High-Speed Laser Plating for Wire-Bonding Pad Formation

Maekawa et al.: High-Speed Laser Plating for Wire-Bonding Pad Formation (1/7) [Technical Paper] High-Speed Laser Plating for Wire-Bonding Pad Format...
Author: Tracy Wright
2 downloads 0 Views 2MB Size
Maekawa et al.: High-Speed Laser Plating for Wire-Bonding Pad Formation (1/7)

[Technical Paper]

High-Speed Laser Plating for Wire-Bonding Pad Formation Katsuhiro 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 nanoparticles. 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 printing, laser-plating parameters, quality of the sintered film, and wire bondability. It is found that both the quality of the sintered 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.

Introduction

However, thermal damage and adhesion to electronic

Conventional fabrication of functional films for elec-

substrates are problems to be solved. Recently, a process

tronic wiring and electrode formation relies on wet pro-

called laser sintering has been developed for gold or silver

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

nanoparticles.[2–4] Densification of metal nanoparticles

troplating, which need plenty of energy and resources. For

consists of such sequences as evaporation of solvents,

example, electroplating includes pre- and post-processing

decomposition of dispersant, necking of adjacent particles

procedures such as alkali degreasing, acid pickling, elec-

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

trolytic cleaning, water washing and drying. Besides, these

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

conventional technologies are not compatible with the

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

need for low-cost production and less environmentally

tion makes the sintered part denser, and interdiffusion

harmful emissions. As an alternative to these wet pro-

between the substrate and sintered part yields firm adhe-

cesses, ink-jet printing with metal nanoparticles together

sion.[5]

with an additional metallization process are attracting

In the present paper, the laser sintering method with Ag

much attention.[1] This printed electronics technology

nanoparticles is proposed as a tool for the formation of

enables us to make conductive patterns by applying a

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

small amount of metal nanoparticles only to the part where

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

the functional film is required.

large as around φ 100 μ m on the leads. Then, metallization

The conductive pattern is mainly obtained by a process

of the paste is completed with laser irradiation of a milli-

of thermal curing, in which a large depression of melting

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

point can be utilized; when the particle diameter is smaller

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

than 5 nm, metallization takes place at a low temperature

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

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

FIB-SIM, TEM, XPS and laser scanning microscope

7

Transactions of The Japan Institute of Electronics Packaging

Vol. 3, No. 1, 2010

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

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

lographic structure are discussed. Then, wire bondability

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

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

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

test. Finally, these experimental results are compared with

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

those of furnace curing and electroplating.

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

2.

High-speed Laser Plating

metallization starting from the paste surface with the visi-

Laser plating is defined as metallization of nanoparticles

ble laser makes the removal of solvent and dispersant dif-

with laser irradiation for the purpose of forming functional

ficult, resulting in an insufficient sintering with large pores.

films. Basically, the process consists of the following:

Near-infrared lasers with little absorption in the paste are

(1)

Metal nanoparticles with dispersant and solvents

more effective than visible ones in obtaining a dense metal

are pasted on the substrate by various methods

structure.

including IJ printing and spin coating. (2)

A short preheating is necessary to remove organic substances in the paste.

3.

Experimental Figures 2 and 3 show the experimental apparatus used

The paste is metallized by a millisecond-order irra-

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

diation of a laser beam under atmospheric condi-

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

tions.

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

Not only metallization by sintering but also inter-

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

diffusion or fusion takes places at the coating-

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

substrate interface, leading to firm adhesion there.

placed on a stage in the Nd:YAG laser equipment

Figure 1 schematically shows the laser plating method

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

with IJ printing. With Ag nanoparticles, we have succeeded

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

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

beam diameter is around 0.2 mm. A continuous-wave

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

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

(3)

(4)

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, insufficient sintering occurs or a porous structure is formed. An appropriate preheating condition was set at 100°C for 1 min on a hot plate when the silver NanoPaste® (NPS-J, Harima Chemicals) was used.[6]

8

Laser wavelength is another factor that influences sinter-

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

Fig. 1

Fig. 3

Schematic of laser plating using Ag nanoparticles.

Laser sintering as part of laser plating.

Maekawa et al.: High-Speed Laser Plating for Wire-Bonding Pad Formation (3/7)

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

Round pad patterns of around φ 100 μ m have to be

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

printed on the leads, and a flat pad surface with a thickness

was placed on the XY table to fix the leadframe.

over 2 μ m is required for wire bonding. However, it is well

Metal nanoparticles prepared by a gas evaporation pro-

known that the “coffee stain phenomenon” takes place as

cess have many advantages such as freedom from contami-

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

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

the contact angle of the droplet is less than 90°, and the

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

ambient conditions encourage droplet drying, the droplet

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

has a maximum evaporation rate at the boundary. Due to

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

temperature and hence surface tension gradients, there

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

results an effective flow of nanoparticles to the boundary.

size of nanoparticles was quite uniform with an average

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

diameter of 5 nm. Neither aggregation nor precipitation

ring-like stain of nanoparticles which decreases in concen-

leading to a broad size distribution was observed. Table 1

tration from the periphery inwards.

summarizes the properties of the paste used in the exper-

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

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

effect by means of controlling discharge rate and substrate

cosity as low as about 9

mPa•s.

temperature during IJ printing, together with varying pre-

The standard method for connecting a bare die to a

heating conditions. Figure 5 illustrates the process of mul-

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

tistep IJ printing to produce discrete pads on the lead-

technique for the production of discrete electrical connec-

frame. An appropriate stage temperature is necessary for

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

controlling the spread of the droplet. The bank formed by

the connections must have suitable contact areas, or so-

the first droplet prevents the second one from overrunning

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

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

sive strength and reliability. Copper leadframes are often

plays the important role of controlling the wettability of the

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

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

lead. Figure 4 shows the Cu leadframe specially designed

sufficient for making a φ 100 μ m flat pad. After each step,

for laser plating. The leadframe consists of Cu/99.28

heating is required for reducing the solvents present in the

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

paste as well as maintaining the pad profile.

having a thickness of 100 μ m, a lead width of 300 μ m, and

The Ag functional film thus obtained can be used as a

a line surface roughness of 0.06–0.07 μ m in Ra and 0.6–0.9

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

μ m in Rz. The leadframe was used as it was without any

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

chemical cleaning or degreasing, or thorough rinsing prior

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

to the IJ printing.

employed to bond an Au wire of φ 25 μ m on the sintered Ag film: ball bonding for the first bond and wedge bonding for

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

Before

After

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

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.

9

Transactions of The Japan Institute of Electronics Packaging

Vol. 3, No. 1, 2010

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

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

wire-bonder used in the experiment. To measure bond

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

strength, the wire bonded on the sintered Ag film was

the scan distance of 100 μ m into account, Ag nanoparticles

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

have been crystallized in the short time of around 75 ms.

Surface observation of the sintered Ag films was carried

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

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

the top surface consists of almost all Ag and the interface

cross-sectional observation of the sintered layers was car-

between the film and the copper substrate contains some

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

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

The sintered film was ion-milled stepwise and tilted by 30°

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

for SIM observation. The sintered film was also analyzed

thin diffused layer, which probably causes firm adhesion to

with TEM to confirm the formation of the Ag crystalline

the substrate used without any pre-treatments.

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

4.2 Multistep printing of 5-nm-particle Ag paste

XPS to measure chemical compositions.

Using the multistep IJ printing as illustrated in Fig. 5, we can overcome the coffee-stain effect and achieve a thin, flat

4.

Results and Discussion

4.1 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 μ m in height; the thickness is 0.2 μ m at the center. Figure 8 shows the cross-sectional image near the pad center: (a) FIBed SIM and (b) TEM images. In these fig(a) FIBed SIM image

ures, the Ag bulk structure and the Ag crystalline lattice

(a) Semiautomatic wire-bonder

(b) Pull test of bonded wire and fracture mode

Fig. 6 Wire bonding and bondability test.

Fig. 7

(a) IJ-printed (b) Laser-sintered Single-step IJ-printed and laser-sintered patterns on

Cu leads.

10

(b) TEM image Fig. 8 Cross-sections of sintered φ 5-nm-particle paste.

Fig. 9

Atomic percent profile of cross-section of sample

in Fig. 8.

Maekawa et al.: High-Speed Laser Plating for Wire-Bonding Pad Formation (5/7)

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

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

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

in the course of testing.

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

Table 2 summarizes the pull strength when the IJ-

Besides, no thermal damage such as oxidation to the lead

printed pads were prepared by preheating at 100°C for 10

takes place after the single-path laser irradiation through

min just before laser irradiation. The number of test sam-

the center of the pads from left to right.

ples was approximately 100. Very few wires separated from

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

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

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

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

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

the minimum one is 7.0 cN.

around 3 μ m. The plateau is as large as φ 100 μ m, which is

In comparison, the electroplated pad has average bond

large enough for a φ 10–25 μ m wire to be bonded. How-

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

ever, a few undesired cracks and voids can be seen on the

2.0 μ m, respectively, being close to the results of the laser-

pad.

plated film with the φ 5-nm-particle paste. In addition,

Figure 12 shows the SIM image of the FIBed cross-

breakage mode in the electroplated pad was more stable;

section. Using the multistep IJ printing method, we can

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

increase pad thickness from 0.2 μ m to 3 μ m. Note that the

at B or C.

cross-section has been tilted by 30° for observation.

Regarding the reliability of the wire-bonded leadframe,

Although full crystallization is not achieved and a porous

the specimen exposed in an atmospheric electric furnace

structure appears in the sintered Ag portion, it seems that

at a temperature of 150°C with a holding time of 1000 h

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

showed no changes in pull strength and fracture modes:

strate.

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

4.3 Wire bondability of laser-plated Ag pads

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

Wire bondability was examined between adjacent leads in the manner illustrated in Fig. 6, in which the symbols,

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 sintering Fig. 10 Laser-plated Ag pads on Cu leads.

Fig. 12

FIBed cross-section of laser-plated Ag pad and Cu

lead.

Table 2

Pull strength of φ 25 μ m Au wire bonded to Ag pads.

Pad formation

Laser plating Fig. 11

LSM image of surface profile of laser-plated Ag pads

on Cu lead tip.

Electroplating

Thickness, μm

Strength, cN Max.

Min.

Avg.

0.2

10.3

6.0

8.2

3.0–3.2

10.5

7.0

8.6

0.2

9.9

7.0

8.4

2.0

10.1

7.6

8.5

11

Transactions of The Japan Institute of Electronics Packaging

Vol. 3, No. 1, 2010

5.

Summary High-speed laser plating for forming wire-bonding pads

on a Cu leadframe using Ag nanoparticles has been proposed. Its novelty lies in the implementation of drop-ondemand laser plating on the specially designed leadframe. Various aspects of the proposed method have been investigated, including experimental set-up, multistep printing, laser-plating parameters, quality of the sintered film with Fig. 13 FIBed cross-sections of furnace-sintered and electro-

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

plated pads on Cu leadframe.

between the Ag pad and Au wire. Experimental results with Ag nanoparticles have been compared with those of

by furnace curing. The leadframe was heated under atmo-

furnace curing and electroplating.

spheric conditions in an electric oven at 220°C for 60 min.

It was found that the structural quality of the sintered Ag

Figure 13(a) shows the FIBed cross-section. Compared

pad was almost the same as that of an electroplated Ag

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

film, so that no difference in wire bondability was obtained

is more porous under the recommended curing condi-

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

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

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

that a dense layer is formed there, which prevents the

nace curing and electroplating, the superiority of the high-

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

speed laser plating was confirmed from the viewpoints of

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

material consumption (picoliter order), necessity of pre-

image.

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

The problem is that the Cu substrate suffers a degree of

strate, and environmental protection.

oxidation during the curing process. An oxygen atmo-

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

sphere is required for curing the nanoparticles because

environmental and endurance tests in addition to the high-

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

temperature storage test mentioned above should be car-

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

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

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

plating for wire-bonding pad formation.

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 problems took place in the laboratory.

Acknowledgments This research was conducted as Practical Application Research and supported by JST Innovation Satellite Ibaraki.

We carried out wire-bonding to an electroplated pad

The authors would like to thank Director Katsutoshi Goto

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

and staff for their assistance. Acknowledgments are

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

extended to Toshiyuki Asano, Ibaraki Prefectural Industrial

crystallization has been achieved, but the Ag surface is

Technology Center for testing and measurements.

rather rough: 0.27 μ m in Ra and 2.75 μ m in Rz, which is significantly higher than the laser-plated one: 0.09 μ m in

References

Ra and 1.51 μ m in Rz (measured area: 100 μ m × 100 μ m).

[1] H. Saito, M. Ueda, Y. Matsuba, and K. Oyama, “Liquid

Various pre- and post-processing procedures such as

wiring by ink-jet printing using NanoPaste®,” The 4th

alkali degreasing, acid pickling, electrolytic cleaning, water

International Workshop on Microelectronics Assem-

washing and drying, are indispensable in conventional

bling & Packaging, Kitakyushu, pp. 31–32, November

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

2004.

back surfaces of a leadframe must be removed before wire

[2] N. R. Bieri, J. Chung, D. Poulikakos, and C. P.

bonding. Otherwise, a masking process must be added to

Grigoropoulos, “Manufacturing of nanoscale thick-

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

ness gold lines by laser curing of a discretely depos-

posed method, however, no such additional operations are

ited nanoparticle suspension,” Superlattices and

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

Microstructures, Vol. 35, pp. 437–444, 2004.

was formed only on the top face of the lead.

12

[3] K.-S. Chou, K.-C. Huang, and H.-H. Lee, “Fabrication

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

and sintering effect on the morphologies and conduc-

Ag Nanopaste Film and Its Application to Bond-pad

tivity of nano Ag particle films by the spin coating

Formation,” Proc. ECTC 2008, pp. 745–1750, 2008.

method,” Nanotechnology, Vol. 16, pp. 779–784, 2005.

[7] Y. Matsuba, “Conductive ink using metal nano parti-

[4] K. Maekawa, M. Mita, K. Yamasaki, T. Niizeki, Y.

cles,” CONVERTECH, Vol. 369, pp. 52–57, 2003.

Matsuba, N. Terada, and H. Saito, “Packaging of Elec-

[8] R. D. Deegan, O. Bakajin, T. F. Dupont, G. Huber, S.

tronic Modules through Completely Dry Process,”

R. Nagel, and T. A. Witten, “Capillary Flow as the

Proc. ECTC 2008, pp. 950–955, 2008.

Cause of Ring Stains from Dried Liquid Drops,”

[5] K. Maekawa, K. Yamasaki, T. Niizeki, M. Mita, Y.

Nature, Vol. 389, pp. 827–829, 1997.

Matsuba, N. Terada, and H. Saito, “Influence of

[9] T. Kajiya, D. Kaneko, and M. Doi, “Dynamical visual-

Wavelength on Laser Sintering Characteristics of Ag

ization of “coffee stain phenomenon” in droplets of

Nanoparticles,” Proc. ECTC 2009, pp. 1579–1584,

polymer solution via fluorescent microscopy,” Lang-

2009.

muir, Vol. 24, No. 21, pp. 12369–12374, November

[6] T. Niizeki, K. Maekawa, M. Mita, K. Yamasaki, Y.

2008.

Matsuba, N. Terada, and H. Saito, “Laser Sintering of

13