During the transfer molding or injection molding process, the epoxy is exposed to elevated temperatures and pressure. At temperature and under pressure, the epoxy mold compound’s viscosity will drop significantly going from a solid at room temperature to a liquid when exposed to temperatures of 160°C to 200°C under 1–2 bars of pressure. Unlike a thermoplastic however, epoxy molding compounds are thermoset plastics. This means that at continuous exposure to temperature, the viscosity will first drop to its minimum achievable viscosity and then rise again as the epoxy cures until the epoxy stops flowing completely. (See Figure 1).
This is an important step in the molding process as it allows the epoxy to flow into the mold cavities and encapsulate the device or component before curing to provide strong mechanical protection. Failure to completely encapsulate the part is called “incomplete fill”. The minimum achievable viscosity can thus be important to ensure that the parts are completely encapsulated and that there are no cases of incomplete fill.
An industry standard test to measure the viscosity is called a Shimadzu viscosity flow test. This test measures the viscosity of an epoxy molding compound at a constant temperature, typically using the epoxy molding temperature of 175°C. The curve in Figure 2 shows how the minimum achievable viscosity is affected after exposure to temperature for extended periods of time.
The difference in minimum achievable viscosity is different depending on the epoxy compound itself and the time and temperature that the epoxy was stored. Measuring this flow at different times and temperatures will therefore give you the extent of this property change.
Please stay tuned for the next part in this series to explore the behaviour of epoxy molding compounds.
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