Airfoil: A new fine line fabrication technology on glass-cloth prepreg without insulating films for PKG substrate

As electronic parts increase its performance and miniaturize the size, package substrates are demanded to be thinner as well as higher in density. Thin packages such as Chip Scale Package (CSP) require high elastic modulus and low coefficient of thermal expansion with high density wiring to reduce warpage. To enable high density wiring, while maintaining crucial properties and minimal warpage, various fine line fabrications applied to glass-cloth prepreg have been investigated. The wiring processes with glass-cloth prepreg have made much progress from the subtractive method to SAP (Semi-Additive Process with primer for glass-cloth prepreg) and MSAP (modified SAP with ultra-thin copper). These have been achieved by decreasing the thickness of the to-be-etched copper layer. In the conventional SAP, the roughened surface of the primer is formed by imprinting the low profile copper (Ra = 0.3-0.5 μm) instead of the chemical etching of insulation film [1]. However, the flatness of the surface of Ra;0.3-0.5 μm makes the wiring under 30 μm pitch difficult. Therefore, we developed the new primer “Airfoil”, that can provide the excellent adhesive property with plated copper via chemical interaction. Airfoil has functional groups interacting with plated copper which provide the excellent peel strength despite of smooth surface (Ra<;0.1 μm). In addition, the high peel strength of Airfoil can be maintained for various types of prepregs by the diffusion controlled adhesive process. By applying Airfoil with glass-cloth prepreg, the wiring of 10 μm pitch was successfully fabricated by the SAP. Since Airfoil is very thin, it does not influence the prime properties of glass-cloth prepreg (e.g. elastic modulus, coefficient of thermal expansion, dielectric properties). Consequently, the package substrates of Airfoil with prepreg show the low coefficient of thermal expansion and high elastic modulus with 10 μm pitch wi- ing.