The document discusses the fundamentals of external thermal resistance in relation to LED substrates, detailing various substrate technologies, including FR4 PCB and metal core PCB, and their thermal conductivity characteristics. It also highlights design considerations such as enlarged solder pads and thermal vias to improve heat transfer and manage junction temperatures. Furthermore, it emphasizes the impact of these factors on LED performance and the importance of careful design to avoid potential issues.

1. LED Fundamentals
External Thermal
Resistance –
Substrates
01-03-2012

2. External Thermal Resistance Rth SB
PCB OPTIONS FOR SMT
TJunction LEDS
FR4 PCB
RthJS LED
TSolder Point
Metal Core PCB
RthSB Substrate Technology
TBoard
Heat Sink Flexible Circuit PCB
RthBA
Ambient
LED Fundamentals | External Thermal Resistance - Substrates| Page 2

3. Enlarged Solder Pads
Influence of Increasing Solder Pad Area
▲Simplest method to
distribute heat
▲No additional costs
▼Low component density
LED Fundamentals | External Thermal Resistance - Substrates| Page 3

4. Thermal Vias
Design Parameters for Thermal Vias
Diameter
Pitch
Cu wall thickness
▲Simple way to increase heat
y
transfer in the vertical direction
▲Low cost
▼Effect of soldering wicking for
g g
open plated vias
Plated through hole Thermal vias filled
thermal vias
th l i with solder paste
ith ld t
LED Fundamentals | External Thermal Resistance - Substrates| Page 4

5. Thermal Vias
effective vertical thermal conductivity
(0,8mm FR4 board, 25µm Cu plating in vias)
120.0
100.0
via diameter
100.0E-6
W/mK]
80.0 200.0E-6
thermal conductivity [W
300.0E 6
300 0E-6
400.0E-6
500.0E-6
60.0 700.0E-6
1.0E-3
40.0
20.0
20 0
-
000 0 0
000.0E+0 200.0E-6
00 0 6 400.0E-6
00 0 6 600 0 6
600.0E-6 800 0 6
800.0E-6 1.0E-3
0 3
via pitch [m]
LED Fundamentals | External Thermal Resistance - Substrates| Page 5

6. Metal Core Printed Circuit Board
Circuit Layer Copper ~ 400W/mK
Dielectric
Di l t i Polymer
P l 0.9 W/mK
0 9 W/ K – 3 W/ K
W/mK
Base Plate Aluminum or Copper 150 W/mK – 400 W/mK
▲Good in plane and through plane
conduction
▼Costs higher than FR4
▼Mostly single sided designs
LED Fundamentals | External Thermal Resistance - Substrates| Page 6

7. PCB heat conduction
FR4 with maximized
ith i i d 2 sided
2-sided FR4
MCPCB Single sided FR4 with thermal vias
solder pads
Aluminium Plate t = 1.5 mm FR4 PCB t = 0.8 mm
Maximized Solder Pads
Solder Pads t = 70 µm
Dielectric t = 75 µm  = 2.2 Wm-1K-1 Solder Pads t = 35 µm Solder Pads t = 70 µm
 OSLON SSL  PD = 1 W  Free convection and radiation
 typ. VF = 3.2 V, IF = 350 mA
yp  Tamb = 25°C  Steady State Solution
y
LED Fundamentals | External Thermal Resistance - Substrates| Page 7

8. PCB Heat Conduction
MCPCB FR4 with maximized
ith i i d FR4 with th
ith thermal
l
Single sided FR4
solder pads vias
RthSB 3K/W 45K/W 28K/W 8K/W
Junction
Temperature 10°C 53°C 35°C 15°C
Rise
LED Fundamentals | External Thermal Resistance - Substrates| Page 8

9. PCB on Heat sink
OSLON SSL Single sided FR4 PCB MCPCB
RthJS = 7 KW-1; PD = 1 W
1
IF = 350 mA, VF = 3.2 V Free Convection and Radiation
Recommended Solder Pads
Copper t = 35 µm Tamb = 25°C Steady State Solution
PCB Technology Junction Temperature at Tamb = 25°C
25 C
Metal Core PCB 63.3°C
Single Sided FR4 PCB
105.2°C
with recommended solder pads
LED Fundamentals | External Thermal Resistance - Substrates| Page 9

10. Thank you for your attention.
LED Fundamentals | External Thermal Resistance - Substrates| Page 10

11. Disclaimer
All information contained in this document has been checked with the greatest care.
OSRAM Opto Semiconductors GmbH can however, not be made liable for any damage
that occurs in connection with the use of these contents.
OSRAM Opto Semiconductor GmbH makes no representations and warranties as to a
possible interference with third parties' intellectual property rights in view of products
originating f
i i ti from one of OSRAM Opto Semiconductor GmbH's partners, or in view of
f O t S i d t G bH' t i i f
products being a combination of an OSRAM Opto Semiconductor GmbH's product and a
product of one of OSRAM Opto Semiconductor GmbH's partners. Furthermore, OSRAM
Opto Semiconductors GmbH cannot be made liable for any damage that occurs in
p y g
connection with the use of a product of one of OSRAM Opto Semiconductor GmbH's
partners, or with the use of a combination of an OSRAM Opto Semiconductor GmbH's
product and a product of one of OSRAM Opto Semiconductor GmbH's partners.
LED Fundamentals | External Thermal Resistance - Substrates| Page 11