The effect of silica on the quality of epoxy molding compounds

With the rapid development of electronic technology, higher requirements are put forward for the packaging materials of electronic products. These packaging materials can provide important support and guarantee for electronic chips and electronic integrated circuits, and help to dissipate heat generated during the operation of electronic integrated circuits. Encapsulation materials can generally be divided into metal-based, ceramic-based and plastic-based. Among them, plastic-based packaging materials occupy most of the packaging material market, and epoxy resin packaging materials account for the largest proportion in this category.

Epoxy resin encapsulation material is an inorganic polymer composite material. Its formula usually uses epoxy resin as the matrix, and contains curing agent, curing accelerator, coupling agent, mold release agent, filler, flame retardant and other additives. At the same time, according to a certain formula ratio, it is prepared by appropriate process mixing, which is called epoxy molding compound, referred to as EMC (Epoxy Molding Compound).

In the formulation of epoxy molding compounds, inorganic fillers are the most abundant components. Among the fillers available for epoxy molding compounds, the most used is silica (silicon micropowder). However, even if it is just silica, there are actually several options. For example, it can be divided into angular and spherical in shape; in internal crystalline form, it can be divided into molten form and crystalline form; in terms of a-ray content, it can be divided into Normal form and low uranium content form.

Although they are different, each type of silica has a common feature when used as a filler, that is, it can improve certain parameters and characteristics of EMC, such as reducing shrinkage, enhancing toughness, enhancing wear resistance, reducing water absorption, increasing heat Deformation temperature, increase thermal conductivity, reduce thermal expansion coefficient, reduce cost, etc. In addition to commonalities, different silicas also have their own characteristics. For example, spherical silica powder has a larger filling amount than angular silica powder, and has good fluidity and crack resistance. Silica has low thermal conductivity and low coefficient of linear expansion; crystalline silica has high thermal conductivity and high coefficient of linear expansion. Therefore, when they are put into application, they will have different effects on epoxy molding compounds, which can be seen below.

1. Effect on viscosity
In order to improve the EMC performance, the researchers tried every means to increase the filling amount of the filler as much as possible. However, increasing the filling amount will also increase the viscosity of EMC, reduce the formability, and have an impact on the gold wire of the integrated circuit, affecting the reliability, so the filling amount should be selected appropriately. At present, the method of increasing the filling amount without changing the viscosity is mainly by adjusting the particle size distribution, increasing the bulk density, selecting spherical or angular and spherical composite inorganic filler technology, and choosing low-viscosity resins. EMC viscosity basically unchanged requirements. The optimal median particle size of the prepared composite inorganic filler is 5-40 microns.

2. Effect on Thermal Expansion Coefficient
The main method to reduce the EMC thermal expansion coefficient is to increase the amount of inorganic fillers. The expansion coefficient of epoxy resin is about 100×10(-6)/°C, while the thermal expansion coefficient of silica is 0.5×10(-6)/°C, the difference between the two is nearly 200 times. However, the increase of filler is limited. Too much filler added will increase the melt viscosity of EMC, resulting in a decrease in fluidity and an increase in E. Therefore, if the main purpose of controlling the thermal expansion coefficient is to control the thermal expansion coefficient, spherical fused silica micropowder is generally used, and the filling amount can be greatly increased, and the filler content can reach 75% to 80%. Wear of the encapsulation material in the mold, etc.

3. Effects on flashiness
Flash is a common form of defect during package formation that affects subsequent solderability and appearance. One of the reasons for the overflow is that the resin viscosity is too low and the particle size distribution of the filler is unreasonable. Therefore, within the allowable range of viscosity, a resin with a larger viscosity can be selected, and the particle size distribution of the filler can be adjusted to increase the filling amount. to improve its anti-flash performance. Japanese researchers found that when using spherical fused silica with a maximum particle size of less than 74 microns and fused silica with a maximum particle size of less than 40 microns, they can be mixed at 55-95% and 45-5% (mass fraction). As a mixed filler with a specific surface area below 3m2/g, the dosage can account for 40-90% of the entire EMC.

4. Effect on Thermal Conductivity
Generally speaking, the thermal conductivity of EMC increases with the increase of the filler content, and the thermal conductivity of crystalline silica is higher than that of fused silica. In order to meet the thermal conductivity requirements of epoxy molding compound for high-power discrete devices, high-heat devices, especially fully-encapsulated discrete devices, crystalline silica can be used as a high thermal conductivity filler.

5. The effect of fillers on EMC water absorption
Since fillers are non-hygroscopic and moisture-permeable, increasing the content of fillers is also an effective method to reduce the water absorption rate of EMC, but there will be a problem of decreased fluidity. At present, the moisture resistance of EMC is mainly improved by adding surface-treated fillers, so that the distance that moisture penetrates into the chip is as long as possible. Generally, angular particle fillers are more effective in improving crack resistance during reflow soldering than spherical fillers.