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Bonding Process

In this chapter, the details of the wire bonding process are discussed. The bonding

wire conﬁgurations used in the electronics industry are listed, and wire parameters

such as wire processing, wire purity, and wire diameter are explained. Since the use

of bare copper wires is a concern owing to the propensity of copper to oxidize,

oxidation prevention technology is needed. The chapter discusses the two oxidation

prevention technologies used in the industry, including cover gas and oxida tion

prevention coating. The wire bonding parameters govern the quality of formed free

air balls (FABs) and subsequent ﬁrst and second bonds. The effect of elect rical

ﬂame-off current and ﬁring time on formed FABs, and the effect of ultrasonic

energy, bonding force, bonding temperature, and time on wire bond quality, is

explained, and the optimal parameter ranges are listed. The bonding tools for

copper wire bonding are explained and the failures resulting from improper bond-

ing are also listed.

2.1 Bond Wire

Wire purity and the processing undergone by the wire during wire drawing opera-

tion determine the mechanical properties of the bonding wire. The mechanical

properties relevant for wire bonding are elon gation percentage and the breaking

strength. Additionally, the stress state of the wire—residual stress or stress free

(fully annealed)—determines the looping characteristics of the wire. The use of

annealed wire reduces the variation in the bond strength and makes the bonding

process less sensitive to thermal effects. A high-purity wire is desired, which is

melted in a high vacuum to obtain a homogeneous microstructure. The resulting

cast bar is subjected to multiple stages of drawing through a series of dies to reduce

the wire diameter for bonding. The thinner the wire requirement, the more stages of

wire drawing the wire undergoes. Wire processing should ensure that the wire has a

clean surface and smooth ﬁnish so that it easily comes out of the spool without

P.S. Chauhan et al., Copper Wire Bonding, DOI 10.1007/978-1-4614-5761-9_2,

Springer Science+Business Media New York 2014

snagging [5]. Bare Cu wires of varying purity (3–6 N) are used in the industry, and

the wire cost is directly proportional to the wire purity. Also, as the wire purity

increases, the wire becomes softer.

Bond wire manufacturers often dope pure Cu wire with additives to enhance the

wire bondability and minimize the Al splash. For the same wire thickness, a Cu

FAB is larger in diameter than the corresponding Au wire. Hence, to achieve the

same FAB diameter, Cu wires are 2.54 μm thinner than Au wire. Bare Cu wires

used in the industry have purities of 3–6 N, with 4 N as the most commonly used

[42, 43]. Srikanth et al. [44] investigated Cu wires of varying purity, including 3, 4,

and 5 N, and 50 μm thickness. They reported that higher purity wires have lower

ﬂow stress than lower purity wires (95.5 MPa for 5 N, 115 MPa for 4 N, and

120 MPa for 3 N), since they have fewer grains. Cu wire of 3 N purity had ﬁne

columnar grains in the initial state, as well as after melting (FAB formation). On

the other hand, wires of 4 and 5 N purity had large grain sizes and few columnar

grains in both the i nitial state and after melting. The grain direction on the FAB

affects the hardness and elastic modulus of the FAB, wherein the lack of direc-

tionality results in softer wires. The low ﬂow stress in 5 N wires was attributed to

the lack of directionality in the grains. Because of the lower ﬂow stress, a lower

bonding force is req uir ed, which result s in a lower Al splash. The high-purity

wires showed high twinning features, which can impai r the deformation processes

during the bonding operation. On the other hand, for 3 and 4 N FABs, the number

of grains in the [ 100] direction was larger than that in the [110] and [111]

directions [44]. The thermomechanical processing that the wire undergoes

determines the residual stress states in the wire and the mechanical properties,

including the hardness and tensile strength. The grain structure of the wire can be

optimized for the individual purity level by the wire processing steps, including

the deformation and annealing steps during wire drawing. Optimization of wire

purity and microstructure can help achieve the desired mechanical properties,

such as wire elongation and tensile strength. Wires with a high elongation strength

have smaller grains and higher tensile strength [45].

Another consideration for the bonding wire is the wire diameter. The choice of

wire diameter depends on the compatibility with the process and the required

current carrying capacity. For consumer electronics, the common wire diameter

range is 18–25 μm, whereas for the power electronics, the common wire

diameter range is 20–50 μm[46].

The use of bare Cu is a concern due to the propensity of Cu to readily oxidize.

The use of oxidation-resistant coatings is one way to address the problem of Cu

oxidation. Al-coated Cu wires for room-temperature wedge–wedge bonding have

been shown to suppress oxidation and have better pull strength, better metallic

contact format ion, and better storage capabilities than bare Cu wires [47]. Al-coated

wire is suited for room-temperature bonding on low-temperature co-ﬁred ceramics

The material purity in % or Nines scale is denoted in terms of purity in parts-per-million (ppm).

For example, 1 N ¼ 90 %, 2 N ¼ 99 %, 3 N ¼ 99.9 % and so on.

12 2 Bonding Process

with silver and gold metallization. Among the oxidation prevention coatings (Au,

Ag, Pd, and Ni), Pd coating on Cu has shown sufﬁcient potential to replace Au

wire due to its excellent bondability and reliab ility at a relatively low cost [48–53].

Pd is a semi-noble metal with similarities to both Ag and Pt. PdCu is oxidation free,

and Pd has good adhesion to Cu wire and higher tensile strength than bare Cu wire

when bonded on Al pads. Table 2.1 shows a comparison of Au, Cu, and PdCu wires.

Because of the Pd layer on Cu wire, there is always a layer of Pd or a Pd-rich

phase that prot ects the bonded ball from an attack of corrosion. Easing the use of Pd

may also ease the stringent molding compound requirement. Pd prevents the

formation of CuO and can form a bond with N

without requiring forming gas.

Figure (2.1a, b) shows a comparison of Cu and PdCu wires bonded on Al. This

comparison of the ﬁrst bonds of PdCu and Cu wires shows that PdCu-bonded ball

has lower Al pad splash than Cu-bonded ball.

Robustness in the second bond is the most important reason to adopt PdCu wires

[52, 55]. This robustness has led to an improved C

(process capability index). The

stitch pull strength of PdCu wire is more than 50 % higher than bare Cu [51]. PdCu

wire on an aluminum bond pad has also been demonstrated to perform better than

bare Cu in high-humidity conditions, such as in the highly accelerated stress test

(HAST), pressure cooker test (PCT) [50], temperature cycling test (TCT), and high-

temperature storage (HTS) test. Table 2.2 shows a bond strength and defective

second bond ratio comparison of Au, Cu, and PdCu wires. The PdCu wires have a

higher secon d bond strength than bare Cu wires and zero defective second bonds

[48]. PdCu also works better at highe r ultrasonic generator (USG) current levels

than Cu wire. It should be noted, however, that due to the higher hardness and

rigidity of PdCu o ver Cu, a higher bonding force is needed for PdCu wires, which

could increase the risk of Al splash and pad damage [56]. Hence, careful optimiza-

tion of bonding parameters is needed for PdCu wires.

Since PdCu wire has a larger diameter than bare Cu wire, the FAB diameter for

PdCu wire needs to be smaller than for bare Cu wire. Because of the Pd layer on the

Cu wire there is always a layer of Pd or a Pd-rich phase that protects the bonded ball

from an attack of corrosion. Easing the use of Pd may also ease the stringent

molding compound requirement. Pd prevents the formation of CuO and can form

Table 2.1 Bonding wire comparison: Au, Cu, and PdCu [54]

Au Cu PdCu

Cost High Low Low (higher than Cu)

Cover gas No need Forming gas Forming gas or N

FAB hardness Compatible to Al ~40 % harder than Au ~10 % harder than Cu

First bond process Good process window Narrower than Au Same or slightly

narrower than Cu

Second bond process Same Same Same

Portability requirement Moderate High High

Reliability Good Good; more stringent

mold compound than

that for Au

Same or slightly better

than Cu

2.1 Bond Wire 13

2.2 Oxidation Prevention Technology

FAB formation requires the generation of high voltage across the EFO gap, causing

a high current spark to discharge and melt the tail of the Cu wire to form a spherical

ball. Oxidation must be avoided in order to obtain a symmetr ical FAB without

deviation in size [60]. Cu oxidation during ball formation inhibits the formation of a

spherical ball, which in turn affects the reliability of the ﬁrst bond. Under high-

temperature and high-humidity environments, copper oxidation at the interface of

the Cu–Al bonding region causes cracks and weakens the Cu–Al bonding. Copper

oxidation typicall y starts at the wire region and then spreads to the upper bonded

area and then to the bonding interface with time. Cu oxidation also causes

corrosion cracks.

Since Cu oxidizes quickly, Cu FABs need to be formed in an inert gas environ-

ment. Oxidation can also occur if the cover (inert) gas ﬂow rate is not sufﬁciently

high to provide an inert atmosp here for the FAB formation [61]. It has been

reported that the use of single-crystal Cu wires eliminates the need for cover gas

during bonding [62]. Requiring inert gas, such as forming gas, to address the

oxidation probl em adds complications to the bondi ng process and results in a

narrow process window.

The oxidation of Cu is prevented in two ways: use of an inert gas (nitrogen or

forming gas) during bonding, and use of oxidation prevention coating on the Cu

wire [42, 63, 64]. The use of N

as the cover/shielding gas has resulted in defective

FABs. Since forming gas contains 5 % H

(95 % N

,5%H

), it has better anti-

oxidation properties than N

and is the cover gas for Cu wire bonding. The main

purpose of injecting forming gas (FG) is to form an inert gas shroud around the

copper tail and the FAB to prevent oxidation prior to bonding. The use of H

has the

twofold purpose of helping to melt the Cu, as well as acting as a reducing agent to

reduce the copper oxide back to Cu [60].

2.3 Free Air Ball Formation

FAB formation starts with the Cu wire being heated and subsequently melted by the

low-energy plasma discharged. The molten wire turns into a spherical ball under the

effect of surface tension. At the end of discharge, the molten spherical ball starts

cooling and then solidiﬁes to form a FAB [65]. Qin et al. [66] compared Au and Cu

balls on Al pads and found that non-optimized bonding condi tions resulted in over-

bonded balls for Au wire and under-bonded balls for Cu wire. Although Cu wire

bonds showed higher pull strength than Au wire bonds, Cu wire bonds had a larger

standard deviation, which indicates that more process development is required to

optimize the process conditions for Cu wire bonding. Figure 2.2(a, b) shows the

FAB micrographs for Au and Cu wires. Even though the ball diameter of Au wire

(39 μm) was higher than that of Cu wire (37 μm), the volume of the bonded ball was

found to be lower in the case of Au. One possible reason is the softness of Au, which

makes Au able to be more easily squeezed into a capillary during the plastic

2.3 Free Air Ball Formation 15

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