Conductive inks for flex circuits: with higher conductivity and lower resistivity, highly conductive silver inks are propelling new flexible applications, from RFID tags to "smart" clothing.

From RFID tags and wearable electronic devices to "thinking" carpets and "smart" clothing, a host of innovative applications rely on highly conductive silver inks to create very flexible printed circuits. New formulations attain resistivity levels an order of magnitude lower than previous materials, delivering higher conductivity and greater stability under thermal and environmental stresses. The resulting array of consumer, industrial and military products would not have been possible just a few years ago.

The manufacture of flexible printed circuits is one segment of the electronics industry in which new materials are enabling an ever-widening variety of products and devices. At the heart of many new designs are highly conductive silver inks that allow engineers to develop smaller, more powerful electronics that can be manufactured in very complex footprints. These formulations offer device manufacturers a way to reduce weight, conserve space and configure unconventional circuit shapes, even on flexible substrates, including clothes and other textile goods, PVC and even paper. To ease integration, the materials employ processing techniques already in common use.

One of the growing applications for highly conductive flexible inks is the group of so-called "intelligent" textiles. With the emergence of new materials, manufacturers have already prototyped innovations such as clothing that can dispense medication based on sensors that monitor the body's need. The same technology can be used to provide a fragrance or cooling action in response to elevated body temperature, deliver insect repellant on demand or even change the color of a garment. Although still in its infancy, studies predict that the intelligent textiles market will be worth more than $1 billion dollars per year by 2007.

At least two large firms have already commercially introduced ski jackets with embedded radios and music players, both of which incorporate Bluetooth mobile phone technology. In the flooring market, one supplier has demonstrated a prototype of an electronically networked carpet that can be used to control lighting, adjust temperature, monitor security and improve fire safety. In the military segment, new designs have been developed to show how electronics and fluidics could transform clothes into biometric bodysuits that respond to a wearer's environment and vital signs. The sensor technology includes pathogen detectors that are more reliable and more sensitive than current devices, permitting a garment to perform such specialized functions as identifying and capturing bacteria, destroying it, then amplifying the bacteria's DNA to look for characteristics of specific pathogens, such as anthrax or smallpox. Future versions could incorporate sensors to monitor a soldier's vital signs and fatigue, and even technologies that would change colors and patterns on the garment to promote superior camouflage.

Alongside this technology, the concept of thermogeneration is also beginning to mature, in which the temperature differential between the human body and the clothing can be used to generate electricity. Already proven in space technology applications, the final goal of this approach is wearable electronics that require no batteries. These applications typically require outstanding flexibility and low, stable conductivity to assure performance and durability, both of which are key features in new highly conductive ink formulations.

Among the growing applications for the highly conductive inks is the manufacturing of smart card antennas and bridges connecting chip to antenna. One of the formulations has also been developed specifically to provide a very smooth surface to enable high-frequency active RFID tag technology.

One of the most promising applications for silver-filled inks is in RFID (radio frequency identification), which until recently has been used primarily in small, closed-loop environments, including libraries and military inventories. The technology is now finding usage in other high-value applications, such as tracking prestige textiles and designer clothes, controlled building access and automated, high-speed fare collection. So far, the potential of low-cost RFID tags to identify everything from pharmaceuticals to fresh produce has been slow in developing, largely because the unit cost remains relatively high. Yet the potential is clear, with applications in medical, the military and elsewhere. As the trend continues, it's expected that annual RFID tag volumes will eventually rise into the billions.

Dow Corning's initial objective in developing highly-conductive silver-filled inks was to deliver a formulation that would provide a step change in performance, one that would address performance gaps and enable a broader range of substrates and applications. When product developers conducted benchmark testing, however, they found that the new formulations demonstrated resistivity a full order of magnitude lower than many competitive inks, which has since been validated in the market.

Perhaps more important, while most silver-filled formulations would typically experience an increase in resistivity over time, the new formulations have demonstrated an initial and predictable decrease, which is then maintained throughout the test. (See TABLES 1 and 2.) As a result, they remain far more stable under thermal or environmental stresses that would tend to degrade the performance of conventional silver-filled materials.

Based on specialty organic chemistry, the newest formulations are designed around silver-filled epoxy polymers and a proprietary additive package. The unique chemistry of the technology relies on a combination of additives that initially clean the silver particles, removing the fatty acids and other lubricants that are typically found on the silver from conventional processing.

Product engineers hypothesize that this cleaning action helps promote a level of cold welding to enhance particle-to-particle contact in the materials, which brings about the significant improvement in electrical performance. The assumption is well supported by thermal and electrical conductivity data and long term environmental stress testing.

In addition, the additive package reduces or eliminates the formation of silver oxides, a common problem with silver-filled materials that can decrease conductivity over time. Typical silver-filled materials are susceptible to dendritic "whisker" growth when exposed to electric current and high humidity, which can bridge adjacent lines and cause a short to occur. By dramatically reducing the effects of acids and the formation of silver oxides, the new materials help prevent the conditions responsible for silver migration. Testing thus far has shown no evidence of dendritic growth after 1,000 hours at 85[degrees]C/85% relative humidity, using a test bias of 100 volts and .050" line spacing. (See FIGURE 1.)

[FIGURE 1 OMITTED]

Processing and Curing

As shown in the tables, the new materials family includes both thermoset and thermoplastic formulations to accommodate a wide range of substrates, performance requirements, cost considerations and process conditions, giving developers the versatility to consider many new applications. Manufacturers can specify low-, mid-or high-temperature curing systems, with physical properties and processing characteristics to suit individual manufacturing operations.

The thermoset and thermoplastic formulations can be screen--or stencil-printed for low-cost, high-volume application using standard production equipment. (Table 1 offers typical process recommendations.) The formulations have been proven effective in high-speed laser ablation, a promising manufacturing technology that can produce feature resolutions as low as .00008" (2 microns). Equally important, laser processing eliminates many of the production steps and virtually all undesirable effluent.

The new inks are typically dried/cured using standard convection and IR reflow ovens or belt furnaces to accommodate high-speed manufacturing operations. Because some formulations can be cured at low temperatures, they may permit the use of low-cost substrates which could not withstand high heat, such as fabrics and inexpensive plastics. With epoxy used to form the polymer matrix, the formulations are robust and difficult to over-cure. The process ease and the performance of the materials contribute to a favorable cost structure.

Because thermoset materials crosslink and shrink during cure, they form an inherently tighter network than thermoplastic formulations and can generally offer superior electrical conductivity. Thermosets are typically specified when chemical--and moisture resistance are needed, as the materials actually form a chemical bond with the substrate that they're applied to.

Depending on the specific formulation, the heat cure cycle can be as short as five minutes at 150[degrees]C or even less using a variable frequency microwave system, which achieves a complete cure within two minutes. These materials have a quick response when cured in a variable-frequency microwave (VFM), which does not affect the substrate, making the technology well suited for manufacturing wearable electronics.

The new thermoplastic formulations offer cost efficiencies, and they attain such a compact structure after solvent removal that their conductivity exceeds the performance of conventional thermoplastics, even rivaling that of many existing thermoset materials. Although mild heat (85-120[degrees]C) is recommended to efficiently drive off the solvent carrier, the thermoplastic formulations can be dried at temperatures as low as 40[degrees]C. This low-temperature processing ability can help enable the use of inexpensive substrates, including phenolics, polyesters (Mylar and other PETs), PVC, styrene and even paper.

As materials technology continues to advance, it brings performance and size advantages that can translate to greater speed and functionality from smaller form factors. Combined with the ability to configure complex shapes and incorporate flexible substrates, along with dramatically lower resistivity, new silver-filled ink formulations are helping engineers develop smaller, lighter, more powerful and more reliable products, while permitting the design of new devices and technologies that were not possible using previous materials.

TABLE 1. Physical properties testing of new thermoset polymeric
interconnect materials illustrates improved resistivity
(negative %DR) after exposure to thermal stress.

                          THERMOSET #1 *           THERMOSET #2 *

Viscosity                   10,400 cps                6,600 cps

Cure                   10 min/180[degrees]C   10 min/120[degrees]C -or-
                                                 20 min/85[degrees]C

Bulk resistivity        3.1 x 10-5 W / cm         4.2 x 10-5 W / cm

Sheet resistivity           7 mW / sq                17 mW / sq

Damp heat,                    -38%DR                   -26%DR
1000 hrs @
85[degrees]C/85%RH

Long-term aging,              -40%DR                   -12%DR
1000 hrs @
125[degrees]C

Thermal shock:                -28%DR                   -14%DR
-55[degrees]C to
125[degrees]C,
100 cycles

Thermal cycling:              -43%DR                   -34%DR
125[degrees]C,
25[degrees]C,
-65[degrees]C,
25[degrees]C,
100 cycles

                          THERMOSET #3 *           THERMOSET #4 *

Viscosity                   21,500 cps               19,000 cps

Cure                   10 min/120[degrees]C     10 min/120[degrees]C

Bulk resistivity        3.3 x 10-5 W / cm         6.2 x 10-5 W / cm

Sheet resistivity           13 mW / sq               25 mW / sq

Damp heat,                    -32%DR                   -28%DR
1000 hrs @
85[degrees]C/85%RH

Long-term aging,              -23%DR                   -12%DR
1000 hrs @
125[degrees]C

Thermal shock:                -18%DR                   -14%DR
-55[degrees]C to
125[degrees]C,
100 cycles

Thermal cycling:              -42%DR                   -38%DR
125[degrees]C,
25[degrees]C,
-65[degrees]C,
25[degrees]C,
100 cycles

                      COMPETING THERMOSET #5

Viscosity                   41,400 cps

Cure                   10 min/180[degrees]C

Bulk resistivity        2.4 x 10-4 W / cm

Sheet resistivity          94.2 mW / sq

Damp heat,                   -1.8%DR
1000 hrs @
85[degrees]C/85%RH

Long-term aging,             +2.2%DR
1000 hrs @
125[degrees]C

Thermal shock:               -1.6%DR
-55[degrees]C to
125[degrees]C,
100 cycles

Thermal cycling:             -1.3%DR
125[degrees]C,
25[degrees]C,
-65[degrees]C,
25[degrees]C,
100 cycles

* Dow Corning materials

TABLE 2. The thermoplastic formulations of the new epoxy-based
materials offer conductivity levels that exceed the performance of
conventional thermoplastics.

                              THERMOPLASTIC #1 *    THERMOPLASTIC #2 *

Viscosity                         64,600 cps            9,520 cps

Cure                         10 min/100[degrees]C  10 min/100[degrees]C

Bulk resistivity              5.6 x 10-5 W / cm     2.0 x 10-4 W / cm

Sheet resistivity                 22mW / sq             81mW / sq

Damp heat,                          -56%DR                -76%DR
1000 hrs @ 85[degrees]C/
85%RH

Long-term aging,                    -55%DR                -59%DR
1000 hrs @ 125[degrees]C

Thermal shock:                      -49%DR                -58%DR
-55[degrees]C to
125[degrees]C, 100 cycles

Thermal cycling:                    -52%DR                -66%DR
125[degrees]C,
25[degrees]C,
-65[degrees]C,
25[degrees]C, 100 cycles

                                  COMPETING
                               THERMOPLASTIC #3

Viscosity                         5,420 cps

Cure                         9 min/120[degrees]C

Bulk resistivity              1.3 x 10-5 W / cm

Sheet resistivity                4.9 mW / sq

Damp heat,                         +41.4%DR
1000 hrs @ 85[degrees]C/
85%RH

Long-term aging,                  +179.6%DR
1000 hrs @ 125[degrees]C

Thermal shock:                     +2.1%DR
-55[degrees]C to
125[degrees]C, 100 cycles

Thermal cycling:                    +7%DR
125[degrees]C,
25[degrees]C,
-65[degrees]C,
25[degrees]C, 100 cycles

* Dow Corning materials

TABLE 3. Typical process recommendations are shown for silver-filled
conductive inks.

Screen mesh (stainless steel)      180-400 mesh
Squeegee type                      60-80 Shore A
Pressure / speed                   Application dependent
Recommended wet print thickness    18-36 mm
Recommended cured thickness        12-25 mm
Demonstrated print resolution      Trace width 0.2 mm; Spacing 0.15 mm

PAUL BERRY is Dow Corning's Global Application Engineering Center coordinator. He has worked in electronics materials application engineering for the past seven years. Berry holds a BSME from Michigan State University and an MBA from Central Michigan University. He can be reached at paul.berry@ dowcorning.com. GREG BUTCH is a principal S&T Technician for Dow Corning's Auburn Site Application Engineering Center. He has been producing, testing and analyzing electronics materials for the past 11 years. Butch has a degree in chemical technology from Delta College. He can be reached at g.s.butch@ dowcorning.com.