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By now a good percentage of the PCB fabrication world has graduated to the use of oxide alternative as a means for enhancing the inter-laminar bond strength between copper circuitry and the resin system. However, conventional brown or black oxide systems, as these are commonly called, have continued to serve the industry well. Indeed, frequent visits to Asia have shown me that oxide chemistry is alive and well. So before dedicating the next few columns to oxide alternative chemistry, it would be appropriate to present the current state of conventional oxide. I can honestly say that the death of conventional-reduced oxide has been greatly exaggerated.
Oxide vs. Oxide Alternative
Anyone involved in PCB fabrication during the last 25 years should understand that standard oxide treatment of innerlayers to enhance bond strength between the copper and resin has served the industry well. Oxidation of the copper surfaces for multilayer board fabrication was borrowed from the general metal finishing industry. Basically, it is a simple process best characterized as the controlled anodic oxidation of copper in an alkaline medium. To refresh the memory, the purpose of the oxidation step serves a two-fold purpose: 1) passivation of the copper surface and 2) enhancement of bond strength between the resin and copper. Look at this as the copper oxide crystals providing a high surface area on the copper so that the resin can flow through oxide crystals and provide a heat-resistant bond. With respect to passivation, the concern is that when B-stage resins and unpassivated copper surfaces are bonded together under high heat and pressure conditions, water can be a by-product of this reaction. With moisture now in the bond line, the water can vaporize, thus weakening 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 properly passivated copper surface is very important. In a future column, I will discuss this subject in more detail, including what constitutes a poorly passivated copper surface.
Read the full column here.
Editor's Note: This column originally appeared in the February 2014 issue of The PCB Magazine.