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New White Paper on LED Thermal Management Basics

PCB designers must focus on more than just the circuit board when it comes to LED applications, according to Clemens Lasance. 

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. 

Or so claims a new white paper from Saturn Electronics Corporation entitled Basics of (PCB) Thermal Management for LED Applications written by Clemens Lasance. 

Incorporate the entire system's thermal behavior
In it, Lasance also stresses thermal management's overall importance; delineates heat transfer basics 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 which has traditionally served as an afterthought in later design stages--if at all; asserts the importance of thermal interface materials before illustrating the cause of dubious thermal data currently circulated by vendors and widely accepted in the industry; in turn, Lasance urges that the reader join him in sweeping the jargon term "thermal impedance" into history's dustbin as he seeks to pass a law banning the use of a term that he feels has more than outlived its usefulness. 

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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.

most important LED features

FIGURE 1 shows the most important features of a typical LED-based product

As a result, they'll be addressing key issues linked to the end product (lifetime, color point, and efficiency) which traditionally are not considered in individual part's design phase. 

Instead of limiting predictions to component temperatures, designers can now begin reducing the marked thermal risk associated with the end product. 

Hence, critical temperature determination becomes vital to overall system assessment; for this purpose, a correct understanding of thermal conductivity (k), heat transfer coefficient (h), and the electrothermal analogue--and its derivative thermal resistance (Rth)--is imperative when performing back-of-the-envelope calculations which, in turn, generate the thermally-focused design feasibility assessments. 

Indeed. Designers with a superior thermal conduction, convection and resistance knowledge are more capable of assessing the PCB's thermal requirements than their peers who find themselves lacking 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 for spreadsheet-based calculating tool. 

For the purpose of real-life applications, Lasance provides concrete examples such as determining the 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; a given light output with a prescribed LED (Luxeon Rebel in this case); a prescribed Metal Core Printed 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. 

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Click here for LED Thermal Management White Paper 

Consequently, the data reveals that the LED itself is the culprit; thus, the designer has the option of choosing 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's no early guidelines for initial determination. 

However, understanding heat spreading physics provides the capability to surmise clever approximations based on single and multiple sourcing; in turn, these capabilities distinguish specific approaches for optimizing heat spreading calculations. 

With this in mind, Lasance addresses heat spreading's effect on simple LED applications--once again, exemplifying definitions through seamless deliberation, examples and equations. 

Moving on to the TIMs contribution in LED thermal management, Lasance concludes that there is much 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 test method. Since the vendor cannot possibly know the application, the designer is responsible for appropriate specifications; simultaneously, reliable vendor data depends on interpreting the customer'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 use it as shorthand for unit area thermal resistance; thus, violating the electrothermal analogy. 

Defining TIMs performance requires a reciprocal conversation based on universally-accepted terminology and used by all. 

Providing two specific reasons and their subsequent effects, he ultimately proposes a solution successful in unifying the building field before the white paper culminates with the proposal for a new law expressly forbidding any further use of this term. 

LED PCB Resource Center
This white paper can be found in SEC's 
Metal Core / LED PCB Resource Center which also includes an on-demand webinarfabrication notesmaterials comparison charts, and calculators all tailored to assisting the OEM, Designer, and Assembler in not only the fabrication of the bare board for LED Applications but the entire supply chain to include the end product. 

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