Electroplating of Palladium for Electrical Contacts
Electroplating of Palladium for Electrical Contacts
When palladium is plated from an appropriate electrolyte the deposit possesses similar physical properties to those of hard gold, making it a very suitable electrical contact material. The relative cost of the two metals helps to account for the growing interest in the electrodeposition of palladium and the properties of the deposits, which are discussed here.
In one of the first appraisals of palladium electroplating it was remarked “that from a position of relative obscurity, palladium plating has advanced within a comparatively short time to a place of considerable importance among platinum metal finishes, particularly in the field of electrical contacts” (1). One of the reasons given for this was that some of the gold electrolytes used at that time—1965—to process printed circuit boards attacked the adhesives, lifting the copper foil in the boards. Another reason, favouring the use of palladium in contrast to gold, is the price per volume. Since the density of palladium is roughly one-half, and the price at present one-third to one-quarter that of gold, an equal thickness of palladium costs only one-sixth to one-eighth that of gold. This advantage is reduced if, for example, the higher forming and operating costs of palladium electrolytes and the refining of scrap are considered, but there is still quite an incentive to use palladium instead of gold. This is also probably the main reason why the interest in the electroplating of palladium and in the properties of the deposits has increased as the price of gold has risen. Today nearly all suppliers of plated connectors and major users have accepted palladium as a reliable connector contact material, and it has been in practical and increasing use for many years (2).
General Properties of Palladium Deposits
In general, one has to keep in mind that palladium is a member of the transition metal group of the periodic system and that gold belongs to the non-transition metals. This is the cause of several basic differences in the behaviour of the two elements, for example, palladium is catalytically very active both in solution and as a metal, while under these circumstances gold is not. Therefore a palladium electrolyte tends to be much more sensitive to impurities or alterations in its composition than a gold bath, and the surface of palladium is well known for exhibiting the so-called “brown powder effect” which originates from the condensation or polymerisation of organic vapours, emanating from plastics in the surroundings. This brown powder increases the contact resistance to intolerably high levels (3,4,5,6) but the effect can be greatly reduced or avoided if care is taken to use only appropriate plastics in the circuitry, or if the contact pressures are high enough, or if a gold flash (≤ 0.1 μ m) is applied on top of the palladium (7). A series of tests with plastics showed that polystyrene and PTFE caused the smallest increase in contact resistance of palladium, but that glass fibre reinforced plastic, polyethylene and especially hard paper (Pertinax) critically increased the contact resistance (3).
The electrical resistivity of electroplated palladium is between 10.7 and 15 μΩ cm, this being higher than that of bulk palladium and four times as high as that of hard gold (gold-cobalt, gold-nickel, gold-iron alloys) layers, which is 2 to 3 μΩ cm, but similar to the value of 18 carat gold-copper-cadmium deposits (8). However for contact applications, the resistivity of the deposits in most cases is not as important as the contact resistance and its change with time under different environmental conditions.
The contact resistance of as-deposited palladium, measured against gold, is in the range of that of hard gold and varies—depending on the measuring method—between 1 and 7 mΩ (1,3,8 to 12). In extensive tests carried out in atmospheres containing various amounts of sulphur dioxide, hydrogen sulphide, nitrogen dioxide and hydrogen it was demonstrated that under these conditions electrodeposited palladium shows only a slight increase in contact resistance of up to about 30 mΩ, at the most (1,3,8,9,10,12,13). Under the same conditions 18 carat gold-copper-cadmium layers exhibit much worse behaviour, their resistance becoming about twice as high as that of palladium (4,10).
The contact resistance of a 2.5 μ m thick palladium layer on copper did not change after a heat treatment of 200 hours at 125°C or one of 15 minutes at 200°C (3,9). However, that of a comparable gold layer had doubled under the same conditions (9), a behaviour confirmed for 2 hours at 165°C and 250 hours at 100°C (12). This is explained by the lower diffusion coefficients of metals such as copper and zinc through to palladium, compared with gold.
The different diffusion rate also plays a role, in the solderability of palladium layers (1,14). In general, dissolution of palladium is considerably slower than that of gold in 60 tin-40 lead solder, but the intermetallic compounds formed at the dissolving interface may result in fracture. It is therefore suggested that the amount of palladium dissolved in the solder be kept below 1 per cent by weight (14). The growth of intermetallic compound layers at room temperature is fairly rapid. After six months its thickness has increased to 3 μ m (14), while after 25 days at 100°C it is between 3 (15) and 60 μ m (14). The compound seems to be a lead-tin phase, with the rejected lead accumulating at the solder interface (14).
The other method of connecting leads to connector pins is by wire wrapping. From practical experience it is known that palladium layers behave in a similar way to hard gold, especially if they are heat treated. The microhardness of the deposits is generally in the range of 140 to 300 HV, this depending on the deposition parameters (8,9,16,17). It increases in the first hours after deposition and stays constant at longer times, or even drops slightly. This is due to the out-diffusion of hydrogen and to the corresponding volume changes (16,17,18).
The internal stress in the deposits is strongly dependent on factors such as the type of electrolyte and the deposition parameters (8,16 to 20). Tensile stresses observed are in the range of 50 to 200 N/mm2, but they increase during storage of the deposits up to, for example, 400 N/mm2 (20).
The wear and friction behaviour has also been studied extensively (1,3,8,9,13,16,17,23). F. H. Reid reports on field tests and production experience up to 1965 (1) and G. Schachmann (24) and G. D. Fatzer (25) on the experience of I.B.M. with palladium plated connectors. All investigations showed that palladium deposited in the form of single or duplex layers compares favourably with hard gold in its wear resistance. A thin gold layer on top of the palladium improves the wear behaviour further, as does a combination of a poor wear-resistant underlayer with a wear-resistant porous top layer (26,27).
Alloys are often heat treated at temperatures of 200 to 300°C, in order to reduce stresses and the tendency to crack. This is at least partly due to the removal of hydrogen, during which tensile stresses change to compressive ones.
Many investigations have been concerned with the porosity of the deposits, but it is very difficult to evaluate the results, since many factors not related to the electrolyte are involved. However, it seems to be established that, if a proper base material and pretreatment procedures are used, a level of porosity can be obtained at a given thickness which is equal to that of cobalt, nickel or iron hardened gold.
Hydrogen in Electrodeposited Palladium
The importance of dissolved hydrogen has been recognised since the earliest deposition experiments, but a method for its quantitative determination was only developed in recent years (18). This method was adopted and developed further for investigations on d.c. and pulse plated palladium (18).
Palladium dissolves appreciable amounts of hydrogen as a face centred cubic a-palladium solid solution at hydrogen to palladium ratios up to 0.03, at atmospheric pressure and room temperature. At higher hydrogen concentrations the face centred cubic β -PdH phase with hydrogen : palladium ≥ 0.57 is observed. Its lattice constant is higher by about 3.8 per cent than that for the α -Pd-H solid solution. These data refer to equilibrium conditions. In highly distorted electrodeposited layers solubility is much higher.
The hydrogen concentration of electrodeposits is important insofar as there is a connection between stress in the deposits and the hydrogen dissolved in the as-deposited material. Furthermore, if during storage hydrogen diffuses from deposits with higher hydrogen : palladium ratios, the decomposition of the β -phase and the accompanying change in volume may cause stresses and cracks in the layers, sometimes even days after deposition took place (18 to 22).
In general, the determination procedure for hydrogen by vacuum extraction is so slow that it catches only part of the hydrogen which is very rapidly diffusing out from the deposit ( cm2/s in β -PdH). Therefore an electrochemical method was developed which is very fast, and permits the first hydrogen measurements to be made within a few seconds of deposition (18). It uses the electrochemical oxidation of the hydrogen diffusing from the palladium layers to determine its amount via the charge Q used: Q = ∫i(t)dt, where i = anodic current and t = time. Figure 1 shows the circuitry used. The anodisation of hydrogen is done in 1N sodium hydroxide at a constant potential of +0.25V, which is high enough to oxidise hydrogen, but low enough to avoid other electrochemical reactions. The anodic current is dependent upon the diffusion rate of the hydrogen to the metal/electrolyte interface and the charge can be measured by integration with respect to time; thus via Faraday’s law the hydrogen quantity is calculated. Certainly there are limitations to this method, such as the loss of hydrogen in the short time between deposition and measurement, but it has proved to be a simple and reliable method for establishing the influence of deposition parameters, at least relatively, for the various electrolytes. Comparative tests with the vacuum hot extraction analysis gave lower values, since some hydrogen had diffused out before it could be measured.
Palladium Electrodeposition Processes
Only a few comparative laboratory investigations have been published which give details of the electrolyte compositions, deposition parameters and properties of deposits (3,9,18 to 23). F. H. Reid (1) tested seven electrolytes: (a) Tetrammino-palladous nitrate, (b) Sodium-pallado-nitrite, (c) Diammino-dinitrito-palladium (P-salt), (d) Dicyano-diammino-palladium, (e) Acid-palladous-chloride, (f) Palladosammine-chloride and (g) Palladium-sulphamate (developed by the Automatic Telephone and Electric Company). Of these electrolytes he found only (c) and (g) to be commercially interesting. Later, electrolyte prototypes (a), (e), (f) and a newly developed acid type bath (17,22) were tested at the Forschungsinstitut für Edelmetalle und Metallchemie (18,20,21,22) and the Technisch-Chemisches Laboratorium of the ETH Zürich (19). The sulphamate electrolyte has recently been re-evaluated (28).
The Acid PdCl2/HCl and Pd(NO3)2/H2SO4 Electrolytes
In the PdCl2/HCl electrolyte deposition occurs from an equilibrium Pd2+ + 4Cl + 2H+ ⇌ H2PdCl4. The palladium is rather loosely bound, so it deposits easily without external current on to less noble metals in a blackish powdery form. At a current efficiency of between 97 and 100 per cent palladium is electrodeposited as a compact crackfree layer, even at higher thicknesses, with a microhardness of about 250 to 350 HV. At current densities between 0.1 and 10 A/dm2 deposition at 10 to 50°C occurs at potentials of +0.5 to +0.1 V/NHE, see Figure 2. The activation energy of the reaction Pd2+/Pd was found to be ▵H = 17.1 ± 1 kcal/mol (18). The hydrogen content of the deposits is less than hydrogen : palladium = 0.03. X-ray analysis of such deposits shows only a slight expansion of the f.c.c. a -palladium lattice. The layers are fine, crystalline and non-oriented, with low internal stresses of about 60 N/mm2, which are reduced to 20 N/mm2 at 7 μ m thickness. This makes the electrolyte suitable for producing thick layers.
Recently the electroplating of palladium-silver alloys from a LiCl containing PdCl2/HCl type electrolyte was reported (29).
Unfortunately the emission of hydrochloric acid vapour and the tendency of these electrolytes to self-decompose makes them unsuitable for industrial production, at this stage of development.
These problems are avoided to a certain extent by a proprietary acid electrolyte based on Pd(NO3)2/H2SO4 (17,22,30), and containing sodium sulphite. Depending on the sulphite concentration and current density, the current efficiency for palladium deposition varies from 108 per cent (0.01 mol/l Na2SO3, 1 A/dm2) nominal (electroless deposition) to 63 per cent (0.2 mol/l, 2 A/dm2). The sulphite concentration of the electrodeposits increases from 0.15 per cent (0.01 mol/l Na2SO3, 1 A/dm2) to 10 per cent (0.2 mol/l Na2SO3, 1 A/dm2) and the H : Pd ratio varies from 0.007 (0.01 mol/l Na2SO3, 1 A/dm2) to 0.0006 (0.2 mol/l Na2SO3, 1 A/dm2) see the Table. The deposits have a microhardness of 350 HV, which can be increased to 430 by annealing for 1 hour at 100°C. After 1 hour at 400°C it dropped to 200 HV (11,22). Deposits show a fibrous texture, the <100> orientation being the preferred crystal axis. Internal tensile stresses are very low at nearly 50 N/mm2 but even at 10 μ m thickness they never exceed 100 N/mm2. Due to their low hydrogen concentration, deposits do not change their stress values during storage. The deposits made at low sulphite levels are bright, but on less noble base metals they need a gold or palladium prestrike.
A common feature of all acid electrolytes of low pH values seems to be their current efficiency of about 100 per cent, and the low hydrogen concentration and low tensile stresses of deposits produced from them. A problem with this type of electrolyte is its tendency to show electroless deposition on base metals, but this can be overcome by using gold or palladium prestrikes.
The Alkaline Pd(NH3)4Cl2 Electrolyte
The alkaline Pd(NH3)4Cl2 complex seems to be completely stable above pH 7.5, but below pH 6 the Pd(NH3)2Cl2 form is the stable one (18). Palladium deposits at 1 to 5 A/dm2 and −300 to −600 mV at a current efficiency of less than 100 per cent. The potential-current density curves show a strong temperature dependence explained by a decreasing stability of the complex, as can be seen from Figure 3. Current efficiency increases from about 60 per cent at 0.2 A/dm2 to 90 per cent at 2 A/dm2 and from 82 per cent at 25°C to about 85 per cent at 50°C. Deposits are highly stressed (100 to 200 N/mm2) and tend to form cracks at high thicknesses. Stresses and hardness values decrease with deposition temperature and thickness. Layers deposited at temperatures below 30°C show age hardening, during which crack formation is often observed, see Figure 4. The hydrogen concentration of the deposits increases with current densities, but decreases with deposition temperature. It depends on the relative movement of the electrolyte with respect to the sample. The hydrogen : palladium ratio of deposits from a still electrolyte increases from about 0.1 at 1 A/dm2 to 1 at 2 A/dm2. Deposits with hydrogen : palladium below 0.1 are bright and exhibit metallic lustre, but for hydrogen : palladium ≥ 0.8 they are powdery black. The layers show a cauliflower-like surface structure shown in Figure 5, and are rather sensitive to finger prints. This structure is strongly influenced by additions of carbonic acias or their derivatives, see Figures 6 and 7. When benzoic and nicotinic acid and their derivatives are added deposition potentials are shifted to more negative values, Figure 8 (20). Citrates have a similar effect. If the citrate concentration is 0.5 mol/l, the hydrogen concentration increases from hydrogen : palladium 0.02 to 0.08 at 2 A/dm2. In general citrate reduces the tensile stresses, but the deposits are still highly stressed (150 N/mm2) and tend to crack as illustrated in Figure 9 (20). Sodium salicylates increase the hydrogen concentration from hydrogen : palladium<0.01 to 0.18 (0.5 molar sodium salicylate at 2 A/dm2). Benzoates and salicylates, as well as heterocyclic N-containing compounds result in a sharp fibrous (110) orientation. At concentrations above 5 mmol/l phenacetine reduces the tensile stress of the deposits to values below those of deposits from addition-free electrolytes (20).
The influence of deposition parameters on the porosity of deposits from a palladosammine solution and the influence of saccharin were investigated by Fatzer (25). Apparently saccharin reduces hardness, but increases the porosity of the layers. The best deposits were obtained at room temperature and 60 g/l Pd(NH3)4Cl2 at a current density of 2.4 A/dm2. Such deposits are glossy or bright and at thicknesses greater than 1.25 μ m are pore free. A soluble anode process with a Na2PdCl2 solution produced highly stressed, porous deposits (25).
The Alkaline Pd(NH3)2(NO2)2 Electrolyte (P-salt Type)
Electrolytes of this type are among the most widely used solutions for palladium electrodeposition. They work at a pH value of between 7.5 and 10, and palladium is electrodeposited at −300 to −700 mV as dense, bright layers. At higher temperatures the deposition potential becomes more positive, and at higher pH values more negative. The current efficiency for palladium deposition increases from about 40 per cent at 20°C to nearly 60 per cent at 60°C; correspondingly the hydrogen : palladium concentration drops from about 0.04 (20°C, 1 A/dm2) to 0.005 (70°C, 1 A/dm2). Tensile stresses of 250 to 300 N/mm2 drop sharply with increasing deposition temperature and thickness, and there seems to be a relationship between hydrogen concentration and stress, see Figure 10 (21). The change in the hydrogen : palladium ratio at various storage temperatures with storage time is shown in Figure 11. It is observed that at pH values between seven and eight, insoluble palladium salts are incorporated in the deposit (21).
The behaviour of a P-salt type electrolyte and the deposits obtained by both pulse and d.c. plating were reported by Locarnini and Ibl (19). In general, the results of Hedrich and Raub (21) are confirmed. Under certain conditions pulse plating produces harder deposits than d.c. plating. For thicknesses of less than 2.5 μ m, crack and pore free deposits are more readily achieved in pulsed than in d.c. plating (19).
Excellent wear resistance and very low transverse porosity was obtained with doublex coatings consisting, for example, of a 1 μ m base layer from a Pd(NH3)2Cl2 type solution, with low wear resistance of its own, and a top layer (2 μ m) from an ammonia free electrolyte believed to contain as the predominant species. This combination proved better than a single layer from a Pd(NH3)4NO2 electrolyte, which worked at a deposition speed of 12 μ m/hour and a current efficiency of 75 per cent at 50°C and 1 A/dm2 (26).
An interesting variation of a palladium electrolyte with ammonia as a complexing agent is the use of higher alipathic polyamides, such as tetraethylenepentamin, as complex formers (31). Another solution based on P-salt was developed by the Automatic Telephone and Electric Company of Liverpool (32). These electrolytes contain 10 g/l P-salt, 100 g/l ammoniumsulphamate and operate at pH 7.5 to 8.3, 32°C and 0.6 to 1 A/dm2, with a cathode efficiency of 70 per cent. Deposits are milky in appearance at least up to 20 μ m, but at this thickness cracks appear. Apparently the current density of 2 A/dm2 is a limit, above which edge blackening of the deposits occurs.
Atkinson patented a process for deposition of palladium and a number of other metals from ammoniacal solutions in a diaphragm-cell (33,34). Further literature on deposition of palladium alloys was reviewed by Brenner (34).
The deposition of a palladium-nickel alloy with 27 per cent nickel from a bath containing 0.05 to 0.1 mol/l Pd(NH3)4Cl2 is discussed by McCaskie, Nobel and Whitlaw (12). Palladium-nickel deposits up to 50 per cent nickel, with or without a final gold flash, were studied by Pike-Biegunski and Bazzone (35). After ageing one month at 125°C in air, the contact resistance of 50 palladium-50 nickel increased to 12 ± 8 μΩ. The films with a microhardness of 240 to 450 Knoop show good wear characteristics, comparable to hard gold. These palladium-nickel alloys seem to be quite interesting as underplates for electrical contacts (35) or for decorative applications.
The electrodeposition of palladium and the properties of deposits obtained are discussed. There are two groups of electrolyte prototypes, one operating at pH values below 7 and the other one at values mostly above 10. The deposits are similar to hard gold in the properties required for electrical contacts. The hydrogen concentration seems to be a critical factor, since it has an influence on both internal stress and porosity. Today, palladium electrodeposits with or without a gold top layer are finding a widening application range as a contact finish for connector contacts.
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