Inner Layer Pre-Clean
Inner layer processing requires Dry Film Photo Resist to be laminated onto the copper surface. The dry film is resolved into a circuit pattern. Copper we want to keep is protected by the circuit pattern from the etchants used to remove the copper. The bond of the dry film to the copper surface must be strong enough so that the film does not come of when in the etchant chemistry. Copper foil such as RTF, Reverse Treated Foil, are rough enough so that the dry film forms a good bond to the copper as is. Many copper foils are not rough enough or are supplied with a contaminated surface. In these cases the foil must be cleaned and roughened prior to laminating the dry film.
There are several chemistries available for this task. The most commonly used chemistry in commercial applications is sodium per-sulfate. Sodium per-sulfate is the low cost solution for surface roughening of the copper surface. It is also the most difficult to control. As the concentration of copper grows in the process bath the reactivity or effectiveness is reduced. As the effectiveness of the bath is diminished more sodium per-sulfate is required in greater amounts to obtain the same results. Where as the chemistry is inexpensive it offers less control and thus lower quality.
Hydrogen peroxide is another chemistry used for cleaning and roughening the copper surface prior to dry film lamination. Hydrogen peroxide offers slightly better performance over sodium per-sulfate. However, this chemistry suffers the same limitations as sodium per-sulfate. As the copper concentration increases more chemistry is required. The effectiveness of the bath is also diminished as the copper concentration increases. The hydrogen peroxide is slightly more expensive than sodium per-sulfate but does offer slightly more consistency over sodium per-sulfate.
Why does the copper concentration in the process bath increase? The answer is simple. In high concentrations both sodium per-sulfate and hydrogen peroxide can be used as etchant. These chemicals dissolve copper. The copper that is etched away remains in the bath. In our application here the chemistries are in low concentration and act as a micro-etch. The chemistries attack the copper surface only roughening it up. Some copper is removed from the surface. Think of the smooth starting copper surface as a plane. Micro-etching removes copper unevenly and forms peaks and valleys within the plane. These peaks and valleys form the topography that increases the bond strength between the dry film and copper. The copper that is removed remains in the bath and builds up at a slow rate.
Copper build up in the bath is a weakness of sodium per-sulfate and hydrogen peroxide. The baths become more unstable as they are used. For commercial applications this is not critical. Baths using these chemistries are typically processed in batch mode. You add water to the process line and enough chemistry to obtain a 15 to 20 micro-inch etch rate. The bath is controlled by measuring a coupon for etch loss. The coupon is weighed, processed through the process line and then weighed again. The weight loss is entered into a formula to determine the depth of etch rate reported in micro-inches. A maintenance add is made and or the process line is slowed down to achieve the desired results. Reworking or reprocessing materials through a second time can be problematic especially for lower copper weights. Processing through a second time results in a more aggressive etch rate.
Special chemistries are available that retain a consistent etch rate regardless of the copper concentration. DuPont for example offers a family of micro-etch chemistries that do just that. The ZA100CL chemistry provides a consistent and uniform topography. The chemistry is also much easier to maintain. As the copper concentration increases as the bath is used, the etch rate remains the same. Consistent rates of replenishment are possible based on usage. The bath can more or less achieve a steady state of operation where the etch rate stays consistent. The following graphic is a comparison from the DuPont ZA product brochure.
The full brochure in pdf format may be downloaded through the following link…
The comparison between the various chemistries makes it is easy to see the difference. Sodium per-sulfate and hydrogen peroxide are less effective as the copper concentration increases. The replenishment adds are also inconsistent. The ZA series chemistry is consistently effective with regards to both the etch rate and replenishment. Consistency leads to yield improvement. Yield improvement leads to profit. As can be expected though, the quality chemistries are more expensive. However, you get what you pay for.
The process baths do not last forever. As these chemistries remove copper and contaminates from the copper surfaces these substances remain in the bath. They build up to the point where the copper starts to crystallize in the bath itself. Crystal build up may start to clog up spray nozzles and manifolds. Gears and rollers may also see buildup that can result in roller skips and ultimately panel jam ups. Residues may also start to re-deposit on the copper surfaces. It is important to dump the bath and clean the line to remove the residues and crystal buildup to keep these non-conformances from happening.