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February 2014 • The PCB Magazine 49 ing. Essentially this is a matter of reaction ki- netics. The key chemical materials for the oxide process are sodium chlorite and sodium hy- droxide. In addition, a buffer in the form of tri- sodium phosphate is also added to the working solution. These chemicals in the proper con- centrations along with operating temperature ultimately control the thickness and uniformity of the oxide coating (and ultimately the peel strength). If a finer grained, short oxide needle structure is desired (Figure 2), then keep the so- dium hydroxide content at 0.6 N or less (not more) and the sodium chlorite concentration greater than 80 grams per liter. One of the control methods used is to em- ploy a standard copper foil coupon of known surface area. It is used much like a rate coupon on the electroless copper plating line. Essen- tially one is measuring weight gain. The recom- mended procedure is as follows: Procedure A. Process the copper coupon down the normal oxide production line. B. Dry the coupon completely, bake at 110°C (230°F) for 10 min, then cool at room temperature for 10 minutes. C. Weigh the copper coupon and record the weight (X) to the nearest 0.001 g. D. Immerse the coupon in 20% sulfuric acid solution at room temperature and strip all oxide off of the coupon. Do so for three minutes. E. Rinse and dry the coupon completely, bake at 110°C (230°F) for 10 min, then cool at room temperature for 10 minutes. F. Reweigh the copper coupon and record the weight (Y) to the nearest 0.001 g. Calculations A. Weight gain (Z) = X – Y B. Coupon area (A) = length x width x 2 (cm 2 ) C. Weight gain per coupon area (mg/cm 2 )= Z x 1000 A D. Weight gain per coupon area (mg/in 2 ) = Z x 6452 A OxIDE vS. OxIDE ALTERNATIvE CHEMISTRy, PART 1 continues The recorded weight gains will serve as a good process control metric and should be measured once per shift. Keep in mind that the more brittle resins such as polyimide perform better in terms of peel strength with thinner ox- ide coatings as opposed to very thick coatings. A guideline for measured weight gains is shown in Table 1. Keep in mind the thinner, lighter coat- ings are favored for polyimide, while the heavier coatings can be used with FR-4 resins. If the sodium chlorite concentration drops or the sodium hydroxide concentration increases, there can be a significant increase in the oxide weight gain. What many people don't realize is that over time, the combination of high temper- atures, sodium hydroxide and oxygen will form sodium carbonate. The concern here is that sodi- um carbonate contributes to the total alkalinity of the oxide solution. If not accounted for, the carbonate will give a false reading of the sodium hydroxide content. Sodium hydroxide influenc- es the oxide crystal growth and overall structure. In the next Trouble in Your Tank, I will cov- er more in-depth information related to oxide chemistry and process control and present the latest thoughts on pink ring and what it really means. PCB Oxide weight Characterization Gain Comments heavy black oxide 0.60 Generally lower peel mg/cm2 strengths, but ok for Fr-4 mid-range oxide 0.45–0.60 needed for higher mg/cm2 performance Fr-4 resins light brown oxide 0.25–0.45 ideal for polyimide mg/cm2 coating; thin and actually looks light brown to bronze in color table 1. michael carano is with omG electronic chemicals, a devel- oper and provider of processes and materials for the electronics industry supply chain. to read past columns, or to contact the author, click here.