Magnetic deposition of Ni/Au coated polymer core particles for flip-chip interconnection

With the increasing demand for high-density interconnections, flip-chip packaging has become increasingly used in the electronics industry. Traditional solder bump based interconnects are however unable to meet the challenge of ultrafine pitch and very high I/O count integration. Consequently, alternative interconnection methods are being investigated for such applications. This paper presents a novel process for flipchip interconnection through the use of Ni/Au coated polymer microparticles, similar to those already widely used in anisotropic conductive adhesives, but selectively deposited into the required locations under the control of a magnetic field. The particles used in this work consisted of a 9.8 μm diameter polymer core coated with layers of Nickel and Gold with thicknesses of 100 nm and 50nm respectively. Nickel is one of the few metals which demonstrate ferromagnetism and is also often used as the bump material on the bond pads of integrated circuits to allow flip chip bonding. In the work presented in this paper, the particles were suspended in an organic solvent, and placed together with a nickel bumped test chip within a uniform magnetic field created by two solenoid electromagnets. The results showed that the Ni/Au particles moved gradually toward the nickel bumped pads under the controlled magnetic gradient, and selectively deposited onto the bumps, providing a promising method to achieve fine or ultra-fine pitch integration.