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Effects of Wire Type and Mold Compound on Wearout Reliability of
Semiconductor Flash Fineline BGA Package
C.L. Gan,1&2, Senior Member, IEEE, C. Francis1, B.L. Chan1, E.K. Ng1, U. Hashim2
1
Spansion (Penang) Sdn Bhd
Phase II Free Industrial Zone, 11900 Bayan Lepas, Penang, Malaysia
2
Institute of Nano Electronic Engineering (INEE)
Universiti Malaysia Perlis (UniMAP), 01000 Kangar Perlis, Malaysia
Email: clgan_pgg@yahoo.com
ABSTRACT
This paper discusses the influence of wire type (Cu and Au) and
the effect of mold compound on package reliability after exposure to
component reliability stress tests. Failure analysis is conducted to
identify associated failure mechanisms, and wearout reliability of
both wire types investigated after biased HAST (HAST) and
Temperature Cycling (TC). Weibull plots are plotted for each
reliability stress. Cu (Copper) ball bonds are found to have better
reliability margin in biased HAST stress compared to Au (Gold) ball
bonds for both mold compounds. Cu wire also exhibits a better
reliability margin compared to Au ball bonds in the TC stress test.
Failure mechanisms of HAST and TC are proposed for both wire
alloys in this paper.
Hence, extended reliability is crucial to determine the lifetime of
gold and copper ball bonds in microelectronics packaging. Cu-Al and
comparable Au-Al IMC growth kinetics have been studied and have
shown Cu-Al growth to be at least 5X slower than Au-Al IMC [16].
However, copper wirebonding still has some reliability challenges
and complex failure mechanisms to understand which are primary
barriers to the complete replacement of gold with copper in
wirebonded packages [17]. To help address this, in this study, we
have prepared 64 ball BGA (Ball Grid Array) packages assembled
with Gold and Pd-coated copper wire and submitted for biased
HAST (HAST) and Temperature Cycling (TC) tests. The test vehicle
construction is indicated in Figure 1.
Key words
Wearout reliability, weibull plot, HAST, TC, wire type
The primary considerations of finding an alternative to replace
gold wirebonding in microelectronic packaging are driven by new
technologies coming to market, in line with ever decreasing cost
expectations [1]. The currently common Au-Al intermetallic
compound (IMC) growth has been widely characterized and
analyzed [4-6]. Work is still ongoing with Au wire, as Zulkifli MN et
al. [7] suggested a new approach of examining the effect of the
individual phases and surroundings on the strengthening produced by
the Au–Al intermetallic compound; combining FEA based on
friction and wire bonding parameters and correlating TEM results
with results obtained from other techniques should enable a more
detailed understanding of the bondability and strength of
thermosonic gold wire bonds. Copper wirebonding, however,
appears to be the best candidate to meet cost and market
considerations, and various engineering studies on copper wire
deployment have been previously reported [2, 3]. Key technical
barriers such as Intermetal dielectric (IMD) cracking due to
excessive bonding, copper ball bond corrosion under moist
environments and extended reliability of copper ball bonds have been
identified accordingly [8-10]. In previous works, Copper ball bonds
have been shown to be more susceptible to moisture corrosion
compared to gold ball bonds, and undergo different corrosion
mechanisms in microelectronic packaging [11-12]. Our previous
studies indicate that Pd-coated copper ball bonds actually outperform
gold ball bonds in biased HAST wearout reliability [13]. Extended
reliability of High temperature storage life (HTSL) of copper ball
bonds in TSOP package is found with apparent activation energy
(E aa ) of ~ 0.70 eV compared to gold ball bonds [14]. Blish et al. [15]
investigated E aa of typical Au-Al IMC of 1.0 eV ~ 1.5eV which is
dependent on Al thickness as well as dopants. A similar range may
be appropriate for Cu as more reliability papers are published.
Figure 1: FBGA Package Interconnect Construction
EXPERIMENTAL RESULTS AND DISCUSSION
Materials used include 0.8 mil Pd-coated Cu wire (Cu) and 4N
(99.99% purity) gold (Au) wire, and 110 nm flash devices packaged
into fortified Fine-Pitch BGA packages with green (< 20 ppm
Chloride in content and pH values of 5.30 to 5.80) molding
compound and substrate. In this study, there are two variables,
namely mold compound type (A or B) and Wire type (Pd-coated Cu
wire and 4N Au wire) yielding four legs, all bonded on Fine pitch
64-ball BGA packages on a 2L substrate. The sample size used was
77 units for HAST and TC stresses, and the corresponding stress
tests conditions are tabulated in Table 1. After electrical test, good
samples were then subjected for Level 3 Preconditioning and 3 times
reflow at 260ºC as described in JEDEC IPC-STD 020 standard.
Each leg then went through HAST stress testing per JESD22-A110 at
130ºC/85%RH, 3.6V as well as TC per JESD22-A104 at -40ºC to
150 ºC. Electrical testing was conducted after several readpoints of
stress as well as to check Au and Cu ball bond integrity in terms of
its moisture and thermo-mechanical reliability.
Table 1: Summary of experimental matrix (for Au and Cu wires)
Compound
Stress Test
Test Conditions
SS
A
HAST
85%RH, 130 ºC, 3.60V
77
A
TC
-40ºC to 150 ºC
77
B
HAST
85%RH, 130 ºC, 3.60V
77
B
TC
-40ºC to 150 ºC
77
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INTRODUCTION

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All reliability data belong to the wearout portion of the reliability
curve (i.e. the bathtub curve) since the Weibull ß (shape parameter)
is greater than 1.0 for all legs. Mold compound B reveals a greater
time to failure in HAST and higher cycle to-failure (t first , t 50 and
t 63.2 ) in the TC test compared to mold compound A, even though
they have similar specifications (See Table 2). Possibly, the slightly
lower pH of EMC A might have played a role in the reduced
reliability as observed in other works [19]. Additionally, Cu ball
bonds show superior extended HAST reliability compared with Au
ball bonds for both mold compounds A and B (See Figure 2).
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FAILURE ANALYSIS AND MECHANISMS
Typical failure mechanisms of both biased HAST (HAST) of gold
(Au) and copper (Cu) ball bonds indicate IMC microcracking along
the AuAl and CuAl interfaces with the ball bonds. Figure 4 shows a
representative image of Cu ball bond microcracking and open after
3000 hours in HAST. We observed crack initiates from the edge of
Cu ball bonds and it appears to propagate towards the center of ball
bond, typically due to the ball bond force by the capillary, and
consistent with literature [9, 10, 13].
Weibull Plot (HAST Wearout Reliability)
0.00
Au_EMC A
Cu_EMC A
Au_EMC B
Cu_EMC B
Weibits
-1.00
y = 4.9585ln(x) - 39.652
R² = 0.8804
-2.00
y = 5.0585ln(x) - 41.954
R² = 0.9477
-3.00
y = 9.9595ln(x) - 80.853
R² = 0.996
-4.00
y = 8.676ln(x) - 72.19
R² = 0.9419
-5.00
500
5000
Hours
50000
Figure 2: Biased HAST Weibull reliability plots of Au and Cu ball
bonds with different EMC (Epoxy Molding Compounds)
Figure 4: Representative SEM image shows Cu ball bond
microcracking along CuAl interface after HAST 3000 hours
The wire type does not appear to be a key factor affecting the TC
reliability performance, although we did observe Cu ball bonds to
have slightly better TC extended reliability performance (higher t first ,
t 50 and t 63.2 ) compared to Au ball bonds (Figure 3). The coefficient
of thermal expansion (CTE) of mold compounds A and B are similar,
but perhaps the CTE difference between the two with respect to
silicon or the pH values (as mentioned earlier) might play a role.
EMC B (with α 2 of 3.0 x 10-5/°C) is closer to silicon die CTE of 3.0
ppm/°C compared with EMC A(α 2 of 0.7 x 10-5/°C) shows higher
TC reliability margin and longer cycles-to-failure for Au and Cu
wires. Increased internal stresses due to CTE mismatch might result
in faster dislocation movement and IMC formation. The key material
properties of EMC A and B are tabulated in Table 2.
Figure 5 shows the proposed Cu ball bond corrosion mechanism in
HAST test. The proposed corrosion mechanism as described in Eq. 1
to 3 below.
Weibull Plot (TC Wearout Reliability)
-1.00
Weibits
-2.00
Au_EMC A
Cu_EMC A
Au_EMC B
Cu_EMC B
y = 8.4311ln(x) - 81.641
R² = 0.9304
-3.00
-4.00
-5.00
2000
y = 11.956ln(x) - 115.45
R² = 0.9754
y = 7.2471ln(x) - 71.315
R² = 0.8618
(1)
(2)
(3)
Cu ball bond corrosion is most probably attributed to Cl- attacking
the edge of Cu ball bond region. Hydrolysis of IMC and AlCl 3 (an
intermediate product) in a moist environment forms Aluminum (III)
oxide which is a resistive layer and ionic Cl- is usually found at the
corroded ball bond [1, 10]. Equation 1 indicates the hydrolysis of
Cu 9 Al 4 into Al 2 O 3 and out gassing. Cracking of the Al 2 O 3 interface
of Cu to the Cu IMC might be due to out gassing of H 2 during
hydrolysis (as shown in Eq. 2 and 3) in between Cu IMC to Cu ball
bonds. Cracking usually starts at the Cu ball bond periphery and will
propagate towards center of Cu ball bond [9, 10, 13]. There is a
possibility that under moist conditions, internal oxidation of
intermetallic phases can result in oxidation of aluminium,
precipitation of the noble metal (Au or Cu), and generation of
hydrogen. Possible reactions resulting in formation of aluminium
oxide that might occur with gold and copper ball bonds are given in
Eq. 4 and Eq. 5 [11, 12].
2Au 4 Al + 3H 2 O  Al 2 O 3 + 8Au + 6H
Cu 9 Al 4 + 6H 2 O  2(Al 2 O 3 ) + 9Cu + 12H
y = 8.347ln(x) - 82.541
R² = 0.9129
Cycles
Cu 9 Al 4 + 6H 2 O  2 (Al 2 O 3 ) + 6H 2 + 9Cu
CuAl 2 + 3H 2 O  Al 2 O 3 + Cu + 3H 2
2AlCl 3 + 3H 2 O  Al 2 O 3 + 6 HCl (acidic)
(4)
(5)
20000
Figure 3: Extended TC Weibull reliability plots of Au and Cu ball
bonds with different mold compounds.
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Temperature cycling was conducted on the 4 legs of Au and Cu
ball bonds to examine its thermo-mechanical reliability performance
assembled with different epoxy mold compounds (EMC). It should
be noted that there is a mismatch in CTE (Coefficient of Thermal
expansion) between both Cu (17 ppm/°C) and Au ball bond (14
ppm/°C) compared to the silicon die (3.0 ppm/°C). This, combined
with the CTE mismatch between Au and Cu ball bonds with Al
bondpad of silicon die, will likely result in differential thermal
stresses during hot cycles (150°C) and cold cycles (-40°C). IMC
formation initiated at the edge of the ball bond (again, due to the ball
bond force of the capillary) and micro cracking was observed after
long cycles of thermal cycling effects. The micro cracking occurred
in between the ball bond and the IMC (as shown in Figure 8).
Figure 5: Proposed HAST failure mechanism of Cu ball bond
Au ball bonds also can exhibit Au oxidation and corrosion during
HAST stress. Figure 6 reveals the representative SEM image of Au
ball bonds and microcracking after HAST 2000 hours stress. Au is
commonly thought to have high corrosion resistance compared to Cu
ball bonds and has been deployed in microelectronics packaging for
more than 25 years. However we observe higher HAST reliability
margin of Cu ball bond compared to Au ball bond (Figure 2). The
AuAl HAST corrosion failure mechanism is illustrated in Figure 7.
Figure 8: Representative SEM image of Cu ball bond CuAl
interfacial microcracking after TC 9500 cycles
Figure 9 indicates the representative AuAl IMC microcracking after
9000 cycles of TC (Temperature Cycling) test in our wearout
reliability study.
Figure 6: Representative SEM image shows Au ball bond
(microcracking along AuAl interface after HAST 2000 hours)
3 Formation of AuAl
IMC micro- cracking
ball bond
Al Bondpad
Al Bondpad
1
Moisture
penetrate from
Edge of Au
ball bond
4 Lifted ball bond
Figure 9: Representative SEM image of Au ball bond AuAl
interfacial microcracking after TC 9000 cycles
ball bond
Al Bondpad
Figure 7: Proposed Failure mechanism of AuAl HAST oxidation
failure.
EFFECT OF MOLDING COMPOUND ON RELIABILITY
The epoxy mold compound (EMC) used in this evaluation are from
suppliers A and B. The key material characteristics of EMC A and B
are shown in Table 2. In general, the material specifications are very
similar between the two compounds. EMC B exhibits a longer timeto-first-failure in HAST extended reliability (Figure 2) and a higher
cycles-to-first-failure TC extended reliability tests (Figure 3)
compared to EMC A. EMC B also shows higher t 50 and t 63.2
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2
AuAl IMC will be
react with oxygen
From moisture>
oxidised AuAlO
layer

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compared to EMC A in HAST and TC stresses. These results are
summarized in Figure 10. It should be noted that both EMCs show
promising extended reliability results which far exceed the typical
requirements of 96 hours of HAST and 1000 cycles of TC according
to the JEDEC JESD47 standard.
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EMC types. This is an interesting finding since the mismatch in CTE
(Coefficient of Thermal expansion) between Cu (17 ppm/°C) is
greater than that of Au (14 ppm/°C). Normally speaking, reducing
CTE mismatch would tend to result in an improvement in reliability,
but there are clearly other factors at work here. Perhaps the fast IMC
growth of the AuAl interface plays a role in this case.
Table 3: Material properties of Au and Cu
Material Properties
Units
Au
Cu
Thermal Conductivity
W/mK
320
400
Electrical Resistivity
10-8 Ωm
2.2
1.72
Young’s Modulus
GPa
78
130
Vickers hardness
MPa
216
369
CONCLUSION AND SUMMARY
Figure 10: Plot of TC (cycles-to-failure) and HAST (hours-to-failure)
against EMC and wire types
Table 2: Key material characteristics of Epoxy Mold Compound
(EMC) A and B
Units
EMC A
EMC B
-5
10 /°C
1.1 +/- 0.3
0.7+/-0.3
10-5/°C
4.5 +/- 1.0
3.0+/-1.0
ACKNOWLEDGMENT
The authors would like to render appreciation to Gene Daszko and
Tony Reyes for their encouragement on the publication of this
technical paper.
REFERENCES
°C
125 +/- 15
125 +/- 15
ppm
< 10 ppm
< 10 ppm
Na content
ppm
< 10 ppm
< 10 ppm
pH
n/a
5.30
5.80
EFFECT OF WIRE TYPE ON RELIABILITY
Many previous studies reported Au wirebonds to have higher
reliability margins compared to Cu wirebonds. However, there is
very little published data on extended reliability of Au and Cu ball
bonds. Table 3 tabulates the major material properties comparing Au
and Cu metals. In our study, Cu ball bonds show significant higher
t 50 (mean-time-to failure) and t 63.2 (characteristic life) in HAST and
TC reliability compared to Au ball bond in FBGA package (as
indicated in Figure 10). Although Cu is commonly oxidized under
moist environments like the HAST test, an optimized package billof-material (such as that being used in our flash memory packages)
show excellent results and exceed Au performance with either EMC.
Similarly, Cu ball bonds exhibit a higher number of cycles-tofailure in TC test compared to Au ball bonds (Figure 3) regardless of
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Material
characteristics
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thermal expansion 1
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Glass transition
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From the study, we analyzed the effects of wire alloy on
extended reliability of HAST and TC stresses. Cu ball bonds show
significant higher HAST reliability compared to Au ball bonds in the
FBGA package. In addition, Cu ball bonds are also superior to Au
ball bonds in an extended TC stress test. Overall, EMC B exhibits
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requirement of 96 hours of HAST and 1000 cycles of TC according
to the JEDEC JESD47 standard.

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