Historical Articles

July, 1953 issue of Plating

 

A Flexible Plated Circuit

---

E.R. Bowerman and R.F. Walton

This paper was presented at the National Conference on Airborne Electronics, Institute of Radio Engineers, Dayton, OH, May 1953.

ABSTRACT
A cloth backed plated circuit is made by a transfer process by which a strippable copper electrodeposit and a thin polyvinyl resist are transferred to a cloth adhesive tape from a stainless steel base. The stainless steel is reusable and its surface may be patterned to identify the wiring on the unit being produced. The transfer method eliminates the need for providing, between the wires, of conductive bridges which have to be removed later. Design changes require only a change in the silk screen used to apply the resist to the stainless steel.

INTRODUCTION
The flexible plated circuit appears to fit nicely into the general methodology of fabrication of electronic equipment. Some of the current forms of printed circuitry are rigid and this is, in fact, made use of in supporting small components. On the other hand, larger equipment is going to be chassis bound for some time to come. Here, experience has shown that a state of fluctuation exists in individual terminal to terminal dimensions and elevations. Individual wires easily take this variation-into account as can pig-tailed components. If the circuit is complex, it is not unusual for a number of small components to be sub assembled on a terminal board where dip soldering can be used to advantage.

THE PROCESS
The production of a flexible plated circuit, simply stated, consists in silk screening a stop-off (i. e., an insulating lacquer) onto a stainless steel surface, copper plating the non stopped-off area, and stripping the plating and stop-off from the stainless surface by an adhesive tape. This is illustrated in the flow diagram (Fig. 1) which will serve as an outline of the description to follow.

Stainless Steel Preparation
Type 316 or 347 stainless steel sheet, free from blemishes, flaws and fissures, is chosen because of its resistance to chemical attack in the copper plating solution and because it can be easily maintained in the passive condition necessary for stripping the copper plating. The surface is given a mirror finish by polishing and buffing and care is exercised in handling to avoid scratches. Surface roughness has a tendency to cause adhesion of copper plating to the steel, possibly by mechanical bonding since the difficulty is noticed even with properly passivated steel. Individual scratches will be faithfully reproduced in the plating and, if sharp, will have a tendency to cause local mechanical weakness in the copper probably as the result of a notch effect. After polishing and buffing, the surface is cleaned and passivated by anodic treatment in a conventional hot alkaline steel electrocleaning solution.

Silk Screening
A reversed silk screen is prepared by conventional graphic arts procedure using a fine mesh wire cloth (140 or 160 US Std) and a film which is resistant to the stop-off lacquer and thinner. The reversed screen is necessary because of the subsequent reversal when the circuit is stripped from the stainless steel.

The stop-off lacquer is a vinyl copolymer whose viscosity is adjusted to approximately 200 poises by means of suitable solvents. Care is used in the screening to avoid holes or discontinuities in the lacquer film other than those of design. The evaporation rate of the solvent is adjusted in a compromise between rapid drying of the screened film and slow drying on the screen.

Fig. 1. Flow diagram.	Fig. 2. Conventional chassis wiring.	Fig. 3. Chassis with plated circuit.

Electroforming Copper Wiring
After the stop-off is dry, the wiring is formed by electroplating between 1 and 3 mils of copper onto the non stopped-off stainless steel in an acid copper sulfate plating solution. A practical lower thickness limit exists in that deposits thinner than 0.7 mils have a tendency to tear when being stripped from the stainless steel. This is postulated to be due to a lower strength within the copper than in the weak bond between the copper and the stainless steel. An upper thickness limit is less easily defined but as the thickness increases the flexibility of the copper decreases and the deposit tends to spread along the surface over the stop-off. If the wiring is closely spaced, bridging may occur between wires due to the spreading deposit. For the circuitry investigated, loss in flexibility gave an upper limit at about 20 mils.

The plating solution used for the investigative work contains 250 g/l CuSO₄ · 5H₂O and 75 g/l H₂SO (sp. gr. 1.83). The solution was filtered through activated carbon before using and commercial electrolytic cathode copper was used as anode. The stopped-off stainless steel is connected as cathode before immersion in the plating solution so that plating starts immediately on the passive surface. The cathode is agitated during plating and a cathode current density of 100 amperes per square foot is used. In 18 minutes a 1.5 mil deposit is obtained which is suitable for 1/16 inch wide wires in receivers having series string heater connections with 150 milliampere currents. After plating the cathode is water rinsed to remove the plating solution and dried.

Stripping the Circuit
An industrial cloth backed adhesive tape is pressed against the copper plating and stop-off with sufficient pressure and time to obtain good contact to the adhesive. The tape and adhered circuit is then stripped from the steel sheet with a rapid, continuous, even pull. The stop-off also transfers and serves to cover the sticky areas of the tape not covered by the copper. The particular tape used was compounded using a thermosetting adhesive which could be cured at 250° F before use or allowed to cure in equipment. After stripping, the tape is trimmed and the required holes punched out. The stainless steel may be reused as long as the surface is maintained smooth, clean and passive.

Installation of the Plated Circuit
The flexible circuit is forced over the fixed component and tie point lugs before the small components are installed. The flexibility of circuit harness speeds up the installation by the give and take possible in cloth fabrics. Some attention to the design of the lugs will pay off in easing the assembly and reducing the necessity for hole punching to accommodate lugs. On the other hand, if highly detailed work is not being produced it is also possible to use a loose woven fabric for the adhesive tape backing which will permit the lugs to pierce it easily.

Coding of the Wiring
Since the electroforming of the copper will faithfully reproduce the surface finish of the stainless steel, it is possible to transfer certain types of designs to the copper surface beyond the obvious capabilities of outline identification techniques. Localized texturing of the stainless steel as, for example, by shot blasting, etching or engraving, will give a visual coding to the plating. Care must be used to employ only rounded and relieved designs to permit parting after plating. In addition, the deposit may have to be made a little thicker to obtain good strength for the parting operation.