Soldering Palladium Plated Electronic Components

AN INVESTIGATION OF THE RELIABILITY OF JOINTS

The use of gold or palladium plated com- ponents to which soldered connections have to be made is now widespread in the elec- tronics industry. Some concern has been expressed from time to time, however, about the reliability of such joints, and the possi- bilities 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 8 Elec- This photomicrograph shows (top) the tin-lead solder, next the growing palladium alloy layer tric Co Ltd (A.T.E. Journal, 1965,21,(I), 2). resulting from 30 seconds in contact with molten This investigation shows that the doubts solder, then the palladium electrodeposit, and hitherto expressed can be largely discounted (below) the connector spring. The alloy layer, although having a rough appearanre at the solder in that during normal soldering of tinned interface, remains adherent ( x 1000) copper wire to flat gold-plated tags there was no evidence of unusual crystal formation sufficient to impair the strength of the joint, those made with gold were obtained using and that normal soldered joints made to this technique. Without pre-tinning, joint palladium are free from abnormal crystal strengths were still considered to be adequate formation and have adequate joint strengths. for most purposes, a conclusion substantiated In the experimental work, connector springs by field experience. as used in production were plated with either A further series of experiments involved gold or palladium to a minimum thickness of solder dipping for periods ranging from I to 0.0002 inch, and were soldered with 60:40 60 seconds. Here extensive alloying of gold solder to 0.036 inch diameter tinned soft was found and acicular and dendritic crystals copper wire, the joints being made approxi- coald be seen throughout the joint under the mately & squwe inch. Shear tests were carried microscope. Palladium plated springs showed, out by pulling the wire in a direction parallel however, that the palladium had not dissolved to the length of the springs. With gold-plated in the solder to the extent found with gold, and also pre-tinned palladium-plated springs, and that the solder layer showed no abnormal in every case the spring fractured before joint crystal formations. Further observations failure could occur. showed that the behaviour of palladium on Palladium is not quite as readily solderable contact with molten solder is quite different as gold-which is outstanding in this respect from that of gold. Whereas gold dissolves -but its solderability is enhanced by pre- almost instantaneously, palladium slowly tinning and joint strengths comparable to forms a thin alloy layer that thickens pro-

Platinum Metals Rev., 1965, 9, (3),90-91 90 gressively, finally-under extreme conditions These conclusions are supported by the of test-becoming detached from the basis company’s industrial experience; in a period metal. During a normal soldering operation, of almost four years in which some millions however, very little alloy formation was found, of palladium plated spring connections used and the authors conclude that negligible in telephone equipment and computers have brittleness would be expected to be present been soldered, no difficulties with joints have in a joint. been reported. J. E. P.

Continuous Temperature Control in the Blast Furnace A NEW IMMERSION THERMOCOUPLE An improved means of controlling the stream of molten iron leaving the furnace. In thermal balance of a blast furnace in order to this construction the couple wires are protec- achieve smoother operation and high-quality ted from the metal by an impervious inner iron has been developed by K. H. Gee and ceramic sheath and an outer graphite sheath. M. H. Schmidt at Bethlehem Steel Company. Compressed air is used to cool and purge the In the course of studies on slag basicity interior of the thermocouple holder. By some few years ago, it was found by taking means of the usual type of recorder, a chart of several immersion thermocouple measure- the iron temperature is made throughout the ments of iron temperatures during each cast duration of a cast. that a pattern developed reflecting the con- This instrument has now been in routine dition of the furnace, suggesting that an use at two of the Bethlehem plants for more accurate method of continuous measurement than three years and has proved its value as a of iron temperature might have operational simple and practical means of control. value. To this end a new design of platinum: Exclusive rights in this thermocouple have rhodium-platinum thermocouple was devised been granted by Bethlehem to Electro-Nite suitable for continuous immersion in the Carbon Company of Philadelphia.

The Gee continuous immersion thermocouple in position in the stream of molten iron Joining from n blast furnace in one of the Bethlehem Steel Company’s plants

Platinum Metals Rev., 1965, 9, (3),91-91 91