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100 PCB007 MAGAZINE I JULY 2022 ers, carriers, levelers), gave improved throwing power. And let's not underestimate the influ- ence that plating cell design, cabling connec- tions from the power supply to the buss bars, and solution movement have on plating distri- bution and the overall quality of the deposit. Additional challenges and complications have been introduced due to the changes in circuit board design technology. Shis in cir- cuit designs toward higher density and ultra- high density have ushered in a new wave of improvements in process technology. Several other electroplating processes are used in the fabrication of the printed wiring boards. Tin/lead or tin is electroplated, occa- sionally also electroless plated, to serve as a metal etch resist. Other electroplated met- als include nickel/gold and palladium (also deposited from immersion baths or electro- less baths) to form suitable surface finishes for insertion connectors or component attach- ment, e.g., wire bonding. An overview of these processes will be presented in a future column. Focusing on the acid copper electroplat- ing process, one would start with the pre- plate cleaning sequence of acid soak cleaner, micro-etch, and acid predip. e acid pre- dip is optional and if rinsing aer micro-etch- ing is sufficient, can be omitted. However, this only applies to using peroxide/sulfuric acid micro-etchants. Use of persulfate-based etchants indicates an acidic pre-dip. Critical acid cleaner variables are chemical composi- tion, concentration, time, and temperature. Regarding the micro-etch, etch depth is the critical parameter, which in turn, is influenced by the chemical composition, concentration, time, and temperature. e acid predip, typ- ically of the same acid and concentration as is used in the plating bath, protects the plat- ing solution from undesirable drag-in, which might adversely affect the composition or con- centration of the bath. us, chemical compo- sition of the predip and its level of contami- nants are critical. Critical parameters of the acid copper bath include electrical, mechanical, physical, and chemical variables. Starting with the electrical parameters, current density and primary cur- rent density distributions affect plating rate, thickness distribution, and the metallurgical properties, e.g., ductility, roughness, and color Figure 1: Throwing power is defined in this graphic.