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August 2015 • The PCB Magazine 73 have a balanced amount of exposed copper on each side. Have your CAD personnel use bleeder strips to even out the sides if necessary. I have used differential plating tanks with two power supplies—one for the back plating and one for the front. This way, you can plate 200 amps on the front and 170 on the back to ensure even plating. In most shops there is a difference in plating tanks—one may have reverse pulse rectification and another work tank with just straight DC. Re - verse pulse plating works better than straight DC in some applications such as copper flatness and high-ratio, deep hole plating. But be warned: There are so many settings on a typical reverse pulse unit that it is easy for a plating engineer who likes to play, to never have it set right. With the push to HDI and flat via-in-pad, many shops are putting in separate hole-fill cop - per tanks. These are special tanks with special chemistry that require a significant amount of testing to get the current and chemistry param- eters right; it is not just a pour-and-plate situa- tion. One of the biggest quality improvements in plating is controlling how much copper is in the hole. Below 1 mil the board may not even make it through three assembly heat cycles if the panel thickness is 40–90 mils. At 1.4 mils you have a reasonable chance of a good PCB that will with - stand a normal 500 thermal cycles. For higher- reliability PCBs, I use 1.6–2 mils of copper in the hole and compensate by starting with a slightly thinner base copper so the overall trace copper thickness is the same as a normal board; there is just more copper in the hole and this thicker copper can pass 2,000 heat cycles. A good opera - tor can have a rough guess at the copper thick- ness in the hole using pin gauges, but pin gauges by design are not accurate enough to properly determine if you have 1 mil or 1.4 mils of copper in the hole; the difference is a PCB that fails or one that works. The only way to see plating qual- ity and thickness is constant cross-sections and solder float tests. When you get the tank work- ing correctly and understand plating speed and currents, your operators can be very close to the desired thickness based on time alone. For more accurate measuring, I use a good, deep-throat digital micrometer (in millionths) and measure the panel before, during and after copper plat- ing. I have the CAD department place a special pad on each side of the panel for use in measur- ing the combined plating thickness of the board. A water break test, for fingerprints or greases, is used to show why you should work hard to keep fingerprints off the bare copper panels. Try it yourself—take two panels, one with the nor- mal fingerprints all over it (that I hate to see) and one that is new, dip each into a clean water tank and remove and hold vertically. As the water sheen drips away it will hold a thin water film unless grease or fingerprints are present; each fingerprint will leave a break in the surface film, while the clean panel will hold a surface film for a long time. The most impressive test is to now take the panel with all the fingerprints and run it through a scrubber or chemical clean line. It will be clean right? WRONG! Do the water break test after you have cleaned it and you will still see most of the fingerprints. Fingerprints are not eas - ily removed by our cleaning process. They cause poor plating because they add an oily surface that the copper ions will have to break through, slowing down the plating process and resulting in thin or spotty plated areas. Once exposed and developed, dry film panels should be covered with black plastic until ready to be plated to stop further hardening of the dry film polymers from the UV plating room lights. UV light hardening can increase dry film chip - ping and reduce adhesion. The copper tank pro- duces hydrogen bubbles as it plates; these bub- bles are powerful enough to break off fine tracks if the dry film gets brittle. The black plastic cov- ering also keeps dust, dirt and any airborne oils off the boards. I hope some of my 52 years of experience in PCB plating will help you, even if it only helps you find and fix one small problem. PCB Bob Tarzwell is a PcB consultant who has spent 50 years in the PcB industry, inventing technology and building almost every type of PcB. He is the co-owner of DB Publishing, publisher of the PcB 101 and Quality 101 handbooks. To contact the author, click here. FeAture PLATING AND QUALITY ARE CLoSE PARTNERS continues

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