The document evaluates four different lead-free solder paste alloys for assembling LED packages onto metal core printed circuit boards with different dielectric materials, finding that the solder alloy had a more significant impact on resulting solder joint void levels than the dielectric type, with the SnBiAg alloy paste and type A dielectric board combination yielding the smallest average and maximum void sizes.

1. Low Voiding Reliable
Solder Interconnects for
LED Packages on Metal
Core PCBs

2. PRIVILEGED AND CONFIDENTIAL MATERIALS
Outline/Agenda
• Introduction
• Assembly
• Components
• Materials
• Process
• Results of Experiments
• Conclusions
• Q & A

3. PRIVILEGED AND CONFIDENTIAL MATERIALS
Introduction – LED Packages
• Lumen maintenance requirement for LED
packages used in commercial & outdoor
residential lighting is 70% lumen maintenance for
50% of population after 35,000 hours and 3 year
warranty (Energy Star A; IESNA LM-80)
• Good thermal management is key to long term
stability of light output (brightness) and color
temperature of LED lights
– Solder joints with low voiding for low thermal
resistance

4. PRIVILEGED AND CONFIDENTIAL MATERIALS
Package Assembly
• For this study used commercially available LED
package, metal core boards and solder pastes
– Luxeon Rebel LED
– Metal Core PCB substrate w/ Aluminum core, Copper
circuitry and 2 dielectric types
– 4 solder pastes – Pb-free; no clean; zero halogen; 4
alloy types

5. PRIVILEGED AND CONFIDENTIAL MATERIALS
LED Package
• Surface mount package w/ ceramic substrate &
metal interconnect
• Solderable to next level substrate - bottom side
with thermal pad under LED, and (2) pads for
electrical connections
• Hard silicone lens
over LED

6. PRIVILEGED AND CONFIDENTIAL MATERIALS
Metal Core PCB (MCPCB)
• Aluminum core; Copper
circuitry and (2) Dielectric
types
– Dielectric A: High thermal
conductivity for good
conduction of heat into
metal core
– Dielectric B: Low modulus
for reducing solder joint
strain, especially for
assemblies w/ large CTE
mismatch between package
and board

7. PRIVILEGED AND CONFIDENTIAL MATERIALS
Solder Pastes
• All with Type 3 lead-free solder powder; no-clean; zero
halogen
ID Alloy Attribute
A SAC305 Broad application range
B Maxrel Superior creep resistance,
temp cycling / vibration
performance
C SACX Plus
0807
Reduced Ag for lower cost
D SnBiAg Low melting point (<140°C)

8. PRIVILEGED AND CONFIDENTIAL MATERIALS
Assembly Process & Testing
• Assembled 36 LEDs per board; (5) boards per
solder paste and dielectric type
• Standard SMT equipment – printing, P/P, reflow
• Solder paste printing: 5 mil stencil, on-contact
printing at 2.54 cm/sec; 268 gr/cm pressure;
0.051 cm/sec stencil release
• Reflow in air with high soak profiles
• % Voids by Xray; 50% solder joints per board
measured

9. PRIVILEGED AND CONFIDENTIAL MATERIALS
Reflow Profile Test
• Pre-screening test with high soak & straight ramp
profiles, SAC305 paste & Type B dielectric MCPCB
board
Str RampHigh Soak
25
20
15
10
5
0
Profile
%Voids
5.875.71
Boxplot of % Voids - MCPCB with Type B Dielectric; SAC305 Paste

10. PRIVILEGED AND CONFIDENTIAL MATERIALS
Reflow Profile – SAC & Maxrel Alloy Pastes
150-200°C, 115 sec soak; 240°C peak; 67 sec
TAL

11. PRIVILEGED AND CONFIDENTIAL MATERIALS
Reflow Profile – SnBiAg Alloy Paste
100-110°C, 75 sec soak; 175°C peak; 60 sec TAL

12. PRIVILEGED AND CONFIDENTIAL MATERIALS
Test Results

13. PRIVILEGED AND CONFIDENTIAL MATERIALS
Main Effects Plot for Dielectric & Paste Alloy
BA
20.0
17.5
15.0
12.5
10.0
7.5
5.0
SnBiAgSACX0807SAC305Maxrel
Board Dielectric
Mean
Paste Alloy
Main Effects Plot for % Voids
Data Means

14. PRIVILEGED AND CONFIDENTIAL MATERIALS
% Voids vs Board Dielectric
BA
35
30
25
20
15
10
5
0
Board Dielectric
%Voids
10.80510.465
Boxplot of % Voids

15. PRIVILEGED AND CONFIDENTIAL MATERIALS
% Voids vs Paste Alloy & Dielectric
Paste Alloy
Board Dielectric
SnBiAgSACX0807SAC305Maxrel
BABABABA
35
30
25
20
15
10
5
0
%Voids
20
Boxplot of % Voids

16. PRIVILEGED AND CONFIDENTIAL MATERIALS
Void Sizes
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
%ofJoints
ZERO 0-4% 4-8% 8-12% 12-16% 16-20% >20%
Void Size as % of Joint Area
LED Assembly Voids Sizes
B Diel/ SAC305
A Diel/ SAC305
A Diel/ SnBiAg
B Diel/ SnBiAg
A Diel/ Maxrel
B Diel/ Maxrel
A Diel/ SACX0807
B Diel/ SACX0807

17. PRIVILEGED AND CONFIDENTIAL MATERIALS
Average Void Sizes
LED Assembly Average Void Size
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
SAC305 Maxrel SACX0807 SnBiAg
Paste Alloy
AvergeVoidSize(%jointarea)
Dielectric A
Dielectric B

18. PRIVILEGED AND CONFIDENTIAL MATERIALS
Maximum Void Sizes
LED Assembly Max. Void Size
0.0
5.0
10.0
15.0
20.0
25.0
30.0
SAC305 Maxrel SACX0807 SnBiAg
Paste Alloy
Max.VoidSize(%jointarea)
Dielectric A
Dielectric B

19. PRIVILEGED AND CONFIDENTIAL MATERIALS
Typical Xray Voids Images
Paste Alloy/
Dielectric Type
SAC305 Maxrel SACX
0807
SnBiAg
MCPCB
Dielectric A
%Voids 9.3 15.8 10.0 11.6
MCPCB
Dielectric B
% Voids 7.2 12.6 12.3 12.3

20. PRIVILEGED AND CONFIDENTIAL MATERIALS
Summary/Conclusions
• (4) different metal alloy solder pastes were evaluated for % voids in
the reflowed joints formed between a LED surface mount package
and MCPCB substrates having (2) different dielectric types
• Overall, the board dielectric type had little effect on the solder joint %
voids, whereas the solder alloy had a more significant effect
• The Type A dielectric boards resulted the lowest average and
maximum void sizes, for all pastes, and the SnBiAg paste resulted in
the smallest void sizes
• Overall, >90% of the solder joints had void sizes ≤4% of the solder
joint area and less than 20% voids on average
• The SAC305 alloy paste combined with the Type B board dielectric
resulted in the lowest % voids (<8.5%)
• The boards from this study will be subjected to further tests including
electrical/optical, die shear, thermal cycling/shock, and solder joint
characterization by cross-sectioning

21. PRIVILEGED AND CONFIDENTIAL MATERIALS
For More Information Please Contact:
Ravi Bhatkal – VP, Energy Technologies
Rbhatkal@cookson.com
+1-908-791-3013
Amit Patel – Technology Analyst
Apatel@cookson.com
+1-908-791-3051