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August, 2017
Managing Thermal Energy with a Finely-Tuned Adhesive
By Laurie Gibbons, Business Development Manager, Permabond
in the design process. Managing heat in processing, heat dissipation, and thermal expansion and contraction are all affected by miniaturization. The dense population of boards, combined
H
with the high-temperature needs of lead-free solder technology has made PCB attachment even more challenging. Prior to wave solder- ing, adhesives used in the past to bond com- ponents in position to the bottom of dual- sided PCBs could not resist the tempera- ture needed for RoHS-compliant wave sol- der. High-temperature-resistant instant component-tacking adhesives have been developed to address that issue.
Thermal Dissipation With miniaturization, heat dissipa-
tion is also a greater concern. However, thermally conductive adhesives can be designed to meet the fire retardancy requirements of UL94 V-0. Single-component, heat-cure epoxies
have been long established as the go-to method of dissipating heat. Grades are formulated to achieve good thermal conductivity of 1.3 W/m-K, for example, and dielectric strength of 45 kV/mm. These single-component epoxies are designed with high viscosity and high green strength to allow further process- ing prior to cure. Thixotropic grades that dispense easily without
stringing are ideal. They are an excellent choice for bonding aluminum heat sinks to ceramic headers, though not all processes allow heat cure.
eat, vibration, moisture, and dust can all wreak havoc in electrical sys- tems. With the miniaturization of components, heat or ther- mal management becomes an even more critical factor
Surface-activated structural acrylics are a room-temperature cure option. Surface-activated structural acrylics are two components, but are not mixed. A “water thin” activator is applied to one surface and the adhesive is applied to the other. The com- ponents are assembled with sufficient force to spread the adhesive thinly. Structural acrylics can develop strength to a
large range of substrates very rapidly. Handling time — the time it takes to achieve 0.3 N/mm2 shear strength at 23°C (73.4°F) — can be reached in less than 10 seconds. They also provide good thermal conductiv- ity. Typical values are 1.1 W/m-K thermal conductivity with dielectric strength at 25 to 30 kV/mm. Both single-component epoxies and
structural acrylic tend to be hard, rigid and non-flexing. With miniaturization, base materials have become thinner and rigid adhesives will not flex with the base material and can cause stress points. Regardless of how high the thermal conduc- tivity is, if the adhesive doesn’t stay bonded to the two components, due to flexing or vibration, the thermal transfer will be adverse-
ly affected and result in reliability issues. Thermal Expansion and Contraction New adhesive options, such as two-component
modified epoxies, provide the flexibility needed to
Permabond 820 instant cyanoacrylate for high-temperature adhesive applications.
maintain contact between the component and the base, even in high-vibration and flexing applications. Two- component modified epoxies are soft and absorb the dif- ferential thermal expansion and contraction between the two substrates.
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