Issue link: https://iconnect007.uberflip.com/i/1514628
82 PCB007 MAGAZINE I JANUARY 2024 It's always funny when someone says, "Why can't we just do this?" Well, you're violating Faraday's Law. ere are only so many fac- tors you can manipulate, and others you sim- ply can't. ere are certain laws, like Faraday's, that you cannot violate. You don't mess with it. erefore, you must manipulate the amper- age, voltage, and other things that make a dif- ference in getting chemistry into the hole. In addition, cathode current density does matter. To improve plating dis- tribution and surface-to-hole ratio (measured plated thickness in the center of the hole divided by the plated thickness on the surface), reduce the cathodic current density. ere are several tips from a mechanical/electri- cal standpoint that influence plating distribution and throw- ing power. For example, I visited a printed circuit board manufac- turing facility several years ago. ey were struggling with plating distribution, particularly with board designs considered higher density. A good trouble- shooter must get to the root cause of the prob- lem. I checked the chemistry. Were the key chemical additives within the process specifi- cation? Yes. So, how to account for the plating distri- bution issue? Remembering Faraday's Law, we weighed a panel without circuity. e test panel was then plated for 60 minutes at 20 ASF (amps per square foot). Aer plating and allowing the panel to dry, the test board was weighed again to determine how much actual copper was deposited. e result was that the actual amount of copper deposited was only 78% of the predicted value from Faraday's Law. What might account for this lack of efficiency in the plating process? We decided to look at the mechanical/electrical aspects of the plat- ing process, and upon inspection of the cables connecting the power supply (rectifier) to the buss bars on the plating cell, we made two observations. First, parts of the cables looked worn and even exhibited some cracks in the cable covering. Second, when we touched the cables, they felt extremely warm, even hot. e concern was that the actual amperage out- put from the rectifier was not reaching the cir- cuit board at near 100% efficiency. e current was being lost between the power supply and the circuit board. It was suggested that the cables be replaced with coaxial cables that had suffi- cient cross-sectional area to carry the current with minimal loss. Once the cables were r e p l a c e d , e f f i c i e n c y improved significantly. The lesson learned was not to overlook what some consider to be less obvious. Another key parameter is the ability of the plating rack to suffi- ciently conduct electricity. ere's a similar principle here: If the plating racks cannot carry the current due to improper size or poor rack- to-board connections, current to the board will also be lost. Hence, the expected plated copper thickness will be less than planned. Finally, I am oen asked whether one should use constant voltage or constant current in the electroplating operation. I prefer constant current density which automatically adjusts the output to match the size of the load in the plating cell. PCB007 Michael Carano brings over 40 years of electron- ics industry experience with special expertise in manufac- turing, performance chem- icals, metals, semiconduc- tors, medical devices, and advanced packaging. To read past columns, click here. There are certain laws, like Faraday's, that you cannot violate. You don't mess with it.