This illustrates that not just the bulk thermal conductivity of the filler material is important. The particle size of the filler determines thickness, and the thickness determines thermal resistance. This relationship is expressed by the formula
R = t/K,
where R is the thermal resistance, t the thickness and K the thermal conductivity of the material. The lower the thermal resistance, the better the heat transfer and heat dissipation. The bottom line is that the thinner the bond thickness, the lower the thermal resistance and the better the heat transfer. An added benefit is with smaller particle size, less material is needed.
POTTING COMPOUNDS
Unlike adhesives, where the main goal is to provide a bond between devices, potting compounds isolate and protect components from outside contact or contamination, both electrical and mechanical. Potting is common for both power and high frequency devices. For potting, the most important parameters to consider for electrical insulation properties are volume resistivity, dielectric constant, dissipation factor and overall dielectric strength. Heat dissipation properties also play an important role, depending upon the application.
From a manufacturing standpoint, other properties to consider are viscosity, or flow properties, and working life or gel time. Typically, lower viscosity helps minimize air bubbles or voids and avoid arcing. Low viscosity is also beneficial in enabling the potting compound to wick in and around intricate components. In a production environment, using syringe applicators and automated dispensers increases the speed of the process, even if it is a two part potting system. Various packaging and dispensing options are available to get the best results.
DESIGN CONSIDERATIONS
What are the different product parameters designers should consider when choosing a thermally conductive adhesive or potting compound? Start with the substrates and the design dimensions, combined with the operating temperature range and processing and handling requirements.
Much depends on design specifics. For example, when bonding two components made of different materials, e.g., a circuit board and a chip or IC, the adhesive must handle mismatches in the coefficients of thermal expansion (CTE). This is important for the end product use, as well as while the product is being manufactured and undergoes processes such as wave soldering and stress testing. Using adhesives and potting compounds with high thermal conductivity can be a particular benefit, because the thermally conductive filler helps to effectively reduce the CTE mismatches between different materials.
From application to application, the requirements for cure time and temperature can vary. Some products can be cured at room temperature, others require a moderately elevated temperature and some high temperatures. Working life/pot life is also an important consideration, especially in larger volume potting applications, where it is necessary to have sufficient working life so the potting compound does not set up too soon. In such cases, compounds with more than 12 to 24 hours of working life at room temperature are available. This provides sufficient time after mixing for de-gassing and application to achieve a void-free bond.
Other production-related factors include compound handling and packaging. Many different packaging options are available, even for two part chemistries. For example, these chemistries can be packaged in pre-mixed and frozen syringes. For production applications potting about 10 cc per assembly, the compound can be packaged as a pre-mixed and frozen system in a syringe, so the required amount of material is used at one time. By eliminating the need for measuring, mixing, de-gassing and other operations, the production process can be faster, while still providing the desired properties.
When selecting a supplier of adhesives or potting compounds, designers should also consider how much design support the supplier is willing to provide. While it is best to contact a supplier early in the design phase, a strong supplier will help at different stages of the life cycle. Even if a system is in production, the supplier can help improve an existing process or product.n
Reference
- “Evaluating Electrically Insulating Epoxies,” MasterBond, www.masterbond.com/whitepaper/evaluating-electrically-insulating-epoxies.
Venkat Nandivada has been the manager of technical support at Master Bond since 2010. He has a master’s degree in chemical engineering from Carnegie Mellon University. His work consists of analyzing application issues and providing product solutions for the aerospace, electronics, medical, optical, OEM and oil chemical markets.
Rohit Ramnath is a senior applications engineer for Master Bond. Also a graduate of Carnegie Mellon University, with a master’s degree in chemical engineering, he analyzes application issues and provides product support to the same industries.
