Encapsulants and potting compounds are designed to insulate and protect electrical and electronic components. They typically provide environmental protection, electrical insulation, and other specialized characteristics.
Encapsulants protect circuit boards, semiconductors, and other electronic components from environmental hazards such as mechanical, chemical, electrical, and thermal stress. Nearly all encapsulants consist of polymeric material, which must fdeliver an adequate balance of mechanical strength, chemical resistance, dielectric insulation, and thermal conductivity within a broad temperature range (generally from -65 to 200° C [-85 to 390° F]).
Encapsulant materials are typically chosen based on the rigors of the intended application. Polyurethanes, polyesters, and polyamides are employed when circuits will be exposed to mild environments and consistent thermal conditions. Thermoplastics, which offer the advantage of easy recycling and reuse, are occasionally used for low-reliability applications. Silicone, having several qualities which are desirable for encapsulating circuits, is commonly used as a pottant or encapsulant.
Internal stress, an important parameter to consider when selecting encapsulants, refers to the stress of the hardened encapsulant on encapsulated components. Internal stress causes die warping, the cracking of encapsulants or chips, and wire shearing.
Some common electronics packages, along with their ideal encapsulant characteristics, are listed below.
Surface-mounted (SMT) packages: Small amounts of encapsulant with high mechanical strength, adhesion, hydrophobicity, and excellent high temperature resistance.
Beam-bonded components: Low maximum processing temperatures.
Large chips: Low internal stress; very low thermal shrinkage.
Memory devices and microprocessors: Uranium- and thorium-free to eliminate radiation-induced errors.
High-power devices: High thermal conductivity; low internal stress.
Encapsulant Systems and Curing
Encapsulants change from a viscous, liquid substance, which can be poured into a circuit, to a solid, protective substance; this process is known as curing. Curing occurs when cross-linkers within encapsulant compounds form solid polymeric or elastomeric bonds. Encapsulants and potting compounds may cure when exposed to UV or visible light, or simply room temperature air (the latter process is known as room temperature vulcanizing, or RTV). Condensation curing involves the use of moisture present in the atmosphere, which is possible only for open-air curing.
The image above and at right shows the curing process of a UV adhesive. When exposed to ultraviolet radiation, photoinitiators within the resin—represented here by small, red circles—trigger cross-linking and subsequent hardening.
The application of various hot-melt encapsulants.
Video credit: PowerAdhesives
Some encapsulants require an additive to cure; these are known as two-part or two-component systems consisting of the resin itself as well as a catalyst such as a hardener or accelerator. Some resins and catalysts are available premixed and frozen for convenience. Two-component resins must be carefully considered, as the curing reaction often produces byproducts which may prove harmful to certain circuit types.
Encapsulants can be generally classified as potting compounds, glob-top encapsulants, and molding compounds.
Potting compounds (or pottants) are designed to be poured into potted circuit boards. Potted PCBs have relatively high-walled sides, with components mounted within the enclosure. Potting compounds are typically applied in a thicker layer than other encapsulants as a result. The image at right shows pottant being poured into a potted PCB.
Most potting compounds are two-component resins and share these common characteristics:
Low viscosity and high thermal expansion coefficients
Moderate cure temperatures
Low internal stress
Low levels of ionic contaminants
Good thermal stability and insulating properties
Glob-top encapsulants are applied directly to microelectronic components which are mounted to a PCB. They have historically been low-precision thermoset resins used to encapsulate inexpensive components, although improved-quality glob tops are becoming more common. All glob tops share some common attributes, such as higher viscosity, quick cure times, mechanical strength, adhesive compatibility, and good electrical insulation properties.
A bare component (left) and the same component encapsulated using a glob top.
Image credit: DIMA Group
Molding compounds take the form of granulated powder tablets which must be mixed with a fluid and heated to form a soft dough. This material then efficiently fills electronics packages using a molding press. The compound itself is typically a one-component formula consisting of finely ground resins, catalysts, and modifiers such as flame retardants.
Encapsulants and potting compounds may be produced, used, and tested according to various published standards and specifications. Examples include:
ASTM F641 Standard specification for implantable epoxy electronic encapsulants
BS EN 60664-3 Insulation coordination for equipment within low-voltage systems: Use of coating, potting, or moulding for protection against pollution
QPL-24041 Polyurethane molding and potting compound
Master Bond / AZOM - Potting and Encapsulants for Electronics
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