Effect of Improving the Slip Properties of the Organic Materials on the Inorganic Filler in Heat Dissipated Pad

The heat dissipated pad is made of composite mixing silicon or epoxy resin with thermal conductive inorganic fillers. The heat-dissipation material improves performance as the amount of thermal conductivity filler increases. However, the optimum recipe should be determined by considering the price and pad formability. In this study, high performance thermal pad is made of silicon resin mixed with Al 2 O 3 as a thermally conductive filler. Since Al 2 O 3 is low cost, it can use much filler. Al 2 O 3 has improved slip-ability with organic coating on it to increase the viscosity of the slurry. The same process and the same recipe, could maximize the amount of the filler. As a result, the thermal conductivity is lower by 10%. But the viscosity is reduced by 60%, too. So form-ability is getting priority.


Introduction
As high-power LED lighting and related electronics industry are growing, the high-performance and high-integrated electronic devices are increasing. Electronic devices can cause system error or shorten the life due to heat generated during the running. Therefore, heat-dissipation, which can prevent fire or explosion, is an important technology in an electronic device [1] [2].
Heat sinks are commonly used but the heat dissipated sheets are also used to reduce the volume [1]. The heat dissipated sheet has classified grade depending on the thermal conductivity, which is used according to the performance of the target [2].
In automotive technology, it is needed to deal with generated heat from electronic control units such as the IGBT (Insulated Gate Bipolar Transistor) and Power IC. Motor output is increasing to improve the performance of HEV or EV. Related to this, power system is minimizing heat accumulation inside by increasing thermal conductivity of silicon in component material. The thermal conductive silicon composite is required to have flow-ability, wettability, adhesion and fast curing. Silicon is a highly reliable and durable material. Therefore, it is possible to apply in various fields such as automotive, electronics and industrial use [1].
In this study, the insulated thermal pad has been manufactured and evaluated.
The organic material coated on the Al 2 O 3 can improve the viscosity to facilitate form-ability. However, thermal conductivity may be lowered by the organic coating layer. Therefore, we proposed a method of developing high performance sheet by control of viscosity and thermal conductivity.

Methods
The heat dissipation pad was produced by mixing silicone resin and alumina powder [1] [2]. The heat dissipated pad is very important choosing of filler because a large amount of filler must be contained. The price of functional filler is a very important factor because it uses approximately 700 to 800 phr in heat dissipated pad. The Al 2 O 3 is both plate and spherical, and the particle size is tens or hundreds micrometers. In case of the plate types Al 2 O 3 , thru-plane thermal conductivity is low. And there may be a disadvantage to forming a thermal path. In order to make the heat transfer path well, a composite material was prepared by mixing plate-shaped and spherical fillers. The voids can be minimized by using various sizes of spherical Al 2 O 3 . By mixing the shape and size in this way, packed property of filler can be enhanced (Therefore, it can be improved packing properties). It is necessary to consider the effect of fillers for conductivity as it is oriented in the process of rolling the thermal pad [1] [2]. The Al 2 O 3 is coated with water soluble acrylic emulsion by Fine Chem Tech Co. The coated Al 2 O 3 and silicon liquid resin (HRS Co.) were mixed and then it was cured to pad. The cured pad was measured its hardness, insulation characteristics and thermal conductivity.
The organic layer on Al 2 O 3 was coated by mixed acrylic emulsion and catalyst.
It was stirred for 1000 rpm during 30 minutes again by adding aqueous acrylic emulsion after stirring 800 rpm for 5 min in distilled water with the catalyst. The coating layer was observed according to curing conditions as like temperature and curing time. We were cured at 60˚C, 100˚C, 130˚C and 150˚C for 30 minutes or 1 hour [1]. The coating layer of each condition was analyzed by FT-IR (IS5 from Thermo) and SEM. It was showed the coating procedure in Figure 1 The thermal pad is made of silicon resin with coated Al 2 O 3 . We used planetary mixer by THIKKY ARE310, for mixing and degassing [3]. It was measured viscosity for slurry and thermal conductivity for sheet. The thermal conductivity was

Materials
The water-based acrylic emulsion used as coating solution has a similar structure to silicon resin. Similarity of molecular structure may increase the compatibility of coating layer and matrix resin. The more filler amount there is, the lower flow-ability it is. The interface of uncoated filler and resin were separated and the void was formed. Thus, the composite has degraded. The organic-inorganic interface could be stabilized by the coating layer [1] [2]. We carried out FT-IR analysis for the coated layer by water-based acrylic emulsion with 5% curing agent.  Because it was used as a weight ratio, coating area of Al 2 O 3 varies depending on the particle distribution. Figure 3 shows the surface before and after the coating

Thermal Conductivity & Viscosity
The thermal conductivity was measured by the heat flow method (Anter Co. UNITHERM ® 2022) in the bulk thru value. The thermal conductivity of thermal pad is important. The thermal pad is important to thermal conductivity. Thermal conductivity for the directional bulk thru is important in the thermal pad.
The compatibility between filler and resin has a major influence on the flowability of slurry [4]. In this study, it was coated with a silicone resins similar material to increase the mixing limit amount of Al 2 O 3 in composite. The viscosity was compared for slurry using uncoated and coated Al 2 O 3 . The thermal path was cut off by the organic interfacial layer [1] [2]. However, the thin coating layer did  Table 1 shows the effect of the coated filler in composite material of its amount. The thermal conductivity and viscosity of composite with 600 -800 phr Al 2 O 3 are shown in Figure 5.
The higher the coated filler content, the lower the thermal conductivity by up to maximum 10% and the lower the viscosity by about 60%. In order to increase thermal conductivity of the thermal pad, the filler content should increase. But the composite of inorganic and organic materials is inhibited its flowability because of its low compatibility. As the amount of filler in composite increases, the viscosity increases [3] [4]. Therefore, it is hard to make the pad. By the coating layer that has a similar property to matrix resin on the surface of the Al 2 O 3 , the flow-ability is increased by the improved compatibility. For higher thermal conductivity, it is effective to increase the filler. The higher thermal conductivity can

Conclusions
Through this study, the following conclusions were obtained.
Slurry composite materials were manufactured using Al 2 O 3 and silicon resin. The improvement of viscosity was shown to be a greater effect than the degradation of thermal conductivity.