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As electronic parts increase its performance and miniaturize in its size, package substrates are demanded to be thinner and higher in density. But higher density substrates using insulating films give higher warpage values when they are very thin, due to its low modulus. Packages with glass-cloth prepreg as its outer-layer have lower warpage, but making a 40 μm pitch package with them are difficult because it cannot adapt to the semi-additive method. Conventional insulating films can be applied to the semi-additive method; however, a process of roughing the surface of the insulating films by chemicals is required, causing the limitation of material of insulating films. Therefore, we developed a new technology of semi-additive primer (SAPP) with glass-cloth prepregs which allows higher density and lower warpage for substrates. Under 30 μm pitch wiring of SAPP applied substrates were easier compared to conventional substrates made from the insulating film semi-additive method. Also, the roughening process (Ra=0.50-0.60 μm) of conventional insulating films to achieve adhesion strength with plated copper makes high density wiring difficult. But the new SAPP system has a surface roughness of Ra <;0.25 μm with high adhesive strength which allows easy high density wiring. It has been made clear that by providing a new adhesive-agent layer, peel strength equivalent to that of the roughened insulating film (0.7 kN/m or more) can be obtained with sufficient reliability. The thinner package boards consisting of our new low-CTE and high modulus insulation prepreg E-700G(R) and SAPP have lower warpage than that of build-up structure and coreless structure using conventional insulating films. The package substrates made of SAPP with E-700G(R) core show low coefficient of thermal expansion (9-10 ppm/°C) and high modulus of tensile elasticity (32 GPa), with 20 μm pitch wiring possible.