The new primer with copper foil corresponding to semi-additive process for package substrates

The demand for downsized and light-weighted electronic parts has become stronger to realize the higher electrical performance and smaller size of electronic equipment. Thus, the package substrate used in electronics is demanded to be thinner and higher in wiring density. Generally, semi-additive process using insulating film is used to form lines finer than line/space: L/S=20/20 μm in the package. The problem is that the thinner insulating film is likely to cause the higher warpage due to the higher coefficient of thermal expansion (CTE) and lower modulus. The applying glass-fabric prepreg to outer-layer of the package may result in the lower warpage, but it is difficult to make finer lines less than L/S=20/20 μm [1]. We started to develop the new primer with a copper foil (PF-EL) having high adhesion property with electrolessly plated copper and glass-fabric prepreg. After laminating the PF-EL with glass-fabric prepreg, the copper foil is etched out. Then, the surface of appropriate roughness made by replica of the copper foil profile is remained on the primer. The primer with appropriate roughness is applicable to semi-additive process, resulting in the higher wiring density and lower warpage for package substrate. In addition, PF-EL can be also applicable to the conventional press process without the laminator process. The peel strength of PF-EL for electrolessly plated copper is equivalent to that of the chemically roughened insulating film (0.7 kN/m or more), the sufficiently enough value for the excellent reliability. This higher peel strength is achieved by chemical functional groups and the optimization of curing degree of primer resin. The line/space of the fine line obtained with the primer is L/S=10/10 μm.