Plating Clinic: Non-Dyed Versus Dyed Acid Copper
Non-Dyed Versus Dyed Acid Copper
Q. What is the difference between non-dyed vs. dyed acid copper technologies?
A. In general, there are three primary types of copper used for decorative plating: cyanide copper, non-dye-based acid copper, and dye-based acid copper. Acid copper baths generate bright, level deposits that are not obtainable with cyanide copper. For easier plating geometries, this translates to parts that do not need to be buffed prior to nickel plating. The bright copper is more economical than nickel and can allow for less nickel thickness to achieve equivalent brightness. While nickel baths can, and do plate bright and level, taking advantage of epitaxial effects from a bright substrate only enhances the nickel specularity.
There are many similarities between non-dyed and dyed acid copper additives. In conventional terms, all decorative acid copper additive systems contain three types of additives: carriers, brighteners and levelers.
Carriers, also referred to as suppressors, are commonly polyethyl (or less conventionally polypropyl) ethers of molecular weights in excess of 3,500. These “carriers” define the cathode diffusion layer path length in which the brightener and carrier work to align copper into the desired orientation.
Typical brighteners contain one or more sulfur atoms, typically without nitrogen atoms, and often belong to families generically described as R’-S-RSO3. Brighteners form a layer on the copper surface where they enter, together with chloride ion, in the one electron transfer Cu2+ ◊ Cu+ ◊Cu0. Brighteners “mask” preferential growth sites and planes. Copper growth has little directional preference, and produces a structure that is semi-amorphous/micro-crystalline. Levelers are more polar than carriers or brighteners, can protonate, and deposit preferentially near the most negatively charged sites of the cathode (protrusions, corners, edges of scratches). The resulting increased diffusion layer thickness, plus the strong copper ion holding force of the strongly complexing leveler, slow down the copper deposition rate at these sites, thus “leveling” the surface. It has been observed that as the levelers are chemically reduced during electrolysis, the dyed type quaternary amines form compounds that are difficult to remove with standard carbon treatments. However, non-dye additives do not tend to form these same types of compounds, rendering non-dyed additives much more amenable to carbon purification. The carriers and brighteners from either type tend to be similar and are readily removed, given that enough carbon is used to provide adsorption sites.
The debate over dyed versus non-dyed chemistry is ongoing. The primary difference between the two systems is the characterization of the chosen leveler. Levelers that have color (purple, blue, yellow, green, etc.) tend to be classified as “dyes.” Non-dye levelers tend to produce additives that are colorless or slightly yellow. Whether dyed or non-dyed, virtually all levelers are quaternary amine compounds. Brighteners and carriers are consistent between the two systems and from supplier to supplier. What does change is the chosen leveler and the ratio of brightener components to leveler components.
The chart below chronicles the similarities and differences between some typical commercially available decorative acid copper systems. What the chart indicates is that there are a lot of different levelers, but not much difference in choices of brighteners and carriers. There is a lot of lore about advantages and disadvantages of the two types of chemistry. Generally, results are skewed by changing bath chemistries and are not additive related. That said, the following differences are repeatedly observed: Non-dye deposits are a little pinker, dye deposits a little redder; throwing power is equivalent and predicated on metal concentration and temperature; non-dye baths tend toward softer deposits (120-140 Hv versus 140-160 Hv for dye-based); hardness can be made to vary dependent upon leveler choice and concentration—dye or non-dye; soft coppers are easier to buff; non-dye baths are generally easier to carbon treat; high current density polarizing is equivalent; non-dye baths minimize anode bag weave plugging; dyed baths have slightly better very low current density leveling; and non-dye deposits are very “clean” on the surface, post plating.
Dye deposits often leave an additive film that needs a post dip to remove prior to nickel plate to improve copper-nickel adhesion (if no buff is employed). Experience has shown that either non-dyed or dyed acid copper technologies can be used for virtually any bright copper plating application. The choice is based on personal preference and not bath performance.
Brad Durkin is director of international product development at Coventya. For information, visit coventya.com.
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