pcb thermal management - kbl circuits thermal-management.pdf · why is the issue of “pcb...

PCB Thermal Managementin the field of LEDs

Albert Schweitzer16.01.2015 Vers. 1.1

Table of Content

Content

LEDs and the influence of heat

Three types of heat transfer

Thermal conductivity and thermal resistance

Basic laws of heat conduction

Thermal management in the field of PCBs

New materials used in the future

Why is the issue of “PCB Heat-Management” in the field of LEDs important?

Although LEDs have a higher efficiency than incandescent lamps, about 80% oftheir electrical power input nevertheless amounts to the heat loss.

Due to the temperature-sensitive light-emitting chip surface area, a heatdissipation and reduction of heat loss isessential for use of LEDs as light sources.

Temperature characteristics of LED Technology

Approximately 80% of the supplied electrical energy is dissipated through the LED package as heat, mainly generated in PN-junction area of the LED chip and about 20% is converted into visible light. As the junction temperature of an LED is increased, both the forward voltage, the lumen output and lifetime of the LED chip decrease. The output wavelength also shifts with a change in junction temperature.

Light

Power

Heat

PN-Junction

LED Chip

LED Package

PCB

Heat Sink

Energy Conversation of Light Sources

Light Sources LED Fluorescent Metal IncandescentTube Halid

(Halogen)

Visible Light 20-30% 21% 27% 8%

Infrared Component ~ 0% 37% 17% 73%

UV Component ~ 0% ~ 0% 19% ~ 0%

Radiation Energy 20-30% 58% 63% 81%

Residual Heat 70-80% 42% 37% 19%

Cause of Electronic Component Failures)In the field of Automotive(

6%

19%

20%

55%

Schmutz

Luftfeuchtigkeit

Vibration

Temperatur

6%

Dirt

19%

HumidityVibration

20%

Heat

55%

What happens in case a LED becomes overheated?

There are mainly three negative effects in case ofoperating a LED outside of LED-manufacturersspecifications. Most important parameter in the fieldof heat is the “Tj ” junction temperature.

� Decrease of lifetime

� Decrease of lumen output = reduction of brightness

� Shift of output wavelength

The lifetime of LEDs decrease of 50% in casethe junction temperature „Tj“ increase of 20°C.

Dependence oflifetime on the junctionttemperature andsolder pointtemperature.

Source: Osram

How does the temperature influence the lifetime of LEDs?

For instance, at tj = 85 °C, the decrease in light output already amountsto 15% in comparison to the starting value provided in the data sheetfor tj = 25 °C.

Dependence of brightness(lumen output) on the junctionttemperature.

Source: Panasonic

How does the temperature influence the brightness of LEDs?

How does the temperature influence the output wavelength of LEDs?

Typical color- and brightness-changes within a range oftemperature between 5°C and 70°C. (If 350mA)

Source: Osram

The consequences of effective thermal design

Given an identical luminaire casingtemperature in bothcases, reducingthe temperature at thetc/tp point from 105 °Cto 46 °C serves toincrease service lifeto more than 60,000hours and brightnessby 17%.

Source: Panasonic

Heat can only be discharged from a warmerto a cooler material. Three different processes are responsible for transferring heat:

• Radiation

• Convection

• Conduction

Processes for transferring heat

General: Types of heat transfer


Radiation

Convection

Conduction

Transport of heat by Radiation:

Heat is transported via electromagneticradiation Wλ, mainly due to the emission of photons, without needing a medium. Radiationalso functions in and through a perfect vacuum.A heat sink or LED housing radiates heat in theform of infrared (IR) radiation.


4 4


Q = heat transfer per unit time [W]A = area of the emitting body [m2]σ = 5.6703 10-8 [W/m2K4] - The Stefan-Boltzmann ConstantTh = hot body absolute temperature [K]Tc = cold surroundings absolute temperature [K]

Transport of heat by Radiation:

[W]

ε = emission coefficient = factor between 0 and 1depending on the surface finish of the heat sink


Transport of heat by Radiation :The emissivity of the surface of amaterial is itseffectiveness inemitting energy asthermal radiation.

Color and emissivity are notrelated!

Examples for Emission Coefficient Ɛ

Aluminium, polished 0,04

Aluminium, untreated 0,09

Mild Steel / Stainless Steel 0,20

Copper, polished 0,04

Ceramic, grey 0,90

Matt Black coated Surface 0,97

Marble, polished 0,92

Asphalt 0,88

Glass smooth, uncoated 0,95

Water pure 0,96

Radiation of heat sinks:


Heat is radiated mainly from the outer surfaces of a finned heat sink because the spaces between the fins are normally too narrow to permit radiant heat to escape and heat exchange by radiation consequently takes place only between the surface of the opposing fins.

Transport of heat by Convection:


Combination of heat conduction and heat transfer via a moving medium like air that transports heated particles to cooler regions, e.g. a heat sink through which ambient air circulates.



Rth , �� =

�

�

[�]

[�]

Rth = thermal resistance� = thermal transfer coefficient, temperature-independent [W/m² K]A = effective surface [m²]

The thermal resistance for the heat transferby convection is dependent on the size ofthe surface and the flow conditions.


Examples for �:


Media α [W/m²K] ConditionAir (3) ..5..(10) Free convectionAir ….50 Forced flow, the more the better

Water 500…. Free convectionWater …5000 Forced flow, the more the better

Improvement of Convection by profiledsurfaces:


In profiled surfaces with cooling fins, and forced convection by fans, improves the heat transfer by convection to a multiple thereof, depending on the increase in surface area or increasing the heat transfer coefficient.

Ratio of Radiation to Convection:


Convection and radiation play for the heat transfer approximately the same role, on condition that:

� The surfaces are got heat emitters(lacquered, coated etc.)

� The surfaces are flat� Allowing the air to circulate freely

Transport of heat by Conduction:


Conduction is the transfer of heat between substances that are in direct contact with each other. The better the conductor, the more rapidly heat will be transferred.

Following we will mainly deal with heatTransfer by Conduction.

How can the negative influence of heat can be avoided?

� Metal-Core PCBs - MCPCB� Copper-Inlay� Thermal-Vias� Heavy Copper PCBs� Special Materials� Combination of different Measures

Basic PCB technology concepts to avoidor reduce temperature problems in electroniccircuits. :

MCPCB:= Metal-Core Printed Circuit Board

Thermal Management by MCPCB

MCPCB is the generic term for all typesof printed circuit boards with one ormore metal layer or a metal core. Alsoincludes IMS PCBs (Insulated metalsubstrate).

Thermal Management by IMS PCB

IMS (Insulated Metallic Substrates) are state

of the art and widely spread for thermal

issues in electronic systems. An IMS consists

of a metallic base material (mostly aluminum

or copper) with a thickness of about 0.5 mm

to 3.0 mm.

Basic Structure of MCPCBs

Metal - Subtrat (Kupfer oder Aluminium)

Dielectric Layer

Heat SourceCopper Layer

Solder Mask

TIM:= Thermal Interface Material = Dielectric Layer

Required characteristics of thermalInterfaces:

� Excellent thermal conductivity� Low thermal resistance� High dielectric strength

The thermal interface TIM

Thermal Conductivity (λ, k oder κ) is a measure of the ability of a material to allow the flow of heat from its warmer surface through the material to its colder surface

The thermal conductivity is the ability of a substance to transport thermal energy byconduction.

Definition Thermal Conductivity

The Thermal Conductivity Lambda is a temperature-independent material constant:

The physical unit of thermal conductivity is given by:

[J/(m·s·K)] with the proviso: [J/s=W] and thus:

Definition Thermal Conductivity

The Thermal Resistance describes the inability totransfer heat; the specific heat resistance is a material constant (reciprocal of the Thermal Conductivity) and counteracts the heat conduction.

Unit of Thermal Resistance:

Definition Thermal Resistance

Rth = �

� ∗ A =

�� ℎ�!��""

� ∗ Area

K

�

Basic laws of heat conduction

the larger the cross-sectional-area is

the shorter the heat path is, and

the better the thermal conductivity is

The heat conduction is better:

The larger the value of Lambda the better is the heat conductivity.

Values below 0.8 W / m * Kthe substance is consideredas heat insulator.

Thermal Conductivity of some important elements/materials in the field of PCBs.

Material �

[W/m*K]

Copper ~ 400Silver ~ 430Aluminium ~ 240Gold ~ 310Tin ~ 70Nickel ~ 85FR4 ~ 0,2TIM Materials ~ 1 -5

This manufacturer‘s list is not exhaustive.

Some important Manufacturer of TIM Materials.

Manufacturer Product Name Arlon 92MCL Bergquist HAT-07006 Denka Hittplate EL-1 DuPont LX03517016 Isola 370HR-106 Iteq IT-859GTA Laird T-lam SS HTD04 Nanya NPMRA Shenyi ST115 Taconic TacLED-20 Totking T112 Ventec V-4A1

Thermally Conductive Materials used by Fine Line suppliers

PermittivityDielectric Thermal Dielectric Dielectric Glass PeelThickness Conductivity Constant Breakdown Transition Strength

Manufacturer Part Number [mil/µm] [W/mK] @ 1Khz/1MHz [kVAC] [°C] [lb/in] [N/mm]Bergquist HT-04503 3/76 2.2 7 6.0 150 6 / 1.1

HT-07006 6/152 2.2 7 11 150 6 / 1.1MP-06503 3/76 1.3 6 8.5 90 9 / 1.6HPL-03015 1.5/38 3 6 2.5 185 5 / 0.9

ITEQ IT-859GTA (V2) 3/76* 1.5 4.8 4.0 105 6 / 1.1IT-859GTA (V3) 3/76* 3 4.8 4.0 105 6 / 1.1

Laird Tlam SS 1KA04 4/102 3 4,3 1.8 105 4,5 / 0.8Tlam SS 1KA06 6/152 3 4.3 2.5 105 4,5 / 0.8Tlam SS 1KA08 8/203 3 4.3 3.5 105 4,5 / 0.8

Polytronics TCB-2L 4/100* 2 4.6 5.0 130 10 / 1.8TCB-2 4/100* 3 4.8 5.0 130 10 / 1.8TCB-4 4/100* 4 4.9 4.5 140 10 / 1.8TCB-8 4/100* 8 5.2 4.5 140 10 / 1.8

Sheng YI SAR 15 4/100* 1.5 5.2 6.0 140 11.5 / 2.1Totking T-110 4/100* 1.5 4.6 6.0 100 11 / 2.0

T-111 4/100* 2 5.2 6.0 100 10 / 1.8T-112 4/100* 3 5.2 6.0 100 9 / 1.6T-113 4/100* 1.8 5.2 6.0 170 7 / 1.3T-311 4/100* 2 5.2 6.0 100 10 / 1.8T-411 4/100* 2 5.2 6.0 100 10 / 1.8T-511 4/100* 3 5.2 6.0 100 10 / 1.8

Ventec VT-44A 4/100* 1 5.0 1.5 130 8 / 1.4VT-4A1 4/100* 1.6 5.0 1.5 130 8.5 / 1.5VT-4A2 4/100* 2.2 5.1 1.5 130 7.5 / 1.4VT-4A3 4/100* 3 4.9 1.5 130 6 / 1.1

*) Dielectric Thickness in various thicknesses available

View of a original Data Sheet

Extract from the Data Sheet: Laird T-lam SS 1KA04

THERMAL PROPERTIES UNITS Tlam SS 1KA04

Thermal Conductiviy W/m*K 3

Thermal Resistance °C-in2/W 0,05

°C-cm2/W 0,34

Glass Transition Temperature °C 105

Operating Temperature Maximum °C 110

Soldering Temperature Maximum °C 288

ELECTRICAL PROPERTIES UNITS Tlam SS 1KA04

Dielectric Contant @ 1KHz/1MHz 4,3 / 4,1

Dielectric Contant Strength VAC/mil (KVAC/mm) 650 (25.6)

Withstand Voltage VDC 1800

PCB Design with respect to thermal characteristics

Basic Measures to minimize Tj:

� Selection of appropriate Material� Optimization of copper area� Size and position of LED Pitchs� Use of heat sinks� Thickness of copper� Thermal Vias

How to measure the Tj temperature?

Determining of the temperature Tj by the followingmethods:

A) Measurement by using a thermocouple at the solder pad of LED cathode.

B) Determining by measuring the forward voltage VF

How to measure the Tj temperature?

Determining by measuring the forward voltage VF

and have a look in LED Data sheet:

Source: Nichia

Measurement with pulsed current: pulsemodulation10ms, Tactile 1:10

PCB Design based on Cree LED XH-G

Keep the pad size as big as possible:

Source: Cree

PCB Design based on Cirrus Logic AN315

More copper layers increase the thermal resistance:

Source: CirrusLogic

2 Layer 4 Layer

PCB Design based on Nichia LED NS6W183

Following, based on Nichia LED NS6W183, some investigations thermal design investigations like:

�Thermal cross talk�Pitch size�Copper thickness�Thermal Via size�Thermal Via count

Source: Nichia


Results of thermal measurements:

Source: Nichia


Results on thermal measurements:

Source: Nichia


Relationship between the thermal resistance and copper thickness, considering the area ofcopper.

Source: TI

Thermal Via Applications

Source: Viasystems


Source: Cree

Filles vs. unfilled Vias

Quelle: Cirrus Logic

Filled vias havebeen found to yieldonly modestperformance gainsfor applications inwhich the power dissipated in thedevice is less than5 W. Also the costincrease with thisprocess can beunacceptable for cost-sensitive designs.

Design Recommendation for Via Cluster

Optimal dimensions for a thermal Via cluster:

Pitch 0.5mmDiameter final hole 0.25mmcopper thickness 25µm

Use of „Carbon Composite Material“

New PCB materials for future use

Quelle: Stablcor

Carbon Composite Material:

More information you find on the following webside: www.stablcor.com)


Nano-Ceramic Material:


Quelle: Nanotherm

Nano-Ceramic Material:


More Information: Cambridge Nanotherm Ltd | www.camnano.com

pcb thermal management - kbl circuits thermal-management.pdf · why is the issue of “pcb...

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