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.
With their ability to support diverse requirements, advanced epoxy systems such as Master Bond EP30LTE-LO find broad applications in assembly fabrication. For example, this epoxy system played a key role in a patented thermal sensor design intended to deliver better performance in a smaller footprint. Here, engineers looked to reduce the size of a thermal sensing system by creating a vertically stacked system with a microelectromechanical system (MEMS) thermal sensor bonded to the top of the ASIC in the final assembly (see Figure 2). In this system, the adhesive needed to achieve a reliable bond without compromising thermal performance. Engineers found that Master Bond EP30LTE-LO met their requirements, delivering the required bond between the dissimilar materials. At the same time, the epoxy system exhibited the low CTE and high thermal conductivity needed to ensure stability and performance of the integrated thermal sensor assembly.
In another application, researchers used the Master Bond EP30LTE-LO system in a study of corrosion control in metallic systems. Here, the research team needed to examine the corrosion in metal samples in high-temperature aqueous solutions. With earlier approaches, mismatches between the CTE of the mounting epoxies and metal systems would cause the epoxy to expand more than the metal sample, eventually breaking the bond and leading to crevice artifacts that would degrade the measurements. Because the CTE of the EP30LTE-LO system was the same or lower than the metal samples, the metal would expand with the epoxy, eliminating any crevice artifacts.
Often, there is difficulty in creating reliable bonds in applications that rely on more specialized materials. For example, precision optics takes advantage of the remarkably low thermal expansion characteristics of invar, a nickel-iron alloy that exhibits a CTE of only 2.56 × 10-6 in./in./°C or even lower in some alloy formulations. On the other hand, optical instrument manufacturers also use materials with significantly higher CTEs, including glass with its CTE of 10.8 × 10-6 in./in./°C and polymer optical materials with CTEs of 60-80 × 10-6 in./in./°C. For these applications, manufacturers commonly rely on adhesives such as Master Bond EP21TCHT-1 and EP30LTE-LO, which offer the required combination of low CTE with high strength and stability.
For applications with unique requirements, adhesives manufacturers can further tune the characteristics of a bonding agent by adding one of many available filler materials. Filler materials include metallic particles, quartz, and ceramic powders. Each filler imparts its unique characteristics to the adhesives. For example, quartz fillers can enhance the compressive strength, while graphite fillers can improve an epoxy’s EMI/RFI shielding capability while still lowering the CTE of the epoxy matrix.
Fillers such as alumina (Al2O3) lower the CTE of an epoxy or silicone, allowing adhesives manufacturers to tune the thermal stability of these systems for specific applications. Some filler materials even exhibit negative CTE characteristics. Using such fillers, adhesives manufacturers can deliver a system that is optimized to meet unique requirements over a specific temperature range, for example. Next-generation filler materials such as nanoparticles open the door to even more robust performance characteristics.
Use of existing fillers already offers significant advantages in a wide range of applications. In building a radiofrequency (RF) coil assembly, engineers used an epoxy with a filler to meet their unique thermal performance requirements. In this application, the RF coil assembly formed the heart of a system built to study high-density gas discharge plasmas for microelectronics processing. Here, the epoxy needed to reliably bond a ceramic-coated RF antenna to a dielectric substrate (see Figure 3). Besides the basic requirement to bond the different materials, the epoxy bond is needed to maintain thermal stability during operation. In this case, the engineering team relied on Master Bond EP 30 AO epoxy with alumina filler, which provided the adhesive strength and stability required to fabricate and reliably maintain operation of the assembly.
Indeed, each assembly application brings its own unique requirements, and manufacturers can typically find available one- and two-component epoxy systems that offer the characteristics required for that application. When application requirements call for more specialized solutions, adhesives manufacturers can augment these systems with a broad array of additives to deliver a solution tuned to those requirements.
Differences in thermal expansion of materials bonded in an assembly can introduce stresses during normal operation. By closely matching the CTE of the bonding adhesive to the CTEs of the assembly components, manufacturers can build assemblies able to retain thermal stability, strength, and reliability. Advanced adhesive systems such as Master Bond EP42HT-2LTE, EP21TCHT-1, and EP30LTE-LO combine exceptionally low CTE with characteristics such as thermal conductivity, electrical insulation, and chemical resistance needed in many applications. As manufacturers encounter more specialized requirements, adhesives manufacturers can draw on filler materials to deliver solutions tuned to those requirements.
This article was written by Venkat Nandivada, Manager, Technical Support at Master Bond. He is located in Hackensack, NJ. For more information, Click Here .