Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 169 of 707

enhanced by the presence of halogen ions. The concentration of these organic additives must also be closely controlled in order to attain the desired deposition properties and surface morphology. In most instances, the recommended working temperature for the sulfuric acid plating solution does not exceed about 27°C, and more typically, plating is car- ried out at room temperature. Because these copper plating electrolytes are designed for use at room temperature, they are not generally suited for plating through-holes at elevated temperatures. On many occasions, the brighteners undergo chemical changes at elevated temperatures and are no longer effective for copper plating. In other instances, the levelers used in combination with the wetter/suppressor additives in the solution present issues leading to the deposition of dull, rough layers, especially inside the through-holes. The thermal charac- teristics of the copper layer deposited at elevated temperatures are adversely affected and reliabili- ty performance decreases. Failure occurs during the sol- dering operation that follows plating.Over the past few years, printed circuit board fabrication has dra- matically increased in geographic areas with hot climates. In order to maintain the desired temperature in these areas, chillers or other cooling methods are general- ly needed. Thus, it is desirable to simplify the process in these warmer areas to elim- inate the need for chillers or other cool- ing methods while still obtaining a desired plating deposit. Figure 1: Thickness of copper plated on the surface and in the through- hole. Figure 2: HT 100 microdistribution on 1.6 mm measured at 24°C.. Figure 3: HT 100 microdistribution on 1.6 mm measured at 35°C. A new direct-cur- rent process for plat- ing smooth, bright, and planar copper layers at tempera- tures up to 40°C (104°F) is described in this article. The method outlined here- in yields excellent microdistribution. The organic brightener used assists in reducing the surface-to-hole thickness ratio, as well as improving the ductility and tensile strength of the copper deposit. Furthermore, the thermal characteristics of the deposited copper are excellent. 168

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