WHY ARE MORE AND MORE FLEXIBLE GRAPHITE USED IN HIGH-END CONSUMER ELECTRONICS?

Introduction

In today's consumer electronic products, the main functions of radiator are as follows:

1. Cool the heating components, such as the processor, to reduce its temperature

2. Reduce the hot spot on the surface of the device for the parts sensitive to temperature, such as the mobile phone case

3. Protect thermal sensitive components from heat sources, such as OLED phone screen from heat generated by processor

4. Reduce the surface temperature gradient of devices sensitive to uneven surface temperature, such as OLED TV screen

Until the beginning of the 21st century, aluminum and various aluminum alloys are the thermal materials for almost all consumer electronic equipment. Plasma TVs, laptops, mobile phones, and early tablets all use relatively inexpensive and abundant metal materials to meet the cooling requirements. First, the obvious disadvantage of using aluminum as a thermal management material is the weight and thickness of the metal. At the beginning of the 21st century, plasma TVs weighing 30 to 70 kg on the wall often fall off. To prevent this problem, professional installers usually provide "incidental services" for purchasing screens. In the early 21st century, the thickness of mobile phones is generally about 20 mm. If cooling is needed, the heating device is usually placed on a metal base, and then the heat is transferred to the whole mobile phone by a metal structure with good thermal conductivity. When the processor and other devices are designed with higher power, thicker and heavier bases are required to compensate for the increased thermal load. When the first smart phone was launched, thin became a major design feature pursued by high-end mobile phones. This trend quickly extended to notebook computers, as thin as possible has become a high-end notebook design requirements. In order to achieve the desired product characteristics, the industry quickly realized that solutions to the problem of aluminum weight and thickness needed to be developed.

In the era of flexible graphite sheet

Flexible natural graphite sheet began to replace aluminum as the thermal management material of plasma TV display screen since 2002. Natural graphite has been widely used in the field of display screen because of its thermal conductivity is three times that of aluminum and its weight is only one third of that of aluminum. Relying on this successful application, the application of graphite extends to other equipment and market categories. Flexible graphite was first used in mobile phones in the early 21st century. By 2015, graphite has become the standard thermal management material for all high-end smart phones. The earliest smartphone models used natural graphite with a thickness of 100 microns, but soon the industry began to use synthetic graphite in the range of 17 to 50 microns. Thinner graphite, such as 10 microns, can also be made, but it turns out that it can't effectively remove enough heat. Over the past 15 years, graphite has replaced aluminum in the vast majority of high-end consumer electronics devices that pursue lightweight designs.

Characteristics of graphite materials

There are two forms of natural graphite and synthetic graphite. Natural graphite is made of flake graphite, and flake graphite deposits are all over the world. Generally, the thickness of natural graphite ranges from 40 microns to 1000 microns, and its thermal conductivity ranges from 300 to 600 W / m · K. Because natural graphite is usually thicker than synthetic graphite, it can transfer more heat even at lower thermal conductivity.

Synthetic graphite sheet is produced by graphitization of the substrate containing carbon atoms at temperatures over 2500 ℃. Synthetic graphite is usually very thin, ranging in thickness from 10 microns to 100 microns, but its thermal conductivity is higher, ranging from 700 to 1950 w / m · K. However, even if it has a higher thermal conductivity, the heat it can take away is limited due to the thickness of the material. Previous attempts have been made to produce synthetic graphite with a thickness of more than 100 microns, but these attempts are difficult to achieve

Both natural graphite and synthetic graphite are stacked by thousands of graphene sheets. These graphene sheets have excellent thermal conductivity, electrical conductivity and sound transmission properties in the inner layer, but the performance is weakened in the interlayer. This results in a 100 to 450 fold anisotropy ratio of the thermal conductivity within and between layers [1]. Manufacturers use the special anisotropy of graphite in heat transfer to reduce the surface contact temperature and protect thermal sensitive devices from the influence of heating devices. Metallic materials, such as aluminum or copper, have the same thermal conductivity in all directions. This makes them less suitable for these applications because the heat applied to one side of the metal surface is quickly transferred to the opposite side of the material.

Current trend of graphite

For some applications, such as tablets, laptops, VR / AR glasses, OLED TV screens, set-top boxes and some mobile phones, natural graphite with a thickness of 40-1000 microns still has a place. As mobile phones, tablets and laptops are moving in a thinner direction, many of these products are turning to synthetic graphite. The industry quickly standardized the thickness of synthetic graphite required for these applications, such as 17 microns, 25 microns, 32 microns, 40 microns, and 50 microns. Because the performance of the equipment is becoming more and more powerful, single-layer synthetic graphite often can not transfer enough heat to meet the thermal design, and there is not enough thickness space to use natural graphite. To solve this problem, the conventional method is to use 5 μ m thick film to compound two or more layers of graphite. This ensures that the radiator maintains a high thermal conductivity even though it is thinner than natural graphite. When all devices use this multi-layer composite scheme for thermal management, its disadvantages are highlighted. The first is the cost of raw materials. Due to its nature, synthetic graphite itself is a relatively expensive and highly customized synthetic material. In order to composite four, five or even six layers of this material, it is necessary to use multi-layer adhesive film at the same time, which makes this method not feasible in cost. Secondly, the complex composite process leads to low yield. The most important issue is to increase the number of splices in each roll. All rolls are more or less spliced to varying degrees, even if it's just from the end of one roll to another. Making a roll of multi-layer graphite and adhesive film material will have many more seams than single-layer material. One seam on any substrate will result in a new joint in the whole multilayer material. When parts need to be die cut, these seams must be removed or discarded. Even if these seams do not affect the quality of the product, they do have a significant impact on cost and production. Interlaminar wrinkles and air bubbles are also common, which can reduce the yield. The final problem is that the multilayer composite is bonded and stacked by plastics and adhesives with poor thermal conductivity. In addition to making the final product thicker, each layer of these materials increases the thermal resistance and prevents the smooth transfer of heat between the graphite layers.

High performance, single-layer thick graphite solution

This project is to solve the problem of multi-layer synthetic graphite. On the basis of being thinner than natural graphite, it also has the thermal conductivity of synthetic graphite. This scheme not only has the expected performance of multi-layer synthetic graphite, but also has the cost advantage of single-layer scheme. This brand new high performance, single layer thick graphite product, has been put into the market. At present, the thickness is between 70μm and 300μm, and the thermal conductivity is between 750 W/m · K and 1200 W/m · K. The first consumer products to use the product are smart phones, laptops and VR / AR glasses.

Conclusion

Graphite radiator has become the standard thermal management material for high-end consumer electronic equipment with light and thin design elements. Single layer synthetic graphite has been the standard solution for most high-end devices, but some devices now require a multi-layer material solution. It is expensive and complicated to produce multi-layer graphite materials. In order to solve this problem, a new kind of high-performance, single-layer thick graphite product has been developed. This material can avoid the cost and yield problems of multilayer synthetic graphite.