[1].U.nited States Patent Office 327832111 3,783,111 GOLD PLATING BATH FOR BARREL PLATING OPERATIONS Maurice Bick, South Orange, and Jean A. Loche4 Metuchen, N.L, assignors to Auric Corporation, Newark, N.L No Drawing. Continuation-in-part of application Ser. No. 273,860, July 21, 1972. This appheation Sept. 11, 1972f Ser. No. 288,020 The portion of the term of the patent subsequent to Dec. 26, 1989, has been disclaimed Int. Cl. C23b 5128 U.S. Cf. 204-46 14 Claims - ABSTRACT OF THE DISCLOSURE An electroplating bath particularly suited for barrel plating operations. The bath includes in addition to an alkali-gold cyanide, a soluble salt of one or more chelating aminopolycarboxylic acids, with quantities of hydrazine, and small amounts of arsenic and/or lead.ion. This application is a continuation-in-part of our copending applications, Ser. No. 273,860, filed July 21, 1972, for "Gold Plating Bath for Barrel Plating Operations" and Ser. No. 176,334, filed Aug. 30, 1971, now U.S. Pat. No. 3,716,463, for "Bright Gold Electroplating Bath," which applications are assigned to the same assignee as the instant application. BACKGROUND OF INVENTION T'his invention relates generally to electroplating baths, and more specifically relates to such baths as are useful in barrel gold plating operations. Gold, within recent years, has become a very important part of the electronics industry. Among those properties recommending its use are its relative inalterability, high solderability, and low contact resistance. in the semiconductor field, gold has furthermore found favor because of its ability to readily form a eutectic alloy with silicon and germanium. In the latter connection, it may be noted that most headers or packages for diodes, transistors, and integrated circuits are gold plated as a preparation for the mounting or , attaching of the semi-conductor devices. For such an application, the gold deposit must be of high purity and deposited as unifornily as possible, in order to readily alloy with silicon or other metalhc contacts. The problem of plating such parts is compounded by the fact that these components are irregularly shaped and of complicated design. Such parts are exemplified by the well-known line of TO-5 and TC)@-8 multi-lead headers. Such headers consist of an eyelet of Kovar metal to which several insulated Kovar leads are attached and sealed in glass. In accordance with known principles in the art, headers of the foregoing type have in the past been plated (among other methods) by so-called barrel plating techniquesthat is, by subjecting such articles to electroplating while a plurality of articles tumble in a barrel. When such articles are thus plated, however, it is found that many leads do not make electrical contact with the remainder of the load. Where such condition obtains during the plating cycle, the portion of the lead closest to the anode becomes cathodic. Such leads become bipolar, and at the anodic portions of ihe leads problems can arise in that (a) the gold may redissolve anodically; and (b) the base metal can be attacked to expose bare sp0t especially where the plating solution includes high concentrations of citrates or phosphates. Should the tumbling action be markedly inadequate, these problems can become quite severe. In the past these problems have parPatented Jan. 1, 1974 2 tially been overcome by incorporating mechanical means for improving the electrical conductivity through the load. Such means have taken the form of metal particles or metal shot. Unfortunately, during the plating operation, the shot itself becomes gold-plated, resulting in loss of gold and attendant increase in the cost of plating the desired objects-i.e. the headers, etc. In our above-cited co-pending application Ser. No. 273,860, we have disclosed our finding that an alkali10 gold cyanide electroplating bath including a chelating phosphonic acid, with additional quantities of hydrazine and small amounts of arsenic and/or lead ion, eliminates or miniiiiizes many of the cited problems, and in fact provides superior performance in the aforementioned 15 barrel @lating operations. The said baths, however, are in at least one important commercial respect less than satisfactory: we refer to the fact that the phosphonic acid compounds identified therein are relatively expensive, which tends to inhibit the otherwise advantageous use 20 thereof. In the above connection it is pointed out that it has long been known (see for example U.S. Pat. No. 2,724,687) that certain of the chelating aminopolycarboxylic acids, such as, for example, ethylene diamine tetra acetic 25 acid (EDTA) are excellent chelating agents for use in gold plating. These compounds, furthermore, in comparison particularly to the phosphonic acid compounds mentioned above, are relatively inexpensive. - Unfortunately, however, the use of these compounds has, in the past, 30 been sharply limited by the fact that they lack stability under anodic oxidation. In consequence of this, decomposition products have tended in the past to form in gold plating baths in which such materials are employed, with resultant contamination of the bath, or of the deposit. 35 in order to rectify such condition it has, in the past, been required to regularly remove the decomposition products, as, for example, by means of activated charcoal, or the like. See, for example, the discussion of such decomposition phenomenon at page 125 of Fischer and Weimer, 40 Precious Metal Plating (Robert Draper, Ltd., Teddington, 1964). This phenomenon in turn has compounded the operating procedures and produced attendant cost and loss of operating time. 45 in accordance with the foregoing it may be regarded as an objert of the present invention to provide electroplating bath compositions for use in barrel plating operations, which are highly effective in reducing effects of bipolarity. 50 It is a further object c>f the invention, to provide gold electroplating baths, for use in the barrel floating of electronic components or the like, which baths display reduced tendencies to attack the base metal of the said components. 55 It is another object of the invention, to provide gold electroplating baths, especially suited for use in barrel plating of electronic components or the like, which prevent or slow down co-deposition with the gold of the troublesome common impurities such as copper, nickel, 60 cobalt, iron and lead. It is a further object of the invention to provide goldelectroplating baths enabling improved metal distribution and superior aesthetic properties in the resultant platings. 65 It is a still further object of the invention to provide Fold elec@troplating baths which incorporate relatively inext)ensive chelating aminopolyearboxylic acids and the soluble salts thereof, and which incorporate additional agents which act to prevent the formation of undesir70 able decomposition products in consequence of anodicoxidation.
[2]31783,111 SUMMARY OF INVENTION Now in accordance with the present invention, it has unexpectedly been discovered that the foregoing and other objects are achieved in anelectroplating bath which includes in addition to an alkali-gold cyanide, a soluble salt of one or more chelatinaminopolycarboxylic acids (or as appropriate the acids themselves), together with quantities of hydrazine and small amourits of arsenic and/or lead ion. The said combination in addition to eliminating or minimizing many of the cited problems previously found to exist during barrel plating operations, also by virtue of its use of the cited chelating agents enables formulations at relatively low cost. The cited chelatidg agents appear to function in the present environment to reduce the tendency of the compositions to attack the base metal, and by virtue of their chelating characteristics slow down or prevent co-deposition with the gold ef the common impurities, such as copper, nie-kel, cobalt, iron and lead. The electrolyte is kept slightly on the reducing side by the addition of the reducing agent, hydrazine. Such agent appears in the combination of the invention to greatly reduce or even eliminate the aforementioned tendency to anodic deplating of the parts being pr(>cessed. Furtliermore, for reasons that are not completely understood, the hydrazine, in the present composition acts to improve the throwing power of the bath. Additionally, as will be discussed in greater detaii hereinbelow, a synergistic effect appears to occur in the present bath between the chclating agents and hydrazine, which acts to stabilize the present bath, well beyond that which might be anticipated on the basis of the individual components. One of the conscquences of such synergistic effect appears to be a marked reduction in the tendency of the chelating a,-ents to anodically oxidize. As a result decomposition products are maintained at minimal levels. The trivalent arsenic ion and/or lead ion serve in the present environment to augment the smoothness of the deposit, yielding platings of superior aesthetic qualities, such as with respect to lustre and color thereof. The addition of as little as .4 mg. of As... or 2 mg. of Pb++ per liter of solution, refines the grain of the deposit and provides an attractive pale yellow semi-bright gold deposit of attractive lemon-yellow color and of low porosity. For reasons that are not completely understood these additives appear in the environment of barrel plating operation, to greatly improve the resulting metal distribution. DESCRIPTION OF PREFERRED EMBODIMENT Among the chelating agents which may be utilized in the baths of the present invention are the bath soluble salts of: ethylenediamine tetra acetic acid (EDTA); trans1,2- diaminocyclohexane tetra acetic acid (CDTA), available as the monohydrate from Geigy Chemical under the trademark "Chel-CD"; nitrilo triacetic acid (NTA); diethylene trian-iine penta acetic acid (DTPA); hyd roxyethylethylene diamine tetra acetic acid (HOEDTA), available from Geigy Chemical under the trademark "ChelDM"; and diaminoethylether tetra acetic acid, available from Geigy Chemical under the designation "Chol-ME." The acids themselves, as opposed to the salts thereof, may also be used where suitable solubility is exhibited by such materials, or combinations of the salts and acids may be utilized in desired formulations. The cited chelating agents are present in the baths of the present invention in a preferred concentration range of from about 60 to 320 g./l. All of the compounds cited exhibit high stability constants, in particular exhibiting relatively high pK values with respect to chelated metallic impurities, such as for example, copper, nickel, cobalt, iron, etc. However, the EDTA is particularly advantageous for use in the present compositions, in consideration of the relatively low rost and availability of such QQMpound. If the said compound is coriipared, for ekample, with the specified CDTA, the latter is considerably more resistant than EDTA; and it is found that the cited decomposition problems (under anodic oxidation) are not as acute with respect to the latter compound. However, the cost of the CDTA is far higher than that of EDTA, and for such reason its use in the past has been rather limited. In accordance with the present invention, however, the tendency of EDTA to decompose under anodic 10 oxidation may be so sharply diminished as to &nable use of EDTA by ;tself in the compositions to be@ set forth; or alternatively optimized combinations of EDTA and CDTA, such as provide at reasonable cost both high chelating abilities and also the absence of decomposition 15 products, may be achieved. Examples 1-3 A series of three gold electroplating baths were prepared in accordance with the present invention, incorporating the following components: 20 Bath 1 Bqth 2 Bath 3 EDTA acid, g./i -------------------------- 75 150 150 EDTA Na4, 9-/l --------------------------- 120 --- ---------------- Hydrazine 64%, mi./i ---------------------- 25 26 25 Au, g./i ----------------------------------- 8 8 2 25 AS, +++mg./l ------------------------------- 2 la phadjustedto ---------------------------- 7.0 71.0 4.5 KOHtopH ---------------------------------------- 7.0 4.5 All of the three baths cited above were operated at 50' C. and at current densities of 5 a.s.f., and were found to 30 be highly suitable for barrel plating of parts such as the cited TO-5 headers. Bath 2 is a preferred formulation; Bath 3 is particularly useful as a strike bath, but is somewhat less stable by virtue of its lower pH. In place of, or in addition to the As... ion, it has been 35 found that small quantities of Pb++ ion may be utilized, with consequent marked improvement in the properties of the deposit-including refinement of grain, production of porosity, and better metal distribution. Resultant iin40 provement of aesthetic qualities, such as lustre and color is similarly evidenced. Where lead is used alone as little' as 2 mg./I. are thus effective (calculated as the metal), with the useful range of addition extending up to about 12 mg./I. The lead may be in the form of bath soluble salts, such as lead nitrate and lead acetate. It may be 45 observed in connection with the present results that it is quite unexpected in view of the fact that lead ion acts as a most undesirable impurity in pure gold deposits plated from citrate or phosphate-citrate systems. 50 Examples 4-5 Bath 4 Bath ODTA,g./l ----------------------------------- Hydrazine 647,, ml./I ------------ ir)o 120 25 25 55 KoHtopH ----------------------------------------- 7.0 4.3 Au, g./i ----------------------------------------- 8 3 As + + +, mg./i ----------------------------------- 1 1 pH adjusted to ---------------------------------- 7.0 4.6 The CDTA cited for Examples 4 and 5 is the "Chej60 CD" material available from Geigy Chemical, as above indicated. Both of the baths cited above were found to be extremely suitable for barrel plating operations when operated at 50' C. and current densities of 5 a.sf. The gold cited for use in all of the foregoing examples is 65 calculated as the metal and is added to the compositions preferably as a standard 68% P.G.C. (potassium gold cyanide) solution. Other alkali-gold cyanide solutions, such as sodium, or ammonium gold cyanide solutions may be used. As metallic gold, the element may be pres70 ent in the approximate range of from about 2 g./l. to saturation, with a practical upper limit being about 32 g./l. The hydrazine 64% (hydrazine hydrate) is a product available from Fairmount Chemical Co., Newark, N.J. Said hydrazine 64% may be present in the bath in 75 the approximate range of from.5 ml,/I. to 100 ml./I.,
[3]31783,111 6 somewhat hi ver@al com- Example 7 ghet. In preparing. the: bath se P;onents are adde-d. to th6.,bat@ 4,n the order indicated, A bath was prepared as follows: V-iith uticieni' wgtei b6i@g added io ptovide one lit6r bf solution. 'fhe f @d '-@@@trAtion of hydra'zine Gold (as P.G.C.) ----- ------------------ g./I-- @8 pre orre conc (ca@6@l@ied as 64% h@dliaz,ine) is abbut 25 ml./]. for tyoiMonosodium phosphate ------------- 7 ---- g./I-- 50 CAI a@olications, such 'as where headers like the TO-5 Citric acid ----------------------------- g./I--. 110 and TO-8 are pr@:t@d@ Contentrations substantially beyond Hydrazine 64% ------------------------ m./I-- 100 this level yield li-ttle.additional benefits. As --- ----------------------------- -- mg./I-- 2 'Tho bat@s of inVention ar@@ typicallk used at a pH pH adjusted to 7.0. 6i a,bo,u@ t, 7"."o'. Good r6sults are ac@ieved Wit@ pHs a low Temperature --- -------------I------------- C-- 50 ag 4.0 but a prefeired 6p6ratifig r@nge is fr6m about 6.0 A panel.wa s plated at, .4 amp/sq. deciinete r in the to 8.0. Typit@@l opeiating temperatures are from about above,solutio n and was acceptable with a pleasing lemon;90' F. t6, 1'3'6",F., with a preferred. operating range being ,@ i ' i2O' to 125' F.' ye.1low color. Efficiency was found to be 127.6 mg./ampfrolli @aboiit min ii, @@ , . . (which indicates an electroless action since the maxias p,4 -pointed Out'previously, 66rAbinations of 1,5 mum efficiency for a goldplating solution is 122.6 mg./ l@ 'D'fA atid CDTA r@ay be'foritiulated in, order to pro"@id' ' p imize resul s M e -o'; t' ' d t fro' b6th a technical an,d e6onomic viewr)oint. Example 6 below sets forth su&h a bath. Ex ample 6 20 FDTA --------- g@/1-- @140 CDTA@ - - ---------- ------------ g.ll-- 10 2 ------------------ ----------- g./I-- 5 @llydrazine,@64% ----------------- ml./I-- 25 25 As --- ------- ----------------------- mg./I- I AU@ ------- -------------------- --g./I-- 8.2 pH adjusted to 7.0. The said bath was operated at 50' C. and at a current 30 density of 5 a.s.f. in@th@,'plating@@of@@-d test panel. After 20 minutes the said,- - ,panel was plated withia lustrous gold n yell6w color. Th pie deposit-, of @i asi g 0 efficiency was fouiid io be; 128 mg./amp,min. Maxi @mum increased' '@'stability of bath8' in accordance 35 -with@ t]@e invention are achioved; in fh@: @' pH rdn-gi-. of 6.0 8.6.-@It will,be appredi@i6d@@@by e skilled @in thd art ,@tfi@a't s6@h result is - unexpedted a'g' the: 1 reducing' @biliiies of hydrazine would normally be anticipated to increase with k @ n@ a 40 increasing pH, iii@: 61 @ll @ r n'ge betweeil 7.0 and 8.0. This point is well illustrated by the Bath 3 set forth above. Such bath is very useful, and is, a preferred strike bath to precoat TO-5 and TO-8 multi-lead headers, by virtue of its pH of 4.5-which is necessary to provide adhesion on Kovar and similar related alloys. 45 (Baths at pH 6 to 8 do not normally provide good adhesion over Kovar and other ferrous alloys.) The eifficiency of the Bath 3 is about 122 mg./amp-min. at 50' C and with current densities of 5 a.s.f. The stability of the bath with a pH of 4.5 is much less than that. of the preferred 50 bath at pH of 6.0 to 8.0. In particular the gold slowly plates out on the side of the tank heaters, anodes, etc. The bath performs satisfactorily provided the gold content is maintained above about 2 g./I.-with the other components being in the range otherwise specified for the 55 invention. However, the bath has a much shorter life and must be changed more often, in contrast to which the baths having pHs of about 6.0 to 8.0 and temperatures of about 50' C., have unlimited life and need be changed only when the level of contaminants (especially organics) 60 becomes excessive. The action of the hydrazine in the present bath appears to be much more than that of a mere redticing agent. Rather it appears that a synergistic action occurs by virtue of the combination of the chelating agent, such as 65 the cited EDTA, with the hydrazine. Thus, it is observed that the cited combinations stabilize the baths well beyond what might be anticipated in the absence of synergistic effects-since gold plating baths with large concentrations of hydrazine are normally destroyed during 70 electrolysis. This aspect of the invention is illustrated by comparing the data for the bath of Example 7 below with the data cited in connection with bath 2, above. These two baths, while both ut@lizin-g hydrazine, differ with respect to the acid additive. 75 am mn . P- i .) Th,e said bath was stable overnight, but after a few days of standing at room temperature the bath bi6ke down and the gold started toprecipitate out. In contrast to this bath 2 was found to be stable and continued to be stable for several weeks. Pho@@hate ion can be added to the baths of the present invention to adjust the conductivity and/or the pH. The id i ny sa on may be typically added in excess and a alkali phosphate is useful, although preferably potassium dip4osphate is so utilized. A typical formulation is illustrated in Example 8 below. Example 8 Potassium diphosphate ------------------- g./I-- 150 EDTA --------------------------------- g./I-- 100 As --- ------------------------------- mg.ll-- Gold ----- g./I-- ----------------------------- Hydrazine ---------------------------- ml./I-- 25 The said bath set forth in this example displays a pH 'of approximately 7.1. Further@in accordance with the present invention, it has been.found that addition of quantities of cyclohexylamine to baths of the foregoing type are useful in adding additional stability to the said baths. Exarnple 9 Monopotassium phosphate ---------------- g./I-- 150 CDTA --------------------------------- g./I-- loo Hydrazine ---------------------------- 25 Cyclohexylamine ----------------------- ml./I-- 25 Gold ---------------------------------- g./I-- 3 pH ----------------------------------------- 4.3 Plating temperature ----------------------- C-- 50 Current density ------------------------- a.s.f-- 5 The bath cited is found to possess a very high chelating ability. Addition of as little as I mg. of arsenic per liter is found to provide a very pleasing Iemon-yellow color in the ensuing plating; it is of interest, however, to note that the chelating ability of the said bath is so high that even with arsenic at levels as high as 8 to 16 mg./I., satisfactory results are still achieved. The bath cited, as is evident from its pH, is particularly suitable for strike purposes. Baths of this general type are normally quite difficult to stabilize over extended period; in contradistinction the bath of this example is found to be stable over extended time durations. A preferable range of addition for the cyclohexylamine is in the approximate range of from about 1 to 50 ml./I. While the present invention has been particularly set forth in terms of specific embodiments thereof, it will be understood in view of the instant disclosure, that numerous variations upon the invention are now enabled to those skilled in the art. The compositions set forth, for example, while being particularly adapted for use in barrel plating operations, also find application in other plating environments, as for example in rack or still plating operations, where it is chiefly the distribution of metal which is of concern. Accordingly the invention is to be broadly construed, and limited only by the scope and spirit of the claims now appended hereto.
