关键词: 电镀添加剂, 脉冲电镀, 高深径比通孔, 正交设计试验, 深镀能力

Abstract:

As an important component in electronic products, printed circuit board (PCB) plays a supporting and interconnecting role for the electronic components in it. With the development of communication technology, electronic products are developing in the direction of “thin, light and small”, and high density interconnection (HDI) comes into being. Due to the high-density interconnection characteristics of HDI boards, the thickness of the board is increasing. At the same time, in order to reserve space for the laying of fine lines on the subsequent board surface, the diameter of the through holes on the board is also decreasing, so the depth-diameter ratio of the through holes is increasing. In order to ensure the electrical interconnection between the middle layers of the HDI board, the through-hole plating technology has become the key. In the process of through-hole electroplating, due to the relatively small diameter of the through-hole, the current density distribution inside and outside the hole is uneven, and the dispersion ability of the plating solution is poor, resulting in uneven copper plating layer and thick surface copper layer, which is not conducive to subsequent fine circuits laying. An effective way to overcome this drawback is to add electroplating additives to the bath, and to use bidirectional pulse electroplating technology. Therefore, 2, 2'-dithiodipyridine (DTDP) with low toxicity and low cost was used as an additive in the plating solution, and its application in pulse plating of high aspect ratio through holes was studied. The additive concentration and pulse plating parameters were optimized. It was concluded that DTDP could be applied to pulse plating of high aspect ratio through holes. And through orthogonal optimization experiments, the optimal concentration of additives suitable for the high aspect ratio (14.5:1) through-hole electroplating was obtained. Finally, the throwing power of the plating solution was measured at about 86%, and the average thickness of surface copper was about 38 μm. It is not conducive to the operation of the subsequent process, and the through hole is in the shape of a “dog bone”; then the single factor analysis of the pulse parameters was carried out, and finally, the optimal single-stage pulse plating parameters were obtained. At this time, the throwing power of the plating solution was about 75%, and the average thickness of the surface copper was about 27.6 μm, which is convenient for the subsequent processing and solves the above-mentioned “dog bone” phenomenon. SEM test and tin immersion thermal stress test were performed on the experimental boards of the two experiments separately. The SEM images found that the coating particles of the two experimental boards were fine and uniform, and relatively flat; the tin immersion thermal stress test results did not find any cracks. These phenomena are all in line with industry requirements, so it provides a certain basis for the research of high aspect ratio through-hole pulse electroplating. Further experiments are needed in the follow-up to make the plating solution higher in throwing power and at the same time reduce the surface copper thickness.

Keywords: plating additive; pulse electroplating; high aspect ratio through hole; orthogonal design tests; throwing power