Fine-Pitch Metal-Laminated Fabric Substrates Using B-Stage Non-Conductive Films (NCFs)

Materials and Fabrication Processes

To fabricate the metal-laminated fabric substrates, 12-pm thick Cu foil, 40-pm thick В-stage NCFs, and commercially available polyester/rayon-woven fabrics were used. The NCFs were prepared by coating NCFs resin on a releasing film. The NCFs resin consisted of epoxy resin, thermoplastic resin for film formability, curing agent, and elastomer to control the materials properties of the NCFs. Three types of NCFs having various elastomer contents were used and the materials properties of the NCFs were summarized in the Table 10.1. As the elastomer contents increased, the NCFs resin viscosity increased due to the high molecular weight of the elastomer; however, the modulus and peel adhesion strength decreased because the relative amount of the epoxy resin decreased.

Figure 10.1 shows the process steps to fabricate metal-laminated fabric substrates. Cu was firstly attached to the NCFs coated on a releasing film. And then, Cu was patterned using conventional photolithography and Cu wet etching processes. For better handling, Cu/NCF/releasing film was temporary attached to a 4-inch silicon wafer using a double-sided tape. Through the patterning processes, the NCFs remained as solid

TABLE 10.1

Materials Properties of the NCFs Used in This Study

Units

NCFA

NCFB

NCFC

Test methods

Elastomer contents

wt%

40

30

0

Storage modulus3

MPa

10

183

1000

Dynamic mechanical analyzer (DMA)

Peel adhesion strength15

gf/cm

583

841

1555.3

90-degree peel test

Minimum viscosity'

Pa»s

14,470

4920

21

Rheometer

■' At room temperature. b Cu/Fabric laminates. c Ramp up speed: 5°C/min.

FIGURE 10.1

Fabrication of metal laminated fabric substrates using В-stage NCFs.

state, and the Cu patterns were successfully fabricated on the NCFs. Finally, the carrier wafer and releasing film were removed and the Cu electrodes on the NCFs were laminated onto the fabrics using a vacuum-lamination method (160°C, 60 min, and 0.14 MPa N2 pressure). By applying heat and pressure, the NCFs resin flow due to the reduced NCFs viscosity caused NCFs permeation into the porous the fabrics which will be discussed later.

NCFs Curing Property Optimization

Normally, conventional patterning processes such as photoresist (PR) baking and metal wet etching are performed at elevated temperatures. As a result, the epoxy-based NCFs can be thermally pre-cured before laminating onto the fabrics. And if they are pre-cured too much, the lamination cannot be possible because of the lack of adhesion. To prevent the NCFs pre-curing, the NCFs curing reaction should take place above the maximum processing temperature, in this case, 110°C. On the other hand, if the curing reaction temperature was too high, the NCFs resin will not be sufficiently cured during the processing. Therefore, the NCFs curing onset temperature should be optimized to prevent NCFs precuring during metal patterning process and NCFs should be sufficiently cured during the lamination process at 160°C. Figure 10.2 shows the degree of cure of the NCFs after the patterning process measured by the Fourier transform infrared (FT-IR) spectroscopy and the peel adhesion strength of the Cu/fabric laminates using pre-cured NCFs by a 90-degree peel test. As the curing onset temperatures increased, the NCFs were severely pre-cured over 50%. However, the NCFs having higher curing onset temperatures did not show any resin pre-curing.

If severely pre-cured NCFs were laminated onto the fabrics, NCFs showed poor adhesion to the fabrics. As shown in the Figure 10.3, NCFs were clearly detached from the fabrics resulting in degraded peel adhesion strengths. In terms of resin pre-curing and lamination temperatures, the onset temperature of the NCFs was optimized at 150°C, which showed less than 10% resin pre-curing after the patterning process, and finally cured after the 160°C vacuum-lamination temperature.

FIGURE 10.2

Degrees of pre-curing after the patterning process and the peel adhesion strength of Cu/fabric laminates using pre-cured NCFs.

Effects of NCFs Viscosities on the Fabric Substrates Morphology

The metal electrodes patterned on the NCFs were laminated onto the fabrics using a vacuum lamination method. During the lamination process, heat and pressure were applied and the NCFs viscosity gradually decreased with heating temperatures. As a result, NCFs resin flow occurred in two directions: 1) in-plane resin flow to fill the space between neighboring electrodes and 2) resin permeation into porous fabrics as shown in Figure 10.4.

Figure 10.5 shows the top-view optical microscope (OM) images of 100-pm-pitch Cu electrodes and cross-section SEM images of the 500-|im-pitch Cu electrodes on the fabric substrates using three types of NCFs. As the minimum viscosity of the NCFs decreased, the Cu electrodes were severely tilted, which could be explained by severe in-plane resin

FIGURE 10.3

Top-view SEM images of fractured surfaces of Cu and fabrics after the peel test.

FIGURE 10.4

Viscosity behavior of the NCFs and NCFs resin flow behavior during the lamination process.

flow to push the electrodes away causing Cu electrodes' displacement. This result shows that the Cu electrodes cannot be laminated on the fabrics as desired.

On the other hand, when the NCFs viscosity increased, NCFs resin extruded between two neighboring Cu electrodes, which can be explained by the NCFs permeation behavior. The resin-permeation behavior through the porous fabric materials can be governed by the Darcy's law,

FIGURE 10.5

Top-view optical microscope (OM) images and cross-sectional SEM images of the fabric substrates using three types of NCFs (red dash line indicates the original metal patterns to be laminated, and red solid line indicates the NCFs resin area between the Cu electrodes).

FIGURE 10.6

Top-view OM images of the ENIG/Cu electrodes and voids on the NCFs.

where Q is the volumetric permeation rate, ДP the pressure difference, rj the viscosity, and К the permeation constant depending on the porosity of the fabric substrates. Under Cu electrodes, NCFs resin was mixed with the fabric materials due to resin permeation. However, when higher-viscosity NCFs A and В were used, the extruded NCFs resin was observed between neighboring Cu electrodes as indicated as red lines in Figure 10.5.

Metal Surface Finish of Cu Electrodes on the B-Stage NCFs

In order to achieve a stable interconnection, Cu electrodes should be protected from oxidation. Therefore, electroless nickel immersion gold (ENIG) metal finish, which is one of the widely used for printed circuit boards (PCBs), was performed on the Cu electrodes after Cu patterning was completed on the NCFs. ENIG metal finish was also performed at elevated temperature (80°C); however, there was no NCFs resin pre-curing occurred, because the optimized onset temperature of the NCFs curing was higher than the ENIG-plating process temperature. However, another problem occurred during the electroless nickel plating process. Since the NCFs were exposed after Cu etching, the NCFs resin flow can be easily occurred at high temperature, as explained before. As a result, the low-viscosity film C showed severe voids in the NCFs, because severe resin flow caused resin agglomeration on the releasing film as shown in Figure 10.6. Therefore, to fabricate ENIG/Cu electrodes on the NCFs, NCFs viscosity should be as high as possible to prevent unstable NCFs morphology and Cu electrode pattern distortion.

 
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