THERMODYNAMIC IN ELECTRONICS THERMAL MANAGEMENT IN ELECTRONIC HEAT SINK

HEAT SINK SELECTION transistor computer LED Water heat sink

WHAT IS HEAT SINK? Heat sinks are devices that enhance heat dissipation from a hot surface, usually the case of a heat generating component, to a cooler ambient, usually air. For the following discussions, air is assumed to be the cooling fluid. In most situations, heat transfer across the interface between the solid surface and the coolant air is the least efficient within the system, and the solid-air interface represents the greatest barrier for heat dissipation. The primary purpose of a heat sink is to maintain the device temperature below the maximum allowable temperature specified by the device manufacturers.

CATEGORIES OF HEAT SINK Passive Heat Sinks Used in natural convection applications or in applications where heat dissipation is not dependent on designated supply of air flows http://www.dansdata.com/images/c3ezra/viasink220.jpg http://www.amatteroffax.com/images/inventoryimages/691916.JPG

CATEGORIES OF HEAT SINK Active Heat Sinks Fans are designated for its own use Reliability is dependent on moving parts One big negative: if your fan dies, you need to replace the entire unit http://cache-www.intel.com/cd/00/00/14/97/149748_149748.jpg http://www.newegg.com/Product/ProductList.asp?Brand=1647&N=2010110062+50001647&Submit=ENE&Manufactory=1647&SubCategory=62

CATEGORIES OF HEAT SINK Semi-Active Heat Sinks - Leverage off of existing fans in the system - Usually a passive heat sink set in front of a fan to produce impingement or vertical flow

TYPES OF HEAT SINK Stampings Casting Copper or aluminum sheets stamped into desired shapes Low cost solution to low density thermal problems Casting High density aluminum or copper/bronze pin fins are produced with sand, lost core, or die casting (May or may not be vacuum assisted) Allows for maximum performance with impingement cooling More expensive but can make odd shapes Al alloys used have lower conductivity than Al alloys used for extrusions

TYPES OF HEAT SINK Extrusions Allows for the formation of elaborate 2-D shapes capable of dissipating large heat loads Cross-cutting to produce rectangular pins may increase performance by 10-20% but the extrusion rate will be slower Design limits are usually set by Fin height-to-gap aspect ratio Minimum fin thickness-to-height Maximum base to fin thickness

TYPES OF HEAT SINK Bonded/Fabricated Fins Thermally conductive aluminum filled epoxy is used to bond planar fins on a grooved extrusion plate Greater fin height-to-gap aspect ratio is achieved with this method. Cooling capacity is increased without increasing volume requirements But epoxy has a much lower thermal conductivity, acting as an increased resistance.

HEAT SINK GEOMETRIES Folded Fins Aluminum or copper sheet metal is folded into fins and then attached to a base plate or directly to the heat surface via brazing or epoxying Due to the availability and fin efficiency folded fins are not suitable for high profile heat sinks. Allows for the fabrication of high performance heat sinks when extrusion or bonded fins are unacceptable

HEAT SINK GEOMETRIES Rectangular Fins Rectangular fins also have better performance than round fins; pressure drop is also higher for rectangular fins. Round Fins - offer highest performance at higher values

HEAT SINK MATERIALS & FINISHES Aluminum (6063 or 6061) is most common, followed by copper (which is 4-6x more expensive, 3x as heavy, by has 2x the conductivity) It is difficult to alter the surface of copper to improve radiation. External finish of aluminum is usually anodize or chromate of various colors.

EXAMPLE Thermal Circuit

Notations and definitions of the terms are: Q = total power or rate of heat dissipation in W, represent the rate of heat dissipated by the electronic component during operation. For the purpose of selecting a heat sink, the maximum operating power dissipation issued. Tj = maximum junction temperature of the device in °C. Allowable Tj values range from 115°C in typical microelectronics applications to as high as 180°C for some electronic control devices.

Tc = case temperature of the device in °C Tc = case temperature of the device in °C. Since the case temperature of a device depends on the location of measurement, itusually represent the maximum local temperature of the case. Ts = sink temperature in °C. Again, this represents the maximum temperature of a heat sink at the location closest to the device. Ta = ambient air temperature in °C.

Where ∆T is the temperature difference between Using temperatures and the rate of heat dissipation, a quantitative measure of heat transfer efficiency across two locations of a thermal component can be expressed in terms of thermal resistance R, defined as: R = ∆T/Q Where ∆T is the temperature difference between the two locations. The unit of thermal resistance is in °C/W, indicating the temperature rise per unit rate of heat dissipation. This thermal resistance is analogous to the electrical resistance Re, given by Ohm’s law: Re = V/I With V being the voltage difference and I the current.

The thermal resistance between the junction and the case of a device is defined as: Rjc = (∆Tjc)/Q = (Tj- Tc)/Q This resistance is specified by the device manufacturer. Although the Rjc value of a give device depends on how and where the cooling mechanism is employed over the package, it is usually given as a constant value. It is also accepted that Rjc is beyond the user’s ability to alter or control.

Similarly, case-to-sink resistance are defined as: Rcs = (∆Tcs)/Q = (Tc- Ts)/Q Here, Rcs represents the thermal resistance across the interface between the case and the heat sink and is often called the interface resistance. This value can be improved substantially depending on the quality of mating surface finish and/or the choice of interface material.

Sink-to-ambient resistance are defined as: Rsa = (∆Tsa)/Q = (Ts- Ta)/Q Where Rsa is heat sink thermal resistance. Obviously, the total junction-to-ambient resistance is the sum of all three resistances: Rja = Rjc + Rcs + Rsa= (Tj – Ta)/Q

Rsa = ((Ts – Ta)/Q) – Rjc- Rcs HEAT SINK SELECTION FIRST STEP: Determine heat sink thermal resistance Determine the heat sink thermal resistance required to satisfy the thermal criteria of the component. By rearranging the previous equation, the heat sink resistance can be easily obtained as: Rsa = ((Ts – Ta)/Q) – Rjc- Rcs Tj, Q and Rjc are provided by the device manufacturer, and Ta and Rcs are the user defined parameters. The ambient air temperature Ta for cooling electronic equipment depends on the operating environment in which the component is expected to be used.

Typically, Ta ranges from 35 to 45°C, if the external air is used, and from 50 to 60°C, if the component is enclosed or is placed in a wake of another heat generating equipment. The interface resistance Rcs depends on the surface finish, flatness, applied mounting pressure, contact area and, of course, the type interface material and its thickness. In selecting an appropriate heat sink that meets the required thermal criteria, one needs to examine various parameters that affect not only the heat sink performance itself, but also the overall performance of the system. The choice of a particular type of heat sink depends largely to the thermal budget allowed for the heat sink and external conditions surrounding the heat sink.

When selecting a heat sink, it is necessary to classify the air flow as natural, low flow mixed, or high flow forced convection. Natural convection occurs when there is no externally induced flow and heat transfer relies solely on the free buoyant flow of air surrounding the heat sink. Forced convection occurs when the flow of air is induced by mechanical means, usually a fan or blower.

DESIGN CONSIDERATION The following cases always increase thermal performance TRUE OR FALSE???? Longer fin heights Longer heat sinks in the direction of air flow Increased number of fins

All of the cases are FALSE Longer fin heights mean increased surface area but with a fixed volumetric flow rate performance may actually decrease with fin height Longer heat sinks in the direction of flow and more fins both mean increased surface area but both have adverse affects on pressure drops and flow bypass, and the average heat transfer coefficient goes down

HEAT SINK ATTACHMENT Clip to device Clip to PCB Snap-on stampings Double-sided tapes Solder or adhesives Thermal interface resistance minimized using grease or pad and by making surfaces as flat as possible