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102 The PCB Magazine • March 2014 Roughness: Current oxide coating processes impart micro-roughness to the copper surface that greatly exceeds the degree of roughness that is obtained with normal copper micro- etchants. In order to match or exceed the bond strength that oxide processes impart to the copper, these microetchants would need to be reformulated to greatly increase the degree of roughness [2] . Not all forms of roughness are the same. For example, sanding the copper surface would impart a great deal of roughness to the surface but the bonding strength would still be very low due to the fact that this roughness would be macro-roughness rather than micro- roughness. Good copper adhesion starts with imparting a micro-roughened surface to the copper. This roughness can only be seen with a SEM at powers of about 2000X or higher. A power of 5000X is commonly used. With such a photograph of the surface we see that a typi- cal oxide process roughens the copper on the order of 0.1–0.5 microns peak-to-peak. Also, the peak-to-valley distance (perpendicular to the copper plane) is greater than the peak-to- peak distance (parallel to the copper plane). The type of micro-roughness (different peak-to-peak and peak-to-valley distances) will influence the bonding characteristics of the copper to the prepreg. Even shape (prism-like, jagged, round, bent, etc.) can influence the bonding character- istics [2] . The very same argument can be made with respect to photoresist adhesion to the cop- per surface. Shape of the peaks and the extent of the overall surface roughness will influence the ability of the resin to properly flow and adhere to the surface. Different prepregs will have dif- ferent flow and wetting characteristics. It is pos- sible that some prepregs will flow and wet out the entire micro-roughened surface from top to bottom. While other prepregs with low flow characteristics may not be able to flow all the way down to the bottom of the valley and may not completely wet out the micro-roughened copper. This leaves a micro-gap at the bottom of the valley where solution can leach into the coating and cause issues, maybe even pink ring (even though the peel strengths are high). So it is conceivable that some prepregs will like a certain type of micro-roughness, and other pre- pregs may do better with another type of micro- roughness [2] . This may explain the differences in interlaminar bond strengths achieved with oxide versus oxide alternatives. Chemistry of Oxide Alternatives The alternative process often referred to as an organo-metallic process, imparts a coating on the copper surface at the same time as etch- ing/micro-roughening the surface. Unlike the inorganic oxide coating, the alternative coating is organic. (Often referred to as an organo-me- tallic, due to the nature of the organic coating forming on the copper.) During the oxida- tion of the copper, a metal/organic coordinate is formed that is insoluble in the process solu- tion. This coating is a vital part of the bond- ing mechanism. Tests have shown that copper surfaces with equal or even greater degree of micro-roughening, but which do not contain a metal/organic coating, do not provide good bonding characteristics. This is especially true with respect to time to delamination. The orga- no-metallic coating increases the bond strength, especially at high temperature stress condi- tions such as solder immersion. This is due to chemical interaction between the copper and the coating (a coordination bond is formed), and the coating and the prepreg (a sharing of pi electrons occurs) [2] . Chemical Reaction: Organic metallic process is a peroxide-sulfuric based etching and coating solution that has been specially and uniquely formulated to impart micro-roughness to the copper surface that greatly exceeds normal etchants and at the same time forms a coating on the surface that promotes adhesion through chemical bonding. Peroxide etches copper by oxidizing Cu° to Cu 2+ . The half-cell reactions are shown below: Cu 0 ® Cu 2+ + 2e - (-0.3419 volts) H 2 O 2 + 2H + + 2e - ® 2H 2 O (+1.776 volts) The net reaction is: H 2 O 2 + 2H + + Cu 0 ® Cu 2+ + 2H 2 O (+1.4341 volts) If sulfuric acid is used then we have: H 2 O 2 + H 2 SO 4 + Cu 0 ® CuSO 4 + 2H 2 O (~+1.4341 volts) OXIDE VS. OXIDE ALTERNATIVE CHEMISTRY, PART 2 continues

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