High thermal conductivity siloxane free pad 【SifreeX-HL】
5G support, AI, IoT, etc. – A side effect of chip performance enhancement thermal management
Releasing more heat but flexible, Sekisui’s new technology leading social development
With ever advancing performance and downsizing in recent years, the amount of heat generation and heat density of electronic components have been increasing and so has the importance of countering them.
As shown in the figure on the right, Thermal Interface Materials(TIM) are inserted between CPU thermal conductive parts such as the heat sink.
The surfaces of the CPU and heat sink are hard and rough, having fine dents and bumps and difficult to be contact firmly. Therefore direct contact of CPU and heat sink still leaves gaps between them; they have a small contact area and this makes it difficult to efficiently conduct heat.
Therefore, we insert TIM such as a thermal conductive sheet, which is soft, conforms to the irregular surface, and has high thermal conductivity, between the CPU and heat sink, to efficiently release heat by using its entire contact surface.
A thermal interface material is a compound of flexible but low thermal conductive resins and hard but high thermal conductive fillers. In general, the resins used include silicone, acrylic, epoxy, etc., of which softness and heat resistance vary depending on the properties of the resin.
Thermal conductive fillers are generally grouped into two types: electrically conductive and electrically insulation.
Electrical insulating. thermal conductive fillers are made of inorganic ceramic materials such as alumina, magnesium oxide, aluminum nitride and boron nitride. Electrically conductive thermal conductive fillers are made of carbon-based materials such as carbon fiber and graphite.
Carbon-based materials are more likely to improve the thermal conductivity. However, they pose have a risk of short circuits due to their electrical conductivity. Therefore, when high electrical reliability is required, thermal conductive fillers made of electrical insulating materials, such as alumina and boron nitride, are preferred.
There is a difference of about 100 times or more in the thermal conductivity between the resins and the thermal conductive fillers. Therefore, the heat in the thermal interface materials is released mainly through the contact points of the thermal conductive fillers. Accordingly, the larger the loading amount of the thermal conductive fillers, the more the contact points and heat release paths, thus the higher the thermal conductivity. On the other hand, however, an increased volume of hard thermal conductive fillers reduces the composite material’s softness. Thus, there is a trade-off between the thermal conductivity and the softness.
A thermal interface material with high thermal conductivity but low softness may not fully exhibit its heat release properties due to its loose contact with the surfaces of the CPU and heat sink and thus increased heat resistance at the interface. Moreover, high stress during compression may cause defects in electronic components and substrates. Therefore, it is important to design a product that achieves both high thermal conductivity and softness.
A technology to overcome this trade-off is to control the orientation of anisotropic fillers to efficiently form heat release paths even with a small amount of fillers.
By exploiting its core technology of molding and controlling the orientation of anisotropic fillers, Sekisui Chemical has developed “SifreeX-HL,” a thermal conductive sheet that achieves both high thermal conductivity of 10 W and softness. Moreover, our proprietary compounding design technology added the features of electrical insulation, Siloxane-free, low outgassing, and low dielectric constant to the product. It is a highly reliable thermal conductive sheet with a low risk of causing defects in electronic components.
By their shape and properties, thermal interface materials are generally divided into three types: sheet, paste, and grease.
It is important to select the most suitable thermal interface material according to the intended application and desired physical properties.
Excellent softness, reduced stress on heat source, and conformity to rough surface
No recovery after compression
Two types: curable and non-curable after dispensing
Low thermal conductivity compared with the sheet and paste types, but good at minimum thickness and softness.
Adaptable to automated production by using a dispenser
High thermal conductivity
Excellent electrical insulation
Low dielectric constant
With its features of high thermal conductivity, softness, electrical insulation, and siloxane-free,
SifreeX-HL contributes to improve performance and reliability of ADAS cameras and HDDs that require high reliability.
High thermal conductivity, softness, electrical insulation, siloxane-free, and low dielectric constant are achieved by our original material design.
Both high thermal conductivity and softness are achieved by controlling the orientation of anisotropic fillers with molding technology and efficiently forming thermal conductive paths even with a low filler amount.
SifreeX-HL is a siloxane-free electrical insulation thermal conductive sheet that combines high thermal conductivity and softness by utilizing Sekisui’s proprietary formulation design technology and filler control technology. While retaining high thermal conductivity of 7 W and 10 W, the product shows better softness than competitors’ thermal conductive sheets of 5 W or less.
1 High thermal conductivity
Why is high thermal conductivity required?
By using a thermal conductive sheet with high thermal conductivity, we can eliminate the bottleneck of heat release, efficiently release heat and reduce the temperature of a heat source such as a CPU.
We verified the heat release effect due to high thermal conductivity by using a simple simulation model below.
We used three thermal conductive sheets with different thermal conductivities of 3, 5, and 10 W/mK for the simulation. As a result, we verified that the system using a thermal conductive sheet with higher thermal conductivity can effectively release heat and reduce the maximum temperature of the CPU.
Sekisui Chemical measures thermal conductivity and thermal resistance by a method based on ASTM D5470, which is considered the closest to the actual performance when used in an actual module.
SifreeX-HL, a thermal conductive sheet developed by Sekisui Chemical, achieves low thermal resistance (which means high thermal conductive) from lower compression ratio compared to competitors’ thermal conductive sheets.
2 Softness and low compression stress
Why is softness required of thermal interface materials?
A thermal conductive sheet is inserted between electronic components such as CPU and thermal conductive components such as heat sinks. To insert the sheet and closely contact each component, the sheet is generally compressed by about 20% to 50%. Therefore, higher compression stress may cause device failure through increased stress on the CPU, warpage of substrates, cracks in solder, etc.
With its low compression stress, SifreeX-HL can reduce stress on the CPU and eliminate the risk of substrate warpage and solder cracks during assembly, thus contributing to the improvement of device reliability. In addition, the low compression stress enables a higher compression ratio, allowing larger absorption of assembly parts tolerances. Therefore, the product can be used for devices with larger assembly tolerances and for many stacked parts.
Why is compression strength used instead of hardness as a softness index?
Softness of thermal conductive sheet is showed by hardness such as Asker’s hardness. However, it is just an index of hardness equivalent to the surface of the sheet compressed by a few percent. It may be different from the stress when the sheet is compressed by about 20 to 50% as actually used with electronic devices.
Therefore, we use the compression strength, which is the stress when compressed by 20 to 50% as actually used with electronic devices, as an index of softness.
Correlation between thermal conductivity and compression strength
In general, to increase the thermal conductivity of a thermal conductive sheet, we need to increase the loading volume of the hard thermal conductive fillers.
Consequently, the higher the thermal conductivity, the higher the compression stress (which means more stress on electronic components).
By controlling the filler orientation, SifreeX-HL realizes lower compression stress than competitors’ thermal conductive sheets of 5 W or lower despite its high thermal conductivity.
3 Siloxane-free/low outgas
What are the risks of low molecular weight siloxane?
When heated, highly volatile low-molecular-weight siloxane gas becomes a silicon dioxide (SiO2) as insulator, which may cause contact failure.
siloxane-free SifreeX-HL does not emit low-molecular-weight siloxane gas, eliminating the risk of contact faults.
What is the risk of outgassing?
A thermal conductive sheet may emit volatile components (outgas) from its base resin, plasticizer, and additives.
This outgas emitted under the operating environment of the thermal conductive sheet adheres to and condenses on the lens of the camera module cooled by the outside air, making the lens haze, a phenomenon called “fogging.”
Reduced visibility due to this fogging is a problem for a device that requires high reliability, such as an ADAS camera. Therefore, a thermal conductive sheet with low outgas and low fogging is important.
SifreeX-HL is siloxane-free, without containing low-molecular-weight siloxane as a volatile component.
Moreover, our original material design and molding technique reduced the total amount of outgas and the risk of lens fogging due to volatile components.
The product is suitable for camera applications that require high reliability.
High mechanical strength and easy handling in assembly.
There are two types of thermal conductive sheets by the electrical characteristics of the thermal fillers to be used: electrical insulation and electrically conductive. The insulation thermal conductive sheet have no risk of short circuit and has excellent reliability, while the conductive thermal conductive sheet has excellent thermal conductivity. The orientation of anisotropic fillers is controllable by forming technology.
Materials of electrical insulating thermal conductive fillers: Alumina, aluminum nitride, boron nitride, etc.
Materials of electrically conductive thermal conductive fillers: carbon fiber, graphite, etc.