Palladium-Nickel As a Gold Substitute an EVALUATION for ELECTRONIC CONNECTOHS
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Palladium-Nickel as a Gold Substitute AN EVALUATION FOR ELECTRONIC CONNECTOHS Probably the first use of palladium plating as reporting an extensive evaluation of coatings a substitute for gold dates back to the 196os, from eight proprietary electrolytes, five for pure when it was successfully adopted for the plating palladium and three for palladium-nickel alloy of printed circuit connectors and allied contacts plating, as possible substitutes for cobalt- by the Automatic Telephone & Electric hardened gold deposits on pin and female Company of Liverpool. Although, even with a electronic connectors (A. H. Graham, M. J. gold price then of $35 per ounce, palladium still Pike-Biegunski and S. W. Updegraff, Plating showed a substantial advantage in terms of Surf. Finish., 1983, 70, (II), 52-57). metal cost, the reasons for its use in this case Comparative assessment against the were technical in that it was used to eliminate reference hard gold deposit was based on solu- cold welding problems occurring in the mating tion stability and on the properties of coatings of connectors plated with a conventional produced under simulated high-speed condi- cyanide gold deposit. At this time a further tions (rotating disc), in terms of contact problem with cyanide gold plating was resistance; electrographic porosity; solderability associated with attack by the plating solution by dip test (Military Standard 202) with prior on the adhesive system used in bonding copper steam exposure to simulate long-term storage; foil to the supporting laminate, which led to ductility, by a bend test; and internal stress, by weakening of the bond and often to exfoliation distortion of plated discs. of the foil. Here again, palladium offered a solu- All coatings studied showed satisfactory tion since the ammoniacal electrolytes then contact resistance and solderability, with the mainly available were only mildly aggressive. exception, in the latter case, of one pure For these reasons palladium found significant, palladium deposit which required a mildly if not extensive, use during this period by a activated flux to achieve the requisite 95 per number of major companies, with satisfactory cent coverage. The others gave satisfactory performance reported in respect of protective performance either with a non-activated or a value, contact resistance and wear properties. mildly activated flux. Outstanding solderability Further development at this time was is reported for the palladium-nickel coatings effectively halted with the development of the with a top coat of 0.I 25 pm of soft gold, giving acid gold systems, which largely eliminated the I 00 per cent coverage. Palladium-nickel also problems of cyanide baths and offered a new scored strongly in porosity tests, on nickel. spectrum of applicational properties combined While coatings from some of the more recently with the cachet of high reliability con- developed pure palladium electrolytes showed ventionally associated with this noble metal. It lower porosity levels than hard gold at all is only with the escalation in the gold price of thicknesses investigated, there was a high recent years, leading to a very active search for standard deviation in lengthy laboratory runs, economic alternatives, that palladium plating indicating sensitivity of the deposits to normal has re-emerged as a leading challenger, as process variations. Palladium-nickel coatings evidenced by the increasing number of showed a porosity level more than an order of literature references relating to process develop- magnitude below that for hard gold, and in ment, and deposit properties and performance. addition the standard deviation of results for The level of activity in this context at the palladium-nickel was much less than for pure present time is well illustrated by a recent paper palladium. In ductility tests based on scanning from the Berg Electronics Division of Du Pont, electron microscope observation of surface Platinum Metals Rev., 1984, 28, (2), 54-55 54 cracking produced by a 180’ bend of coatings 0.3 mohm) of data points. Similar tests on tin- on 0.64 mm square wires around pins of lead/palladium-nickel and tin-lead/hard gold varying radius, palladium-nickel also out- couples gave erratic results with high deviation, performed all others, initial cracking occurring indicating the inadvisability of these combina- at a bend radius of less than 1.27 mm, com- tions in practice. In creep corrosion studies sul- pared with 6.4 mm for hard gold. In internal phide creep across hard gold was 5 mm in a five stress observations on thick deposits on disc year simulation, compared with only 0.90 mm specimens, discs carrying palladium-nickel coat- for palladium-nickel. Interestingly, similar good ings were flat as plated and remained so after results were reported with a soft gold top coat, prolonged storage, while similar samples plated which is apparently without effect on the creep with the “best” pure palladium coating, resistance of the underlying alloy coating. although flat as plated, with little porosity and No difference was found between palladium- good ductility, curled into a U-shaped bend nickel and hard gold coatings in the first after only one day, with severe microcracking at 100 to 200 wear test cycles, a typical purchaser the bend, caused by out-gassing of hydrogen. requirement. After 500 cycles the contact resistance of bare palladium-nickel increased by Product Evaluation I mohm, which is still below the maximum On the basis of the above results, three specified in Military Standards. However, for systems were selected for product evaluation on critical applications the authors suggest the use fabricated connectors, namely: [i] 0.75 pm of palladium-nickel with the soft gold top coat, palladium-nickel, [ii] 0.75 pm palladium-nickel which shows equivalent performance to that of + 0. I 25 pm soft sold and [in] 0.75 pm hard gold. hard gold. In the light of the results reported, Tests included on-going exposure in a variety the conclusion that the preferred palladium- of industrial atmospheres; accelerated nickel alloy is a viable alternative to gold on laboratory environmental tests in mixed water electronic connectors is not surprising. vapour, chlorine, hydrogen sulphide and nitrogen dioxide atmospheres; and exposure to Conclusion high hydrogen sulphide environments in a While this paper represents a valuable con- study of creep corrosion, that is the creep of tribution to the literature on palladium plating, corrosion products from exposed substrate particularly in furnishing much-needed across the plated surface. Cyclic wear tests were quantitative information on service perfor- also carried out in which plated pins were mance of deposits, it might have been of con- mated with hard gold plated female connectors. siderably more general interest had the authors In atmospheres with high chlorine content specified, if only by general type, the palladium contact resistance of hard gold was unchanged plating baths investigated. For example, in the during a test period simulating ten years’ case of pure palladium solutions, reference to exposure, while bare palladium-nickel showed a solutions developed prior to and since 1980 is small increase of about I mohm, well within the not too informative, particularly in view of the acceptable specified maximum. Palladium- level of activity in process development of nickel with a soft gold top coat performed recent years. Naturally enough, emphasis is to as well as hard gold. In a high hydrogen sul- be expected on successes rather than failures in phide environment all coatings showed constant an evaluation of this kind, aimed at a specific contact resistance after a similar exposure application, but some comparative results on period. In galvanic compatibility tests, perfor- the pure palladium baths could have been of mance of a palladium-nickel hard gold couple possible interest to workers pursuing this line, was reported as excellent, with no change in notwithstanding that palladium-nickel alloy contact resistance during a simulated ten year appears to come out as the clear leader at the period, and a low standard deviation (only present time. F.H.R. Platinum Metals Rev., 1984, 28, (2) 55 .