KR102860259B1 - Tape for electrical circuits having rose gold contact pads and method for manufacturing such tape - Google Patents

Abstract

A tape for manufacturing an electrical circuit having electrical contact pads, comprising: - a flexible dielectric substrate (4); - a copper foil (10) at least partially covering the dielectric substrate (4), the copper foil (10) having an inner surface (18) facing the dielectric substrate (4) and an outer surface (19) opposite the inner surface (18); - an intermediate layer comprising at least a nickel-based layer at least partially covering the outer surface (19) of the copper foil (10). A gold-copper alloy layer (13) is deposited on the intermediate layer. A method for manufacturing an electrical circuit comprises the steps of providing a tape having conductive pads, and plating the pads with a gold-copper alloy layer electrodeposited from an electrodeposition solution.

Description

Tape for electrical circuits having rose gold contact pads and method for manufacturing such tape

The present invention relates to an electrical circuit having contact pads. For example, such an electrical circuit is used to manufacture a module for a smart card, such as a module comprising electrical contact pads designed to be connected to a connector of a smart card reader (such smart cards are used, for example, for banking applications and identity documents).

A connector module for a smart card comprises a dielectric substrate supporting electrical contact pads (see, e.g., document WO2019051712A1). The electrical contact pads are etched from a conductive layer supported by the flexible dielectric substrate (e.g., by photolithography and etching techniques). Alternatively, the electrical contact pads are cut from the conductive layer before being co-laminated onto the flexible dielectric substrate (e.g., by lead frame techniques). In both cases, the contact pads supported by the dielectric substrate are produced as a flexible tape. More generally, this type of flexible tape having electrical circuits can be used to manufacture electrical contacts of connectors for memory keys or disks, etc. Therefore, electrical circuits produced in the field of the present invention are of particular interest when they are intended to be at least partially visible during their daily use.

The present invention is illustrated below using an example of an electrical circuit intended to form contacts for a smart card connector, but this example should not be construed as limiting, since the deposition process described below may generally be applied to any type of metallic conductive support.

In the smart card space, card manufacturers often want to match the color of the card body to that of the card module. For example, for aesthetic and/or customization purposes, it may be appropriate to have electrical contact pads with a so-called "rose gold" color (i.e., a pink-gold color) on the visible side of the module. However, producing electrical contact pads with new colors should not compromise their quality, such as in terms of electrical contact resistance, resistance to weathering, corrosion resistance, or fatigue resistance. The colored contacts must meet the requirements of the standards and specifications in effect for the intended use case.

An object of the present invention is to obtain an electrical circuit comprising conductive tracks or pads having a visible rose gold color in the finished product, while maintaining electrical and mechanical properties suitable for use as contacts intended to be electrically connected to a connector, particularly during a large number of connection and disconnection cycles.

This object is at least partially achieved by the tape according to claim 1 and the method according to claim 12. Other features of this tape and this method, which may be considered independently of one another or in combination with one or more others, are set out in the dependent claims.

In order to provide electrical contact pads having a “rose gold” color and complying with demanding specifications in terms of electrical contact resistance, resistance to climatic tests, resistance to corrosion, resistance to fatigue, etc., these electrical contact pads are composed of a gold-copper alloy having a specific copper concentration and a specific thickness and covered with at least one metal layer deposited with the help of an electrodeposition process.

Other characteristics, purposes and advantages of the tapes and methods described above will become apparent upon reading the detailed description that follows and upon reference to the accompanying drawings, which are provided by way of non-limiting example.
FIG. 1 schematically illustrates a perspective view of a smart card including an example of a module having an electrical circuit manufactured from a tape according to the present invention.
Figure 2 schematically illustrates a plan view of a portion of a tape according to the present invention.
Figure 3 schematically illustrates in cross-section an example of a laminated layer that can be obtained by the method according to the present invention.
Figures 4a to 4k schematically illustrate steps of an implementation example of a method according to the present invention.

An example of an electrical circuit manufactured from a tape according to the present invention is described below. While this example is taken from the field of smart cards, those skilled in the art will be able to adapt this example to other electrical circuit applications without requiring ingenuity. The present invention is particularly advantageous in any application where the use of pink-colored conductive contacts or tracks can provide aesthetic value (e.g., for connectors on SD memory cards or connectors on USB keys).

As illustrated in Fig. 1, a smart card (1) comprises a module (2) having, for example, a connector (3). The module (2) is typically manufactured in the form of individual elements cut from tape. These elements are inserted into a cavity formed in the card (1). This element comprises a typically flexible substrate (4) (see Fig. 2), such as PET, glass-epoxy, etc., on which a connector (3) is formed, which is then connected to a chip (not illustrated).

Fig. 2 illustrates an example of an electrical circuit portion. This electrical circuit portion is a printed circuit (5) having six connectors (3). Each connector (3) comprises eight electrical contact pads (6) (the connector (3) is essentially a module (2) without a chip, without a connection between the chip and the contact pads (6), and without an encapsulating resin for protecting the chip and the connection). The contact pads (6) are cut (e.g., punched or etched) from an electrically conductive sheet (10).

A schematic cross-sectional view of an example of a multilayer structure forming a tape for manufacturing a connector (3) is shown in Fig. 3. This multilayer structure comprises a substrate (4), an adhesive layer (9), an electrically conductive sheet (10) (made of, for example, copper or a copper alloy), and a stack of more or less numerous layers (A, B and/or C), as well as a gold-copper layer (13).

The electrically conductive sheet (10) has an inner surface (18) facing the dielectric substrate (4) and an outer surface (19) opposite to the inner surface (18). More specifically, the electrically conductive sheet (10) has an inner surface (18) attached to the dielectric substrate (4) (e.g., adhered or laminated with an adhesive layer). The inner surface (18) appears at the bottom of a blind hole (14). The blind hole (14) is intended to establish an electrical connection with a chip (e.g., connected by a wire bonding technique) possibly accommodated in the cavity (15). The outer surface (19) corresponds to a contact surface intended to establish an electrical connection with another connector. The layers (A, B, C) deposited on the inner surface (18) and the outer surface (19) do not necessarily have to be of the same type and of the same thickness.

The table below illustrates possible layer stacks:

The presence of a glossy layer beneath the gold-copper alloy layer (13) (beneath does not necessarily mean in contact with the gold-copper alloy layer (13) immediately beneath) results in an upper surface having a brighter appearance and a more intense pink color. This glossy sublayer can be obtained by electroplating copper and/or nickel and/or a nickel alloy onto an electrically conductive sheet (10), which may be prefabricated, for example, from copper or a copper alloy. Alternatively, the glossy appearance can be obtained by a polishing and/or electropolishing process performed on at least one underlying layer selected from the list comprising copper, nickel, nickel alloys and copper-tin alloys.

For example, by the process according to line 6 of the above table, the electrically conductive sheet (10) is a copper sheet. An electroplated layer (A) of copper is deposited on at least a portion of a free surface of the copper sheet (the other surface being attached to the dielectric substrate). This electroplated layer of copper (A) allows to reduce the roughness of the surface of the pad (i.e., the free surface of the layer of copper-gold alloy) in the finished product. For example, this roughness is equal to or close to 0.45±0.15 micrometers (Rz measurement). In the absence of the electrodeposited electroplated layer (A) of copper, the roughness of the upper surface is equal to or close to 0.90±0.20 micrometers (Rz measurement).

The gloss layer deposited beneath the copper-gold alloy layer is, on average, the free surface roughness of the copper-gold alloy layer divided by two. This allows for a semi-gloss finish to be obtained on the pad surface, instead of a matte finish without the underlying gloss layer (underlying does not mean in contact with the gold-copper alloy layer (13) directly below).

The gold layer (C) is optional. When deposited, the gold layer (C) is in the form of a "flash", i.e., deposited in a thickness of 0 to 15 nanometers.

When selective masking of the inner surface (18) is not expected, particularly when depositing a gold-copper alloy layer (13), a thin gold layer (C) is advantageously deposited on the inner surface (18) in the blind hole (14) before deposition of the gold-copper alloy layer (13) to increase the traction strength of the wire bonded on this inner surface (18).

Figures 4a to 4k schematically illustrate steps of an example of a method for manufacturing a tape as illustrated in Figure 2.

These steps can be performed through the reel-to-reel process:

- A step of providing a dielectric substrate (4), for example, a step of providing a dielectric substrate made of epoxy glass, polyimide, PET, etc. (Fig. 4a);

- A step of coating the surface of the substrate (4) with an adhesive layer (9) (Fig. 4b);

- A step of punching the substrate (4) with an adhesive layer (9) to form holes (14) and possibly cavities (15) in which chips can be accommodated in a later step (Fig. 4c);

- A step (Fig. 4d) of co-laminating a substrate (4) provided with an adhesive layer (9) with a sheet of an electrically conductive material (10) (e.g., a copper sheet), heating the complex thus obtained to achieve thermal cross-linking of the adhesive layer (9), and deoxidizing the complex thus obtained; a step in which the hole (14) is now a blind hole (14);

- A step of laminating a dry photosensitive film (16) on the outer surface (19) of an electrically conductive sheet (10) (Fig. 4e);

- A step of exposing a photosensitive film (16) through a mask (Fig. 4f),

- Step of developing a photosensitive film (16) (Fig. 4g);

- A step of forming conductive tracks and/or contact pads (6) by etching the electrically conductive sheet (10) in an area not protected by the photosensitive film (16) (Fig. 4h);

- Step of removing the photosensitive film (16) (Fig. 4i);

- A step of depositing a conductive layer on at least a part of the contact pad (6) and/or at least a part of the track obtained after etching the electrically conductive sheet (10); this deposition can be performed in one or more steps to form a stack of layers as described above with respect to FIG. 3; for example, this stack comprises a nickel layer (11) and a gold layer (12) (FIG. 4j); for example, the nickel layer (11) is deposited on the outer surface (19) to a thickness of 2 micrometers and on the inner surface (18) to a thickness of 5 micrometers; and the gold layer (12) is deposited on the outer surface (19) to a thickness of less than 15 nanometers and on the inner surface (18) to a thickness of 0.3 micrometer;

- A step of depositing a gold-copper layer (13) to form a surface layer having a gold-pink color (Fig. 4k); for example, the copper-gold layer (13) is deposited on the outer surface (19) to a thickness of 0.1 micrometer or more; for example, the copper content of the layer (13) of the gold-copper alloy is less than 40% Wt (by weight), preferably 35% Wt or less, more preferably 30% Wt or less, for example, 10 to 20% Wt.

The inner surface (18) can be protected, for example, by selective masking, at least during the deposition of the copper-gold layer (13) (i.e. the surface is intended to be invisible on the finished product when not covered by the copper-gold layer (13). Only the gold layer (12) and/or nickel and/or other alloy layer (11) covers the inner surface (18) of the blind hole (14). Thus, the packaging process for connecting the chips remains unchanged compared to a standard process.

Electrodeposition of a gold-copper layer (13) to form a rose gold surface layer:

○ As a gold cyanide complex, for example, the gold cyanide complex is KAu(CN) ₂ or KAu(CN) ₄ , a gold cyanide complex

○ As a copper cyanide complex, for example, the copper cyanide complex should be carried out by a specific deposition of a binary alloy together with a plating bath of an electrolyte containing the copper cyanide complex, K ₃ Cu(CN) ₄ .

For example, the gold concentration in the plating bath is 0.6 to 1.0 grams per liter and the copper concentration in the plating bath is 0.6 to 0.8 grams per liter. The bath has a pH of 11 to 13. The temperature of the bath is controlled at 50 to 60°C. Knowing that the cathode yield is about 5 to 15 mg/min, the current density in the plating bath and the speed of the tape are adjusted as a function of the available length of the plating cell to obtain the target thickness.

Advantageously, during the deposition of the copper-gold alloy, the electrodeposition bath does not contain any other metal compounds (based on zinc, nickel, silver or others).

The color of gold is typically calibrated according to the Swiss standard "1-5N", which ranges from a specific pale yellow to a pink gold alloy. The process disclosed above allows for obtaining a pink color outside this range, i.e., a color that would be 5N++ (i.e., a more intense pink color). The entire surface of the contact pad (6) is rose gold in color. Alternatively, the use of masking and/or etching techniques allows for coloring only a portion of this surface (e.g., for a logo).

For example, the pink color obtained by the process according to the present invention for the layer structure (copper sheet (10)/nickel/nickel alloy/gold (flash)/copper-gold alloy) corresponding to the second line of the above table has the following colorimetric parameters:

- L ^* = 63±1

- a ^* = 7.1±0.3, and

- b ^* = 6.9±0.3.

Product performance can be summarized in the table below:

Additionally, the visual appearance of the gold-copper layer (13) after the climatic tests mentioned in the table above can be improved by top surface protection. For example, such protection can be achieved by immersing the metal surface in a solution containing a thiol compound (e.g., 1-dodecanethiol).

Alternatively to the aforementioned method, a lead frame technique may be implemented to manufacture the tape. Subsequently, a gold-copper layer (13) may be plated on the conductive layer (10) before or after co-laminating the dielectric substrate (4) and the conductive layer (10).

Claims (16)

A tape for manufacturing an electric circuit (5) having an electric contact pad (6),
- Flexible dielectric substrate (4),
- A copper foil (10) that at least partially covers a dielectric substrate (4) and includes an electrical contact pad (6) cut within the copper foil (10), wherein the copper foil (10) has an inner surface (18) attached to the dielectric substrate (4) and an outer surface (19) opposite to the inner surface (18),
- In a tape comprising an intermediate layer including at least a nickel-based layer (11) at least partially covering the outer surface (19) of a copper foil (10),
A tape characterized in that a gold-copper alloy layer (13) is deposited on an intermediate layer, the intermediate layer comprises a thin gold layer (12) having a thickness of 15 nanometers or less and at least partially covering the nickel-based layer (11), the gold-copper alloy layer (13) is deposited on top of the thin gold layer (12), and the copper content of the gold-copper alloy layer (13) is greater than 10% Wt. In the first paragraph, the tape has a copper content of the gold-copper alloy layer (13) of less than 40% Wt. In the second paragraph, the tape has a copper content of the gold-copper alloy layer (13) of less than 30% Wt. In the third paragraph, the tape has a copper content of the gold-copper alloy layer (13) of less than 20% Wt. A tape according to any one of claims 1 to 4, wherein the layer of the gold-copper alloy layer (13) has a thickness of 0.1 micrometer or more. A tape according to any one of claims 1 to 4, comprising blind holes (14) formed by copper foil (10) at least partially covering holes formed through a dielectric substrate (4), wherein these blind holes (14) have a bottom surface corresponding to an inner surface (18) of the copper foil (10), and this bottom surface is not covered by a gold-copper alloy layer (13). A tape according to any one of claims 1 to 4, comprising blind holes (14) formed by a copper foil (10) at least partially covering a hole formed through a dielectric substrate (4), wherein these blind holes (14) have a bottom surface corresponding to an inner surface (18) of the copper foil (10), and wherein this bottom surface is covered by several layers including at least a thin gold layer and a gold-copper alloy layer (13), wherein the gold-copper alloy layer (13) is deposited on the thin gold layer, and the thin gold layer has a thickness of 15 nanometers or less. A tape according to any one of claims 1 to 4, wherein the gold-copper alloy layer (13) is at least partially covered by a protective layer comprising a thiol compound. A tape comprising a gloss layer under a gold-copper alloy layer (13) according to any one of claims 1 to 4. In claim 9, the tape is an electroplated layer of a metal selected from the list consisting of copper, nickel and nickel alloys. In claim 9, the tape is a polished or electropolished copper foil (10). A method for manufacturing an electric circuit (5),
- a step of providing a tape including a flexible dielectric substrate (4) supporting an electrically conductive pad (6), and
- A method comprising the step of plating a pad (6) with an intermediate layer including at least a nickel-based layer (11),
A method comprising: a step of depositing a gold-copper alloy layer (13) on an intermediate layer from an electrodeposition solution, wherein the intermediate layer comprises a thin gold layer (12) having a thickness of 15 nanometers or less; and a step of at least partially covering a nickel-based layer (11), wherein the gold-copper alloy layer (13) is deposited on top of the thin gold layer (12), and wherein the copper content of the gold-copper alloy layer (13) is greater than 10% Wt. A method according to claim 12, wherein the electrodeposition solution comprises a gold cyanide complex and a copper cyanide complex. A method according to claim 12, wherein the electrodeposition solution does not contain a cyanide compound. A method according to any one of claims 12 to 14, comprising the step of electroplating a glossy layer made of a metal selected from a list consisting of copper, nickel and nickel alloys under a gold-copper alloy layer (13). A method according to any one of claims 12 to 14, comprising a step of polishing or electropolishing a copper foil from which an electrically conductive pad (6) is manufactured prior to electrodeposition of a gold-copper alloy (13).