Thermally Conductive-Silicone Composites with Thermally Reversible

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Thermally Conductive-silicone Composites with Thermally Reversible Cross-links Jason T. Wertz, Joseph P. Kuczynski, and Dylan J. Boday ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03065 • Publication Date (Web): 25 May 2016 Downloaded from http://pubs.acs.org on May 27, 2016

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ACS Applied Materials & Interfaces

Thermally Conductive-silicone Composites with Thermally Reversible Cross-links J. T. Wertz†, J. P. Kuczynski‡, and D. J. Bodayǂ* †IBM Corporation, Poughkeepsie, NY12601 ‡ IBM Corporation, Tampa, FL33607 ǂ IBM Corporation, Tucson, AZ 85744

* To whom correspondence should be addressed. Email: [email protected]

Abstract Thermally conductive-silicone composites that contain thermally reversible crosslinks

were

prepared

by

blending

diene-

and

dienophile-functionalized

polydimethylsiloxane (PDMS) with an aluminum oxide conductive filler. This class of thermally conductive-silicones are useful as thermal interface materials (TIMs) within Information Technology (IT) hardware applications to allow rework of valuable components. The composites were rendered reworkable via retro Diels-Alder cross-links when temperatures were elevated above 130 °C and required little mechanical force to remove, making them advantageous over other TIM materials.

Results show high

thermal conductivity (0.4 W/m·K) at low filler loadings (45 wt%) compared to other TIM solutions (>45 wt%). Additionally, the adhesion of the material was found to be ~7 times greater at lower temperature (25 °C) and ~2 times greater at higher temperatures (120 °C) than commercially available TIMs.

Keywords: Thermal Interface Materials, Reversible, Diels-Alder, Silicones, Thermally Conductive

ACS Paragon Plus Environment


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Demand for high performance computing has led to an increase in device operating temperatures with heat fluxes routinely observed in excess of 100 W/cm2.1 Increasing operating temperatures provides significant challenges associated with thermal management, device reliability, and performance issues. Typical thermal management of high performance computing devices is handled through the use of thermal interface materials (TIMs) that adhere an IT hardware component to a heat sink while allowing the dissipation of the heat generated from the component. TIMs are typically used in two separate locations for device cooling applications. In these applications, there is a TIM placed between the die and the chip lid (TIM1) and a TIM placed between the thermal solution (e.g., heat sinks) and the chip/hardware component lid (TIM2). TIM compositions can be altered to have better characteristics related to flow and coverage, but the issues are two-fold in that high filler loadings provide high thermal conductivity but with low flowability and poor surface coverage while the opposite is true for low filler loadings. With these issues, there is a fine line between allowing for which the TIM composition can change as it can lead to thermal “pump out”2 of the material. For rework purposes and our further discussion, the TIM2 location is normally the area requiring the most rework and of most interest for siliconebased thermally reversible materials. As high performance computing continues to require more computing power, the device density has significantly increased with 20 billion transistors predicted with the upcoming POC chip technology. 3 In a common application, a single heat sink will connect to numerous devices. If one of the devices fail, the heat sink must be removed in order to rework the failed device on the board. In many applications, the thermal solution


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is placed over multiple devices and requires localized rework that is not possible with today’s current TIMs due to the need for mechanical force detachment. During this removal process, the thermal interface material can adhere and cause physical damage to the electrical circuitry of the devices. A typical removal process would involve the use of solvents to aid the removal process, but this presents its own challenges in IT hardware manufacturing as improper solvents could be used (e.g., conductive) and/or concentrate conductive materials (e.g., dust) to electrically sensitive components. If the adhesive strength of the TIM is low during thermal cycling of the device, the heat sink will separate from the TIM and create air gaps which leads to poor thermal management. Here, we have prepared thermally conductive silicone composites which contain thermally reversible Diels-Alder cross-links. 4 These thermally reversible Diels-Alder cyclo-addition cross-links have been used previously as a means to mend damaged polymers 5 or used as a means to prepare removable materials6. Several other reaction pathways exist that can allow for rework of adhesives through reversible bond formation including coumarin photodimerization7, thermal weak links8, and disulfide formation9. Here, Diels-Alder was selected due to its thermal retro bond formation allowing for a more simple rework process of the TIM2. The resulting composites developed are thermally conductive at standard operating temperatures (< 130 °C), but also allow rework at elevated temperatures (> 130 °C), by reducing the TIM to an oil with little to no adhesive properties. To achieve a thermally conductive-silicone composite that can easily be reworked, base polymers were first synthesized. Through dienophile chemistry 10 , a silicone copolymer was formed that included amine functionalities by reacting aminopropyl-



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polydimethylsiloxane (PDMS) and furan protected maleic anhydride (3,6-epoxy-1,2,3,6tetrahydrophthalic anhydride). Here, protected maleic anhydride was used to prevent Michael addition from occurring.

At temperatures greater than 125 °C, the maleic

anhydride becomes unprotected, but it was found that either version (protected or unprotected) of the polymer was acceptable for use.

A second silicone copolymer

containing diene functionalities by reacting aminopropyl-PDMS with furfuryl isocyanate. After formation of the two silicone copolymers, they were then blended together in a 1:1 ratio and cured at 70 °C affording equal diene and dienophile pendent functionalities. In order to render the new polymer thermally conductive, ~50 nm aluminum oxide (0 – 45 wt%) was blended with the parent material at elevated temperature (130 °C) to render the gel material an oil allowing for mechanical mixing of the blend. After blending, the mixture was cooled to ~70 °C for 2 h for reformation of the Diels-Alder cross-links5. Other

thermally conductive

fillers

(silica,

alumina,

boron

nitride,

diamond,

etc.)11,12,13,14,15,16,17 could be added even at higher weight percents, which is beyond the scope of this work. Rework of the new TIM2 material was easily performed by merely heating the new polymer above its softening temperature (~ 130 °C),which was required in order to induce the retro Diels-Alder reaction (Figure 1). Rework of the new TIM material was characterized by analyzing the probe normalized height where 100% refers to the TIM having good adhesion at low temperatures and 0% referring to when the adhesion has been greatly reduced at elevated temperatures where rework can easily be done. The softening temperature was measured using thermomechanical analysis (TMA) using a ramp rate of 5 °C/min with a 200 mN load and a probe diameter of 20 mm. Here, the


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adhesion becomes very low as the gel transforms to an oil-like material. The viscosity of the material during the application of the material onto the test fixtures was found to be easily changed by merely adding or subtracting the amount of aluminum oxide filler within the composite material. With an increase of filler material, the coefficient of thermal expansion (CTE) was found to be reduced due to the reduced amount of organic content within the composite material. As with the retro Diels-Alder reaction, the new polymer is easily reapplied using the reverse approach (Diels-Alder reaction) by first heating the polymer composite above its rework temperature, applying the thermal solution, then cooling the combined structure down to room temperature to generate adhesion between the TIM2 material and the thermal solution (Figure 2) and reforming the Diels-Alder product. The effectiveness of this process was shown using TMA where the sample was pulled in tension (8000 mN) at 175 °C followed by cooling to 60 °C for 30 mins where it was placed under load (3000 mN). As shown in Figure 2, this process can easily be transferred to a manufacturing environment for removal and replacement of a TIM2 material.



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Figure 1. Rework temperatures of thermally reversible cross-linked silicone composites in relationship to various filler weight percents as analyzed via TMA under 200 mN load with a 5 °C/min ramp rate.


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Figure 2. Reworkability of thermally reversible-silicones by (retro) Diels-Alder reactions as shown on a TMA plot. Sample pulled in tension at 8000 mN at 175 °C followed by cooling to 60 °C for 30 mins and then placed under a 3000 mN tensile load. As with any TIM material, the adhesion strength between the thermal solution and the chip is extremely important as well as the conductivity of the material. Testing of the adhesion strength was performed on the highest loading composite that was adhered to a fixed plate at various temperatures (25-185 °C) using tensile force measurement (Figure 3). Testing was performed using a modified adhesive peel test (ASTM D3330), wherein the sample was pulled at a rate of 5 mm/s and the max load was recorded. Results of the test indicate that as higher forces are applied to the TIM material, the lower the temperature needed to remove the composite material from the fixed plate, thus making these advantageous over commercial TIM solutions (typically require mechanical force detachment).

Results showed a linear relationship between force applied and



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temperature. Compared to commercially available TIM material, the new thermally reversible-silicone composite was found to have superior adhesion strength (approximately 2 – 7x) across a wide range of temperatures (20 – 190 °C). Testing of the thermal conductivity of the new composites was performed using an internal test vehicle which included copper columns with copper thermal measurement elements attached. Testing was performed in accordance with ASTM D5470, by sandwiching the new conductive-silicone TIM material (Figure 4a) between the two copper elements. In the setup, the upper column is heated and the bottom column is cooled and the temperature gradient is measured from top to bottom using two sets of four evenly-spaced thermocouples. Sample size of the TIM material was 22 x 32.5 mm. For each weight percent filler material, thermal conductivity was measured (Figure 4b).

Thermal

conductivity was found to be linear with a maximum thermal conductivity (0.4 W/m·K) achieve at 45 wt% alumina as would be expected with a higher loading of conductive filler. The parent polymer was found to have a thermal conductivity of approximately 0.15 W/m·K without conductive filler, which indicates the need for the filler in order to remove heat from the device lid.


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Figure 3. Adhesion strengths at various temperatures (25-185 °C) of thermally reversible-silicone composites. The TIM samples (Diels-Alder Thermally Reversible (DA-TR) shown in circles and commercially available shown in squares) were placed between a fixed plate and a pull rod (right figure), heated, and then pulled (5 mm/s) using and Instron to determine the maximum force before the TIM separated.

Figure 4. Thermal conductivity (b) of alumina filled thermally reversible-silicone composites tested using thermal conductivity test vehicle (a).



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In conclusion, we have developed a new thermally conductive-silicone TIM2 material that can easily be attached and reworked through the use of reversible Diels-Alder crosslinks. The new TIM material was shown to adhere well within the typical operating range (

Thermally Conductive-Silicone Composites with Thermally Reversible

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