Platinum Metals Rev., 1965, 9, (3), 90
Soldering Palladium Plated Electronic Components
An Investigation of the Reliability of Joints
The use of gold or palladium plated components to which soldered connections have to be made is now widespread in the electronics industry. Some concern has been expressed from time to time, however, about the reliability of such joints, and the possibilities of failure due to the formation of brittle constituents produced by alloying of the tin and lead in the solder with the gold or palladium of the electrodeposit.
A valuable study of this subject has now been reported by J. Whitfield and A. J Cubbin of the Materials and Processes Laboratory of Automatic Telephone & Electric Co Ltd (A.T.E. Journal, 1965, 21, (1), 2). This investigation shows that the doubts hitherto expressed can be largely discounted in that during normal soldering of tinned copper wire to flat gold-plated tags there was no evidence of unusual crystal formation sufficient to impair the strength of the joint, and that normal soldered joints made to palladium are free from abnormal crystal formation and have adequate joint strengths.
In the experimental work, connector springs as used in production were plated with either gold or palladium to a minimum thickness of 0.0002 inch, and were soldered with 60:40 solder to 0.036 inch diameter tinned soft copper wire, the joints being made approximately square inch. Shear tests were carried out by pulling the wire in a direction parallel to the length of the springs. With gold-plated and also pre-tinned palladium-plated springs, in every case the spring fractured before joint failure could occur.
Palladium is not quite as readily solderable as gold—which is outstanding in this respect —but its solderability is enhanced by pre-tinning and joint strengths comparable to those made with gold were obtained using this technique. Without pre-tinning, joint strengths were still considered to be adequate for most purposes, a conclusion substantiated by field experience.
A further series of experiments involved solder dipping for periods ranging from 1 to 60 seconds. Here extensive alloying of gold was found and acicular and dendritic crystals could be seen throughout the joint under the microscope. Palladium plated springs showed, however, that the palladium had not dissolved in the solder to the extent found with gold, and that the solder layer showed no abnormal crystal formations. Further observations showed that the behaviour of palladium on contact with molten solder is quite different from that of gold. Whereas gold dissolves almost instantaneously, palladium slowly forms a thin alloy layer that thickens progressively, finally—under extreme conditions of test—becoming detached from the basis metal. During a normal soldering operation, however, very little alloy formation was found, and the authors conclude that negligible brittleness would be expected to be present in a joint.
These conclusions are supported by the company’s industrial experience; in a period of almost four years in which some millions of palladium plated spring connections used in telephone equipment and computers have been soldered, no difficulties with joints have been reported.