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PCB-Aug2017

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82 The PCB Magazine • August 2017 Sulfuric acid copper baths are heavily used in the PCB industry due to their low maintenance cost as compared to other acids like MSA (meth- ane sulfonic acid). Typical sulfuric acid copper baths contain copper sulfate, sulfuric acid, chlo- ride ions, and organic additives. These additives play a crucial role in controlling the deposit dis- tribution as well as the properties. To meet spe- cific objectives of a plating process these addi- tives should be monitored and controlled prop- erly. The additives work in combination and when controlled within a given range improve thickness distribution, mass transfer, eliminate nodule formation and can fill blind vias. Name- ly these additives are levelers, brighteners, and suppressors. The leveler is a mild suppressor that adsorbs onto specific locations such as cor- ners and peaks of base materials [2] . In the presence of a micro profile the dif- fusion layer tends to be thin at the peaks and thick at the valleys. In this case, if plated with- out a leveler the micro profile will be exagger- ated. On the other hand, if a leveler is present the plating on the peaks will be suppressed and the micro profile will be diminished. Brighten- er is also called an anti-suppressor and as the name implies it reduces the suppression. Most importantly, it also acts as a grain refiner to de- posit copper with a fine grain structure in ran- dom orientation [4] . Therefore, brightener has the most influence on final structure and phys- ical properties of the deposit such as tensile strength and elongation. The suppressor works in the presence of chloride ion to adsorb onto the cathode and in- crease the effective thickness of the diffusion lay- er [3] . Consequently, the plating current increas- es and the deposit becomes more uniform and a densely packed copper deposit can be obtained without burning. This modified diffusion layer improves the distribution of the deposit espe- cially in high throw applications. Owing to the growth of high aspect ratio circuit board manu- facture, the demand for high throw acid copper electrolytes has increased dramatically over the past few decades. Especially DC copper plating for high aspect ratio electroplating is extreme- ly desirable due to simplicity of the process and inexpensive equipment requirements [1, 2] . In this work, we present an innovative DC high throw acid copper electroplating system with high thermal reliability and even copper metallization in high aspect ratio PCBs. This system also allows the plating to be done at high current density without surface imperfec- tions. The allowable CD ranges from 10–30 ASF and the micro distribution(MD) measured is ≥ 85% for the AR 8:1. HIGH-THROW DC ACID COPPER FORMULATION FOR VERTICAL CONTINUOUS ELECTROPLATING PROCESSES Table 1: Bath components.

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