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48 The PCB Magazine • February 2014 the bond significantly. A properly passivated copper surface will resist water formation, thus eliminating delamination from occurring (with respect to moisture). Note this warning: A prop- erly passivated copper surface is very important. In a future column, I will discuss this subject in more detail, including what constitutes a poor- ly passivated copper surface. While it is true that most of the early mar- ket penetration for conventional oxide coating rested in FR-4 materials, the need for higher reliability for the military/aerospace segment necessitated a slightly different approach. Spe- cifically, the military/aerospace market used polyimide resin materials due to the need for higher reliability. In effect, the temperature pro- file employed for polyimide multilayer lamina- tion is much more severe than that of FR-4. It was determined that when fabricators used con- ventional black oxide, the bond strengths dete- riorated significantly on polyimide resins com- pared to FR-4. It was assumed that the large frag- ile oxide crystals of the standard oxide coating easily fractured during multilayer lamination. With the higher temperatures and pressures in- herent in polyimde fabrication, the oxide crys- tals easily fractured significantly reducing the bond strength. Higher lamination pressures and temperatures also share some the blame for the failures to the polyimide itself. It is well known that polyimide cured materials are more brittle than FR-4 resins and thus exhibit a much lower bond strength in general. However, the solution to this issue was to develop an oxide coating that was thinner and denser (Figure 2). As a re- sult of a modification of the chemistry, the ox- ide coating was much denser, thinner and the copper oxide crystals were much shorter. All of these attributes combined to provide a superior heat resistant bond not only for FR-4 but for polyimide as well. The reformulated oxide chemistry was de- signed to limit the oxide crystal growth and thus the overall thickness of the coating. Thin and more uniform coatings provided a more stable innerlayer bond between the copper and the resin. So in this case, thinner is better. When the oxide chemistry is properly formulated, the coating is self-limiting. Essentially the structure and thickness of the oxide coating is dependent on several factors: • Operating temperature of the oxide solution • Concentration of the chemicals' reactants of the oxide solution • Dwell time in the solution From my standpoint, the goal is to grow the oxide structure quickly and uniformly. I want to avoid low temperatures and excessive dwell times that can leave too thick of an oxide coat- Figure 1: conventional oxide chemistry formulation. Figure 2: reformulated oxide chemistry, more applicable to polyimide. OxIDE vS. OxIDE ALTERNATIvE CHEMISTRy, PART 1 continues