers can achieve bonds without locking the assembly into a rigid structure. In other cases, the application might require a very rigid assembly, calling for low CTE adhesives. In practice, manufacturers need flexible and toughened epoxy systems able to meet specific combinations of requirements for strength and resistance to mechanical and thermal stress.
Differences in CTEs in the assembly materials compound the challenges facing manufacturers. In bonding materials with different CTEs, manufacturers must further consider the impact of temperature changes on those bonds in particular and on the integrity of the assembly as a whole. Indeed, any application will face temperature changes related to its operating environment. In others, such as electronics, the assembly will face temperature changes during the normal course of its operation as components cycle through different power states. In either case, the stresses at the bonded junction due to unequal expansion and contraction can work to weaken the bond, introduce cracks, or even create separations in the bond that cause the assembly to fail.
Besides stresses related to differential thermal expansion of opposing surfaces in a bond, the changes in temperature will induce changes in the characteristics of the bonding agent itself. An adhesive’s CTE is itself temperature-dependent, typically increasingly monotonically with temperature. In fact, operating continuously at elevated temperatures can potentially cause changes to some its fundamental properties. At a particular temperature, called the glass transition temperature (Tg), the structure of a bonding agent transitions to a more rubbery/soft state (see Figure 1). For example, an epoxy that exhibits high strength and stiffness below its Tg can become weaker and more pliable as the temperature rises past its Tg.
Above Tg, epoxies can exhibit loss in structural performance, as well as lose some of their thermal, electrical, and chemical characteristics. As a result, adhesives manufacturers typically specify Tg as the maximum continuous operating temperature for these materials. Fortunately, manufacturers can generally find an adhesive with the required Tg characteristic. For example, available epoxy compounds offer Tg characteristics ranging from as low as 50 °C to above 250 °C. In certain cases, however, silicones or other flexible/toughened adhesive formulations with low or even negative Tg characteristics might provide a better solution: For example, in an application that requires a compliant bond joint in an assembly operating at high temperatures, a silicone adhesive or a toughened/flexible epoxy formulation can deliver both strength and flexibility while remaining relatively unaffected by thermal cycling.
Although low CTE can be a critical requirement in many applications, it is rarely the sole concern in assembly fabrication. Manufacturers typically require low CTE in combination with other key characteristics including thermal conductivity, electrical insulation (or conductivity), chemical as well as heat resistance and more. In fact, manufacturers can find adhesive systems able to address a wide range of application requirements, including the ability to join materials with different CTEs. For example, Master Bond EP42HT-2LTE is a two-component epoxy system that features a CTE of 9–12 in./in./°C — one of the lowest available for epoxy systems. Used as an adhesive, sealant, coating, and even as a casting system, this epoxy bonds reliably to a wide range of materials including metals, composites, ceramics, glass, and many plastics. EP42HT-2LTE cures at room temperatures and exhibits continued dimensional stability with less than 0.01 percent linear shrinkage on curing.