Enhancing the reliability of power semiconductors!
High Tg, Heat-Resistant Epoxy Molding Compounds

Power semiconductors handling large currents and high voltages deliver excellent energy-saving performance, and are used in a wide range of applications that include home appliances, electric vehicles, industrial equipment, trains, solar cells, and communication base stations. However, power semiconductors’ higher operating temperatures (Tj: junction temperature), required to meet the demand for higher power output and smaller sizes, face the challenge of passing reliability tests, such as power cycle tests (localized heating by turning devices on/off) and temperature cycle tests (heating/cooling of the entire package).

Silicone resin, a conventional material used to encapsule power semiconductor packages, is too soft to provide restraining force on chips and other components, and has failed to restrain the expansion and contraction of chips and wires in power cycle tests with high Tj, resulting failure.

To overcome these challenges, Resonac proposes its high Tg (glass transition temperature), heat-resistant epoxy molding compound <CEL-400> series. Our product is hard enough to restrain components and their expansion, thereby improving power cycle test results. The <CEL-400> series also helps avoid delamination to improve temperature cycle test results by balancing adhesive strength and stress relaxation, with these epoxy resin properties generally considered trade-offs with high Tg.

Innovative Solution

Optimizing resin compounds and additives to achieve high Tg, decomposition restraint, delamination prevention, and stress relaxation

The coefficients of thermal expansion (CTE) of power semiconductor package components are shown in the figure below. The difference between wires (Au, Cu) and chips (Si) is particularly large, and the thermal expansion difference increases as Tj rises. Soft silicone resin encapsulants fail to restrain chips and wires, resulting in wire lift-off and breakage due to their inability in high-temperature power cycle tests to restrain the concentration of stress on connections generated by differences in the expansion and contraction of these components.

Epoxy resin is hard below Tg, a characteristic that enables the restraining of components and improves power cycle test results by preventing wire lift-off and breakage. As this resin becomes soft above Tg, it is necessary to choose an encapsulant with Tg≧Tj. Higher Tg can help reduce CTE mismatches in the high temperature range. On the other hand, raising Tg in epoxy resin requires a rigid resin structure, but the rigid structure is associated with such trade-offs as easy decomposition at high temperatures, poor adhesion, and greater internal stress, and the resulting delamination leads to failures in temperature cycle tests.

Resonac’s high Tg, heat-resistant epoxy molding compound <CEL-400> series features Tg (TMA: thermomechanical analysis) of 175 to 190°C as well as the capacity to restrain decomposition, prevent delamination, and relax stress. Our product helps avoid delamination in temperature cycle tests by achieving the balance of optimally compounding epoxy and phenol used as a curing agent, adding adhesion promoters, and blending low-stress materials. With other properties of epoxy resin encapsulants affected by changing the resin structure compound, the types of additives, or filler combinations, Resonac proposes its optimally designed encapsulants to meet customers’ reliability requirements by adjusting high Tg, decomposition restraint, adhesion strength, and stress relaxation while balancing other physical properties.

Product Features

Balancing high Tg and decomposition restraint

High Tg epoxy resin can restrain CTE differences in the high temperature range, but it tends to break easily as higher Tg makes the resin structure rigid and the molecular chains difficult to move, causing stress on the chain connections at high temperatures. The chart(1) "Weight loss@250℃ " shows the weight change of various epoxy encapsulants in the Tg oscillations when left under high temperatures while using different types of epoxies and curing agent phenols. The weight loss indicates resin decomposition, and leads to resin embrittlement, cracking, and delamination as the decomposition progresses. There is a general trade-off between high Tg and decomposition restraint, with the figure on the chart(2) : "Tg vs Weight loss"  illustrating this relationship.

Resonac’s <CEL-400> series balances high Tg and decomposition restraint by optimally selecting and compounding epoxy and phenol. The green and blue in the chart(2) : "Tg vs Weight loss"  represent conventional epoxy molding compounds, and the red indicates the <CEL-400> series. There is also a trade-off with the <CEL-400> series, but Resonac has raised the trade-off relationship to the higher Tg side.

• ※ The data shown are representative values that represent examples of the results of measurements, calculations, etc., and are not guaranteed values.

High adhesive strength by adding adhesion promoters

Adhesion is the second trade-off property with high Tg. High Tg epoxy resin cannot be oriented flexibly to the adherend surface as its molecular chains become difficult to move, and its adhesive strength also becomes weak as it is difficult for functional groups to appear on the surface. Weak adhesive strength causes delamination between components with large CTE differences in temperature cycle tests.

The figures below show the simulation results of adhesion energy to various adherend surfaces. Resonac’s <CEL-400> series achieves higher adhesive energy by adding adhesion promoters.

Blending low-stress materials to achieve stress relaxation and low elastic modulus

Stress relaxation is the third trade-off property with high Tg. High Tg epoxy resin causes delamination as it is susceptible to stress from CTE differences due to its restricted molecular chain mobility and high elastic modulus.

The <CEL-400> series can control elastic modulus by adjusting the blending amount of S-silicone as a low-stress material as shown in the chart below.

Resonac also offers a wide range of agents and compounding technologies that add various physical properties such as ion trappers and adhesion promoters for copper wires, and cure shrinkage restraint. Resonac proposes our optimally designed molding compounds that balance various properties and characteristics to meet customers’ reliability requirements. For more information, please refer to our datasheet and do not hesitate to contact us.

Update date: 6th March, 2024