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.

1. Free White Paper Incorporates Thermal Behavior of Entire System for LED PCB Design
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.
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 stage—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.
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 product
Figure 1 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

2. 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.
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.
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 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.
Click here to get your free copy of White Paper