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88 PCB007 MAGAZINE I AUGUST 2020 • Low loading with intermittent use (<3 sq. ft./gal/shift) will lower the amount of active stabilizer in the bath and decrease stability. When this occurs, excessive copper loss is expected. • The specific gravity of the electroless copper plating solution should be monitored each shift. As specific gravity increases, copper loss or plate-out increases (Figure 1). Note: Consult the supplier of the particular electroless copper process in use. There may be differences in specific gravity recommendations. • The process tank containing the electroless copper solution should be etched out every few weeks, depending on operation and maintenance. Copper deposits that form on the tank walls and heaters due to nor- mal plate-out must be removed as part of process maintenance. It is good practice to have two electroless copper plating tanks in the PTH line. As one goes offline for main- tenance, the second one is ready to go. • Sporadic short-term fluctuations in temperature have no adverse effect on stability. Electroless Copper Solution Life As much as we would like them to, plating solutions do not last forever. There are numer- ous factors that negatively impact the useful life of a plating solution. The electroless cop- per solution is no exception. Factors affecting electroless copper solution life may include: • Tank condition (is there copper plate-out on the tank walls?) • Bail-out schedule • Filtration (are the copper particles being effectively removed) • The amount of time spent operating bath out of chemistry and temperature specifications • Extended idle periods at temperature and concentration • Improper bath turnover from low loading • Poor rinsing/drag-in of catalyst and accelerator • The buildup of formate and other by-products The bath ages with the buildup of formate, carbonate, and chloride as by-products. These by-products are measured with specific gravity but are not individually differentiated. Chloride increases linearly with the consumption of cu- pric chloride (if a copper sulfate electroless pro- cess is in use, by-product build up rates will be different). Carbonate is another by-product and is formed as carbon dioxide in the air reacts with sodium hydroxide in the bath. Assuming air flow is constant, carbonate will also build linearly. Formate also tends to grow linearly at low levels through several reactions: Excessive formate is known to promote the creation of additional formate through the breakdown of formaldehyde resulting in the generation of copper(I) oxide (reaction 2), which will, in turn, create copper dust through the following reaction: Therefore, as conventional electroless cop- per solutions age with increased metal turn- overs, and normal plating solution age, there is a higher formate buildup. Therefore, there Figure 1: Example of copper plate-out.

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