JP2012231159A - Printed wiring board and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a printed wiring board on which a barrier layer surely covering an entire surface of a wiring circuit conductor is formed.SOLUTION: A manufacturing method of a printed wiring board includes the steps of: 1) preparing a copper clad laminate having a copper foil at least one surface of an insulating base material; 2) forming a wiring circuit 2a with respect to the copper foil laminate; 3) forming a first conductive barrier layer 4a on a surface of the wiring circuit 2a; 4) forming an insulating layer 6 on the wiring circuit 2a on which the first conductive barrier layer 4a is formed; 5) selectively removing the whole or a part of the insulating base material by etching; and 6) forming a second conductive barrier layer 4b connected to the first conductive barrier layer 4a on the surface of the wiring circuit 2a exposed as a result of the removal by etching.

Description

  The present invention relates to a method of manufacturing a printed wiring board, and more particularly to a method of manufacturing a printed wiring board that ensures insulation in fine wiring.

  In recent years, with the miniaturization and multi-functionalization of electronic devices, there is a demand for miniaturization and high density of printed wiring boards used inside the devices. For this reason, the miniaturization of the wiring circuit formed on the printed wiring board is in progress. Currently, the finest single-sided printed wiring board that can be mass-produced has a wiring pitch of 40 μm (reference: JEITA issued, 2007 edition “Japan The packaging technology roadmap ") or below.

  In addition, some single-sided wiring boards such as COF tapes have begun to appear with a wiring pitch of 30 μm. A printed wiring board having such ultrafine wiring has a very narrow gap between wirings of 20 μm or less, and it is extremely difficult to ensure insulation reliability between the wirings. In particular, many insulation deteriorations due to copper ion migration in a high temperature and high humidity environment have been reported, and it is an urgent task to ensure migration resistance between fine wirings.

  It is known that the migration resistance between such fine wirings is greatly influenced by the cover material formed on the wiring circuit. Currently, solder resists for printing with extremely high migration resistance are being released for single-sided ultra-fine wiring boards such as COF tape. However, the cover material used for the fine printed wiring board is required to have high resolution performance (the ability to form a minute opening of φ100 μm or less), and a solder resist for printing cannot be used.

  For this reason, even if the migration resistance is somewhat sacrificed, a photo solder resist with high resolution performance must be used. From the above, there is a need for a technique that can ensure high insulation reliability without depending on the cover material.

  As a method of forming a wiring circuit with high insulation reliability against this situation, a plating mask is formed again in the circuit gap and a barrier layer is formed on the circuit surface of the wiring circuit created by the semi-additive method. (See Patent Documents 1 and 2).

  In this method, insulation reliability can be ensured by uniform formation of the barrier layer, but it is necessary to accurately align the plating mask to be formed again and the wiring circuit. For this reason, it is difficult to apply to a flexible printed wiring board having dimensional expansion / contraction variation between processes and a thin single-sided printed wiring board having elasticity.

  Patent Document 3 provides a method for forming a barrier layer (gold in the embodiment) on the surface of a wiring circuit. Thus, the printed wiring board which forms a barrier layer on the surface of a wiring circuit is produced by the following processes.

  That is, as shown in FIG. 3 (1), a single-sided copper clad laminate 43 having a copper foil 42 on one side of an insulating base material 41 made of polyimide or the like is prepared. Next, as shown in FIG. 3 (2), the wiring circuit 42a is formed by using an etching method based on a normal photofabrication method.

  Next, as shown in FIG. 3 (3), a barrier layer 44 for ensuring migration resistance is formed on the surface of the wiring circuit 42a using a technique such as electrolytic plating or electroless plating. The barrier tank 44 is preferably made of gold.

  Subsequently, as shown in FIG. 3 (4), the coverlay 47 having the adhesive layer 46 on one side of the insulating material 45 made of polyimide or the like is subjected to external processing, and then the wiring circuit 42a of the single-sided copper-clad plate 43b. The wiring circuit 42a and the barrier layer 44 are covered by positioning toward the side and thermocompression bonding.

  In the printed wiring board manufactured by such a method, a barrier layer is not formed on the entire surface of the wiring circuit 42a, and a boundary B between the insulating base material 41 and the barrier layer 44 is necessarily formed. At the boundary B, there are always irregularities on the surface of the insulating base material 41, irregularities on the wiring circuit 42a, etc., and it is difficult to form the barrier layer 44 uniformly without any gaps.

  Therefore, when a voltage is applied between the wiring circuits 42a in a high-temperature and high-humidity environment, the copper forming the wiring circuit 42a ionizes and flows out from the boundary B portion on the anode side, and finally ion migration Causes insulation deterioration due to From the above, the method of forming the barrier layer only on the surface of the wiring circuit cannot solve the problem.

  Patent Document 4 discloses a method of forming a similar barrier layer on the circuit surface and bottom. Thus, the printed wiring board which forms a barrier layer on the surface of a wiring circuit is produced by the following processes.

  That is, as shown in FIG. 4 (1), a copper foil 61 having a thickness of about 70 μm is prepared, and a pattern of a plating resist 62 made of a photosensitive dry film is formed on one surface thereof. Next, as shown in FIG. 4 (2), the first barrier layer 63 is formed by electrolytic gold plating or the like, and then the wiring circuit 64 is formed by electrolytic copper plating.

  Next, as shown in FIG. 4 (3), the plating resist 62 is peeled off, and a titanium layer serving as a second barrier layer is formed over the entire surface using a sputtering method. The layer is changed to a titanium nitride layer 65. Subsequently, as shown in FIG. 4 (4), the coverlay 66 is bonded together, and the copper foil 61 is removed by etching.

  Thereafter, as shown in FIG. 4 (5), the titanium nitride layer 65 is removed by etching. At this time, the titanium nitride is also removed by a considerable amount at the boundary C portion between the first barrier layer 63 made of gold and the second barrier layer 65 which is a titanium nitride layer. As a result, a small gap is formed.

  Next, as shown in FIG. 4 (6), a solder resist layer 67 is formed at a necessary location. In the printed wiring board manufactured by such a method, a barrier layer is not formed on the entire surface of the wiring circuit 64, and there is always a small gap at the boundary C between the first barrier layer 63 and the second barrier layer. I can do it.

Therefore, when a voltage is applied between the wiring circuits 64 in a high-temperature and high-humidity environment, the copper forming the wiring circuit 64 ionizes and flows out from the boundary C portion on the anode side. Causes insulation deterioration due to From the above, the method described in Patent Document 4 cannot solve the problem.
JP 1997-064493 Japanese Patent Laid-Open No. 2001-345540 Japanese Unexamined Patent Publication No. 2004-119589 JP 2004-235554 A

  As described above, a printed wiring board having a fine wiring circuit has a very narrow gap between wirings, and it is extremely difficult to ensure insulation reliability between wirings. In particular, many insulation deteriorations due to copper ion migration in a high temperature and high humidity environment have been reported, and it is an urgent task to ensure migration resistance between fine wirings.

  Printing solder resists with high migration resistance are also on the market, but printing solder resists have low resolution performance and cannot be used for fine printed wiring boards that require high resolution performance. For this reason, there is a need for a technique that can ensure high insulation reliability without depending on the cover material.

  The present invention has been made in consideration of the above-described points, and an object of the present invention is to provide a method for manufacturing a printed wiring board in which a barrier layer that reliably covers the entire surface of a wiring circuit conductor is formed.

In order to achieve the above object,
A method of manufacturing a printed wiring board,
1) Step of preparing a copper clad laminate having a copper foil on at least one side of an insulating base material
2) Step of forming a wiring circuit on the copper clad laminate
3) A step of forming a first conductive barrier layer on the surface of the wiring circuit
4) A step of forming an insulating layer on the wiring circuit on which the first conductive barrier layer is formed.
5) a step of selectively etching away all or part of the insulating base material
6) A method of manufacturing a printed wiring board, comprising: forming a second conductive barrier layer connected to the first conductive barrier layer on the surface of the wiring circuit exposed by the etching removal. Method,
I will provide a.

  Due to these features, the present invention has the following effects.

  According to the present invention, by forming a barrier layer that covers the entire surface of a fine wiring circuit, a printed wiring board having good insulation reliability between wirings can be obtained inexpensively and stably without depending on the cover material. Can be manufactured automatically.

  The best mode for carrying out the present invention will be described below with reference to the drawings.

  1A and 1B are conceptual cross-sectional views showing a manufacturing process of a printed wiring board in the first embodiment of the present invention. This printed wiring board is manufactured by the following steps.

  Specifically, as shown in FIG. 1A (1), a single-sided copper-clad laminate 3 having a copper foil 2 having a thickness of about 6 μm is prepared on one side of an insulating base material 1 made of polyimide, PET or the like and having a thickness of about 25 μm. To do. The material of the insulating base material 1 is not limited to polyimide or PET, and the present invention can be implemented as long as it is an insulating member that can be selectively etched with respect to the copper foil.

  Next, as shown in FIG. 1A (2), the wiring circuit 2a is formed by using an etching method or the like by a normal photofabrication method. In Example 1, a wiring circuit 2a having a wiring pitch of 30 μm (inter-wiring gap of about 17 μm) was formed by optimizing processes such as a photosensitive dry film, exposure, and etching. It is extremely difficult to ensure the insulation reliability of a product having such a fine inter-wiring gap.

  The formation of the wiring circuit 2a is not limited to the etching method, and it may be formed using other methods such as a semi-additive method of forming wiring by plating. The single-sided copper-clad plate 3a on which the wiring circuit is formed is obtained by the above process.

  Next, as shown in FIG. 1A (3), a first barrier layer 4a for ensuring migration resistance is formed on the surface of the wiring circuit 2a by using a technique such as electrolytic plating or electroless plating. For the barrier layer formed at this time, a metal such as platinum, palladium, or gold having a low ionization tendency, an insulator, or the like can be used.

  However, it is considered preferable to use a gold plating process in consideration of the processing generally used in the printed wiring board manufacturing process and the extremely high migration resistance. The barrier layer 4a needs to be formed uniformly without generating pinholes or the like. For this reason, in Example 1, the barrier layer 4a having a thickness of about 1.5 μm was formed by electrolytic gold plating.

  Note that when the wiring circuit 2a is subjected to soft etching or the like in the pretreatment of gold plating, since the wiring is fine, it is desirable to set the processing weak and set the etching amount to about 1 μm or less.

  Also, the barrier layer can be formed in a plurality of layers, and may have a structure of two or more layers, such as general nickel and gold, and an electrodeposition method or the like may be applied to the surface of the gold plating process. A structure in which insulating resins are stacked may be used. Through the above steps, a single-sided copper-clad plate 3b in which a barrier layer is formed on the wiring circuit is obtained.

  Subsequently, as shown in FIG. 1A (4), a coverlay 7 having an adhesive layer 6 using a resin such as acrylic or epoxy on one side of an insulating material 5 made of polyimide or the like having a thickness of about 12 μm is provided on one side. Thermocompression bonding is performed toward the wiring circuit 2a side of the copper-clad plate 3b to cover the wiring circuit 2a and the barrier layer 4a, and subsequently, a resist 8 for protecting the insulating material 5 from a resin etching step to be performed later is bonded.

  At this time, it is possible to prepare a resist 8 and a coverlay 7 integrated in advance and attach them to the single-sided copper-clad plate 3b. The coverlay 7 used at this time needs to function as an insulating base material that supports the wiring circuit in the step of performing resin etching later, and therefore is a material that has some resistance to bending and pulling (for example, polyimide or the like). The adhesive layer 6 is preferably selected in consideration of the adhesive strength with the barrier layer 4a.

  When the adhesion with the barrier layer 4a is insufficient, the surface of the barrier layer 4a is roughened (for example, a thin matte type electrolytic gold plating is formed), or a silane coupling agent is added to the barrier layer 4a. It is also possible to improve the adhesion force chemically by applying to the surface or by adding it in the adhesive layer 6 in advance. Note that when the insulating material 5 that is resistant to the resin etching step to be performed later is used, the resist 8 need not be formed.

  Next, as shown in FIG. 1B (5), the insulating base material 1 is entirely removed by etching (for example, wet treatment with a resin etching solution made of an alkali metal and an amine compound), and then the resist 8 is peeled off. Do.

  When the consumption of the liquid due to the entire surface removal is significant, the consumption of the liquid can be suppressed by forming a resist on the frame portion outside the product. In addition, if you are extremely disliked by liquid consumption or if you want to use the insulating base material 1 as a carrier, you can improve insulation reliability by forming a resist that opens only the fine wiring in the product. The present invention can be selectively applied only to the power part.

  Subsequently, the wiring circuit 2a is soft etched by about 1 to 2 μm to remove the wiring circuit 2a at the boundary A with the barrier layer 4a and expose the barrier layer 4a. By doing so, a gap can be eliminated when the second barrier layer is formed later.

  Thereafter, as shown in FIG. 1B (6), using a method such as electroplating or electroless plating, the back surface of the wiring circuit 2a (the surface exposed by the resin etching) is secured to prevent migration. Two barrier layers 4b are formed. The second barrier layer 4b wraps the entire wiring circuit 2a together with the first barrier layer 4a without a gap.

  In Example 1, a barrier layer 4b having a thickness of about 1.5 μm was formed by electrolytic gold plating. Since the wiring circuit 2a is soft-etched as described above, there is no gap at the boundary A between the barrier layers 4a and 4b, and the entire surface of the wiring circuit 2a can be reliably protected.

  As described above, the entire surface of the wiring circuit 2a is covered with the barrier layers 4a and 4b, so that elution of copper due to ion migration is prevented, and good insulation reliability between the wirings can be ensured.

  Subsequently, as shown in FIG. 1B (7), a solder resist 9 is formed at a necessary location, and solder plating, tin plating, nickel plating, gold plating is applied to the terminal surface of a component mounting land or connector as necessary. Surface treatment such as plating is performed alone or in combination, or solder paste printing is performed.

  As the solder resist 9 used at this time, a photo solder resist having high resolution performance can be freely selected according to the application without particularly considering migration resistance. In addition, since the wiring circuit 2a formed by the etching method has a trapezoidal shape, component mounting from the back surface is very advantageous because a wide circuit top bottom is secured.

  By forming the barrier layer on the entire surface of the fine wiring circuit by the above process, good insulation reliability between the wirings is ensured without depending on the cover material, and it is advantageous in the subsequent mounting process. A wiring board can be manufactured inexpensively and stably.

  FIG. 2A is a conceptual cross-sectional view showing a printed wiring board manufacturing process according to the second embodiment of the present invention. In the second embodiment, the present invention is applied to a multilayer printed wiring board having two or more layers. This printed wiring board is manufactured by the following steps.

  That is, first, as shown in FIG. 2A (1), a double-sided copper clad laminate 24 having copper foils 22 and 23 having a thickness of about 6 μm is formed on both surfaces of an insulating base material 21 made of polyimide or the like and having a thickness of about 25 μm. prepare.

  The material of the insulating base material 21 is not limited to polyimide, and the present invention can be implemented as long as it is an insulating member that can be selectively etched with respect to the copper foil. However, since this insulating base material is not necessarily removed afterwards, it must be an insulating material that can satisfy the required characteristics required for the product, such as bending resistance and heat resistance.

  Next, as shown in FIG. 2A (2), using one side of the double-sided copper clad laminate 24 (in this example 2, the copper foil 22 side), using an etching method or the like by a normal photofabrication method. Then, the wiring circuit 22a is formed. At that time, the copper foil 23 side is protected using an appropriate etching resist (not shown).

  In Example 2, the wiring circuit 22a having a wiring pitch of 30 μm (inter-wiring gap of about 17 μm) was formed by optimizing processes such as a photosensitive dry film, exposure, and etching. Thus, it is extremely difficult to ensure the insulation reliability of a product having a fine gap between wirings.

  The formation of the wiring circuit 22a is not limited to the etching method, and the wiring circuit 22a may be formed using other methods such as a semi-additive method of forming wirings by plating. Through the above steps, a double-sided copper-clad plate 24a having a wiring circuit formed on one side is obtained.

  In the second embodiment, an interlayer electrical connection method is used later, but an interlayer connection structure such as a through hole or a via hole is previously formed on the double-sided copper-clad plate 24, and a cover formation or build-up method is subsequently formed. Multi-layering can also be performed.

  However, when a multilayer printed wiring board having three or more layers is formed later via an interlayer adhesive, a series of steps such as formation of holes for conduction, desmearing, conducting, and electrolytic copper plating are performed twice. Therefore, it is disadvantageous in terms of cost.

  Next, as shown in FIG. 2A (3), a barrier layer 25a for ensuring migration resistance is formed on the surface of the wiring circuit 22a using a technique such as electrolytic plating or electroless plating. For the barrier layer formed at this time, a metal such as platinum, palladium, or gold having a low ionization tendency, an insulator, or the like can be used.

  However, it is considered preferable to use the gold plating process in consideration of the processing generally used in the manufacturing process of the printed wiring board and the extremely high migration resistance. The barrier layer 25a needs to be formed uniformly so that pinholes and the like do not occur. Therefore, in Example 2, the barrier layer 25a having a thickness of about 1.5 μm was formed by electrolytic gold plating.

  In addition, a plurality of barrier layers can be formed, and a structure of two or more layers such as general nickel and gold may be used, or an electrodeposition method or the like may be used on the surface of the gold plating process. It can also be formed by stacking insulating resins. When the barrier layer 45a is formed, the copper foil 23 side is protected using an appropriate resist or the like (not shown). Through the above steps, a double-sided copper-clad plate 24b in which a barrier layer is formed on the wiring circuit is obtained.

  Subsequently, as shown in FIG. 2A (4), a single-sided copper-clad laminate 28 having a copper foil 27 of about 6 μm thickness is attached to one side of an insulating base material 26 made of polyimide or the like about 25 μm thick. Adhering to the double-sided copper-clad plate 24b through the agent 29. At this time, no wiring circuit is formed on the copper foil 27, and alignment between the base materials is unnecessary.

  When the present invention is applied to a double-sided printed wiring board, a general polyimide coverlay or solder resist may be used instead of the single-sided copper-clad laminate 28 and the interlayer adhesive 29. When it is desired to increase the thickness, a double-sided copper-clad laminate or a multilayer printed wiring board can be bonded. When bonding a double-sided copper-clad laminate or a multilayer printed wiring board, it is necessary to previously form a wiring circuit on the surface to be bonded.

  Thereafter, as shown in FIG. 2B (5), a conductive hole is provided on the surface of the substrate using a conformal laser processing method using a carbon dioxide gas laser, followed by desmearing and conducting treatment, followed by electrolytic copper. By performing a plating process or the like, the copper foils 23 and 27 are electrically connected to the inner copper foil 22a. The conformal laser processing method refers to a method of selectively removing a copper foil by an etching method using a photofabrication method and providing a conduction hole using the copper foil as a laser mask.

  The formation of the hole for conduction is not limited to the carbon dioxide laser conformal processing method. The direct laser processing method is used in consideration of the required processing width, processing length, throughput, processing accuracy, quality, etc. It can be used alone or in combination with UV-YAG laser processing or drilling.

  Further, the conduction hole is not limited to the non-through type, and a general through hole structure can be formed for the through hole. Through the above-described steps, the multilayer wiring substrate 30 with the electrical connection between the layers is obtained.

  Subsequently, as shown in FIG. 2B (6), the wiring circuits 23a and 47a are formed on the copper foils 23 and 27 on both surfaces of the multilayer wiring substrate 30 by using an etching method or the like by a normal photofabrication method. Form. It should be noted that the copper foil 23 at a location where it is necessary to form a barrier layer in the wiring circuit 22a by etching away the insulating base material 21 later is removed in the etching step. Through the above steps, the multilayer printed wiring board 20a on which the wiring circuit is formed is obtained.

  Next, as shown in FIG. 2B (7), a resist 31 is formed on necessary portions of both surfaces of the multilayer printed wiring board 30a, and then a resin etching process (for example, a resin etching solution made of an alkali metal and an amine compound). The insulating base material 21 is selectively removed by etching to expose the back surface of the wiring circuit 22a.

  Next, as shown in FIG. 2B (8), after the wiring circuit 22a is soft-etched, using a technique such as electrolytic plating or electroless plating, the back surface of the wiring circuit 22a (the surface exposed by the resin etching) A barrier layer 25b for ensuring migration resistance is formed.

  In Example 2, the barrier layer 24b having a thickness of about 1.5 μm was formed by electrolytic gold plating. By performing soft etching on the wiring circuit 22a in advance, the barrier layers 25a and 25b are formed without gaps, and the entire surface of the wiring circuit 22a is reliably protected.

  Thereby, elution of copper due to ion migration is prevented, and good insulation reliability between wirings can be ensured. Subsequently, the resist 31 is peeled off to obtain a multilayer printed wiring board 30b in which a barrier layer for ensuring migration resistance is formed on the entire surface of the fine wiring portion.

  After that, as shown in Fig. 2B (9), solder resist 52 is formed at the required location, and solder plating, tin plating, nickel plating, gold plating is applied to the terminal surfaces of component mounting lands and connectors as necessary. Apply surface treatment such as At this time, as the solder resist 52 to be used, a photo solder resist with high resolution performance can be freely selected according to the application without particularly considering migration resistance.

  In addition, since the wiring circuit 52a formed by the etching technique has a trapezoidal shape, component mounting from the back surface is very advantageous because a wide circuit top bottom is secured.

  By forming the barrier layer on the entire surface of the fine wiring circuit by the above process, a good insulation reliability between the wirings can be ensured without depending on the cover material, and it is advantageous in the subsequent mounting process. A printed wiring board can be manufactured inexpensively and stably.

Sectional drawing which shows the manufacturing process of the printed wiring board by 1st Example of this invention. Sectional drawing which shows the manufacturing process of the printed wiring board by 1st Example of this invention. Sectional drawing which shows the manufacturing process of the printed wiring board by the 2nd Example of this invention. Sectional drawing which shows the manufacturing process of the printed wiring board by the 2nd Example of this invention. The process drawing of the manufacturing method of the conventional printed wiring board. The process drawing of the manufacturing method of the other conventional printed wiring board. The process drawing of the manufacturing method of the other conventional printed wiring board.

1 Insulation base material 2 Copper foil
2a Wiring circuit 3 Single-sided copper-clad laminate
3a Single-sided copper-clad laminate with wiring circuit
3b Single-sided copper-clad laminate with a barrier layer formed on the wiring circuit
4a First barrier layer
4b Second barrier layer 5 Insulating material 6 Adhesive layer 7 Coverlay 8 Resist 9 Solder resist
21 Insulation base material
22 Copper foil
23 Copper foil
24 Double-sided copper-clad laminate
24a Double-sided copper-clad laminate with wiring circuit formed on one side
24b Double-sided copper-clad laminate with a barrier layer in the wiring circuit
25a First barrier layer
25b Second barrier layer
26 Insulation base material
27 Copper foil
28 Single-sided copper-clad laminate
29 Interlayer adhesive
30 Multilayer wiring substrate
30a Multilayer printed wiring board with wiring circuit formed
30b Multi-layer printed wiring board with barrier layer formed on wiring circuit
31 resist
32 Solder resist
41 Insulation base material
42 Copper foil
42a Wiring circuit
43 Single-sided copper-clad laminate
43a Single-sided copper-clad laminate with wiring circuit
43b Single-sided printed wiring board with barrier layer formed on wiring circuit
44 Barrier layer
45 Insulation material
46 Interlayer adhesive
47 Coverlay
61 Copper foil
62 resist
63 gold
64 Wiring circuit
65 Titanium nitride
66 Coverlay
67 Solder resist

Claims (5)

A method of manufacturing a printed wiring board,
1) Step of preparing a copper clad laminate having a copper foil on at least one side of an insulating base material
2) Step of forming a wiring circuit on the copper clad laminate
3) A step of forming a first conductive barrier layer on the surface of the wiring circuit
4) A step of forming an insulating layer on the wiring circuit on which the first conductive barrier layer is formed.
5) a step of selectively etching away all or part of the insulating base material
6) A method of manufacturing a printed wiring board, comprising: forming a second conductive barrier layer connected to the first conductive barrier layer on the surface of the wiring circuit exposed by the etching removal. Method. In the manufacturing method of the printed wiring board of Claim 1,
The method for manufacturing a printed wiring board, wherein the insulating layer is formed by providing an insulating cover. In the manufacturing method of the printed wiring board of Claim 1,
The method for manufacturing a printed wiring board, wherein the insulating layer is formed by bonding printed wiring boards together with an interlayer adhesive. It is a manufacturing method of the printed wiring board according to claim 1,
A printed wiring board manufacturing method comprising performing soft etching of a wiring circuit before forming the second conductive barrier layer. It is a manufacturing method of the printed wiring board according to claim 1,
A method of manufacturing a printed wiring board, wherein at least a part of the first and second conductive barrier layers is formed by electrolytic gold plating or electroless gold plating.

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