High Conductivity Nickel-Fiber ACF
Jay Browne
BtechCorp

Btechcorp has recently completed full commercial qualification testing of a unique Z-axis film for microelectronics packaging. Originally developed for advanced USAF fighter radar displays, TP-1 Anisotropic Conductive Film (ACF) aligns fine nickel fibers in a thermoplastic adhesive matrix to achieve unprecedented Z-axis thermal and electrical conductivity.

Z-axis adhesives are widely used to connect display devices with their control circuits. Current adhesives typically contain small electrically conductive spheres. When the circuits are aligned and pressed together with the adhesive in between, the particles will only make electrical contact with the circuit line directly above and below them. This individually connects the parallel circuit lines with no shorting between the circuits.

These Z-axis electrical interconnect adhesives face two problems. Because the circuit surfaces can’t be perfectly flat and the conductive particles can’t be perfectly sized, the particles must be physically deformed in order to get enough of them in electrical contact with both circuits. This requires up to 7 MPa (1000 psi) bonding pressure and leads to losses of display devices due to breakage. The second problem is that current Z-axis adhesives have a limit to how fine a circuit they can interconnect.

Our TP-1 adhesive solves these problems. The adhesive requires only 0.35 MPa (50psi) bonding pressure to achieve a good contact with the substrate that has a relatively low thermal and electrical interface resistance. The fineness of the circuit that can be joined with a Z-axis fiber adhesive is limited only by the diameter of available fibers, filaments or wires. With a 10mm fiber one could expect to be able to vertically interconnect circuits as fine as 20mm.

Our thin Z-axis composite has high thermal and electrical conductivity fibers aligned from the top to the bottom of the adhesive film. The fiber ends make good contact with the bond pads because their natural spear shape pushes through the surrounding polymer, making a tight contact with the surfaces as pressure is put on the bond. An insulating coating prevents cross connection of the wires. Figure 1 shows some 40% nickel fiber volume (8 micron fiber diameter) with oxide coating.

The angle of the fiber to the surface of the adhesive is critical. It must be slightly less than 90° so that the fiber consolidates along the bond line under moderate pressure. Too steep an angle makes the fiber act like a rigid column; too shallow allows it to be pushed aside.

Using continuous fibers as conducting paths instead of the metal-coated polymer spheres used by conventional ACF eliminates internal film interface resistance. It also allows a much higher bump pitch density of 11μm compared with 75μm or more for today's ACF.

TP-1 ACF provides a number of significant manufacturing advantages over current ACF materials. It needs much lower cure pressure, typically less than 50psi compared with greater than 300psi, allowing the use of plastic display substrates as well as glass.

Since thermoplastics can be re-bonded many times, instead of only once for thermoset and epoxy material, staged assembly bonding is now possible. For example, flex circuits can be laminated with TP-1 prior to singulation, then flip chip die can be bonded at the final assembly stage.

Perhaps the most dramatic impact on production process throughput is laminating TP-1 to a wafer prior to dicing. This eliminates the extra pick-and-place handling of film pre-forms and is especially useful for small die such as LED and RFID, where small film pre-forms are not practical.

Since TP-1 replaces the bump/underfill process used in flip chip packaging, and the material cost is less than half of competitive ACF, total process cost can be reduced by as much as 65% to 80%, with considerably increased production throughput. The high electrical and thermal conductivity and low cost also make TP-1 an attractive lead-free solder replacement.

Production qualification programs and evaluations currently underway include flip chip on flex, RFID tags, LED flip chip packaging, RF amplifier large area lead free solder bond replacement, and high density, hybridized CMOS imaging.

• Electrical Resistance
  Z-axis: <50 microhms (1.5 cm², 0.004" thick)    X-Y plane: 20 megohms

• Z-axis Connection Density 11 micron pitch

• Z-axis Thermal Resistance
  0.20° C-cm²/W (0.004 inch thick bond)

• Coefficient of Thermal Expansion
  Z-axis: 15 ppm/° C.     X-Y plane: 45 ppm/° C

• Tensile Modulus <10 Ksi (0.06 GPa)

• Ionic Purity
  Hydrolyzable Chloride <5 ppm    Hydrolyzable Sodium <2 ppm

• Rework Temperature 155° C

Product Form   Film pre-forms for reel supply or wafer back application prior to dicing. 2-10 mils (0.05-0.25 mm) thick, ± 0.1 mil
Cure Cycle  50 psi bond compression (<3 sec) at 150° C (resin temperature)
Storage Life  6 months at 27° C (80° F)

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