GB2093862A

GB2093862A - Selective plating

Info

Publication number: GB2093862A
Application number: GB8106686A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1981-03-03
Filing date: 1981-03-03
Publication date: 1982-09-08
Also published as: NL8200758A; ES8302804A1; ES510110A0

Abstract

A selective plating method involves the following steps: (a) the region or regions to be plated is or are masked by the forcible application thereto of a flexible or pliable material, e.g., in the form of a strip or belt. The material is just soft enough to prevent the ingress of liquid when the surface is immersed in liquid. (b) electro-painting the unmasked surface in an electrolytic bath, e.g., with an epoxy-phenolic paint. (c) removing the flexible or pliable material so that the region(s) to be plated is or are left exposed. (d) electro-plating the region or regions with the precious metal. (e) optionally removing the electro- paint. The process may be used for plating connector contacts or relay contact springs with gold.

Description

SPECIFICATION Selective plating The invention relates to the selective plating of metal parts with precious metals.

In the making of such metallic parts as electrical contacts for connectors and contact springs e.g. for relays it is usual to plate the contact-making areas of the parts with a precious metal because of the good contact-making properties of such metals. Since such metals are expensive it is desirable to use as little as possible of such metals.

One known method of selective plating of metal parts is described in British Patent Specification No. 1194826 (Ronson Products Limited), where the method is used for gold plating defined areas of such articles as cigarette lighters. In this method the region to be plated is first masked with a non-conductive material, whereafter a coating of paint is applied to the unmasked region by an electro-painting process. Then the pair is at least partly cured, and the non-conductive masking material is removed from the region to be plated. Next the new unmasked region is electroplated with gold, the paint previously applied by the electro-painting process serving as a plating mask.

We devised an improved method of selective plating which, like the process-of the abovementioned Patent Specification No. 11 94826 uses electro-painting, and our method is described and claimed in British Patent Specification No.

1 562179 (H. F. Stirling et al 75-1). Claim 1 of this specification reads as follows: "A method of selective plating of a metal surface with a precious metal, which includes the steps of printing a removable mask on a metal surface to be plated, which mask covers the area to be plated, electro-painting the metal surface in an electrolytic bath with an epoxy-phenolic paint, said paint covering the portion or portions of the metal surface not covered by said removable mask, stoving the coating of epoxy-phenolic paint, removing the removable mask so as to leave the area to be plated with the precious metal unmasked, and electro-plating the unmasked area of the metal surface with the precious metal." The use of epoxy-phenolic paints was found to give results considerably better than the paint referred to in the Ronson Patent Specification, giving a strongly adherent and chemically resistive coating, but later work has revealed further improvements to this method. One of these improvements relates to the method used for applying the mask used prior to the electropainting process, and it is this which forms the main inventive feature of the method to be described herein.

According to the present invention there is provided a method of selective plating of a metal surface with a precious metal, which includes producing a mask by placing in forceful contact with the portion or portions to be plated a region or regions of a flexible or pliable material so that the region or regions to be plated is or are masked by said material, said material being just soft enough to effectively seal the masked region or regions from the ingress of liquid when the metal surface is immersed in liquid, electro-painting the metal surface where it is not masked by the flexible or pliable material in an electrolytic bath with a suitable paint, the paint thus covering the region or regions of the metal surface not masked by the flexible or pliable material, separating the flexible or pliable material from the metal surface so that the region or regions to be plated is or are left exposed, and electro-plating the region or regions of the metal surface not masked by the electro paint with the precious metal.

We now describe in some detail precious metal plating of electrical contacts for use in connectors, which method embodies the invention.

The first stage includes masking the base metal strip with a suitable medium, the masked areas coinciding with the areas to be gold-plated. In the method of our above-mentioned Patent Specification this masking was done by painting a iayer of a suitable medium on the areas to be gold plated. In the present method, a different technique is used for this masking step as will be seen below.

In the second stage the strip is electro-painted in a bath of an epoxy-phenolic paint, the deposition taking place on the exposed areas of the base metal, but not on the areas masked in the first step, no deposition occurring on the mask as it is an electrical insulator. This electro-painting takes place by a complex process based on electro-osmosis and electro-phoresis. Further information on electro-painting can be obtained from the book "Electro-painting" by R. L. Yeates, published 1 966 by Robert Draper Ltd. After the electro-painting the strip is rinsed.In the third stage the strip is heated to a temperature in the range of 1000C to 2500C to stove and cure the second medium to give a thin coating which can be mechanically deformed without flaking off, and is highly chemically resistant, especially to the chemical mixes normally used in electro-plating processes.

In the next stage the initial, i.e. pre-electropaint coating, is removed, whereafter the treated strip, which is not coated with the electro-paint in the region(s) other than those to be plated, passes into the gold electro-plating sequence, in which it forms the cathode in a gold plating bath. Then the gold is deposited only on the regions which were originally masked with the first, i.e. pre-electrocoat, mask.

We have found that with certain types of connector strip, which need to be selectively gold plated, another method of applying the initial i.e.

pre-electro-painting, mask can be used, which method may be preferred in production.

The method is based on the use of a strip or belt of flexible or pliable material (from soft rubbers to material liks polyethylene or polytetrafluoroethylene) which can be placed in forceful contact with the surfaces to be protected from electro-paint deposition. The mask material is chosen to be just soft enough to effectively seal and prevent ingress of electro-coating medium on flat connector blades, for example, a Shore hardness of D range 50/70. On the other hand, for contact fingers having a rounded surface, much softer materials may be chosen, having for example, a Shore hardness of A range 30/50.

In a step and repeat mode, the lengths of connector contacts are clamped (by jigging) between suitably shaped strips of the masking material and then immersed in the electropainting bath. Care is taken to electrically connect only the connector strip to the power supply. The rest of the jig assembly may be metal because during this electro-painting process no deposition takes place on electrically isolated metal parts.

A more important use of this method applies to continuous plating processes where the connector strip traverses all the necessary processing steps from reel to reel. In this application, and taking advantage of the flexibility of the connector strip, the masking belt or belts take the form of an arc of section of a circle having a suitable radius, pressure being applied against a circular drum.

The method is particularly useful in electropainting on the inner facing sides of the contact fingers. This prevents gold being subsequently plated in those areas.

The use of flexible belts and masks is well known in selective gold plating, in which case the belt or mask is forcibly pressed against the parts of the connector contact where no gold is required so that the gold containing electrolyte cannot reach these areas. In the present method the belt or mask is pressed against those parts of the connector contact where gold IS required. That is, the mask is positioned to allow the electrocoat masking film to be deposited where gold is not required. Thus the use of flexible belts or strips in the present case is the inverse of that normally used in selective plating.

The novei method prevents gold deposition, on the inside edges of contact fingers.

The improvements in the selective plating process described and claimed in our Application No. 8017563 (H. F. Sterling et al 81-2) can also be used in conjunction with the pre-electro-paint masking technique just described. Thus we initially apply a nickel flash, i.e. a very thin nickel coating, before printing the masking pattern and applying the electro-paint. This flash gives an electrochemically uniform surface which is a good recipient for the electro-painting, and which is also a suitable surface for gold plating. The flash can be another element, e.g. gold.

The other improvement derived from the above-mentioned application is in the region of the actual gold plating. In the method of the above-mentioned application, tne mask which limes the gold plating to the desired areas forms an integral part of the structure to be plated, so that an "under-solution" jet plating process is advantageous. This permits high throughput rates of plating reel to reel while maintaining good quality plating. The jet system is in the form of a long slot in a plastics housing, so that a narrow horizontal sheet of plating liquid impinges onto the seiected region of the contact fingers or other work pieces at very close range and at a high speed. Thus the plating tank can be very narrow by normal standards with one plating anode mounted around the jet. The opposite side of the tank can serve as a support and positioning device for the travelling strip so that the high jetting speed does not bend the contacts.

In some cases it is required to remove the cured electro-paint mask after the gold plating, and we have found that the electro-paint can be removed by a cathodic electrolytic treatment in an alkaline cleaning solution used hot.

In the arrangements described above we have used an epoxy-phenolic paint to produce the mask. However, other "electro-paint-able" materiais can be used, e.g. epoxy, epoxymelamine, acrylate polymers, and acrylatemelamine co-polymer.

Yet another usable "electro-paint" material is polybutadiene.

Claims

1. A method of selective plating of d metal surface with a precious metal, which includes producing a mask by placing in forceful contact with the portion or portions to be plated a region or regions of a flexible or pliable material so that the region or regions to be plated is or are masked by said material, said material being just soft enough to effectively seal the masked region or regions from the ingress of liquid when the metal surface is immersed in liquid, electro-painting the metal surface where it is not masked by the flexible or pliable material in an electrolytic bath with a suitable paint, the paint thus covering the region or regions of the metal surfaces not masked by the flexible or pliable material, separating the flexible or pliable material from the metal surface so that the region or regions to be plated is or are left exposed, and electro-plating the region or regions of the metal surface not masked by the electro-paint with the precious metal.

2. A method as claimed in claim 1, and in which the flexible or pliable material is a strip or belt.

3. A method as claimed in claim 2, and in which the strip or belt is urged against the metal surface to be plated, which surface is itself urged against the curved surface of a drum.

4. A method as claimed in claim 1,2 or 3, and in which before the electro-painting step a thin coating or flash of metal is applied to the region or regions to be plated.

5. A method as claimed in claim 1,2 or 3, and in which the electro-plating is effected by applying the plating material via a thin narrow jet of a suitable electrolyte, which jet-plating is effected with the metal surfaces to be plated immersed in the electrolyte.

6. A method as claimed in claim 1, 2, 3, 4 or 5, in which the precious metal is gold.

7. A method of selective electro-plating of metal surfaces with a precious metal, substantially as herein described.

8. A method as claimed in any one of the preceding claims, and in which the electropainting applies to the metal surfaces an epoxyphenolic paint.

9. Apparatus for implementing the method of any one of claims 1 to 8 above.

New claim filed on 2 May 1981.

New or amended claims:

10. A method of selective plating of a metal surface with a precious metal such as gold, which includes applying a thin coating or flash of a metal such as nickel to the metal surface so as to render that surface smooth plating-compatible, masking the region or regions of the metal surface to be plated by placing in forceful contact with said region or regions to be plated a flexible or pliable material, said material having a softness such as to effectively seal the masked region or regions from the ingress of liquid when the metal surface is immersed in liquid, immersing the metal surface with the region or regions to be plated masked in an electro-painting bath so that the portions of the metal surface which are not to be plated are electro-painted, the electro-painting material being an epoxy-phenolic paint, separating the flexible or pliable material from the metal surface so that the region or regions to be plated is or are left exposed, and electro-plating the region or regions of the metal surface not masked by the electro-paint with the precious metal.