GB2181743A

GB2181743A - Metal plating of through holes in printed circuit boards

Info

Publication number: GB2181743A
Application number: GB08515048A
Authority: GB
Inventors: Kay Kazuo
Original assignee: Individual
Current assignee: Individual
Priority date: 1985-07-16
Filing date: 1985-06-13
Publication date: 1987-04-29
Also published as: DE3525342A1; GB8515048D0

Abstract

A plating (electroless or electrolytic) bath (11) is divided into two compartments (13a, 13b) by the circuit board (15) and means such as a pump (P) generates a pressure difference between the two compartments to cause plating solution to flow-through the through-holes (16a). The flow of plating solution enhances the uniformity of the deposit. <IMAGE>

Description

SPECIFICATION Method and apparatus for metal plating The present invention relates to metal plating, and relates particularly to a method and apparatus for forming uniform plating layers on minutethrough holes in wiring substrates such as printed circuit boardsforexample.

Recently, in order to meetthe rapidly growing demand for the electronic industry, the printed wiring substratetendsto be compact, high-densitypackaged and highly reliable following materialization of electronic equipment of high speed, high performance and high reliability. Concretely, a one-side copper plated laminated sheet, a both-side copper plated laminated sheet, a both side copper-plated through hole laminated sheet, andtechanical change from such plane two-dimensional wirings to the stereoscopicthree-dimensional wirings, such as multilayer printed wiring board, have been developed.In this case, through hole connection is usually adopted for conductive connection between each layer not only ofthe both-side copper plated laminated sheet but also of the multilayer printed wiring substrate.

Generally, both the electroless plating (chemical plating) and electroplating are often used for carry ingoutthethrough hole plating on the aforesaidwiring substrate.

For example, in explanation of copper plating, the wiring substrate having through holes is dipped into an electroless plating bath containing copper sulfate as a metal source and formalin as a reducing agent to thereby form achemical copper-plated layer on the inner surface of each through hole. Also, the wiring substrate is dipped into an electro-plating bath comprising a copper sulfate solution and subjected as the cathode to copper electroplating, thereby forming the copper electroplating layer onto the copper chemcial plating to complete the plating.

Conventionally, for the aforesaid electroless plating and/or electroplating, the wiring board merely is dipped into the plating bath orthe like and laterally swing therein, which does not permit smooth flow of the liquid intothethrough holes. Especially,thewir- ing board, when swung laterally, instead often makes the plated layer larger in thickness, whereby there has been the defect in the variation is aptto create in thickness distribution of plated layer atthe inner surface of each through hole to thereby hinder the complete uniform electrodeposition property.In order to elminate such defect, stirring for the plating bath by use of air foams has been put to practical use, in which the plating liquid lessflowsintothethrough holes and the electroplating is limited to its application onto the through holeofdiameterofabout0.6 mm or more, sothatthe desired object could notbe said to be attained. Especially,thethrough hole substrate has hitherto been demanded to be highdensity-packaged and the through holes each become at present smaller in diameter, so that the longer an axial length becomes, the more the thickness distribution of plating layer varies as in the multilayer printed wiring board, whereby the conventional plating method has merely increased in technical difficulty.

Afirst object of the invention is to provide a plating method for a wiring substrate having through holes, which can attain a uniform thickness distribution of plated layer on the inner surface of each through hole.

A second object of the invention is to provide a plating method for a wiring substrate having through holes, which is attainable of a desired object to make compact and high-density-packaged the through hole wiring substrate from the aspect of plating.

In orderto attain the above objects, the plating method of the invention for the wiring substrate having through holes provides a difference in liquid pressure of plating bath at both lateral sides ofthe wiring substrate so that the metal plating is applied within the through holes while flowing the plating liquid from the high pressure side to the low pressure side of plating bath.

The invention will now be described by way of example only, with reference to Figures 1 to 3 of the accompanying drawings, of which: Figure 1 shows the stages in a method of plating through holes; Figure2 is a diagrammatic cross-section of a plating bath for carrying outthe plating method ofthe invention, and Figure3 is a diagrammatic cross-section showing a modification of the plating bath of Figure 2.

Referring to Figure 1, an example of plating process for a wiring substrate having through holes as illustrated, in which (a) designates a both surface copper coated laminated board comprising a substrate 1 of epoxy resin or phenol resin material and copper foil 2 of about 35##thick attached onto both the surfaces of substrate 1, and (b) is a process to mask the wiring portion with a resistfilm 4 produced by screen printing or epoxy series ink, thereby obtaining a wiring pattern by etching as shown in (c).

Next, a through hole 5 is bored as shown in (d), and on the surface of the substrate exceptforthethrough hole 5 are printed in the screen printing a conductive inklayer6 as a conductivethinfllm and an acid-proof or alkali-proof ink layer 7 on the ink layer 6, thereby masking the overall wiring portion exceptforthe through hole portions as shown in Figure 1case). In this case, an ultraviolet sensitive film or the like may be used instead ofthe ink layers 6 and 7, thereby for- ming the mask layer by photoetching. In addition, a waterproof ink is selected not to flow out when rinsed at need, the waterproof ink being usable of a mixture of, for example, modified phenolic resin, epoxy resin, carbon powder, rosin series resin, thinner or alcohol series resin, and pigment.Also, the material of acid-proof or alkali-proof layer7 laminated on the conductive layer 6 is selected depending on the pH value of plating liquid.

Afterthe masking of ink, the electroless plating process (process (5)) for the through holes 5 begins, in which a liquid to be used includes coppersulfate as metallic salt and formalin as a reducing agent and deposits by a chemical plating mechanism a copper thin film 8 of about several microns as substrate metal. In addition, in this process, the pretreatment of surface activation of course is carried out by use of palladium solution. Next, the conductive ink layer 6 printed atthe process (e) is transferred asthecathod to an electroplating process (g). In the process (g), since the conductive ink layer 6 covers the overall surfaces of substrate 3 exceptforthe through holes 5, the copper thin film 8 on the inner surfaces of all the through holes 5 are conductive to the conductive ink layer 6.Hence, the electroplating is applied for a proper time to form a copper plated layer 9 of a pred eterminedthickness (about 35 microns) on the portion exceptforthe surface of substrate masked with the acid-proof or alkali-proof ink layer 7, in other words, only on the copper thin film 8 formed by the electroless plating. After such conductive through holes are completed, the conductive ink layer 6 and acid-proof or alkali-proof layer 7 laminated in the process (e) are removed by a proper solvent, so that a wiring substrate 10 having the predetermined wiring pattern and completing through holes as shown in a process (h) is obtained.

Figures 2 and 3 are structural views of a plating bath used in the electroless plating process (5) and electroplating process (g) of the principal portion of the invention.

In Figure 2,11 is an electrode vessel, in which copper pole plate 12a and 12b as the anode and a substrate supporting insulating plate 14 is disposed to separate laterally thereof plating baths 1 3a and 13bfrom each other, the plating baths 13a and 13b containing therein plating liquids respectively. A rectangular opening 15 is provided at the center of sub strate supporting insulating plate 14 and a wiring substrate 16 is supported by grips 17 vertically into the opening 15. The Figure 2 shows the copperelectroplating, in which the substrate 16 is electrically conductive to a minus terminal at a power source 18 through a lead wire (not shown) provided within the grip 17 and insulating panel 14, the copper pole plates 12a and 12b connecting with an anode of power source 18.

A pump P is disposed atthe bottom of each plating bath 13a or 13b and forciblyflows the plating liquid in the direction ofthe arrow rfrom the plating bath 1 3a to that 13b. Hence, the liquid level 19b in the plating bath 13b is higherthan that 19a in the plating bath 13a to thereby create a difference of liquid pressure at both lateral sides of substrate 16. As a result, the plating liquid flows through the through holes 16a.

Accordingly, the plating liquid in each through hole 16a is always exchanged to a fresh liquid, thereby uniforming thickness of plated layer and strictly depositing the film onto the inner surface of each through hole.

Now, when the pump P is driven in the predetermined one direction, the plating liquid flows in the direction of the arrows only, but when the same is driven in the alternate directions, the plating liquid flows in each through hole leftwardly and rightwardly, thereby enabling realization of the lateral symmetrization of hydrodynamic mechanism of plating liquid around the surface of substrate, and improvement in further uniformity in thickness of plated layer and electrodeposition.

In explanation of copper electroplating in Figure 2, when the copper electroless plating is carried out, the copper electrodes 12a and 12b and DC power source 18 are removed sothatthe plating liquid need only be replaced by an electroless plating liquid.

Also, the plating liquid is moved by the pump asthe same as above-mentioned, whereby the uniform copper electroless plating layer can be formed on the through holetothereby imrpovethe uniform ac curacyforthe subsequent copper electroplating.

Figure 3 shows a modified embodiment of the invention, in which the components identical with those in Figure 2 are designated with the same reference numerals and omitted of explanation, butthe necessary portions only are to be described. In Figure 3, C is a hydraulic control device of a kind of pressure control system, and pistons 20a and 20b are disposed at the liquid levels in the plating baths 13a and 13b and pressure-controlled by the hydraulic control device C. For example, when the piston 20a is urged and that 20b is not so (or when the urging force is small), the liquid pressure in the plating bath 13a becomes largerthan that in the plating bath 13b so that the plating liquid flows through the through holes 1 6a in the direction ofthe arrow t.Also, when the hydraulic control device C alternately urges the pistons 20a and 20b, the liquid flows within the through holes 16a in the alternate directions. While flowing the plating liquid as above, the electroless plating (chemical plating) or the electroplating is carried out as the same as in Figure 2, thereby enabling the uniform plated film thickness to be attained.

In addition, the present invention is not limited to the aforesaid embodiments, but is usable of a mechanism even otherthan the pump and pressure control device, which provides a difference of liquid level at both lateral sides of wiring substrate so as to flow the plating liquid within the through holes.

As seen from the above detailed description, this invention provides a difference between the liquid pressures of plating baths at both lateral sides ofthe wiring substrate so that each through hole is subjected to the metal plating while flowing the plating liquid in each through hole, whereby the thick dis- tribution of plated layer on the inner surface of each through hole can be uniform, resulting in that the substrate forthe through holes can meetthedemand of being compact and high-density-packaged to thereby form the through holes stable and of high reliability.

Claims

1. A plating method for a wiring substrate having through holes, which dips into a plating bath said wiring substrate having through holes to apply metal plating within said through holes, said method characterised in that said plating baths at both lateral sides of said wiring substrate are different in the intensity of liquid pressure from each other so that said through holes are subjected therein to metal plating while flowing a plating liquid within said through holes repectively.

2. A plating method for a wiring substrate having through holes according to Claim 1, wherein said plating baths at both lateral sides of said wiring sub strate are changed alternately in the intensity of liquid pressure to thereby allow said plating liquid to flow alternately in the opposite directions within said through holes at said wiring substrate.

3. A plating method for a wiring substrate having through holes according to Claim 1 or 2, wherein the liquid levels of said plating baths at both lateral sides of wiring substrate are different in head from each othertotherebycreateadifferencebetweenthe intensities of liquid pressure at said plating baths at both lateral sides of said wiring substrate.

(4) A plating method for a wiring substrate hav ingthrough holes according to Claim 1 or2,wherein a pump is used to pump up said plating bath from one lateral side to the other lateral side of said wiring substrate, so that said plating baths are different in the liquid levels at both lateral sides of said wiring substrate.

(5) A plating method for a wiring substrate havingthrough holes according to Claim 1 or2,wherein a pressure control device is disposed at the liquid level of each of said plating baths at both the lateral sides of said wiring substrate so that urging forces against said liquid levels are different in the intensity, thereby creating a difference of liquid pressure between said plating baths at both the lateral sides of said wiring substrate.

6. A method of depositing a metal plating on a member pierced by at least one aperture, wherein a metal plating solution is caused to flowthrough said aperture and to deposit metal on the interior surface thereof.

7. A method according to claim 6 wherein said member is a printed circuit board and said metal is deposited on said interior surface to form an electrical connection between plated areas on opposite sides of said circuit board.

8. A method according to claim 7 wherein said metal is initially deposited on said interior surface by chemical deposition from said plating solution and is subsequently deposited on the intially deposited metal by electroplating.

9. A method of depositing a metal plating on a through hole substantially as described hereinabove with reference to Figures 1 and 2 or 1 and 3.

10. A printed circuit incorporating a metal plated through holeformed by the method of any of claims 1 to 5, 7, 8 and 9.

11. Apparatus for carrying out the method of any of claims 6to 9 comprising a bath of plating solution, means for holding said member in said bath so asto divide said bath into two compartments which communicate through the or each said aperture, and pumping means for maintaining a difference in head of plating solution between said compartments so as to cause said plating solution to flowthroughtheor each said aperture.

12. Apparatus for carrying out the method of any of claims 6 to 9 comprising a bath of plating solution, meansforholdingsaid member in said bath so asto divide said bath into two compartments which com municate through the or each said aperture, and hydraulic means for applying a differential pressure between said compartments to cause said plating solution to flow through the or each said aperture.

13. Apparatus substantially as described here inabove with reference to Figure 2 or Figure 3 of the accompanying drawings.