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[Technical Paper]

High-Speed Laser Plating for Wire-Bonding Pad FormationKatsuhiro Maekawa*, Kazuhiko Yamasaki*, Tomotake Niizeki*, Mamoru Mita**, Yorishige Matsuba***,

Nobuto Terada***, and Hiroshi Saito***

*Ibaraki University, 4-12-1 Nakanarusawa, Hitachi 316-8511, Japan

**Mita Engineering Office, 3-16-14 Tajiri, Hitachi 319-1416, Japan

***Harima Chemicals, Inc., 5-9-3 Tokodai, Tsukuba 300-2635, Japan

(Received July 24, 2010; accepted September 22, 2010)

Abstract

The present paper proposes high-speed laser plating for forming wire-bonding pads on a Cu leadframe using Ag nano-

particles. The novelty of the process lies in the implementation of drop-on-demand laser plating on the specially designed

leadframe. Various aspects of the proposed method are investigated, including experimental set-up, multistep ink-jet print-

ing, laser-plating parameters, quality of the sintered film, and wire bondability. It is found that both the quality of the sin-

tered Ag pad and wire bondability are comparable to those of an electroplated Ag film when the near-infrared CW laser

irradiates the pad for a short time of milliseconds. The superiority of the high-speed laser plating is confirmed from the

viewpoints of material consumption, the necessity of pre- and post-processing, thermal damage to the pad and substrate,

and environmental protection.

Keywords: Laser Sintering, Metal Nanoparticles, Metallization, Plating, Ink-jet Printing, Patterning, Wire Bonding,

Leadframe

1. IntroductionConventional fabrication of functional films for elec-

tronic wiring and electrode formation relies on wet pro-

cesses such as liquid cleaning, chemical etching, and elec-

troplating, which need plenty of energy and resources. For

example, electroplating includes pre- and post-processing

procedures such as alkali degreasing, acid pickling, elec-

trolytic cleaning, water washing and drying. Besides, these

conventional technologies are not compatible with the

need for low-cost production and less environmentally

harmful emissions. As an alternative to these wet pro-

cesses, ink-jet printing with metal nanoparticles together

with an additional metallization process are attracting

much attention.[1] This printed electronics technology

enables us to make conductive patterns by applying a

small amount of metal nanoparticles only to the part where

the functional film is required.

The conductive pattern is mainly obtained by a process

of thermal curing, in which a large depression of melting

point can be utilized; when the particle diameter is smaller

than 5 nm, metallization takes place at a low temperature

of below 250°C with a holding time of around 60 min.

However, thermal damage and adhesion to electronic

substrates are problems to be solved. Recently, a process

called laser sintering has been developed for gold or silver

nanoparticles.[2–4] Densification of metal nanoparticles

consists of such sequences as evaporation of solvents,

decomposition of dispersant, necking of adjacent particles

and grain growth. Near-infrared lasers with little absorp-

tion in the paste heat the substrate first, and develop metal-

lization up to the paste surface. As a result, easy evapora-

tion makes the sintered part denser, and interdiffusion

between the substrate and sintered part yields firm adhe-

sion.[5]

In the present paper, the laser sintering method with Ag

nanoparticles is proposed as a tool for the formation of

wire-bonding pads on a copper leadframe. The 5-nm-particle

silver paste is uniformly coated with ink-jet (IJ) printing as

large as around φ100 μm on the leads. Then, metallizationof the paste is completed with laser irradiation of a milli-

second order. We name this functional-film formation pro-

cess high-speed “laser plating” as an alternative to electro-

plating. The laser-plated film is observed and analyzed with

FIB-SIM, TEM, XPS and laser scanning microscope

Maekawa et al.: High-Speed Laser Plating for Wire-Bonding Pad Formation (1/7)

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(LSM), and its thickness and flatness as well as its metal-

lographic structure are discussed. Then, wire bondability

between the Ag pad and an Au wire is examined by a pull

test. Finally, these experimental results are compared with

those of furnace curing and electroplating.

2. High-speed Laser PlatingLaser plating is defined as metallization of nanoparticles

with laser irradiation for the purpose of forming functional

films. Basically, the process consists of the following:

(1) Metal nanoparticles with dispersant and solvents

are pasted on the substrate by various methods

including IJ printing and spin coating.

(2) A short preheating is necessary to remove organic

substances in the paste.

(3) The paste is metallized by a millisecond-order irra-

diation of a laser beam under atmospheric condi-

tions.

(4) Not only metallization by sintering but also inter-

diffusion or fusion takes places at the coating-

substrate interface, leading to firm adhesion there.

Figure 1 schematically shows the laser plating method

with IJ printing. With Ag nanoparticles, we have succeeded

in wiring on polyimide.[5] Padding on a Cu leadframe is

the subject of the present study. In comparison with elec-

troplating, no special attention is paid to pretreatment

before the laser processing, whereas chemical cleaning

and degreasing, and thorough rinsing of the substrate

prior to electroplating are essential.

Previous findings show that sintering is largely affected

by paste composition prior to laser irradiation; especially,

the content of dispersant and solvents.[4] Bulk growth is

boosted with less organic substances. Otherwise, insuffi-

cient sintering occurs or a porous structure is formed. An

appropriate preheating condition was set at 100°C for 1min on a hot plate when the silver NanoPaste® (NPS-J,

Harima Chemicals) was used.[6]

Laser wavelength is another factor that influences sinter-

ing.[5] The Ag microstructure sintered by visible lasers,

488 nm and 532 nm in wavelength, was more porous than

that sintered by near-infrared ones, 980 nm and 1064 nm

in wavelength. The specific resistivity of the Ag film sin-

tered by the near-infrared laser was about 5 μ Ω•cm, whichis smaller than that produced by the visible one. The rapid

metallization starting from the paste surface with the visi-

ble laser makes the removal of solvent and dispersant dif-

ficult, resulting in an insufficient sintering with large pores.

Near-infrared lasers with little absorption in the paste are

more effective than visible ones in obtaining a dense metal

structure.

3. ExperimentalFigures 2 and 3 show the experimental apparatus used

for laser plating. The IJ printer consists of IJ head, XY

stage, ink reservoir, and controller, having 128 micro noz-

zles, an ink discharge volume of 11 pL and a resolution of

1200 dpi. As soon as printing finishes, the sample is heated

on a hot plate to remove solvents in the paste. Then, it is

placed on a stage in the Nd:YAG laser equipment

(SOS8956QSS, LASER SOS Ltd.) and sintered by moving

the XY stage under a laser wavelength of 1064 nm; the

beam diameter is around 0.2 mm. A continuous-wave

mode is preferable to a pulsed output of beam power.[4]

Fig. 1 Schematic of laser plating using Ag nanoparticles.

Fig. 2 Ink-jet printing as part of laser plating.

Fig. 3 Laser sintering as part of laser plating.

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The stage was scanned once at 4 mm/s in an argon atmo-

sphere with a flow rate of 3 mL/min. A stainless-steel jig

was placed on the XY table to fix the leadframe.

Metal nanoparticles prepared by a gas evaporation pro-

cess have many advantages such as freedom from contami-

nation, narrow size distribution, and broad range of met-

als.[1, 7] The silver NanoPaste® (NPS-J, Harima Chemicals)

was used in the experiment. The nanoparticles, being cov-

ered by a protective compound, or an amine-type disper-

sant, are very stable. The TEM image revealed that the

size of nanoparticles was quite uniform with an average

diameter of 5 nm. Neither aggregation nor precipitation

leading to a broad size distribution was observed. Table 1

summarizes the properties of the paste used in the exper-

iment. It has a metal content of around 65 mass%, and a vis-

cosity as low as about 9 mPa•s.

The standard method for connecting a bare die to a

board is the chip-and-wire technique. Wire bonding is a

technique for the production of discrete electrical connec-

tions, generally from a chip on a substrate. Wire bonding

the connections must have suitable contact areas, or so-

called pads. The pad or thin film serves to increase adhe-

sive strength and reliability. Copper leadframes are often

used as a substrate, and the pad is formed at the top of the

lead. Figure 4 shows the Cu leadframe specially designed

for laser plating. The leadframe consists of Cu/99.28

mass%, Cr/0.27 mass%, Sn/0.25 mass% and Zn/0.2 mass%,

having a thickness of 100 μm, a lead width of 300 μm, anda line surface roughness of 0.06–0.07 μm in Ra and 0.6–0.9μm in Rz. The leadframe was used as it was without anychemical cleaning or degreasing, or thorough rinsing prior

to the IJ printing.

Round pad patterns of around φ100 μm have to beprinted on the leads, and a flat pad surface with a thickness

over 2 μm is required for wire bonding. However, it is wellknown that the “coffee stain phenomenon” takes place as

a droplet of a nanoparticle colloidal solution dries.[8, 9] If

the contact angle of the droplet is less than 90°, and theambient conditions encourage droplet drying, the droplet

has a maximum evaporation rate at the boundary. Due to

temperature and hence surface tension gradients, there

results an effective flow of nanoparticles to the boundary.

When the droplet completely dries out, we are left with a

ring-like stain of nanoparticles which decreases in concen-

tration from the periphery inwards.

In order to form a thick, flat pad, we can make use of this

effect by means of controlling discharge rate and substrate

temperature during IJ printing, together with varying pre-

heating conditions. Figure 5 illustrates the process of mul-

tistep IJ printing to produce discrete pads on the lead-

frame. An appropriate stage temperature is necessary for

controlling the spread of the droplet. The bank formed by

the first droplet prevents the second one from overrunning

it, and the third one fills the interior space. The first step

plays the important role of controlling the wettability of the

second droplet as well as making a bank. Three steps are

sufficient for making a φ100 μm flat pad. After each step,heating is required for reducing the solvents present in the

paste as well as maintaining the pad profile.

The Ag functional film thus obtained can be used as a

wire-bonding pad. A thin gold-nickel or silver electroplated

pad is commonly used to increase bond strength. A ball-

and-wedge semiautomatic wire-bonder (HB10, TPT) was

employed to bond an Au wire of φ25 μm on the sintered Agfilm: ball bonding for the first bond and wedge bonding for

Table 1 Properties of Ag-nanoparticle paste before/after fur-nace curing.[7]

Before

Appearance Dark blue

Particle diameter 3–7 nm

Metal content 62–67 mass%

Solvent Tetradecane

Viscosity 7–11 mPa•s

Specific gravity 1.8–2.2

Curing temperature 220°C

Curing time 60 min

After

Appearance Silver gray

Electric resistivity 3 μ Ω•cm

Metal content 99 mass%

Fig. 4 Cu leadframe for laser plating.

Fig. 5 Schematic of multistep IJ printing.

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the second one. Figure 6(a) shows the semiautomatic

wire-bonder used in the experiment. To measure bond

strength, the wire bonded on the sintered Ag film was

pulled by a hook with a load meter, as shown in Fig. 6(b).

Surface observation of the sintered Ag films was carried

out by an LSM (VK-8700, Keyence Corporation), and

cross-sectional observation of the sintered layers was car-

ried out by FIB (FB-2100, Hitachi High Technologies).

The sintered film was ion-milled stepwise and tilted by 30°for SIM observation. The sintered film was also analyzed

with TEM to confirm the formation of the Ag crystalline

lattice and lattice spacing in the sintered Ag layer, and with

XPS to measure chemical compositions.

4. Results and Discussion4.1 Single-step printing of 5-nm-particle Ag paste

Figure 7 shows the appearances of single-step IJ-printed

and laser-sintered patterns: (a) an IJ-printed droplet after

preheating at 100°C for 1 min, and (b) a laser-sintered one.The coffee-stain effect takes place: Ag concentrates at the

periphery, forming a slight bank of around 0.5 μm inheight; the thickness is 0.2 μm at the center.

Figure 8 shows the cross-sectional image near the pad

center: (a) FIBed SIM and (b) TEM images. In these fig-

ures, the Ag bulk structure and the Ag crystalline lattice

are visible; a lattice spacing of 0.2 nm can be seen in the

TEM image. Taking the beam scan speed of 4 mm/s and

the scan distance of 100 μm into account, Ag nanoparticleshave been crystallized in the short time of around 75 ms.

As can be seen in Fig. 9, an XPS analysis revealed that

the top surface consists of almost all Ag and the interface

between the film and the copper substrate contains some

oxygen. We have not yet identified the source of this oxy-

gen. Some diffusion takes place at the interface to form a

thin diffused layer, which probably causes firm adhesion to

the substrate used without any pre-treatments.

4.2 Multistep printing of 5-nm-particle Ag pasteUsing the multistep IJ printing as illustrated in Fig. 5, we

can overcome the coffee-stain effect and achieve a thin, flat

(a) Semiautomatic wire-bonder (b) Pull test of bonded wire and frac-ture mode

Fig. 6 Wire bonding and bondability test.

(a) IJ-printed (b) Laser-sinteredFig. 7 Single-step IJ-printed and laser-sintered patterns onCu leads.

(a) FIBed SIM image

(b) TEM imageFig. 8 Cross-sections of sintered φ5-nm-particle paste.

Fig. 9 Atomic percent profile of cross-section of samplein Fig. 8.

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pad on the lead. Figure 10 shows the shape-controlled

pads positioned at the Cu lead tips. The SEM image also

shows that no silver adheres to the side of the lead.

Besides, no thermal damage such as oxidation to the lead

takes place after the single-path laser irradiation through

the center of the pads from left to right.

A more detailed profile of a lead and pad is shown in Fig.

11. The LSM images make clear that the coffee-stain effect

is almost resolved to yield a flat pad with a thickness of

around 3 μm. The plateau is as large as φ100 μm, which islarge enough for a φ10–25 μm wire to be bonded. How-ever, a few undesired cracks and voids can be seen on the

pad.

Figure 12 shows the SIM image of the FIBed cross-

section. Using the multistep IJ printing method, we can

increase pad thickness from 0.2 μm to 3 μm. Note that thecross-section has been tilted by 30° for observation.Although full crystallization is not achieved and a porous

structure appears in the sintered Ag portion, it seems that

firm adhesion is obtained at the sintered Ag and Cu sub-

strate.

4.3 Wire bondability of laser-plated Ag padsWire bondability was examined between adjacent leads

in the manner illustrated in Fig. 6, in which the symbols,

A to E, in the figure indicate where breakage takes place

in the course of testing.

Table 2 summarizes the pull strength when the IJ-

printed pads were prepared by preheating at 100°C for 10min just before laser irradiation. The number of test sam-

ples was approximately 100. Very few wires separated from

the pads; almost all broke at B or C. In the case of the mul-

tistep-printed pad, the average pull strength is 8.6 cN, and

the minimum one is 7.0 cN.

In comparison, the electroplated pad has average bond

strengths of 8.4 and 8.5 cN for the pad thickness of 0.2 and

2.0 μm, respectively, being close to the results of the laser-plated film with the φ5-nm-particle paste. In addition,breakage mode in the electroplated pad was more stable;

every wire breakage occurred at the middle of the wire, or

at B or C.

Regarding the reliability of the wire-bonded leadframe,

the specimen exposed in an atmospheric electric furnace

at a temperature of 150°C with a holding time of 1000 hshowed no changes in pull strength and fracture modes:

i.e., the average pull strength of 8.5 cN was maintained,

and separation did not take place at the pad; in all cases of

breakage only the wire broke.

4.4 Comparison with furnace curing and electro-plating

The IJ-printed Ag-nanoparticle paste can be metallized

(a) IJ printing (b) Laser sinteringFig. 10 Laser-plated Ag pads on Cu leads.

Fig. 11 LSM image of surface profile of laser-plated Ag padson Cu lead tip.

Fig. 12 FIBed cross-section of laser-plated Ag pad and Culead.

Table 2 Pull strength of φ25 μm Au wire bonded to Ag pads.

Pad formationThickness,

μmStrength, cN

Max. Min. Avg.

Laser plating0.2 10.3 6.0 8.2

3.0–3.2 10.5 7.0 8.6

Electroplating0.2 9.9 7.0 8.4

2.0 10.1 7.6 8.5

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by furnace curing. The leadframe was heated under atmo-

spheric conditions in an electric oven at 220°C for 60 min.Figure 13(a) shows the FIBed cross-section. Compared

with Fig. 12, the structure does not become fully dense but

is more porous under the recommended curing condi-

tions. Curing is more developed at the paste surface, so

that a dense layer is formed there, which prevents the

evaporation of solvents. As a result, the interior of the pad

is more porous, as can be seen in the cross-sectional

image.

The problem is that the Cu substrate suffers a degree of

oxidation during the curing process. An oxygen atmo-

sphere is required for curing the nanoparticles because

carbon in the dispersant is removed with oxygen.[7] We

suggest that Cu may come up to the surface due to diffu-

sion, leading to poor bondability. In fact, wire bonding to

the furnace-cured Ag pad was not successful, and most of

the wires easily separated from the pad at A or E in Fig. 6.

In the case of the laser-plated leads, no such thermal prob-

lems took place in the laboratory.

We carried out wire-bonding to an electroplated pad

using the same apparatus. Figure 13(b) shows the FIBed

cross-section of the electroplated Ag pad and Cu lead. Full

crystallization has been achieved, but the Ag surface is

rather rough: 0.27 μm in Ra and 2.75 μm in Rz, which issignificantly higher than the laser-plated one: 0.09 μm inRa and 1.51 μm in Rz (measured area: 100 μm × 100 μm).

Various pre- and post-processing procedures such as

alkali degreasing, acid pickling, electrolytic cleaning, water

washing and drying, are indispensable in conventional

electroplating. In particular, the film plated on the side and

back surfaces of a leadframe must be removed before wire

bonding. Otherwise, a masking process must be added to

the leadframe in the course of electroplating. In the pro-

posed method, however, no such additional operations are

required; the leadframe was used as it was, and the pad

was formed only on the top face of the lead.

5. SummaryHigh-speed laser plating for forming wire-bonding pads

on a Cu leadframe using Ag nanoparticles has been pro-

posed. Its novelty lies in the implementation of drop-on-

demand laser plating on the specially designed leadframe.

Various aspects of the proposed method have been inves-

tigated, including experimental set-up, multistep printing,

laser-plating parameters, quality of the sintered film with

FIB-SIM, TEM, XPS and LSM, and wire bondability

between the Ag pad and Au wire. Experimental results

with Ag nanoparticles have been compared with those of

furnace curing and electroplating.

It was found that the structural quality of the sintered Ag

pad was almost the same as that of an electroplated Ag

film, so that no difference in wire bondability was obtained

when the near-infrared CW laser was irradiated for a short

time: a millisecond order per lead. In comparison with fur-

nace curing and electroplating, the superiority of the high-

speed laser plating was confirmed from the viewpoints of

material consumption (picoliter order), necessity of pre-

and post-processing, thermal damage to the pad and sub-

strate, and environmental protection.

As for the reliability of the wire-bonded leadframe, other

environmental and endurance tests in addition to the high-

temperature storage test mentioned above should be car-

ried out to confirm the advantages of the high-speed laser

plating for wire-bonding pad formation.

AcknowledgmentsThis research was conducted as Practical Application

Research and supported by JST Innovation Satellite Ibaraki.

The authors would like to thank Director Katsutoshi Goto

and staff for their assistance. Acknowledgments are

extended to Toshiyuki Asano, Ibaraki Prefectural Industrial

Technology Center for testing and measurements.

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