Thermal spreader

At a microscopic level the contact surface is restricted by peaks and valleys and even highly polished surfaces may exhibit a high peak to valley ratio. This makes it necessary to use an extremely flat contact surface between the heat source and thermal spreader to guarantee an efficient transfer of heat. Chemical vapor deposition diamond with a thickness of 1000 pm has been used for Multi Chip Modules (MCM) for this purpose. Heat spreaders are used in the electronic industry for IC packaging and solid-state lasers. 

Figure 1. Design of thermal spreaders A - horizontal two-phase thermosyphon with internal PIN-stmcture B - horizontal two-phase heat pipe with internal PIN-structure C - horizontal liquid TS with internal PIN-structure D - horizontal hquid TS with internal PIN-structure...

TRANSPORT PHENOMENA IN TWO-PHASE THERMAL SPREADERS 129 Then, the pressure drop in eapillary stmeture is... 

The geometry of thermal spreader could be optimized to match one of the following criteria ... 

In order to validate the present approach, the experimental sample of two-phase thermal spreader was designed and manufactured for cooling of high thermal power chip placed in the horizontal position. 

Eigure 4 presents design of the experimental sample of thermal spreader. Temperatures were measured in heat input and heat output zones. 

Figure 7. The effect of heat input on the entire temperature drop in the two-phase thermal spreader.

Further studies in the field of geometric optimization for two-phase thermal spreaders ean be elassified into two categories modeling investigations, and validation studies. 

Figure10.2 Longitudinal temperature profiles at autothermal operation for a H2-air mixture of equivalence ratio 0.6 in (a) a 250 pm gap size ceramic-frame microreactor and (b) a 300 pm gap size thin stainless-steel-based frame microreactor, for different thermal spreaders (material indicated) of thickness 3.2 mm adhered to the framework (redrawn from [6, 7]).

Thermal design is often important in an MCM. An MCM will have a higher heat density than the equivalent PCB, sometimes necessitating a more complex thermal solution. If the MCM is dissipating more than 1 W, it is necessary to check if any heat sinks and/or thermal spreaders are necessary. 

In all cooled appliances, the heat from the device s heat sources must first arrive via thermal conduction at the surfaces exposed to the cooling fluid before it can be transferred to the coolant. For example, as shown in Fig. 2.2, it must be conducted from the chip through the lid to the heat sink before it can be discharged to the ambient air. As can be seen, thermal interface materials (TIMs) may be used to facilitate this process. In many cases a heat spreader in the form of a flat plate with high thermal conductivity may be placed between the chip and the lid. 

High thermal conductivity CVD-diamondfilms deposited on heat spreaders or heat slugs to dissipate the heat of high-density integrated circuits. 

A cross-sectional schematic of a monolithic gas sensor system featuring a microhotplate is shown in Fig. 2.2. Its fabrication relies on an industrial CMOS-process with subsequent micromachining steps. Diverse thin-film layers, which can be used for electrical insulation and passivation, are available in the CMOS-process. They are denoted dielectric layers and include several silicon-oxide layers such as the thermal field oxide, the contact oxide and the intermetal oxide as well as a silicon-nitride layer that serves as passivation. All these materials exhibit a characteristically low thermal conductivity, so that a membrane, which consists of only the dielectric layers, provides excellent thermal insulation between the bulk-silicon chip and a heated area. The heated area features a resistive heater, a temperature sensor, and the electrodes that contact the deposited sensitive metal oxide. An additional temperature sensor is integrated close to the circuitry on the bulk chip to monitor the overall chip temperature. The membrane is released by etching away the silicon underneath the dielectric layers. Depending on the micromachining procedure, it is possible to leave a silicon island underneath the heated area. Such an island can serve as a heat spreader and also mechanically stabihzes the membrane. The fabrication process will be explained in more detail in Chap 4. 

Another possibihty to improve the temperature homogeneity is to introduce an additional polysiHcon plate in the membrane center. The thermal conductivity of polysilicon is lower than that of crystalline siHcon but much higher than the thermal conductivity of the dielectric layers, so that the heat conduction across the heated area is increased. Such an additional plate constitutes a heat spreader that can be realized without the use of an electrochemical etch stop technique. Although this device was not fabricated, simulations were performed in order to quantify the possible improvement of the temperature homogeneity. The simulation results of such a microhotplate are plotted in Fig. 4.9. The abbreviations Si to S4 denote the simulated temperatures at the characteristic locations of the temperature sensors. At the location T2, the simulated relative temperature difference is 5%, which corresponds to a temperature gradient of 0.15 °C/pm at 300 °C. 

The most common type of boiler configuration to bum hog-fuel is the spreader stoker type, although some overfeed stokers also exist. Spreader stoker boilers can bum fuel with high moisture content, are relatively easy to operate, and have relatively high thermal efficiency. Overfeed stoked boilers have lower particulate emissions relative to spreader stoker boilers because less combustion occurs in suspension.13... 

Some of the present industrial uses of diamond coatings include cutting tools, optical windows, heat spreaders, acoustic wave filters, flat-panel displays, photomultiplier and microwave power tubes, night vision devices, and sensors. Because its thermal conductivity and electrical insulation qualities are high, diamond is used for heat sinks in x- ray windows, circuit packaging, and high-power electroific devices. Moreover, the high chemical stability and inertness of diamond make it ideal for use in corrosive environments and in prosthetic devices that require biocompatibility. 

Kang S., Tsai S., Ko M., (2004), Metallic micro heat pipe heat spreader fabrication. Applied Thermal Engineering, 24,299 - 309. 

At the chip level, thermal management solutions need high performance heat spreaders to minimize thermal contact resistance ... 

Spreader plate needed to be attached to box here, for thermal reasons... 

Diamond has the highest thermal conductivity when compared to other substances. For type la natural diamond the thermal conductivity is about 2000-4000W/m/K. For type Ila it is up to 17500W/m/K. A major problem in heat transfer is an effective heat sink that depends on the effective contact area, forces between both the materials, and the gap interface. Diamond exhibits the best characteristics for a heat spreader, which is the interface material that transfers heat between heat source and heat sink. 

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