Product reliability is an absolute must for manufacturers to succeed in today’s competitive marketplace. As customers demand ever-increasing levels of performance at lower unit costs, it becomes increasingly challenging to ensure that electronic circuits operate as designed for a prolonged period of time. Delicate components, densely populated printed circuit boards (PCBs), confining packaging, and highly demanding service environments have the potential to lead to increased failure rates as a result of excessive heat build-up and electrical interference.
Electronic equipment is often subjected to one or more stresses that may impair performance. Prolonged exposure to high temperatures or cryogenic conditions, chemicals, rigorous thermal cycling, mechanical shock, thermal shock, and other conditions can adversely affect electronic devices, causing them to fail.
During manufacturing and assembly, components must often withstand high soldering temperatures, and are sometimes exposed to cleaning agents and other chemicals that can interfere with circuit operation. Additionally, electronic assemblies are commonly subjected to rigorous qualification testing. This includes challenging conditions such as extreme heat, chemical exposure, thermal cycling, thermal shock or mechanical shock, and excessive vibration — all of which can cause product failure. Often qualification testing is more rigorous than the actual operating conditions.
Potting Isolates Electronics from the Environment
Potting and encapsulation compounds impart the highest level of protection from environmental, thermal, chemical, mechanical, and electrical conditions. Designed to completely encapsulate a component, module, or PCB, these specially formulated compounds effectively shield the unit from its surroundings while providing structural support. Potting compounds offer a higher level of physical, chemical, electrical, and temperature protection than do conformal coatings. The flip side is potting can often add extra processing time, cost, and weight.
Epoxies and silicones are among the most frequently used potting compounds. These can be applied to PCBs, capacitors, power electronics, LED lighting, sensors, and more. Some of the desired properties include good adhesion, excellent electrical insulation, thermal stability, superb chemical resistance, low shrinkage upon curing, an appropriate coefficient of thermal expansion (CTE), and suitable viscosity for the specific application. Formulations can be blended to provide a balance of these and other properties through careful selection of resin, agent, and fillers.
Potting Formulations Suit Diverse Applications
A critical part of choosing the right product is prioritizing the requirements and realizing that tradeoffs are associated with choosing a material. That is to say, a balance must be developed between the final properties with handling and processing issues such as open time, viscosity, cure schedule, and other factors.
Epoxies are the most commonly used material for potting and encapsulation, due to their wide range of properties and unmatched versatility. Epoxies offer exceptional chemical resistance, excellent physical properties, and strong adhesion to metals, most plastics, ceramics, and composites — materials commonly used for potting housings. They typically have superior dielectric properties and are normally thermally insulative, but can be made thermally conductive and electrically insulative when needed. They can also be formulated to withstand thermal cycling, stresses, and shocks while retaining their excellent dielectric properties. Although epoxies are usually considered to be rigid and permanent in nature, they can be made more flexible when required, with some grades exhibiting enough flexibility to allow for possible retrieval of a component. However, when epoxies are formulated to be more flexible, chemical and temperature resistance are invariably compromised.
Epoxies can be engineered for use in potting applications that call for very specific characteristics, such as optical clarity, flame retardancy, thermal conductivity, or low outgassing properties, while retaining their electrical isolation capabilities. Specially formulated flameresistant epoxies that are certified to meet UL 94V-0 specifications are selfextinguishing and offer superior electrical insulation properties, making them ideal for potting power supplies, signal transformers, and other high-power electronic devices.
The unparalleled versatility of epoxies is exemplified by their wide service temperature range. Some grades can withstand cryogenic conditions while others resist temperatures up to 500 °F. However, unique B-stage epoxy compounds offer high temperature resistance and are more flexible than typical high-heat-resistant epoxies. Normally, epoxies that resist higher temperatures are rigid. They can withstand rigorous thermal cycling and thermal shock, but require more involved processing in order to realize their special properties. For example, they must be converted to liquid from a solid state. Most importantly, B-stage materials have lower exotherm than typical one-part epoxies, and are suitable for larger castings. Fillers can be added to achieve other performance properties such as thermal conductivity and enhanced dimensional stability.