Epoxy Molding Compounds

Epoxy Mold Compounds (EMC) are formulations of thermosetting solid epoxy polymers, additives, and silica-based fillers.

EMCs are used in industrial molding applications and commonly come in the form of compressed solid powder pellets which are then heated into a liquid and molded to encapsulate or “package” a semiconductor or electronic device.

As thermosets their final molded form is permanent

They won’t melt again even at higher temperatures than their original melting point. This makes them perfect for high-temperature applications including circuit board devices which have to go through a solder reflow process of up to 260°C.

Epoxy is produced by combining epichlorohydrin with a Curative (hardener).

The most common curatives are Bisphenol-A (DGEBA), Bisphenol-F (DGEBF). However, other curatives are also used: including Phenols, Thiols, Anhydrides, Amines, Aliphatic Alcohols. Depending on the curative and the ratio of curative to epichlorohydrin the resulting resins can have a variety of molecular weights, viscosity, and final material properties.

The final result is usually a complex crosslinked long-chain molecule polymer network with great strength, and high thermal and chemical resistance, fast cure time, and low viscosity. Combining it with a silica filler dramatically lowers CTE, increases density, and creates a mechanically strong and incredibly insulating composite perfect for insulating and protecting even the most sensitive electric devices.

EMC performance determining variables: Curatives.

Different curatives produce different kinds of epoxy resins with different properties. For example, Bisphenol epoxy resins are most commonly used as a base for adhesives and coatings. Epoxy Mold Compounds (EMC) on the other hand are usually a formulation of either anhydride epoxy resin or phenolic epoxy resins, along with other additives, and including a high percentage silica-based filler (60%--90%).

Epoxy resins cured with anhydrides have low viscosity, high latency, and have amazing cured strength

This makes them perfect for Industrial EMC for molding larger and more massive parts that require higher mechanical strength. However, anhydride epoxies generally have high moisture absorption which makes them less suited to more sensitive electrical applications like semiconductors.

Phenol curatives form highly crosslinked polymer networks: and so have outstanding high temperature and chemical resistance and lower moisture absorption.

That's why phenolic epoxy resins are perfect in Semiconductor Epoxy Mold Compounds because they can easily survive even SAC solder reflow temperatures (260 °C). This makes phenolic epoxy resin based EMC the standard for semiconductor encapsulation and other high-temperature applications.

EMC performance determining variables: Fillers

Different filler types also dramatically affect the final properties of the resulting EMC composite. For example, adding iron filler creates EMC with higher thermal conductivity and other magnetic properties, whereas silica filler increases the strength and insulation of the final composite.

One important reason silica fillers are the most used mineral filler in EMC composites is that silica has an incredibly low coefficient of thermal expansion (CTE).

Epoxy resin by itself has a CTE of 300ppm/°C, while silica has a CTE of 3ppm/°C

Combining the two creates a final composite with a CTE of around 60ppm/°C in a ratio of 80% filler to 20% epoxy resin.

Having a low CTE EMC is important to ensure that the Epoxy Mold Compound packet and the semiconductor or electric device inside expand and contract at the same rate under thermal stress otherwise delamination will occur.

Silica filler comes in two main types: Fibreglass and silica sand.

Fiberglass, like all glass, is silica based and generally has a higher particle size. Fiberglass fibers create an interlocking structure giving increased mechanical strength, however, it has a much larger particle size and higher viscosity because fiber is unable to flow very well in the molten composite.

Not just the filler type, but filler particle size and ratio have an important impact on the final performance of any given EMC.

Silica sand, on the other hand, has a smaller particle size, and particles that are more spherical and so flow better when the composite is molten. Smaller gate size 100 microns, a general rule filler particle size needs to be less than 1/3 the gate size.

Filler particle size may be the most important variable when it comes to selecting an Epoxy Mold Compound

In the EMC molding process, the solid epoxy molding compound is heated to become a liquid and then must flow through runners and gates of the mold plate that direct it into the mold cavity. Transfer molding plates for semiconductors can have gate sizes as small as 100 microns. Fiberglass filler can have particle sizes of 100 microns so are clearly incompatible. Silica Sand can have particle sizes of less than a micro making it the perfect choice for semiconductor encapsulation.

Industrial EMC VS Semiconductor EMC

So as you can tell Epoxy Mold Compounds tend to fall in two groups.

There are Industrial Epoxy Mold Compounds that are composites of Anhydride epoxy resins with fiberglass filler. There are best suited for molding more massive parts that require high mechanical strength.

And there are Semiconductor Epoxy molding compounds: usually a composite of Phenolic epoxy resins with silica sand filler which are best suited more demanding and complex semiconductor encapsulation. The smaller particle size of the silica sand being able to pass through the smaller gate size of transfer mold is not the only reason this is so.

Phenolic epoxy resins also have better temperature and chemical resistance than Anhydride epoxy resins making them able to withstand the solder reflow heating process better. They also have better lower moisture absorption.

EMC Semiconductors have to meet much higher requirements formalized in the industry as MSL standards.

Moisture absorption is one the most important factors: trapped moisture within the epoxy molding compound expands explosively as steam and literally cause the encapsulated semiconductor to explode like a popcorn.

That's why depending on epoxy resin type and silica filler type, epoxy mold compounds are suited for either industrial product molding or semiconductor molding.