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(11) | EP 0 028 684 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Manufacture and aftertreatment of an article plated with gold, and article obtained thereby |
| (57) This invention relates to the fabrication of articles at least a portion of which
has an electroplated gold coating thereon having a Knoop hardness of at least 100.
The articles are produced by cold forming a material at least a portion of which has
an electroplated Additive-Free Hard Gold deposit possessing said hardness but including
no more than 0.1 percent of hardening metallic ingredients. Relatively high ductility
of the electroplated Additive-Free Hard Gold permits article fabrication which involves
cold deformation of plated members. Permitted cutting, stamping and other cold post-
plating procedures allow fabrication approach not possible with alloy hardened gold. |
Bakcground of the Invention
A Technical Field
[0001] The invention is concerned with fabrication of devices including gold surfaced regions. More particularly, devices of greatest immediate concern are those in which gold surfaces are electrical contact surfaces produced by electroplating.
B. History
[0002] Gold, early of interest for a number of uses based on its excellent resistance to corrosion, was found to have nonideal characteristics. Flexibility of processing was thought possible on the basis of gold characteristics, per se. Gold is a pliable metal, and, generally, is forgiving during a vast variety of processing steps. Wear resistance of pure gold however was found to be poor. This problem was solved by admixture with a variety of alloying ingredients.
[0003] Alloy hardened gold is now in prevalent use. Alloying ingredients, such as cobalt, nickel, cadmium, and arsenic, are used in very small amounts, sometimes tenths of a precent by weight, to markedly improve wear qualities. .These materials are in worldwide use for ornamentation as well as in electrical contacts in a vast variety of device designs. Unfortunately, alloying admixture has been found to have an undesirable side effect. Resulting material is quite brittle--specifically, is of very low ductility--and this restricts the type of handling after initial formation. Gold electroplating, for example, must be carried out on already formed substrates, since any attempt to substantially alter shape or to cut to size may result in cracking or even in flaking.
Summary of the Invention
[0004] A new type of hard gold electroplating is described in copending U. S. application, Serial No. 073,066, filed September 6, 1979 (Blessington-Buckley-Koch-Okinaka-Sard Case 1-2-1-16-6). In accordance with that application, smooth hard gold electroplate of quality quite similar to that produced from alloy hardened gold, results from plating from electrolyte containing no alloy hardening additives. Hardness of the electroplating is attributed to a temperature range of electroplating which is lowered relative to soft gold plating, while smoothness results from providing adequate solution agitation for given plating current.
[0005] In accordance with this invention, it is found that such Additive Free Hard Gold (hereinafter referred to as AFHG) electroplate, while sharing the hardness and smoothness of conventional alloy hardened gold, has an unexpectedly high tolerance for cold work.
[0006] This characteristic, which, inter alia, takes the form of elongations of as much as 30 percent without cracking, for the first time permits a variety of processes. Processes include those in which plated members are permanently deformed by strains larger than the elastic limit, as well as those in which deformation is transitory--i.e., short of the elastic limit at least for substrate material so that the part returns to its original shape.
[0007] Operations which are so permitted include, for example, those in which plated foils are wrapped around a mandrel. Another important category of procedures involves stamping and/or cutting operations in which plated sheets are divided into members of smaller device dimensions. Such a category includes switch contact paddles in which economies are realized by plating large sheets.
Brief Description of the Drawing
[0008] The Figure is a schematic view of a device including gold switch contacts which are cold worked subsequent to electroplating.
Detailed Description
I. The Figure
[0009] The Figure depicts a prototype remanent reed structure of a type described in U. S. Patent No. 3,059,075. The Figure depicts a glass envelope 1 containing two reeds 2 and 3 each of which is provided with contacting regions 4 and 5 of electroplated AFHG, respectively. The larger part of each of reeds-2-and 3 is flattened from its initial round wire configuration and unflattened regions 6 and 7 are hermetically sealed at glass sealed regions 8 and 9. Coils 10 and 11 are arranged to produce magnetizations of directions which oppose or cooperate to permit use of the switch in a crosspoint array.
[0010] The remanent reed structures 2 and 3 are, at this time, made of Remendur, a remanent magnetic material containing cobalt, iron, vanadium, and manganese. Development of appropriate square loop and remanent magnetic properties require a critical set of processing steps terminating in annealing and cold working. Contact material at 4 and 5 is electroplated hard gold which, in the past has been plated on already cold worked reed structures 2 and 3. With the advent of AFHG, small gold plated members, serving as contacts 4 and 5, may be formed by electroplating on Remedur sheets and the entire composite bodies may subsequently be stamped or cut to paddle shapes.
II. Processing Conditions
A. General
[0011] Conventional electroplated alloy hardened gold is extremely brittle and is chacterized by cracking if elongated by more than about 1 percent (J. M. Deuber and G. R. Lurie, Plating, 60, 715 (1973)]. AFHG, on the other hand, is characterized by reduced brittleness and freedom from cracking for elongation in a range up to about 30 percent as measured in terms of cross-sectional area reduction at the point of fracture during simple uniaxial tensile test. The invention may be described as permitting cold working operation subsequent to hard gold plating.
[0012] While detailed considerations of distortion so made available are complicated, they may be simplified by making certain assumptions. The gold surfacing layer is assumed to have a thickness of the order of one or a few micrometers while the substrate is at least twenty-five times thicker. The strain produced in the surfacing layer can be sufficiently well approximated by a classical strain analysis of the substrate alone, i.e., with a coating of negligible thickness. The case where an initially flat substrate is deformed to a certain radius of bending (RB) is an example of practical importance. If RB is less than about fifty times the substrate thickness (ts), cracking of conventional hard gold on the convex surface of the bent substrate is observed. In contrast, a value of RB less than ten times ts is required to produce cracking of an AFHG surfacing layer. The critical strain, approximately calculated from the formula, E = RB/2ts, is about 1 percent for conventional hard gold and 5 percent for AFHG. These are actual experimental values reduced from simple elongation value by extraneous factors.
[0013] Shearing and stamping manufacturing operations invariably produce a deformation of the substrate. One such operation involves shearing of epoxy-fiberglass printed circuit boards. The epoxy-fiberglass substrate is momentarily bent into a cylindrical surface over an area along the line of shearing and extending 1 or 2 mm away from the line of cutting. The radius of bending is several times 25 millimeters (several inches) and produces cracking of conventional hard gold plating which has been applied to form contact fingers within the deformed area of the printed circuit board. The critical radius for bending of AFHG in this application is only 25 millimeters (one inch) [ten times the board thickness of 2.5 millimeters (0.1 inch)] and therefore the deformation involved in shearing does not produce cracking of AFHG contact fingers.
B. Plating
[0014] AFHG plating is described in detail in the above-mentioned copending U. S. application Serial No. 073,066. Attainment of Knoop hardness numbers of 100, or even 200, if needed, results from plating at temperatures reduced with relation to those already used for soft gold plating from the same types of bath compositions. Temperatures are necessarily below 50 degrees C and, preferably, below 45 degrees C to result in the desired degree of hardness which is considered to derive from inclusion of gold cyanide in the plating. The cyanide, as retained, is believed to serve as a grain refining agent and to stabilize submicrometer grains. Smoothness, usually described in terms of deviations no larger than 0.5 µm, peak to valley, over regions of 5mm by 5mm, results from control of bath agitation. The key here is to operate with high agitation resulting in electrolyte flow rates preferaby of greater than 100 cm/sec-- sufficient to result in turbulence.
[0015] As described and claimed in the copending U. S. application, Serial No. 073,066, plating conditions usefully employed for AFHG plating include
(1) making the surface to be electroplated cathodic relative to an anode;
(2) wetting the said surface and the said anode by a gold containing aqueous ionic fluid;
(3) plating under conditions in accordance with which the said fluid has a flow rate of at least 50 cm/second in the vicinity of the said surface;
(4) in accordance with which said fluid defines a path for ionic transport between the said surface and the said anode;
(5) the said ionic fluid contains cyanide in amount such that total cyanide units, with each cyanide unit containing a single carbon atom and a single nitrogen atom however charged at least equal to the total number of gold atoms in the fluid, however charged, in solution;
(6) the said fluid has a pH of a maximum of 7.5;
(7) the said fluid contains a total amount of nonnoble metal however charged which does not exceed 0.5 percent of the said gold content of the said fluid;
(8) in which the temperature of the said fluid in the vicinity of the said surface is maintained at a temperature which is at a maximum of 50 degrees C;
(9) in that current flow resulting-from maintaining the said surface cathodic relative to the said anode is at a maximum value no greater than 0.9ii where i1 is defined as the mass transport limited current which is the current beyond which further increase results in no further increase in gold plating rate;
(10) whereby the resulting gold plating on the said surface is smooth over at least 95 percent as defined by a maximum tolerable deviation from flat of 0.5 µm within a sampling area of 5µm2.
C. Plastic Deformation
[0016] Contemplated procedures involve any processing which results in residual deformation of the already plated member. A simple form of deformation involves parts plated in a flat condition and subsequently bent around a mandrel to produce a curved surface in the final member. Other procedures involve drawing (e.g., to reduce diameter of a wire), flat rolling (e.g., to increase dimension of an already flat surface), roll flattening (e.g., for producing a tape from wire), swaging (e.g., to increase dimension by a hammering action), bending as well as subdividing as by shearing or blanking. It is likely that the elastic limit of concern is that primarily of the substrate. Significant operations permitted by use of AFHG are those in which the deformation, however, produced entails more than the 1 percent elongation permitted for alloy hardened gold.
D. Elastic Deformation
[0017] Under many processing circumstances, deformation producing failure may not be apparent in whole or in part from the cold-worked member. Such operations as carried out on a substrate of high elastic deformability may result in relaxation after deformation to return the plating to its near unworked state.
E. Stock Material and Processing Requirements
[0018] The inventive teaching entails electroplated AFHG. AFHG electroplating is described in detail in the above-mentioned copending U. S. application Serial No. 073,066.
[0019] Briefly, coatings of concern are primarily gold but may contain specified amounts of metallic ingredients. Total content of hardening ingredients, i.e., of cobalt, nickel, arsenic, cadmium, etc. is at a maximum of 0.1 weight percent based on the total coating. Other ingredients predominantly carbon, hydrogen, nitrogen and oxygen, are generally present as contaminants may be at a somewhat higher level, but the totality does not exceed about 1 weight percent.
[0020] AFHG is further characterized by a Knoop hardness number of at least 100. Smoothness generally measured over at least 95 percent of a major face is within a 5 µm square sample area. This deviation is generally measured as a peak-to-valley dimension and is less than 0.5 µm.
[0021] Thickness of AFHG coatings is generally sufficiently small that distortion measurements are not complicated by internal shear stress. Device grade coatings are likely to be nor more than a few m. Substrates on the other hand are likely to be relatively thick--at least thousands of micrometers (mils) so that the relevant distortion of the coating is easily determined. It is the general thesis that failure is due to the coating itself since most substrate maerials have substantially greater elongation and other distortion limits.
CHARACTERIZED BY
conducting said fabricating by first electroplating at least a portion of stock material to produce a coating of gold having said Knoop hardness of 100 but containing no more than 0.1 percent of hardening metallic ingredients and, subsequently, subjecting the electroplated stock material to cold working during which the electroplated portion may be elongated to greater than 1 percent,
wherein the said coating is being produced by electroplating under the following conditions:
(1) making the surface to be electroplated cathodic relative to an anode;
(2) wetting the said surface and the said anode by a gold-containing aqueous ionic fluid;
(3) plating under conditions in accordance with which the said fluid has a flow rate of at least 50 cm/second in the vicinity of the said surface;
(4) in accordance with which said fluid defines a path for ionic transport between the said surface and the said anode;
(5) the said ionic fluid contains cyanide in amount such that total cyanide units, with each cyanide unit containing a single carbon atom and a single nitrogen atom however charged at least equal to the total number of gold atoms in the fluid, however charged, in solution;
(6) the said fluid has a pH of a maximum of 7.5;
(7) the said fluid contains a total amount of nonnoble metal however charged which does not exceed 0.5 percent of the said gold content of the said fluid;
(8) in which the temperature of the said fluid in the vicinity of the said surface is maintained at a temperature which is at a maximum of 50 degrees C;
(9) in that current flow resulting from maintaining the said surface cathodic relative to the said anode is at a maximum value no greater than 0.9i1 where in is defined as the mass transport limited current which is the current beyond which further increase results in no further increase in gold plating rate;
(10) whereby.the resulting gold plating on the said surface is smooth over at least 95 percent as defined by a maximum tolerable deviation from flat of 0.5pm within a sampling area of 5µm2.
CHARACTERIZED IN THAT
the said cold working includes bending.
CHARACTERIZED IN THAT
the said bending results in elongation beyond the elastic limit of the said member so that the shape is permanently changed.
CHARACTERIZED IN THAT
the said bending is within the elastic limit for at least a portion of the said member so that such member relaxes at least partially subsequent to cold working.
CHARACTERIZED IN THAT
the stock material is divided into individual members, the cold working resulting during the division.
the division is by cutting.
the division is by blanking.