WO1999027760A1

WO1999027760A1 - Method and apparatus for manufacturing double sided or multi-layered printed circuit boards

Info

Publication number: WO1999027760A1
Application number: PCT/GB1998/003512
Authority: WO
Inventors: Karl Michael Wallis
Original assignee: Karl Michael Wallis
Priority date: 1997-11-26
Filing date: 1998-11-26
Publication date: 1999-06-03
Also published as: AU1440899A; GB0015393D0; GB2349279A; GB2349279B

Abstract

A method and apparatus for through plating printed circuit boards. The method involves applying a liquid, particularly a curable polymer, including a conductive material to a board having a hole or holes drilled therethrough. Excess liquid is then removed from the board to leave a liquid coating on the inside surfaces of the hole(s). The liquid coating is then dried and, optionally, cured to leave an electrically conductive coating on the inside surface of the holes to provide an electrically conductive path from one side of the board to the other. The apparatus is arranged to carry out the method. Boards travel on a conveyor (20) under a liquid spray (22), excess liquid is then removed by squeegees (24) and air knives (26). The boards then pass through under heated air knives (26), to dry the liquid, under a rotating brush (27), which removes excess dried material, and into an oven (28) to cure the liquid.

Description

METHOD AND APPARATUS FOR MANUFACTURING DOUBLE SIDED OR MULTI-LAYERED PRINTED CIRCUIT BOARDS

The present invention relates to a method of and apparatus for the manufacture of double sided and multi-layer printed circuit boards.

Printed circuit boards are well known and widely used in the electronics

industry. Known single sided boards comprise an electrically insulating board, often a glass fibre material, with a network of conductive tracks, on the surface to electrically connect electronic components which are soldered to the board.

In order to produce more compact electrical circuits it is often preferred

to produce double sided printed circuit boards. The tracks on either side of these boards are usuaiiy formed by starting with a board coated with copper on both sides, forming an image on the copper with a resist material,

electroplating the uncoated copper with a tin lead alloy and removing the resist

material and unplated copper . With double sided boards it is often necessary,

or desirable from the point of view of circuit design, to electrically connect

portions of the circuit on opposite sides of the board. Such connections are

achieved by drilling a hole through the insulative board and then electroplating

the inside surface of the hole with tin lead alloy, referred to as 'through

plating'. In order to enable the inside surface of a hole through an insulating board to be electroplated it is first coated with a layer of copper, by chemical

deposition.

Conventional processes for the through plating of double sided printed circuit boards rely on the chemical treatment of the boards to enable a layer of copper to be chemically deposited into the holes. The processes involve many

stages, are time consuming and use expensive and hazardous chemicals some

of which need to be maintained within close temperature limits, require

continuous stirring and present a disposal problem.

For example, in one existing process after a board has been drilled it is

immersed in a cleaning tank, maintained at 80 ° C, and then passed through

conditioner, microetch, and cataiyst tanks ail with intermediate rinsing stages.

The inside surfaces of the drilled holes are then ready to be coated with

copper, using an 'electroless' copper solution. The solution must be maintained

at between 38 and 42.5 °C and stirred continually, 24 hours a day. The

concentration of the solution must also be carefully controlled, usually requiring

a chemist to be present on site. Generally it is necessary for a titration to be

performed before each batch of boards is immersed in the solution, to check

that a sufficient concentration of copper is present. The boards are immersed

in the solution which deposits a layer of copper onto the surface of the board.

The aim is to deposit a layer of around 2.5 microns in depth. To monitor the depth of copper deposited a sample board of known weight and surface area

is also immersed in the solution. This board is periodically removed and

weighed to determine how much copper has been deposited. This is

inconvenient.

The 'electroless' process coats the board all over with copper, not just

the inside surface of the drilled holes, as well as the tank holding the solution

and jig on which the boards are mounted. Only a small proportion of the copper in the solution, therefore, is usefully deposited onto the inside of the drilled holes. This leads to use of a considerable excess of solution.

The build up of copper on the equipment is usually periodically removed

and sold as scrap.

Once the board has been coated with copper then it is briefly etched to

prepare the copper for electroplating. This process needs to be very carefully

controlled to ensure that copper deposited on the inside surface of the holes

is not removed. A problem can arise if the thin layer of copper around the edge

of the holes is etched away. So called 'rim loss' occurs and the electrically

conductive path between opposite sides of the board is lost.

Once a layer of copper has been deposited to the inside surfaces of the

holes then a circuit design can be formed onto the sides of the board in the conventional way. Exposed copper tracks on the board and the copper in the

holes is then electroplated in the usual way. After electroplating, circuit boards

are usually briefly heated by dipping in hot oil to flow the tin lead alloy.

It is an object of the present invention to provide an improved, more

convenient and more economic method for the production of double sided and

multi-layer printed circuit boards.

According to a first aspect of the present invention there is provided a method of manufacture of a printed circuit board comprising the steps of:

providing a board having a hole formed therethrough;

applying a liquid including an electrically conductive material to the

board; removing excess iiquid from the board to leave some liquid on the inside

surface of the hole; and

drying the Iiquid to form an electrically conductive coating on the inside

surface of the hole.

Preferably the electrically conductive coating is present on substantially

all the inside surface of the hole.

This method provides a convenient way to coat the inside surface of a

hole with an electrically conductive material, this may then be electroplated in the conventional way. Any number of holes may be present as required by a

particular circuit design.

Preferably the board comprises a sheet of electrically insulating material for example fibreglass, both sides of which are coated with an electrically

conductive material, preferably a metal, for example copper, as conventionally

used to produce double sided printed circuit boards. The holes are preferably

formed by drilling after which the board may be washed to remove any swarf or other debris. Subsequent to any washing it is preferred that the board is dried.

The Iiquid may be applied using any convenient technique, it is preferred

however that the board is immersed in or coated all over with the Iiquid and

then the surplus Iiquid removed, to leave a iiquid coating on the inside surfaces of the hole(s) as desired.

Excess Iiquid is preferably removed from both sides of the board using

a squeegee and then the hole(s) blown through with compressed air. The air

preferably being supplied at a high volume and low pressure. The squeegee

and blow through processes may be repeated as required.

It is preferable that the Iiquid coating applied to the inside surface of the

hole(s) is sufficiently thin to leave an exposed hole through the board, into which an electronic component may subsequently be inserted.

The Iiquid preferably comprises a curable Iiquid polymer or resin

containing some electrically conductive material for example a metal which may be silver. It may be an electrically conductive ink. A suitable Iiquid is a mixture

of one part of the conductive ink sold under the trade mark Electrodag (RTM)

926 SS by Acheson Colloids Company, England, thinned down with five parts

of the thinner sold under the trade mark S103 Fast Thinner by Sericol Limited, England. The resultant Iiquid having a viscosity similar to that of cream.

Once the Iiquid has been applied and any excess removed then it is

preferred that the board is dried for between 10 and 1 5 minutes, in an oven,

at approximately 80 °C. After the drying stage it is preferred that the surfaces of the board are cieaned with an abrasive to remove any traces of dried liquid.

A suitable abrasive is wet and dry paper, used wet, preferably with water. This

process helps to give a 'clean' edge around the hole(s).

Following the abrasive cleaning stage, or after drying if this stage is not

performed, the conductive coating is then preferably cured. This preferably

takes place in an oven at a temperature of between 60 and 1 80 ° C, more

preferably 100 to 1 0°C for a period of approximately 30 minutes, although

the preferred curing conditions will depend upon the Iiquid used. Alternatively the Iiquid could be cured during the drying stage.

The board may now optionally be plated all over, 'panel plated', to

provide a stronger key for subsequent processes, especially down the holes,

before a circuit design is formed on the board and electroplated in the normal

way.

Two or more circuit boards may be used together to form a multi-layered circuit board.

The present invention affords numerous advantages over existing techniques for the production of double sided printed circuit boards. The inside of the hole(s) may be through plated more quickly, conveniently and cheaply

than with existing techniques. There is no need to provide or maintain the

expensive chemicals employed by existing methods, nor is there a need to

dispose of toxic waste which can be costly as well as harmful to the

environment.

The method of the present invention only coats the inside surface of the

hole(s) with a conductive material, unlike existing methods which involve

coating the board all over. For an average circuit board the ratio of area of the

inside of the holes to the total surface area of the board is of the order of 50: 1 ,

hence a commensurate saving in conductive material is made. Any excess Iiquid applied to the board, and subsequently removed can be reused.

In summary the present invention enables a reduction in process time,

chemical consumption and waste, energy costs, personnel costs (as a chemist should not be required) and capital outlay in terms of both equipment and materials. Overall, these and other benefits enable an approximate 90% reduction in production costs for circuit boards per unit area compared to

conventional processes.

In addition, circuit boards produced by the method of the invention are found to perform well in industry standard tests, particularly the thermal shock

test which involves rapidly heating the circuit board to 288 °C to simulate the

thermal shock experienced when a plated board is heated to flow the

electroplated tracks. In order to gain good thermal performance it is important

to minimise entrapped air in the conductive Iiquid as this can expand on rapid

heating causing the conductive coating to be damaged. It is important

therefore that rapid stirring of the Iiquid is avoided.

Another benefit of the method of the present invention is that it can be

used as a continuous, through process whereas existing methods are batch

processes. This allows production of circuit boards to be more flexible

depending on demand. According to a second aspect of the invention, therefore, there is

provided apparatus for the manufacture of double sided printed circuit boards

comprising:

a means for applying a Iiquid including an electrically conductive material

to a board having a hole formed therethrough; a means for removing excess Iiquid from the board to leave some Iiquid

on the inside surface of the hole;

a means for curing the liquid to form an electrically conductive coating on the inside surface of the hole.

In one embodiment blank, drilled boards are conveyed on a moving belt

through:-

1 . a spray of Iiquid;

2. spring loaded squeegee biades; 3. one or more air knives;

4. a rotating brush; and

5. a hot air oven.

Preferably the excess Iiquid removed from the boards by the squeegee

and air knives is returned to the sprayer for coating further boards.

This allows for convenient and quick through plating of boards. in order that the invention may be more clearly understood embodiments

thereof will now be described, by way of example, with reference to the

accompanying drawings in which:-

Fig .1 is a flow diagram, illustrating the steps involved in one method

according to the invention;

Fig.2 shows a plan view of a part of a blank circuit board;

Fig.3 shows a perspective cross-sectional view of the board illustrated

in Fig.1 , the cross-section having been taken along the line Ill-Ill of Fig.1 ;

Fig.4 shows a cross-sectional view of a similar board to that illustrated

in Figs. 2 and 3;

Fig.5 shows a similar view to Fig.4 where the inside surface of the hole

through the board is coated with conductive material;

Fig.6 shows a similar view to Fig.5 where the whole surface of the board

has been panel plated; and

Fig.7 shows a cross-sectional diagrammatic view of apparatus for the

through plating of circuit boards, according to the invention. The same reference numerals are used in Figs. 2 to 6 to identify

corresponding parts of the various illustrated boards.

Referring to the drawings Fig.1 illustrates the sequence of operations for

producing a through plated circuit board.

Initially a blank board is provided, step 1 , comprising a non-conductive

fibreglass sheet coated on opposite sides respectively with a layer of copper.

One or more holes are then drilled in the board as required, step 2, depending on the circuit to be produced. The resulting board is then of the form illustrated in Figs. 2 to 4 which show a glass fibre board 13 opposite sides of which are coated with copper 1 4 with a hole 1 5 drilled therethrough.

After driiϋng the board is washed, step 3, to remove any swarf or other

debris and then dried, step 4. The board is then coated with or immersed in

a curable Iiquid containing conductive material, step 5, to allow some Iiquid to

flow into the hole or holes drilled in the board. A suitable Iiquid is a mixture of

approximately five parts of S103 Fast Thinner supplied by Sericol Limited,

England and one part Electrodag 976 SS supplied by Acheson Colloids

Company, England, by volume. Other suitable liquids may be used.

Electrodag 976 SS comprises very finely divided silver particles dispersed

in a thermosetting resin. Excess iiquid is then removed from opposite sides of the board using a

squeegee, step 6, this may also assist in causing the Iiquid to flow into and

fully coat the inside surface 16 of the hole 17.

A flow of compressed air is then directed through the hole, step 7, to

blow out excess Iiquid to leave a thin coating of Iiquid on the inside surface of

the hole.

The excess Iiquid removed from the board during steps 6 and 7 is retained for reuse.

Steps 6 and 7 may be repeated as required, it is usual to perform these

steps twice.

The board is then placed into an oven at a temperature of approximately

80 ° C for a period of between 10 and 1 5 minutes to dry the Iiquid, step 8.

Once the Iiquid is dry then any remaining deposits on the surface of the board

are removed by cleaning with an abrasive, step 9. A suitable abrasive is wet

and dry paper used with water.

After cleaning, the board is heated in an oven to approximately 120°C

for around 30 minutes to cure the Iiquid in the hole, step 10. The board is

gradually heated, to prevent any damage to the conductive material though any remaining solvent evaporating or entrapped air expanding quickly, in the

material.

The result is shown in Fig.5 where the inside surface 16 of the hole 1 5

is coated with a generally cylindrical conductive layer 17 which contacts the

copper layers disposed on opposite sides of the fibre glass sheet 1 3. Step 9,

cleaning the board with abrasive leaves a 'clean' edge around the hole 15 without excessive material build up. That is, the conductive layer 17 does not extend beyond the copper layer 14.

The board is then panel plated, step 1 1 , that is plated all over with copper. This process, whilst not essential, provides an improved surface onto which a circuit can subsequently be plated and etched in stage 1 2. Fig.6

shows a board panel plated with copper 18.

The etching of a circuit, subsequent electroplating and then heating to

flow the electroplate is well known and will therefore not be described in any

detail. Briefly however, a light sensitive resist material is applied to the board

and a negative image projected onto the material to selectively harden it. The

unhardened material is washed away to leave exposed copper including that

on the inside surface of the hole. The exposed copper is then electroplated

with a tin lead alloy. The hardened resist material and underlying copper is

then etched away and the board heated to flow the tin lead alloy to ensure good electrical conductivity.

The above described method is considerably quicker and more economic

than existing methods for through plating boards and the need to provide,

maintain and dispose of expensive chemicals is dispensed with. Also, as the

conductive coating is applied only where required, that is on the inside surface of holes in the board rather than over all the surface of the board, typically a

50 fold reduction in coated surface area, considerable material savings can also be made.

Referring to Fig. 7 there is shown apparatus for the through plating of circuit boards.

The apparatus is arranged for the continuous production of boards by a

through process. Drilled boards (not shown) pass through the apparatus on a

moving conveyor 20. This could comprise two continuous belts or any other

suitable arrangement on which opposite sides of the boards can rest so that

their upper and lower surfaces are exposed as they pass through the apparatus.

First the boards are sprayed with a suitable curable Iiquid containing

some electrically conductive material which is stored in tank 21 and supplied

through spray head 22 by way of pump 23. Excess Iiquid drains directly back

into the tank 21 . The boards then pass between wiper blades 24 which are resiliency

biassed against the boards by springs 25. The wiper blades may be produced

from rubber. The wiper blades wipe excess Iiquid from the surface of the

boards and may also assist in guiding iiquid through the holes in the boards. Excess Iiquid removed by the wiper blades 25 drains back into the tank 21 .

Next the boards pass under four sequential jets of compressed air 26 or

air knives which blow excess Iiquid from the holes in the boards which then flows back to the tank 21 .

The first air knives under which the board passes direct cold air onto the board. Subsequent air knives direct heated air, at approximately 80° to dry the

Iiquid on the board.

The boards then pass under a revolving brush 27 supplied with a

continuous flow of water to remove any dried Iiquid from the surface of the

board, before moving into a continuous hot air oven (which may include hot air

knives) 28 maintained at around 120° C, to cure the Iiquid to leave an

electrically conductive coating on the inside surface of the holes in the board.

The boards spend approximately 30 minutes in the oven. The size of the

apparatus and speed of the belt are arranged to ensure the boards spend

sufficient time in the oven to allow for curing of the Iiquid. This apparatus allows continuous automated production of through

plated circuit boards whilst avoiding many of the problems associated with

existing techniques.

The apparatus may have a track for the belt of variable width.

The above embodiments are described by way of example only, many variations are possible without departing from the invention.

Claims

1 . A method of method of manufacture of a printed circuit board comprising

the steps of:

providing a board having a hole formed therethrough; applying a Iiquid including an electrically conductive material to the board; removing excess Iiquid from the board to leave some Iiquid on the inside

surface of the hole; and drying the Iiquid to form an electrically conductive coating on the inside surface of the hole.

2. A method according to claim 1 comprising the additional steps of washing and subsequently drying the board before applying the Iiquid.

3. A method according to either claim 1 or claim 2, wherein the Iiquid is

applied by immersing the board in the Iiquid.

4. A method according to any of claims 1 to 3, wherein the step of

removing excess Iiquid comprises removing Iiquid by means of a squeegee.

5. A method according to any preceding claim, wherein the step of

removing excess Iiquid comprises removing liquid by blowing air through the

hole.

6. A method according to any preceding claim, wherein the step of drying

the Iiquid comprises heating the board to approximately 80 °C for between 10

and 1 5 minutes.

7. A method according to any preceding claim comprising the additional

step of cleaning the board using an abrasive, after the drying step.

8. A method according to any preceding claim comprising the additional step of curing the conductive coating.

9. A method according to claim 8, wherein the coating is cured by heating

the board to between 60 and 180 ° C for approximately 30 minutes.

10. A method according to either ciaim 8 or claim 3, wherein the iiquid is cured during the drying stage.

1 1 . A method according to any preceding claim comprising the additional

step of panel plating the board after the conductive coating has been formed.

1 2. A method according to any preceding claim, wherein the board

comprises a sheet of electrically insulating material both sides of which are

coated with an electrically conductive material.

13. A method according to any preceding claim, wherein the liquid comprises

a curable polymer or resin.

14. A method according to claim 13, wherein the electrically conductive

material is silver.

1 5. Apparatus for the manufacture of double sided printed circuit boards

comprising: a means for applying a Iiquid including an electrically conductive material

to a board having a hole formed therethrough;

a means for removing excess Iiquid from the board to leave some Iiquid on the inside surface of the hole; a means for curing the Iiquid to form an electrically conductive coating

on the inside surface of the hoie.

1 6. Apparatus according to claim 1 5, wherein the means for applying

comprises a means for spraying.

17. Apparatus according to either claim 1 5 or 16, wherein the means for

removing comprises a squeegee.

1 8. Apparatus according to any of claims 1 5 to 17, wherein the means for

removing comprises an air knife.

1 9. Apparatus according to any of claims 1 5 to 1 8, wherein the means of

removing comprises a brush.

20. Apparatus according to any one of claims 1 5 to 1 9, wherein the means

for curing comprises an hot air oven.

21 . Apparatus according to claim 1 5, wherein boards are successively

conveyed through

1 . a spray of liquid; 2. spring loaded squeegee blades;

3. one or more air knives;

4. a rotating brush; and

5. a hot air oven.