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Alpha®, the world leader in the production of electronic soldering materials, recently presented a paper on Low Voiding Reliable Solder Interconnects for LED Packages on Metal Core PCBs. The paper was presented by Andy Yuen, Technical Services Director of Alpha Southern China, at this year’s SMTA China Conference in Shenzhen. The presentation received the award of “The Best Paper of Technology Conference Three SMTA South China 2012” on August 29, 2012.
Citation preview
Low Voiding Reliable
Solder Interconnects for
LED Packages on Metal
Core PCBs
PRIVILEGED AND CONFIDENTIAL MATERIALS
Outline/Agenda
• Introduction
• Assembly
• Components
• Materials
• Process
• Results of Experiments
• Conclusions
• Q & A
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
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
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
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
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)
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
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
% V
oid
s
5.875.71
Boxplot of % Voids - MCPCB with Type B Dielectric; SAC305 Paste
PRIVILEGED AND CONFIDENTIAL MATERIALS
Reflow Profile – SAC & Maxrel Alloy Pastes
150-200°C, 115 sec soak; 240°C peak; 67 sec TAL
PRIVILEGED AND CONFIDENTIAL MATERIALS
Reflow Profile – SnBiAg Alloy Paste 100-110°C, 75 sec soak; 175°C peak; 60 sec TAL
PRIVILEGED AND CONFIDENTIAL MATERIALS
Test Results
PRIVILEGED AND CONFIDENTIAL MATERIALS
Main Effects Plot for Dielectric & Paste Alloy
BA
20.0
17.5
15.0
12.5
10.0
7.5
5.0SnBiAgSACX0807SAC305Maxrel
Board Dielectric
Me
an
Paste Alloy
Main Effects Plot for % VoidsData Means
PRIVILEGED AND CONFIDENTIAL MATERIALS
% Voids vs Board Dielectric
BA
35
30
25
20
15
10
5
0
Board Dielectric
% V
oid
s
10.80510.465
Boxplot of % Voids
PRIVILEGED AND CONFIDENTIAL MATERIALS
% Voids vs Paste Alloy & Dielectric
Paste Alloy
Board Dielectric
SnBiAgSACX0807SAC305Maxrel
BABABABA
35
30
25
20
15
10
5
0
% V
oid
s 20
Boxplot of % Voids
PRIVILEGED AND CONFIDENTIAL MATERIALS
Void Sizes
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
% o
f J
oin
ts
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
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
Averg
e V
oid
Siz
e (
% jo
int
are
a) Dielectric A
Dielectric B
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
Ma
x. V
oid
Siz
e (
% jo
int
are
a) Dielectric A
Dielectric B
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
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
PRIVILEGED AND CONFIDENTIAL MATERIALS
For More Information Please Contact:
Ravi Bhatkal – VP, Energy Technologies
+1-908-791-3013
Amit Patel – Technology Analyst
+1-908-791-3051