Advanced Fine-Line Thick-Film Conductors with High ... · Advanced Fine-Line Thick-Film Conductors...

40

Transactions of The Japan Institute of Electronics Packaging Vol. 2, No. 1, 2009

Advanced Fine-Line Thick-Film Conductors with High Conductivity

and Soldering Capability Built by Screen-PrintingTakashi Yamamoto* and Dominique Numakura**

*NY Industries, 1-3 Kuribayashicho, Otsu-shi, 520-2151 Japan

**DKN Research, 62 Adams St., Haverhill, MA, 01830 U.S.A

(Received July 28 2009; accepted November 19, 2009)

Abstract

Previously, the common understanding with traditional polymer-based thick-film flexible circuits is that their low circuit

density with low electrical conductivity is because of an organic matrix in the conductor materials. The organic matrix

does not allow any soldering for the polymer thick-film circuits. It is the major reason why thick-film circuits could not

be the mainstream technology in the printed-circuit-board industry and semiconductor substrate industry even though

the technology provides a much lower manufacturing cost and high productivity without wet chemical waste compared

with traditional copper-etched circuits. However, advanced screen-printing processes using new conductive materials are

making remarkable improvements to overcome the technical barriers, and are generating application opportunities as

new electronic packaging technologies.

Keywords: Silver Conductors, Thick Film Circuits, Fine Lines, High Conductivity, Binderless, Printable

Electronics, Soldering, Migration

1. IntroductionThe low electrical conductivity of polymer thick-film

traces is caused by the basic construction and materials, as

shown in Fig. 1. The conductive inks, mixtures of conduc-

tive particles, binder resins and organic solvent, are

printed on the substrates through an appropriate printing

process. After the printing process, the solvent is removed

by low-temperature drying.

The binder resins work as pressure generators for the

conductive particles. After an appropriate curing process,

the binder resins shrink and generate compression pres-

sure that makes electrical contacts among the conductive

particles. This is the basic mechanism of the electrical con-

ductivity of polymer thick-film conductors. The electrical

currents flow through the contact points of the conductive

particles. Because of the small sizes of the contact points

between the conductive particles and longer current paths,

the conductivity of the traditional thick-film conductors is

three to four orders lower than that of solid copper metal

conductors.

The resolution of the thick-film conductors depends on

the total balance of the ink materials, substrate materials,

and capabilities of the printing equipment. Recent screen-

printing and ink-jet printing processes are capable of gen-

erating lines finer than 20 microns. Affinities between the

ink materials and surface conditions of the substrate have

been becoming the bottleneck to generating finer line con-

ductors for thin flexible substrates.

The major barrier for soldering polymer-based thick-film

conductors is the organic resin binder for the conductive

metal particles. As shown in Fig. 1, the majority of the sur-

face on traditional thick-film conductors is covered with an

organic polymer based binder resin. It is the major reason

why the thick-film conductors cannot be wetted with mol-Fig. 1 Conducting model of the traditional polymer thick-filmconductors.

41

ten solder.

Nano pastes that use nanometer size conductive par-

ticles can improve the conductivity and trace resolution of

the thick-film circuits; however, they are not effective for

soldering, unless organic base binders are used for the

conductor traces.

The situation with ceramic-based thick-film circuits is

basically the same in terms of soldering. The majority of

the trace surfaces are covered with a glass matrix that

works in a manner similar to binder resins. In the case of

ceramic-based thick-film circuits, however, it is possible to

conform the conductor traces without the glass matrix and

the traces are available for soldering, although noble

metals such as gold, platinum and palladium should be

used instead of silver. During high-temperature firing,

these noble metal particles melt at the contact points and

diffuse into each other. This process results in metal-metal

bonding between the conductive metal particles and pro-

vides a relatively large cross-section for the conductor

traces.

2. Basic processUnfortunately, the same metals with the same firing

process as the ceramic-based thick-film circuits are not

available for the polymer-based thick-film circuits because

of the limited heat resistance. All of the organic molecules

decompose and vaporize at the firing temperature of the

noble metal particles. A modified idea was introduced to

make thick-film conductor reducing binder resins from

silver conductor ink. An organic silver molecule was

employed as the basic material of the conductive ink for

the thick-film flexible circuits. The ink paste can be applied

using standard screen-printing equipment. Under the high-

temperature baking process, the organic silver molecules

decompose under reduction circumstances and become

metallic silver particles. The organic components, includ-

ing the binder resin and the solvents of the conductive ink

are removed as vaporized gases, and the conductor traces

do not have significant amounts of organic components.

The amount of plastic resin that works as the binder of the

conductor traces could be less than 2% by volume. For this

reason, the conductors are defined as “binderless”.

The silver particles have active surfaces after the decom-

position reaction. The decomposed organic fatty acid

allows reduction around the silver particles. During the

baking process, the silver particles contact and diffuse into

each other making the metal-metal bonding as shown in

Fig. 2. The contact-area size per particle can be much

larger than with the conductors made by traditional thick-

film processes. The effective cross section of the traces for

the electrical current is one order larger than that of tradi-

tional thick-film silver conductors. One issue with these

conductors is the bond strength of the metallic conductors

on the organic substrate; therefore, an appropriate surface

treatment such as plasma treatment must be conducted

before building the conductors. An under coating of adhe-

sive resin, such as epoxy resin, is another choice to

achieve high bond strength of the conductor traces. (Fig.

3)

The resolution of the traces made by screen-printing

depends on the affinities between the conductor inks and

the surface conditions. The particle sizes or resolution of

the screen masks are not the major factors for the resolu-

tion of the final thick-film traces. The surface tensions and

contact angles cannot be the major factors to generate fine

traces either. Appropriate material combinations have to

be chosen with good interface affinities through many trial-

and-error experiments.

As shown in Fig. 2 and Fig. 3, the majority of the surface

of the low-resistance conductors could be covered with

metallic silver that can be wetted with molten solder.

3. Trials and resultsPowder mixtures of the silver fatty-acid compounds and

Fig. 2 Conducting model of the binderless thick-film conduc-tors with surface treatment.

Fig. 3 Conducting model of the binderless thick-film conduc-tors with bonding layer.

Yamamoto and Numakura: Advanced Fine-Line Thick-Film Conductors with High (2/6)

42


silver oxide were prepared as the primary conductive

material for the thick-film traces. The powder size distribu-

tion is between 0.1 and 1.0 microns. The conductor ink

paste was prepared adding small amounts of low molecular

weight polyester resin to stabilize the printable ink. The

polyester resin has similar characteristics to the binder

resin; however, most of the resin is removed from the con-

ductor traces by decomposition and vaporization during

the high-temperature baking process. Mixtures of organic

solvents were added to get an appropriate viscosity for the

screen-printing.

50-micron-thick polyimide films and PEN (Polyethylene

Naphthalate) films were employed as the flexible circuit

substrates to ensure adequate heat resistance for the high-

temperature baking and soldering process. Several surface

treatments were conducted on the surfaces of the sub-

strate films before the conductor generation to guarantee

good affinities between the substrates and traces.

A series of trials were conducted changing screen

meshes, surface treatments, baking conditions and more.

Crosshatch tests were conducted to evaluate the effects

of the surface treatments. The results are summarized in

Table 1. The binderless conductors have very low bond

strength with the polyimide films and PEN films without

surface treatments. Plasma treatment and coating of the

bonding resins provide good bond strength between the

binderless conductors and the film substrates. However,

the plasma treatment did not give good resolutions; there-

fore, it was omitted from the further study of fine line

traces. A coating of polyester resin was employed as the

standard surface treatment for the binderless thick film

conductors.

The effect of the screen meshes for the fine-line genera-

tion was evaluated with the binderless conductors. A fine-

line test pattern down to 15-micron lines and spaces was

used for the evaluation. Table 2 summarizes the test

results. The #500 mesh provides the finest resolution of 30-

micron lines and spaces. Finer meshes did not make any

improvement of the fine resolution. This shows that the

resolutions and meshes are not the key factors to generate

fine-line thick-film traces. No significant differences were

observed by changing the film substrates.

A series of trials of the baking conditions were con-

ducted to optimize the conductivity of the binderless con-

ductors. Table 3 summarizes the results. A JIS standard

test pattern was used for the test. The data indicates that

the higher temperatures and longer baking times produce

lower conductor resistance. The high temperature condi-

tions could reduce the amount of organic resins between

the silver particles, and accelerate the diffusion of the

silver atoms. Accordingly, the high temperature conditions

provide lower conductor resistance.

The tests were conducted with polyimide films and PEN

films. Both of these films have similar conductivity trends.

However, PEN films have different other mechanical prop-

erties such as dimensional stability; therefore, a baking

condition of 180 degrees C for 30 minutes was employed

as the standard for the further studies.

Figure 4 shows an example of the fine thick-film traces

generated on the polyimide film with the binderless silver

paste. The fine screen-printing process could produce 30-

micron lines and spaces using a #500 mesh screen mask.

The screen mask could have sharp line patterns for 30-

micron traces with 30-micron spaces. (The photo shows

50-micron lines and spaces as the minimum because of the

limited resolution of the camera.) The traditional silver

Table 1 Effects of surface treatment.

Treatments Substrates Bond Test

No Polyimide 0/100

No PEN 0/100

Plasma Polyimide 100/100

Polyester Polyimide 100/100

Epoxy PEN 100/100

Table 2 Effects of screen meshes.

Substrates Mesh Min. Line

Polyimide #150 150 microns

PEN #150 150 microns


PEN #350 80 microns


PEN #500 30 microns


PEN #800 30 microns

Table 3 Effects of baking conditions.

Temperature(degrees C)

Time (min.) Resistance(ohm*cm)

150 30 7 × 10–6

150 60 5 × 10–6

180 30 4 × 10–6

200 30 3 × 10–6

43

paste could produce 100-micron lines and spaces as the

finest resolution. There were no significant resolution dif-

ferences observed between the polyimide films and PEN

films. The binderless silver paste provides more than three

times finer resolution compared to the traditional silver

paste.

Figure 5 shows the width dependency of the conductor

resistance for the 100-mm-long traces. The traditional

silver paste (B) cannot generate finer traces than 100-

micron lines and spaces on the flexible substrates even

though a high-resolution screen mask is used. On the

other hand, the binderless silver paste (A) provides finer

resolutions than 30-micron lines and spaces.

The graph does not show the inverse proportions

between line width and resistance, especially in the fine

line area. This was caused by thinness and non-rectangular

shape of the fine conductors. This kind of graphed data is

more valuable than physical properties such as volume

resistivity for the actual design process of the thick-film

circuits. The designers do not need to consider the correc-

tion factors for shape and thickness of the conductors.

Figure 6 shows a SEM photo of the 50-micron-wide

thick-film conductors. Because of the extreme thinness of

the conductors, the SEM views shadow images of the con-

ductor instead of 3D images. A thickness of less than 2

microns can be estimated from the limitation of the SEM

capabilities.

Figure 7 shows an example of the 3D measurement of

the conductor. The z-axis scale of the figure is ten times

larger than x- and y- axes. The cross sections of the con-

ductors are not rectangular. The average thickness of the

70-micron-wide trace is 4 to 5 microns. For the lines finer

than 50 microns, 3D analysis is not available. The thickness

of the 30 to 50 micron traces could be below 3 microns,

which is the lower limit of the 3D measuring equipment.

Several soldering tests were conducted for the thick-film

traces. As is well known, thick-film traces made of tradi-

tional silver paste cannot be wetted with molten solder

under any conditions at all. On the other hand, the binder-

less thick-film traces could be wetted with both eutectic

and lead-free solder without any pre-treatment such as flux

coating.

Table 4 shows a comparison of the soldering capabilities

of the thick film conductors built on thin plastic films. The

standard eutectic solder paste for the soldering test was

coated without additional flux on the 1-cm-square pads

made using the thick-film process and placed on a heat-

plate tester at 230 degrees C for 20 seconds. The coverage

of the conductor pads by molten solder was checked after

cooling down.

Fig. 4 Example of fine line traces with binderless silver Paste(50-micron L/S).

Fig. 5 Comparison of the conductor resistance.

Fig. 6 SEM view of the binderless fine-line conductors.

Fig. 7 3D dimensional analysis of the fine-line traces.


44


As was expected, the thick-film conductors made using

the screen-printing process with traditional silver paste do

not show any soldering capabilities. On the other hand, the

thick-film pads made with binderless conductors show

excellent affinities with the molten solder. 100% of the pad

surfaces are covered during the short heating period.

Figure 8 shows a typical example of the soldering cover-

age of the thick film pads made with binderless silver.

4. Discussion4.1 High Conductivity

Figure 9 shows a cross section photo of the thick-film

traces made using a screen-printing process. The wide

trace made of the traditional silver paste is thicker than 10

microns. On the other hand, the thickness of the narrow

binderless conductors could be less than three microns

with higher conductivities. The volume resistance of

binder-less conductors calculated simulating rectangular

cross-sections is on the order of 10–6 ohm centimeter, that

is, one or two orders lower than the traditional silver paste.

It is two orders higher than the volume resistance of solid

metallic copper (10–8 ohm centimeter). An appropriate

selection of the undercoating of the adhesive resin pro-

vides excellent performances for both the fine line capabil-

ities and bond strength of the conductors. These advan-

tages of the binderless conductors will generate a lot of

new application opportunities, especially with printable

electronics and flexible electronics.

4.2 MigrationThe basic conducting mechanisms of the binderless

conductors do not positively reduce the migration phe-

nomena without additional surface treatment. Electroless

plating of copper and nickel reduces migration apprecia-

bly. However, there are several limitations to their applica-

tion. Traditional screen-printing of coverlay with epoxy

resin and polyester resin is recommended for the non-

electrical contact areas to eliminate migration issues.

Screen-printing of carbon paste is recommended for the

bare conductor areas.

4.3 Soldering capabilitiesAs was explained previously, the major applications of

the advanced screen-printing are assumed to be new

devices of the printable electronics and flexible elec-

tronics, in which soldering is not the major termination

method. The use of conductive adhesive resins and

mechanical connections could be practical solutions for

volume productions. However, soldering is preferred for

reliable connections with other devices or printed circuit

boards, even though standard SMT soldering process is

not available.

The soldering capabilities of the binderless thick-film

conductors are still valuable even though the soldering

conditions are limited because it cannot be applied to tra-

ditional thick-film circuits.

Figure 10 shows a cross-section photo of a sample-sol-

dered thick film pad. The thickness of the traces is much

less than the solder layer (less than 5 microns). The solder-

ing should be conducted with controlled temperatures and

limited heating times. Detailed soldering conditions need

to be determined for each circuit construction and solder-

ing material used. Because of the thinness of the thick-film

Table 4 Soldering capabilities with eutecticsolder paste.

Substrates Conductor Coverage

Polyimide Traditional 0%

PEN Traditional 0%

Polyimide Binderless 100%

PEN Binderless 100%

Fig. 8 Soldered pad of the thick-film traces with binderlesssilver.

Fig. 9 Cross section photo of the thick-film conductors.

45

conductors, the silver metal will be absorbed by molten sol-

der at high temperatures or longer soldering conditions.

5. ConclusionA series of screen-printing trials have shown that bind-

erless silver conductor provides not only fine traces with

high conductivity, but also soldering capabilities. The capa-

bilities will be valuable to generate many kinds of new elec-

tronic devices for printable and flexible electronics. The

new thick-film circuits built on flexible substrates will

widen the applicable ranges of the circuit technology.

The technology considered in this study could be a very

basic part of the whole printable electronics or flexible

electronics. The via-hole capabilities should be considered

for double and multi-layer constructions in future studies.

Further capabilities to build embedded components

should be considered to complete the whole printable elec-

tronics.

References

[1] “Advanced Screen Printing Process –Practical

Approaches for Printable & Flexible Electronics”,

Dominique Numakura, 3rd IMPACT and the 10th

EMAP, Taipei/Taiwan, October 2008.

[2] “Manufacturing process of the Printable Electronics”,

Dominique Numakura, Nikkan Kogyo Shinbunsha,

January 2009.

[3] “Practical Printable Electronics Produced by Screen

Printing Process”, Masafumi Nakayama and Domin-

ique Numakura, Japan Photo Fabrication Association

Seminar, February 2009.

[4] “Embedded passives Components built on Flexible

Substrates”, Dominique Numakura, IPC International

Conference on Flexible Circuits, April 2009.

[5] “Flexible LED Arrays Made by All Screen Printing

Process”, Dominique Numakura, IPC EXPO and

APEX 2009, April 2009.

Fig. 10 Cross section photo of the soldered thick-film con-ductor.


Advanced Fine-Line Thick-Film Conductors with High ... · Advanced Fine-Line Thick-Film Conductors...

Documents

Transcript of Advanced Fine-Line Thick-Film Conductors with High ... · Advanced Fine-Line Thick-Film Conductors...