Heat spreader thickness

Numerical simulations of the junction temperature increase of a laser diode array have been performed for a package configuration as illustrated in Fig. 33. Calculations have been performed with and without a CVD diamond heat-spreader. The decrease in thermal resistance has been calculated for different values of the heat-spreader thickness, depth and thermal conductivity. The simulations have been performed for an array of laser diodes which are individually 200 pm wide... 

Figures 34 and 35 show that the thermal resistance decreases as a function of heat-spreader thickness and depth. Heat dissipation through the rear (or depth) of the heat spreader is important, because in an array of diodes there is thermal cross-talk between junctions in which the temperature of one jimction is affected by that of adjacent junctions. Figure 34 shows that heat-spreader depths between 2 and 3 mm are required for a thickness of 300 pm. Also, because of thermal cross-talk, the...

A more exact spreading angle model uses 26.6° for cases when the ratio of heat-spreader thickness to heat-dissipating-element side is less than 2 [13]. 

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. 

Assuming Mi = Ri, M2 = V or H, where R, Fi, and H are entire thermal resistance in the main heat spreader s volume, and its real volume or its height (thickness), the key issue of the method appears as determination... 

Figure 34. Thermal resistance decrease as a function of the thickness of the CVD diamond heat spreader for the configuration shown in Fig. 33.

The analysis discussed above is an ideal case in which the effects of intermetalic layers is ignored. If the heat spreader is not very flat there will be differences in metal layer thickness between different sections of the array, leading to a non-uniform temperature distribution. This effect is thought to account for differences in thermal resistance observed between different heat-spreaders [79]. It is estimated [80] that the required flatness should be better than 1 pmcm . 

The thickness of a single piece Ud is typically less than that of the heat spreader/stiffener combination. Changing the stiffness of the package could alter its out-of-plane warpage, which... 

The medium is prepared on a master form, consisting of a heavy fabric belt, surfaced on one side with a layer of rubber filled with small round pits imiformly spaced. These pits are 0.020 in. deep, and the number per unit area and their surface diameter determine the porosity of the sheet. A thin layer of latex is fed to the moving belt by a spreader bar so that the latex completely covers the pits, yet does not run into them. This process traps air in each pit. The application of heat to the under-surface of the blanket by a steam plate causes the air to expand, blowing little bubbles in the film of latex. When the bubbles burst, small holes are left, corresponding to the pits. The blown rubber film, after drying, is cooled and the process repeated until the desired thickness of sheet is obtained. The sheet is then stripped off of the master blanket and vulcanized. 

The purpose of the spreader or torpedo is to reduce the thickness of the material to facilitate heat transfer. Heated torpedoes are available in some machines to further increase the heat transferred to the material as it passes between the torpedo and cylinder wall. Materials with high thermal conductivities can benefit from heated torpedoes. 

Variations on these machines include installing a heated torpedo or spreader within the cylinder to provide two side heat transfer and minimize the thickness through which the heat must be transferred. This in turn improves the temperature uniformity in a shorter period of time (Fig. 10.2). 

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