Issue link: https://iconnect007.uberflip.com/i/857644
August 2017 • The PCB Magazine 83 Numerous tests were performed to obtain high MD at various current densities with en- hanced mechanical properties of the plated metal such as tensile strength and elongation. Thermal reliability and structure of plated cop- per was also studied. Conditions and Bath Components Table 1 shows the operating conditions and optimum additive levels. Typically, the high throw bath has high acid to achieve higher con- ductivity inside the holes. Test Vehicles Various test panels with different thick- nesses and hole diameters were used covering a range of aspect ratios. The test vehicles used in the process evaluation were 1.6 mm, 2.4 mm and 3.2 mm thick boards and the through-hole diameters were 0.2 mm, 0.25 mm and 0.35 mm. The through-hole aspect ratio (AR) varied from 4.6:1 to 16:1. All geometries for each test board thickness were plated at the same time in the same tank and throw power was later calculated by using cross section analysis. The process flow included the following op- erations: • Acid cleaner—wets the hole and remove any organic contaminants • DI water rinse • Micro-etch—further smooths the surface and ensures excellent copper to copper adhesion • DI water rinse • Acid dip—acidifies copper surface prior to plating • Electroplating of copper in acid copper bath Cross-Section Analysis Cross-section analysis was started with the sample preparation process by punching or rout- ing sections from a desired area on the board or test panel. Pre-grinding of the coupon was done to get a flat surface closer to the through-holes. Plastic index pins were used to align the cou- pon vertically to the grinding surface. A fast- cure acrylic resin was used to mount the cou- pons. A ratio of 1-to-1, hardener-to-resin, was used to provide optimum penetration and a quick cure rate (10−15 minutes). After the sec- tions hardened they were subjected to grinding, polishing, and microscopic inspection to obtain micro distribution. Figure 1 shows a cross-sec- tion of a through-hole indicating the points of thickness measurements. Microdistribution The micro distribution is defined as the ratio of the average copper deposit thickness in the center of the through-hole to the average cop- per deposit thickness at the surface. It is calcu- lated according to the following equation: Results Microdistribution Microdistribution (MD) is the ratio between the average plated thicknesses in the middle of the hole to the average plated thickness on the surface as shown in Figure 1. Care should be tak- en when using MD % due to the difference in the board thickness, as the same diameter hole will be more difficult to plate in the thicker board as shown in Figures 2, 3, and 4. At the same current density, the same diameter hole has lower MD in a thicker board. For instance, a 0.2 mm hole in a 1.6 mm board at 10 ASF gave 90% MD measure- ment, while a 0.2 mm hole in a 2.4 mm board at 10 ASF gave only 75% MD measurement. Fur- thermore, a 0.2 mm hole in a 3.2 mm board gave 61% MD. Therefore, aspect ratio should be used to define the difficulty of the plating. HIGH-THROW DC ACID COPPER FORMULATION FOR VERTICAL CONTINUOUS ELECTROPLATING PROCESSES Figure 1: Cross-section of plated panel. Eq. 1: Microdistribution = (C+D) / 2 (A + B + E + F) / 4 x 100%