Electroplating of copper, part 5: organic addition agents influence secondary current distribution and control the physical properties of the copper deposit.
During the normal performance of a printed circuit board, the copper deposit will undergo flexing. An example would be a satellite orbiting the globe, being intermittently heated and cooled as the device makes its way around the earth. The copper deposit within the printed circuit board will undergo flexing, which leads to fatigue in the deposit. The result of this fatigue is cracking of the deposit within the barrel of the hole. The deposit must have sufficient tensile strength to be able to withstand the flexing that can induce fatigue cracking. It is well understood that the organic addition agents utilized in acid copper plating processes influence the physical properties of the deposit and provide improved characteristics.
Acid copper additives contain at least one material described as a brightener and a second material that is described as a high molecular weight polymeric compound. Optionally, the additive package can contain a leveling agent.
A brightener is classified as a nitrogen or sulfur containing compound, that by coulombic attraction, forms a layer on the copper (Cu) surface where it enters, together with chloride ion ([Cl.sup.-]), in the one electron transfer [Cu.sup.++] [right arrow] [Cu.sup.+] [right arrow] [Cu.sup.0]. Brighteners mask preferential growth sites on the planes. Even with the brightener, the growth of the plated copper shows little directional preference. The grain structure of the copper is semi amorphous and/or microcrystalline. In the absence of these brighteners, the copper deposit will have a large-grain structure with little refinement. In order for the brightener to function as it is intended, a carrier component must be present in the plating solution.
[FIGURE 1 OMITTED]
Carriers are a class of high molecular weight materials that are designed to exert a strong plating suppression effect on the high current density areas of the cathode. These polyethers are generally in the range of 6,000 to 30,000 in molecular weight. The mechanism of the suppression effect is believed to be caused by the stronger adhesion forces between the high molecular weight polyethers and the copper surface through copper complex formation and the stronger intermolecular forces between the polyether molecules. One can describe this as a smoothing effect that allows for a more uniform diffusion layer across the surface.
Leveling agents are designed to give a very strong suppression effect on the plating deposit. When the plating solution is in the proper concentration, these additives have a tendency to smooth out imperfections in the laminate foil and areas in the via that have some drill gouges. An example of surface roughness without leveling is shown in FIGURE 1. The roughness presents several issues, especially on fine pitch BGA designs and features that will have wire bonded components. This is why excellent leveling is necessary to create a smooth topology as shown in FIGURE 2.
[FIGURE 2 OMITTED]
MICHAEL CARANO is vice president for OM Group, Inc. and can be reached at firstname.lastname@example.org.
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|Title Annotation:||POSITIVE PLATING|
|Publication:||Printed Circuit Design & Fab|
|Date:||Jan 1, 2009|
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