Electroplating Copper onto Tin-Lead Solder Alloys

There are many reasons why one might want to electroplate copper onto solder. Several of these include: Improving conductivity of microwave plumbing, Increasing fatigue resistance of soldered connections, Facilitating over-soldering multiple connections where a melting heirarchy is impracticable, Improving wetting in over-soldering, Facilitating gold plating for contacts, Reducing mechanical friction, Preventing leaching of lead in biological applications. And, increasing the strength of adhesive bonds.

The difficulty with plating copper onto alloys of tin and lead results from the fact that they self-plate due to the difference in their electronegativity with respect to copper. Self-plating results in a weak voided interface. Alkaline cyanides have long been used to avoid this problem since a cyanide complex rather than copper is reduced at the cathode. Cyanides are difficult to handle and alkaline cyanides dissolve gold plating, bonding leads etc. Since gold may be essential to electronic circuitry, cyanides may be unusable. Reducible amine and citric complexes of copper are known that are capable of electroplating use. [C Aravinda, S. Mayanna, V. Muralidharan, "Electrochemical behaviour of copper complexes", Proc. Indian Acad. Sci. (Chem. Sci.)112 (2000) 543-550.]

Assorted cyanide-free copper plating solutions have been described over many decades. They tend to be complicated. The solution given at the right is very simple. It is suitable for laboratory or shop use under circumstances where speed is not too important. A copper strike can be obtained in 15 min. Without annealing, the deposited copper is hard. Increased lead content appears to require increased cell voltage. For applications requiring a polished surface, rouge polishing at least once per hour is needed. Flat surfaces can simply be burnished when plating is finished. Where adhesion enhancement is desired the surface should be left as plated. Higher voltages give rougher surfaces. With cell voltages of several volts, the deposition takes place in front of the cathode rather than onto it. Fractal structures and quasi-colloidal copper can be obtained this way.

Reference Strength: CuSO₄ 0.05M
Copper Sulfate¹ 7.49 gms
Citric Acid 31.52 gms
Sodium Hydroxide 30.00 gms
(Minimum NaOH 18.00 gms)
Water to make 600 mL

Cell voltage: 1.15 to 1.35
pH = 11
¹The copper sulfate used was approximately 50% anhydrous.