Thermal management of electronics is becoming an important and concerned issue due to the compactness, complexity of new generation of electronic devices. An electronic device fails to fulfil its intended function when its application or environmental condition exceeds its application limit. Theoretically, electronic components are said to be reliable at recommended operating temperatures if they can be operated continuously for a long duration. However, adverse environment and unusual operation reduces the effective operating time. It has been found that a 1°C decrease in a component temperature may lower its failure rate by as much as 4% and 10°C to 20°C increase in component temperature can increase its failure rate by 100%. Hence, there is a tremendous need for innovative cooling technologies.

Various cooling techniques are broadly classified into two groups, viz. (a) active cooling and (b) passive cooling. Active thermal management requires external energy to be applied to remove heat from an electronic device. The following types of techniques can be classified as active thermal management:

  • Forced convection air
    • Forced convection liquid
    • Liquid flow through modules
    • Heat sinks with forced convection cooling
    • Thermoelectrics
    • Micro-refrigerators

In case of passive techniques, the removal of heat from an electronic device does not need any external energy to maintain the coolant flow. Different passive cooling techniques such as heat sinks cooled by natural convection, heat pipes, thermosyphons and phase change cooling are used to satisfy performance, reliability and ergonomic constraints. Phase change cooling is one of such technique, which has been widely used as an alternative cooling method for various applications such as wearable computers, power electronics, communication equipment, space craft and avionics etc., where heat dissipation is time-varying or periodic. Phase change material (PCM) plays the key role in the phase change cooling technique.

PCMs are highly effective heat storage materials that undergo a phase change at a certain key temperature and are commercially available for a range of phase change temperatures. Typical phase change temperatures range from –15 to 190°C, giving a wide choice of PCMs for any specific cooling requirement. The kind of heat that is stored in PCMs is the latent heat of fusion along with sensible heat. The latent heat of fusion of PCMs is relatively high. Consequently, only a small quantity is needed to meet storage capacity requirements for a majority of applications.

PCM-based Cooling Technique

The phase change process of PCM is shown in figure 1. When a PCM is heated, the temperature of PCM rises and it absorbs heat as sensible heat. Once the melting temperature of PCM is reached, the PCM starts melting and absorbs heat as latent heat. During this period, the PCM temperature remains constant. After the completion of PCM melting, the temperature of liquid PCM rises again. Hence, the stabilization period can be obtained until the PCM melts completely. The same process is followed in reverse direction during solidification of PCM. Generally electronic devices do not dissipate high heat rates for all the time which is ideal for PCM application. Therefore, PCMs are used in constant power/cyclic cooling for short term thermal management. It cannot be used for the equipment which is in continuous operation.

Figure 1: Phase change process of PCM

Phase Change Materials

Phase Change Material (PCM) used in cooling applications should possess some desirable thermophysical, kinetics and chemical properties. The PCM melting temperature should be below the device’s maximum operating temperature. The latent heat of fusion must be high, so a small amount of PCM can store a large amount of energy. It should have high specific heat which will provide additional sensible heat storage capacity. High thermal conductivity is desirable which makes the PCM melting and solidification homogenous and could also prevent potential PCM overheating. PCM should be chemically stable, so that it will not be changed periodically. The PCM must be non-poisonous, non-flammable, and non-explosive. The most critical properties are PCM’s melting temperature and latent heat of fusion while selecting PCMs for a particular application.

There are a large number of PCMs available either commercially or technical grade for a wide range of melting point and these can be broadly grouped into organic and inorganic. The classifications of PCMs which undergo solid to liquid phase change are shown in figure 2. The organic based PCM is paraffin which is flammable and cannot be exposed to high temperature. Several paraffin and non-paraffin based PCMs have a melting point within the desired range and high latent heat would satisfy the storage requirements. Inorganic PCMs include salts hydrates which are non-flammable, have high of heat fusion, and their melting temperatures range from 18.5°C to 116.0°C making them ideal for thermal design considerations of electronic devices. However, salts hydrates are highly corrosive in nature and a special attention is required while selecting the storage container. A few of PCMs along with their properties are listed in table 1.

Figure 2: Classification of PCMs undergoing solid–liquid phase change

Design of PCM Heat Sink

The design of a PCM based heat sink needs a careful approach. A typical PCM based heat sink used for electronic cooling is shown in figure 3. The heat sink is a rectangular cavity which is fabricated by gluing thin metal or insulation sheets to the metallic base plate. The top surface can be either covered or exposed to ambient depending on the application. The amount of PCM, to be poured, is governed by the time required to stabilize the chip temperature. An important issue in designing PCM based heat sink is leakage of molten PCM at high temperature as the attached sheet may separate. The most convenient way of filling of PCM based heat sink is to keep the PCM on the heat sink and then heat it such that the material melts and fills the heat sink. Care has been taken to accommodate volume change of PCM due to phase change and expansion of the liquid subsequently. Hence, sufficient space has to be provided to accommodate the PCM in liquid state (coefficient of thermal expansion of n-Eicosane is 1.0 ×10-3/°C). The filling is done to no more than 90% of the enclosed volume for the purpose of containment.

The PCM-based heat sink sits over an electronic chip which generates heat. Initially, the whole system is at the ambient temperature. The heat sink is considered to be subjected to either constant power or cyclic loading from the chip. The process of heating causes (i) sensible heating of PCM for a short duration, (ii) melting of PCM, and (iii) sensible heating of the melt. During the cooling period, the same processes occur but in reverse order.

Performance of PCM Heat Sink

A comparison can be drawn between two cases; (i) chip attached with a base plate and (ii) chip mounted under the PCM based heat sink. Figure 5 shows the temperature histories of chip generating 4 W of constant power. The PCM used in this case is n-Eicosane and the thickness of PCM is 5 mm. The heat sink dimension is 42 × 42 × 7 (height) mm with the base plate thickness of 2 mm. The chip is 3 mm thick. The base plate is made of Aluminium. The chip temperature at 900 s in case of PCM based heat sink is lower than the without PCM case as the PCM absorbs major portion of heat while melting and keeps the chip temperature lower.

Figure 3: Schematic diagram of a PCM based heat sink

Figure 4: Chip temperature histories for three cases (a) Without PCM, (b) with PCM and (c) with PCM and 10% of TCE

Enhancement techniques of heat transfer in PCM

One important issue needs to be addressed, is the thermal conductivity of organic PCM that is low (~0.2 W/m.K) and as a result, heat transfer rate is slow within PCM during melting and solidification. The improvement of heat transfer can be achieved by inserting high thermal conductivity materials, known as thermal conductivity enhancer (TCE) into the PCM. The TCE could be in the form of metal matrix, hollow spherical metallic balls, plate or pin fins and graphite flakes. Figure 5 shows the picture of typical TCE used for heat transfer enhancement in PCM.

A small percentage (10%) of aluminium particles is uniformly dispersed in the PCM-based heat sink to augment the heat transfer. Figure 5 shows the variation of chip temperature with time for without and with TCE distribution in PCM. With the addition of TCE, the chip temperature decreases as the effective thermal conductivity of PCM increases.

Thermal Management

In electronic applications, the chip normally operates on a base load and does not produce high heat rates all the time. However, the chip may dissipate extra heat load for a certain period of time for various reasons. This extra high heat rate needs to be transferred from the chip efficiently to maintain its temperature below the critical limit above which chip will start malfunctioning. A cooling system designed both for base load as well as peak load tends to be overdesigned, bulky and expensive. Therefore, special and efficient cooling techniques can be used to tackle that extra high heat load. The PCM based cooling technique can be effectively employed in this type of application. Figure 6 shows such an example where the chip was operating on a base load and suddenly there is a peak load of 10 W for 90 seconds. In this example, the thickness of PCM (n-Octadecane) is 1 mm and the base load is taken as zero for simplicity. The design of PCM based heat sink for such application is critical and should be optimized to meet the need of the requirement.

Figure 5: Typically used TCEs

Figure 6: Variation of chip temperature for (a) Without PCM, (b) with PCM under peak load


As the chip level heat flux has gone up significantly, conventional air cooled designs are no longer adequate and an increasing number of packaging failures are directly linked to inadequate thermal management. Hence, researchers are investigating several alternative cooling techniques those can be used in cooling of electronics equipment. In the recent years, PCM based cooling has emerged as a potential technique that can be applied to dissipate heat from the chip effectively. The main advantages of PCM based cooling technique is that its light weight compared to metal, abundant availability and large heat storage density per unit mass. However, unfortunately the most organic PCMs possess low thermal conductivity which needs to be enhanced by incorporating high thermal conductivity materials. This technique can be used to keep the chip temperature at desired limit for certain period under constant power operation. Also, it can be employed during peak load where extra high heat load can be absorbed by PCM for safe operation of chip.

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