Probably the first use of palladium plating as a substitute for gold dates back to the 1960s, when it was successfully adopted for the plating of printed circuit connectors and allied contacts by the Automatic Telephone & Electric Company of Liverpool. Although, even with a gold price then of $35 per ounce, palladium still showed a substantial advantage in terms of metal cost, the reasons for its use in this case were technical in that it was used to eliminate cold welding problems occurring in the mating of connectors plated with a conventional cyanide gold deposit. At this time a further problem with cyanide gold plating was associated with attack by the plating solution on the adhesive system used in bonding copper foil to the supporting laminate, which led to weakening of the bond and often to exfoliation of the foil. Here again, palladium offered a solution since the ammoniacal electrolytes then mainly available were only mildly aggressive. For these reasons palladium found significant, if not extensive, use during this period by a number of major companies, with satisfactory performance reported in respect of protective value, contact resistance and wear properties. Further development at this time was effectively halted with the development of the acid gold systems, which largely eliminated the problems of cyanide baths and offered a new spectrum of applicational properties combined with the cachet of high reliability conventionally associated with this noble metal. It is only with the escalation in the gold price of recent years, leading to a very active search for economic alternatives, that palladium plating has re-emerged as a leading challenger, as evidenced by the increasing number of literature references relating to process development, and deposit properties and performance.

The level of activity in this context at the present time is well illustrated by a recent paper from the Berg Electronics Division of Du Pont, reporting an extensive evaluation of coatings from eight proprietary electrolytes, five for pure palladium and three for palladium-nickel alloy plating, as possible substitutes for cobalt-hardened gold deposits on pin and female electronic connectors (A. H. Graham, M. J. Pike-Biegunski and S. W. Updegraff, Plating Surf. Finish., 1983, 70, (11), 52–57).

Comparative assessment against the reference hard gold deposit was based on solution stability and on the properties of coatings produced under simulated high-speed conditions (rotating disc), in terms of contact resistance; electrographic porosity; solderability by dip test (Military Standard 202) with prior steam exposure to simulate long-term storage; ductility, by a bend test; and internal stress, by distortion of plated discs.

All coatings studied showed satisfactory contact resistance and solderability, with the exception, in the latter case, of one pure palladium deposit which required a mildly activated flux to achieve the requisite 95 per cent coverage. The others gave satisfactory performance either with a non-activated or a mildly activated flux. Outstanding solderability is reported for the palladium-nickel coatings with a top coat of 0.125 μ m of soft gold, giving 100 per cent coverage. Palladium-nickel also scored strongly in porosity tests, on nickel. While coatings from some of the more recently developed pure palladium electrolytes showed lower porosity levels than hard gold at all thicknesses investigated, there was a high standard deviation in lengthy laboratory runs, indicating sensitivity of the deposits to normal process variations. Palladium-nickel coatings showed a porosity level more than an order of magnitude below that for hard gold, and in addition the standard deviation of results for palladium-nickel was much less than for pure palladium. In ductility tests based on scanning electron microscope observation of surface cracking produced by a 180° bend of coatings on 0.64 mm square wires around pins of varying radius, palladium-nickel also outperformed all others, initial cracking occurring at a bend radius of less than 1.27 mm, compared with 6.4 mm for hard gold. In internal stress observations on thick deposits on disc specimens, discs carrying palladium-nickel coatings were flat as plated and remained so after prolonged storage, while similar samples plated with the “best” pure palladium coating, although flat as plated, with little porosity and good ductility, curled into a U-shaped bend after only one day, with severe microcracking at the bend, caused by out-gassing of hydrogen.