Lecture 7. Lecture 7. Characterization of Components. Cooling at Component Level. Cooling at Component Level. Cooling at Component Level

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1 Characterization of Components Hans Jonsson Heat flow from chip to heat sink - Thermal Component Models Interface resistance (Contact resistance) TIM Thermal Interface Materials Project assignment The active part of the component is the silicon chip which is much smaller than the component shell. The chip temperature should be held below a certain temperature level (around 100 C, varies between C depending on the application and the type of component). Older type of component where the signals were transferred to the PCB by lead frames BGA (Ball Grid Array) - Modern components have a large number of balls (around 600) underneath to transfer the signals to the PCB. Power amplifiers are often mounted on a metal plate to enable mounting on a heat sink. 1

2 How do you model a component thermally? Many models and possibilities exist, but. Simple model less general and harder to define. On the other hand, complex models are general but very hard to develop. The JEDEC model is one of the models that are used in industry. The DELPHI model was developed in a large joint european research project in the early ies. The goal was to be able to use the model in commercial computational tools. Exam problem Heat flow from chip to heat sink -PCB - Thermal Component Models Interface resistance (Contact resistance) TIM Thermal Interface Materials 2

3 The heat flow between two seperate layers in thermal contact with each other is dependent on the surface roughness and how plane the surfaces are. Surfaces that look smooth often have considerable microscopic roughness If the two layers are pressed together, a large amount of air will be captured in the enclosures and lead to poor thermal conductivity. This gives rise to a temperature difference between the two layers that theoretically should not exist if the surfaces were smooth and plane. This temperature difference can be considerable! It is important to minimize this temperature difference, and to take into account the remaining part in the thermal design. The air pockets (enclosures) can be eliminated by the introduction of a thermal interface material (TIM). By only apply thermal grease, the temperature difference can be reduced by a factor 5 compared to a case with no TIM. Contact resistance is also dependent on the contact pressure. 3

4 How to Determine? Consider a case where we shall connect a silicon chip with an aluminium cold plate (t Q& = bottom The total resistance from top to bottom can be written: tot Silicon top Interface t R R = R + R + R tot ) A Alu min ium How to Determine? Resistance at the interface: R interface : R = R + R + R Interface Si,contact The thermal resistance for heat conduction through a material can be written: R = The Interface resistance can now be expressed: R Interface = RSi,contact + R TIM + R Al,contact = = R contact + TIM TIM Al,contact How to Determine? How to Determine? The total thermal resistance from top to bottom, R tot can now be expressed: R tot = Silicon + R contact + TIM + λ A Al Heat flow from chip to heat sink -PCB - Thermal Component Models Interface resistance (Contact resistance) Thermal Interface Materials TIMs - additives By choosing TIMs containing small balls of conductive material, the thermal resistance can be decreased. Material used for increase in thermal conductivity in TIM are: - Aluminium oxide - Magnesium oxide - Aluminium nitride - Boron nitride - Diamond powder - Silver 4

5 Types of TIMs Thermal grease Pads Tape Phase changing material (PCM) Gel Glue Brazing Thermal grease Consists of silicon or hydrocarbon oils with different additives, R = K cm²/w. - good heat conduction - eliminates air pockets since the viscosity goes down at high temperatures and thereby fills air gaps (enclosures) - Messy - Spring load to ensure good contact pressure (300 kpa) - migration of the grease at temperature cycling (pumpout) Pump-out Pump-out Pads Consists of silicone rubber reinforced by a glass fiber mesh. Thickness ca 0.25 mm, R = 1 3 K cm²/w. - easy to handle not messy, and are manufactured in sizes that match the component size. - No pump-out! - Spring load (or screws) to ensure good contact pressure (700 kpa) - high thermal resistance limits their applicability. Tape Pressure Sensitive Adhesive on a polyamide film, a glass fiber mesh or an aluminium foil. Used primaily when you want the tape to join the two parts. R = 1 4 K cm²/w. - easy to handle - does not need any spring load once mounted. - high thermal resistance limit the application. 5

6 Phase Changing Materials PCMs Wax with a melting point of C. In practice, the material has good thermal conductivity in both solid and liquid state. R = K cm²/w. - easy to handle solid state when they are mounted. - Contact pressure (300 kpa) needed for good contact in liquid form. - cannot be disassembled Gel Acts like paste, but solidifies after a certain time and thereby eliminates the pump-out effect. R = K cm²/w. (- Contact pressure (300 kpa) needed for good contact in liquid form). Glue Epoxy or silicon based adhesives with different additives. Give very strong mechanical bond, and only need a small thickness. R = K cm²/w. - good mechanical contact - Cannot be disassembled requires reliable electronics Brazing A metallic interface material (solder) with very good thermal conductivity, R < 0.05 K cm²/w. - requires high temperature at mounting, and can therefore not be used generally - cannot be disassembled. Further Reading on TIM 6