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Basic Thermal Management of LED Applications for PCB Designers

Understanding and subsequently predicting thermal behavior in PCB design for LED applications requires identifying the relationship between additional aspects such as thermal interfaces, heat sinks, luminaires and the LED itself.

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Basic Thermal Management of LED Applications for PCB Designers

  1. 1. Free White Paper Incorporates Thermal Behavior of Entire System for LED PCB DesignUnderstanding and subsequently predicting thermal behavior in PCB design for LED applicationsrequires identifying the relationship between additional aspects such as thermal interfaces, heatsinks, luminaires and the LED itself.Or so claims a new white paper from Saturn Electronics Corporation entitled Basics of (PCB) ThermalManagement for LED Applications written by Clemens Lasance.In it, Lasance also stresses thermal management’s overall importance; delineates heat transferbasics for applied use; introduces a new calculator promising to alleviate front-end design problems;examines the vital heat-spreading process, which deserves mindshare from the very start but whichhas traditionally served as an afterthought in later design stage—if at all; asserts the importance ofthermal interface materials before illustrating the cause of dubious thermal data currently circulatedby vendors and widely accepted in the industry; in turn, Lasance urges that the reader join him insweeping the jargon term “thermal impedance” into history’s dustbin as he seeks to pass a lawbanning the use of a term that he feels has more than outlived its usefulness.Although LED applications designer are designated responsibility of a single part (PCB, heat sink,etc.), this does not constitute their full responsibilities.Designers must address the entire system’s thermal behavior. Figure 1 shows the most important features of a typical LED-based productFigure 1 Typical LED-based productAs a result, they’ll be addressing key issues linked to the end product (lifetime, color point, andefficiency) which traditionally are not considered in individual part’s design phase.Instead of limiting predictions to component temperatures, designers can now begin reducing themarked thermal risk associated with the end product.Hence, critical temperature determination becomes vital to overall system assessment; for thispurpose, a correct understanding of thermal conductivity (k), heat transfer coefficient (h), and theelectrothermal analogue--and its derivative thermal resistance (Rth)--is imperative when performing
  2. 2. back-of-the-envelope calculations which, in turn, generate the thermally-focused design feasibilityassessments.Indeed. Designers with a superior thermal conduction, convection and resistance knowledge aremore capable of assessing the PCB’s thermal requirements than their peers who find themselveslacking in these thermal basics.Lasance then defines and exemplifies these thermal basics responsible for determining heat sink,convection mode, and dielectric (or PCB) enabling designers to identify the input needed forspreadsheet-based calculating tool.For the purpose of real-life applications, Lasance provides concrete examples such as determiningthe ideal heat sink and necessary convection code for dissipation of a 5W LED.For example, assume a designer is trying to solve for the following: 10cm2 PCB area per LED; agiven light output with a prescribed LED (Luxeon Rebel in this case); a prescribed Metal CorePrinted Circuit Board (MCPCB); and a prescribed thermal interface material. According to Lasance,even with ideal heat sink and liquid cooling (Rth = 0) the required 5W cannot be reached.Consequently, the data reveals that the LED itself is the culprit; thus, the designer has the option ofchoosing an LED with a (much) smaller thermal resistance or using two LEDs.Next, Lasance addresses heat spreading’s role by asserting that it is no trivial issue.Although Designers should be privy from the get-go as to the role heat spreading will play, there’sno early guidelines for initial determination.However, understanding heat spreading physics provides the capability to surmise cleverapproximations based on single and multiple sourcing; in turn, these capabilities distinguish specificapproaches for optimizing heat spreading calculations.With this in mind, Lasance addresses heat spreading’s effect on simple LED applications--onceagain, exemplifying definitions through seamless deliberation, examples and equations.Moving on to the TIMs contribution in LED thermal management, Lasance concludes that there ismuch dubious information that has spread industry-wide regarding thermal interface materials.First of all, he notes the difficulty of reproducing operation contact resistances in a standardized testmethod. Since the vendor cannot possibly know the application, the designer is responsible forappropriate specifications; simultaneously, reliable vendor data depends on interpreting thecustomer’s minimum value requirements, and only when a certain pressure is provided.Finally, Lasance tees off on what has seemingly become his personal mission: eliminating the term“thermal impedance” from industry jargon.Lasance blames unscrupulous vendors for exacerbating this meaningless word by claiming they useit as shorthand for unit area thermal resistance; thus, violating the electrothermal analogy.Defining TIMs performance requires a reciprocal conversation based on universally-acceptedterminology and used by all.Providing two specific reasons and their subsequent effects, he ultimately proposes a solutionsuccessful in unifying the building field before the white paper culminates with the proposal for anew law expressly forbidding any further use of this term.This white paper can be found in SEC’s Metal Core / LED PCB Resource Center which also includes an on-demand webinar, fabrication notes, materials comparison charts, and calculators all tailored toassisting the OEM, Designer, and Assembler in not only the fabrication of the bare board for LEDApplications but the entire supply chain to include the end product.Click here to get your free copy of White Paper