Microstructure evolution of Sn-Ag-Cu solders
after thermal excursion
Stephen Merkley, Chien-Hung Wen, Zhe Huang, Indranath Dutta
Method of Calculating Eutectic Channel Width and Interparticle Spacing
During service, Sn-Ag-Cu (SAC) solders undergo
temperatures ranging from 0.4 to 0.8 of the melting
point. These temperatures accelerate diffusion
processes, causing the solder microstructures to coarsen.
This limits the reliability of the microelectronic
package.
Solder balls
Hexagonal array
d 2 1
3 ( ) 
2
6
Afproeu 
3
(d  L)2
4
d
L

L(
a
2 3A
Proeutectic region
proeu
f
 1)d
d 2
d 2
( )
 ( ) L: Channel
2
width between
Afproeu  2 2 
2
(a )
( d  L) b-Sn
L

L(
Pad

4A

 vs t
 vs t
5
5
SAC105
3 2

4
SAC105
Hexagonal Distribution
Cubic Distribution
4
4
3
3
2
2

r
in eutec
Aprec
 2
f



r
prec in eutec
2r

 1)d
proeu
f
1
6
2
r
prec in eutec
3 Af
d: Diameter of
Precipitate
b-Sn
Cubic array
Cubic array
d
in eutec
Aprec

f
2r
Afproeu: Area
fraction of
proeutectic
area
3  r 2 
 (m)
Hexagonal array
Inter-particle spacing, 
 (m)
Introduction
2
1
1
H-max^2
H-min^2
H-mean^2
0
-50
0
50
100
150
200
C-max
C-min
C-mean
250
0
-50
0
Aging time (hours)
Af
SAC105-Aging at 150 oC
Average Diameter (m)
2
L (m)
1.5
Cubic
0
50
100
150
200
12
16
12
Before aging
0
50
100
150
200
14
Mean
Median
Std Deviation
12
18.04
18.87
5.09
10
Count
Count
Count
Channel width
0
0
4
8
12
16
20
24
8
8
6
6
4
4
2
2
0
28
Diameter of b-tin dendrite (m)
0
4
8
12
16
20
24
0
28
0
Diameter of b-tin dendrite (m)
4
8
12
16
20
24
28
Diameter of b-tin dendrite (m)
Eutectic channel width and average diameter of proeutectic
b-tin grain don’t change.
10m
20m
110 hours at 150oC
Particle size, dp and particle size fitting
SAC105-Aging at 150 C
1.4
o
SAC105
Particle diameter, d p (m)
0.8
10m
Minor axis
0.2
0.2
50
100
150
200
250
Major
Minor
Mean
y = 0.29317 + 0.52497x R= 0.98145
0.2
0.4
0.6
0.8
0
1
(Dt)1/2 (m)
Aging Time (hrs)
0
50
100
150
200
Aging time t (hrs)
Particle size distribution
100
Mean
Std Deviation
Median
Variance
Major axis Minor axis
0.62
0.93
0.27
0.48
0.56
0.78
0.08
0.23
Minor
Major
50
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6
Particle Size (m)
200
150
o
SAC105-Aging220hrs 150 C (AR)
Mean
Std Deviation
Median
Variance
Major axis Minor axis
0.66
1.03
0.29
0.56
0.60
0.89
0.08
0.31
100
100
Minor
Major
50
10m
150
o
SAC105-Aging110hrs 150 C (AR)
Particle Count
Mean
Std Deviation
Median
Variance
Major axis Minor axis
0.28
0.41
0.07
0.15
0.28
0.38
0.00
0.02
200
Particle Count
Particle Count
150
SAC105-Before aging (AR)
3
500
400
800
Major
Minor
600
Mean
Std Deviation
Median
Variance
SAC 387 Joint
Copper Side: Ag Sn
3
Major
Minor
Major axis
Minor axis
1.49
1.23
1.12
1.52
0.56
0.42
0.43
0.17
300
600
400
Major axis
Minor axis
Mean
1.44
0.53
Std Deviation
Median
Variance
0.88
1.16
0.79
0.31
0.43
0.10
Not much
difference
in particle
size on both
sides
0
0.8 2 3.2 4.4 5.6 6.8 8 9.2 10.411.6
Particle Size (m)
0
0.8 2 3.2 4.4 5.6 6.8 8 9.2 10.411.6
Particle Size (m)
0.5
0
300
SAC 387 Joint
Nickel Side: Ag Sn
Key Findings
y = 0.42507 + 0.84459x R= 0.99048
200
The particles coarsen after aging
2
0.4
700
1000
1
Minor axis
0.4
150oC
20m
Major axis
0.6
0
220 hours at
1
0.8
0.6
20m
(m )
Major axis
y = 0.06 + 0.0027273x R= 0.94491
d 2-d 2
p
0
Particle diameter, dp (m)
1.2
1
Aging time (hours )
100
y = 0.042467 + 0.0016236x R= 0.92465
1.2
15
1/2
200
y = 0.085 + 0.0040455x R= 0.94939
SAC105-Aging at 150 C
1.4
10
200
dp2-d02 vs t
1.5
o
800
Particle Count
Grain
diameter
5
The effect of Cu and Ni bond-pads on coarsening has
also been studied for SAC387. Near the interface where
the solder is connected to Ni, Cu is depleted at its most.
The coarsest Ag3Sn particles are expected to be found
here, since depletion of Cu from the Sn-matrix is
thought to cause faster Ag3Sn growth.
4
2
0
Additional Research
SAC305 After aging 220 (hrs)
18.31
19.70
5.28
10
6
-5
250
10
8
0
15
Interparticle spacing increases with the square root of
the aging time.
16
SAC305 After aging 110 (hrs)
Mean
Median
Std Deviation
10
15
Aging Time(hrs)
14
5
C-max
C-min
C-mean
Aging time (hours )
250
15.54
15.26
5.51
0
1
1/2
5
-50
SAC105 Before aging
Proeutectic region
 (m)
-5
16
Eutectic region
2
0
20
Average Diameter of proeutectic b distribution
Mean
Median
Std Deviation
3
H-max^2
H-min^2
H-mean^2
Aging Time (hrs)
14
3
1
10
0.5
0
-50
4
2
25
Hexagonal
1
4
SAC105 b-Sn diameter
2.5
There are four parameters being calculated: (1)eutectic
channel width between proeutectic b (L), (2)diameter of
proeutectic b-tin grain (d), (3)particle size (dp), (4)interparticle spacing between particles (). After aging for
110 and 220 hours at 150oC, we compare these
parameters.
Cubic Distribution
Particle Count
Objective
30
 (m)
d vs t
250
SAC105
Hexagonal Distribution
L vs t
200
5
SAC105
Average Diameter of
proeutectic b
150
 vs t1/2
5
Eutectic channel
width of b-tin, L
100
Aging time (hours)
 vs t1/2
2a
3
50
Minor
Major
50
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6
Particle Size (m)
250
During isothermal aging:
•Eutectic channel width L is more or less the same
•Diameter of proeutectic b-tin grain does not change
•Particle size increases proportionally to the effective
diffusion distance (Dt)1/2
•Interparticle spacing  is proportional to square root of
aging time
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6
Particle Size (m)
Particle size coarsens after aging with
respect to the square root of Dt.
•The particle size is very similar in SAC387 for both
sides possibly because the joint-scale size was too
small
This work was supported by the National Science
Foundation’s REU program under grant number 0755055