Plating method

#The present invention is directed to an improved plating method. More specifically, the present invention is directed to an improved plating method where adjustments of the plating cycle improve throwing power and reduce nodule formation.

#Generally, plating substrates with metals includes passing a current between two electrodes in an electrolyte where one of the electrodes is the substrate to be plated. Electrolytes for depositing a metal on a substrate typically include one or more metal ions, a soluble salt in a sufficient amount to impart conductivity to the electrolyte, and additives to improve plating uniformity and quality of the metal deposit. Such additives may include brighteners, levelers, suppressors, antioxidants, and surfactants.

#In many conventional plating processes electrodes (cathode and anode) of an external circuit are immersed into the electrolyte and a DC (direct current) is applied across the electrodes. This causes an electrochemical reaction or reduction resulting in deposition of a metal or metal alloy onto a cathode from metal ions in the electrolyte. Current density profile and primary distribution across the cathode varies according to the geometric path or distance between the anode and the cathode leading to deposit thickness variation according to shape and location of the cathode relative to the anode. This effect is most apparent when high applied average current densities are used. Therefore, in order to obtain the best uniformity of metal distribution, low applied average current densities are used.

#FIGS. 1A-B are photographs of a center section and upper section of a through-hole plated with copper having an average throwing power of greater than 0.9:1.

#FIGS. 2A-D are photographs of four sections of a surface of a copper plated circuit board showing nodules.

#As used throughout the specification the following abbreviations have the following meanings, unless the context clearly indicates otherwise: ° C.=degrees Centigrade; mA=milliamps; cm=centimeter; V=volts; hr=hours; min.=minutes; wt %=percent by weight; mm=millimeters; g/L=grams/Liter; mils=0.001 mils/inch; inch=2.54 cm/inch; ms=milliseconds; and SEM=scanning electron micrograph.

#The terms “printed wiring board” and “printed circuit board” are used interchangeably throughout the specification. “Depositing” and “plating” are used interchangeably throughout the specification. “Multilayer” refers to two or more layers. “Throwing power” is defined as the ratio of the metal deposit thickness at the center of a hole to the metal deposit thickness at the surface of the hole. “Aspect ratio” means substrate thickness divided by aperture diameter. “Aperture” means hole such as a through-hole or a depression such as a via in the surface of a substrate.

#All percentages are by weight, unless otherwise noted. All numerical ranges are inclusive and combinable in any order, except where it is logical that such numerical ranges are constrained up to 100%.