Rhodium Plating to Specification By li. K. Benham, F.R.I.C. .John~on,Matthey & Co., Limited The Ministry of Aviation has recently issued Process Specijcation DTD 931, covering the rhodium plating of silver, copper, steel and aluminium components primarily for electrical and electronic applica- tions. This article reviews the specijication and outlines recommended procedures for the successful preparation of rhodium electrodeposits. The electrodeposition of rhodium is of rhodium, coupled with its excellent resist- conducted on normal plating lines and, at ance to abrasion, provided just the right least for the thinner deposits, there are no combination of properties required in certain special difficulties. It is some twenty-five types of sliding or rubbing contacts in years since the process was first carried out on electronic equipment, and a demand arose a commercial scale. Originally it was chiefly for thicker deposits than had formerly been used on jewellery to provide a white non- needed. The Second World War, with its tarnishing surface of high reflectivity; for this rapid development of radio communication purpose deposits of only 0.000005 to o.oo0010 and radar, led to an enormous increase in the inch were adequate, and fortunately were amount of rhodium used for this type of relatively free from porosity. In addition the application and to the appearance on many great hardness of electrodeposited rhodium engineering drawings and individual specifica- gave an excellent resistance to wear. tions of a note calling for such-and-such a Within a few years it was found that the thickness of electrodeposit. complete freedom from tarnish characteristic In using a high-cost metal such as rhodium A group of typical electrical and electronic components with rhodium plated contact surfaces Platinum Metals Rev., 1961, 5, (l), 13-18 13 Masking of surfaces not required to be plated is carried out with a chlorinated rubber paint a good deal of discrimination naturally needs surface was required without wear-resisting to be used in deciding on the appropriate qualities, the thickness of rhodium was thickness of deposit to meet any particular specified as a minimum of O.OOOOI inch. This set of design requirements, and in the earlier specification made no reference, however, to days of its employment there was sometimes the methods or procedures to be adopted in a tendency for users to settle upon one producing rhodium electrodeposits. arbitrary thickness to meet a variety of con- The recent issue by the Ministry of Avia- ditions. Much more information is nowadays tion of Process Specification DTD 931, available on the thickness of rhodium “Rhodium Plating” fills this gap and pro- suitable for a particular application, and it is vides both users and electroplaters with a well understood that thicknesses may range comprehensive guide to the factors involved from a minimum of, say, O.OOOOI inch for the in the successful use of rhodium. protection of a silver surface from tarnish up to something over 0.001 inch where heavy Basis Metals and Undercoats mechanical wear is to be met with. Most of the metals and alloys commonly This variation in optimum thickness was used may readily be rhodium plated directly, recognised to some extent in the Ministry of although for reasons of skin conductivity, Supply Inter-Service Specification RCS/IOOO exposure to high temperatures or other special (now replaced by Defence Specification requirements it is more usual to deposit first DEF-~OOO),“General Requirements for an undercoating of silver or less frequently of Service Telecommunication Equipment”, nickel. which called for a rhodium thickness of The exceptions to this include steel and 0.00015 inch over a silver undercoating, but aluminium, which are readily attacked by the added that special requirements might be strongly acid plating bath unless a dense non- demanded where unusually heavy wear was porous undercoating of silver or nickel is first to be encountered. Where an untarnishable applied. Masking materials such as lacquers Platinum Metals Rev., 1961, 5, (l), 14 or chlorinated rubber paints are not sufficiently The choice of undercoat for rhodium is protective when the rhodium plating time is often dictated by the application. Thus, as more than a few minutes. For the deposition already mentioned, silver is specified when of say 0.00025 inch of rhodium this may be the plated component is to be used in a high- of the order of I to I* hours, and much damage frequency circuit. Here, particularly at very could result in this time unless the steel or high frequencies, the “skin effect” is pro- aluminium were completely protected. ‘The nounced, the current being concentrated more specification therefore calls for a deposit of in the outer skin of the conductor as the silver, copper or nickel not less than 0.001 frequency increases. In certain very high inch in thickness for steel or 0.0015 inch for frequency circuits the thickness of rhodium aluminium after polishing. may therefore have to be restricted because Tin-lead solders also present a problem in of its inferior conductivity compared with that that unless they are completely covered with of silver (35 per cent IACS against 106 per a non-porous undercoat before immersion in cent). The choice of rhodium thickness then the rhodium bath a dense black stain is becomes a compromise between the con- produced on the solder and over an appreci- flicting requirements of long life and con- able area around the joint. A similar require- ductivity. ment of a minimum deposit of silver, copper Silver is also the preferred undercoat to or nickel is therefore specified on all areas of minimise the possibility of cracking in thick soft solder. rhodium deposits. Where the application As soldered joints are frequently located on involves conditions of exposure to high temp- the inside corners of assemblies to be rhodium eratures, however, such as in reflectors of plated, it is essential that allowance be made various kinds, a nickel undercoat is to be for the fact that less silver will deposit in ‘such preferred. areas by comparison with the average thickness over the whole component. It would appear to be the tin content of the solder that gives rise to the problem, and where possible the use of a tin- silver-lead solder containing no more than 5 per cent tin is to be preferred. The writer has found that only 0.0001 inch of silver is adequate before rhodium plating over joints made with this alloy, while they may even be plated directly with no more than a very slight yellow- ing of the rhodium over the sol- dered area. A straight 5 per cent silver-lead solder is completely satisfactory for direct rhodium plating but is rather more difficult to use. Rhodium plating a batrh of electrical contart springs Platinum Metals Rev., 1961, 5, (l), IS Preparation for Plating paint should never be immediately adjacent The degreasing and alkaline cleaning to the essential contact area, but should be procedures given in the DTD specification brought to within about one-sixteenth inch follow the normally accepted practice for of this area. Rhodium deposits adjacent to these operations, but with rhodium plating stopping-off materials are sometimes found very great care must be exercised to ensure to be defective and this small margin is thorough cleaning, or poor adhesion of the helpful in reducing the possibility of failure deposit may result. When thick deposits of on this score. When using chlorinated rubber rhodium are to be applied an additional paint, stoving for two or three hours at etching step may well be incorporated, such 160°F effectively removes solvents in the as the anodic etch in cyanide for silver and paint film and also reduces the possibility of the immersion in a ferric chloride-hydro- failure. When parts are stopped-off it is also chloric acid solution for nickel described in advisable to use a cold cleaner to prevent the an appendix to the specification. Activation paint or lacquer from “frilling” at the edges. of nickel by cathodic treatment in sulphuric acid (say 10 per cent by volume) at 20 to 25 Plating Procedure amperes per square foot for half a minute is The concentration of rhodium in the also beneficial. Work so treated should be plating bath, its temperature, and the current transferred without rinsing with as little delay density at which it is operated may be varied as possible into the rhodium solution. It is over quite a wide range. For the production the writer’s experience, however, that ad- of thick deposits, a high metal content and a hesion on nickel is inferior to that to be low current density are to be preferred, obtained on silver even when these etching especially when pinholes in the deposit must treatments have been used. be avoided. This particular defect is also For stopping-off materials, either chlorin- minimised by regular tapping of the work bar ated rubber paints or special cellulose lacquers to dislodge hydrogen bubbles adhering to the are to be recommended, but wax is not surface. Alternatively, it is possible to fit a satisfactory. The edge of the stopping-off cam, operated by a fractional H.P. motor, to Recommendations for Rhodium Thickness and Electrolyte Composition Minimum Thickness Recommended on Agreed Recommended Minimum Significant Rhodium Sulphuric Class General Type of Surfaces Content, Acid Content, No. Application (inch) gmllitre millitre Rh 15 Tarnish protection . 0.0000 I5 12 20 Rh 25 Reflectors .. .. .. 0.000025 I Rh 100 Light mechanical loading and infrequent use .. .. 0.000 I 20 R h 200 Medium mechanical loading.. 0.0002 Rh 400 Heavy mechanical loading . 0.0004 Rh 800 Very heavy loading conditions 50 in special applications only 0.0008 Platinum Metals Rev., 1961, 5, (1) 16 lift the work bar say one inch and then drop failure to meet specification.