GB2277202A

GB2277202A - Electrical circuit board manufacture

Info

Publication number: GB2277202A
Application number: GB9405608A
Authority: GB
Inventors: Peter Leslie Moran
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-03-22
Filing date: 1994-03-22
Publication date: 1994-10-19
Also published as: GB9405608D0; GB9305930D0

Abstract

In the manufacture of an electrical circuit board a high pressure liquid jet 30, optionally mixed with an abrasive material, is used to drill holes for through connections. Stacked circuit boards may be drilled. <IMAGE>

Description

In the manufacture of printed wiring boards used for the interconnection electronic components one key step is the drilling of the insulating part of the board in order to facilitate the electrical connection between the various layers. Such wiring boards may be double sided, in which case the hole is used to facilitate the mechanical and electrical connection of the electrical traces on each side of the board, or they may be multilayer in which case electrical connections are also made to the sides of the the hole in the centre of the board as well as around the surface perimeter of the hole. Recent developments have attempted to avoid the need to drill holes by the use of so called sequential multi layering techniques where the insulating layer is deposited on top of a conducting layer either with the deposition process itself defining the through holes or the holes being etched in the insulating layer by chemical or physicalchemical means after deposition. However the practical application of these latter techniques at the time of this application are minimal. The vast majority of circuit boards require holes to be drilled in the insulator and this is likely to remain the case for the foreseeable future.

The types of insulator used for circuit boards can be organic based or inorganic based. Typical organic boards use epoxy resin with glass reinforcement, polyimide with and without glass reinforcement and PTFE with glass or ceramic reinforcement. The insulator of inorganic boards may be alumina ceramic, silica, glass ceramics and other ceramics such as aluminium nitride. The organic boards are normally drilled with a carbide or similar twist drill and for the most part this is satisfactory, although there are a number of problems related to resin smear and drill wear. Inorganic boards are normally drilled using a laser or sometimes an ultrasonic drill with abrasive assistance. The laser process is very expensive since the cutting of the ceramic is slow. The ultrasonic drill requires equally expensive mechanical tooling for multihole drilling or is extremely slow if single hole drilling is used.

The invention described in this application relates to a new method of drilling such holes in circuit boards. It is particularly appropriate to ceramic and other inorganic insulators, but also has benefits for organic boards in certain situations.

DESCRIPTION OF E INVENTION According to the present invention a method for the manufacture of a printed wiring board consists of or includes the following steps 1 A sheet of insulating material is drilled in the locations where electrical connections from one side of the sheet to the other are required. The method of drilling consists of firing a quantity of liquid at high pressure, which may optionally be mixed with an abrasive material, through an orifice at the surface of the insulator, the pressure of the liquid and diameter of the orifice being such as to drill the hole rapidly through an abrasive type of action.

2 Coating the surface of the drilled hole and the surfaces of the insulating sheet with an electrically conducting material.

3 Patterning the conductor on the surfaces of the insulator to form the desired electrical interconnection pattern including electrical connection from one surface of the insulating sheet to the other by means of the conductor deposited on the surface of the drilled hole.

In certain circumstances it is possible to deposit conductor in a predefined pattern and in which case steps 2 and 3 would be combined into one but the end result would be identical.

In certain other circumstances the insulating material would be clad or coated on its surfaces with a layer or layers of conducting material before drilling. This conductor may itself be prepatterned.

However in all cases this invention would be characterised by drilling the hole or holes by firing a quantity of liquid at high pressure, which is optionally mixed with an abrasive material, at the surface of the insulator and subsequently coating the surface of the drilled hole with a conducting material to form an electrical connection from one surface of the insulator to the other.

Another example of the use of this invention is to manufacture multilayer circuit boards. In this circumstance the starting insulator material is in fact a laminate of insulating and prepatterned conducting layers. The drilled hole would then pass through the insulating layers and conducting layers and optionally make contact to one or more buried conductor traces. When the drilled hole is coated with conducting material, electrical contact is made to the buried conductor traces that have been drilled.

Still further according to this invention before any drilling of the holes destined to become the electrical through connections, a number of the insulator sheets are stacked and aligned together and when drilled the burst of high pressure liquid cuts through all the boards. The individual sheets of insulator are then processed as if they had been drilled separately one at a time.

PREFKRRED EMBODIMFINT The preferred embodiment of the invention will now be described with reference accompanying diagrams.

Figure 1 shows the insulator and drill orifice Figure 2 shows the high pressure liquid drilling the hole Figure 3 shows the drilled hole Figure 4 shows the hole coated with conductor and some accompanying tracks Figure 5 shows a stacked assembly of insulator sheets just prior to drilling.

A sheet of 96% alumina (Figure 1 10), for example Coors(TM) ADS96R, 0.5mm thick was placed under an orifice (Figure 1 20) made from carbide material approximately 0.5mm diameter. Water (Figure 2 30) at a pressure of approximately 3000 bar and containing a mixture of powdered abrasive materials was then fired through the orifice at the surface of the ceramic sheet. A hole (Figure 3 40) approximately 0.5 mm diameter was then cut in the alumina in a little under 1 second. A thick film conductor (Figure 4 50), for example ESL (TM) 9635, was then printed on the surface of the alumina using conventional screen printing apparatus. The equipment was adjusted to coat the surface of the hole at the same time as the material was deposited in a predefined pattern on the surface. The conductor was then dried in an oven at 1500C for 30 minutes. The same conductor deposition process was then repeated on the other side of the insulator and the entire assembly was then fired in a conveyor belt furnace at approximately 8500C. As in well known in the industry, this latter firing caused the conductor material to sinter and adhere to the ceramic hence forming a circuit board with electrical traces on both sides and electrically conductive connections from one side to the other through the holes drilled with the high pressure water and abrasive mixture.

In another embodiment the same process was used with the exception that a stack of 6 alumina sheets (Figure 5 60) were drilled simultaneously. It has been found that a reasonably large number of sheets may be stacked and drilled with no apparent deterioration in the hole quality or significant increase in the time taken to drill the hole. However it has also been noted that a certain amount of shell-out occurs at the exit of the hole on each sheet. In order to overcome this, before drilling, each ceramic sheet was coated on both sides with wax or adhesive (Figure 5 70). The stacked assembly was then laminated together under vacuum while heat was applied to melt the wax or adhesive.

This had the effect of making the stack into a solid sheet with minimal voiding. When this stack was subsequently drilled, only the rear sheet suffered any shell out and this was then subsequently discarded. The quality of the entrance sheet was not as good as the remainder and so this may optionally be discarded.

Alternatively lower cost thinner materials may be used for the entrance and exit sheets. The sheets of alumina were separated by immersing the stack in a suitable solvent for the wax or by heating the ceramic to approximately 700or for a few minutes to burn off the remnant organic material. They were then processed as if they had been drilled individually.

One particular aspect of water drilling needs observing when drilling stacked boards. During the drilling process the hole becomes progressively filled up with abrasive. The action of this filling of the hole is to prevent the fresh abrasive reaching the bottom of the hole whilst under sufficient pressure to drill the hole. It is necessary therefore to provide some means of allowing the abrasive to escape from the hole after its energy has been dissipated in drilling the hole. This may be achieved by moving the board in an oscillating motion during the drilling process, thus allowing the abrasive to escape up the sides of the slightly larger hole, or by switching or pulsing the abrasive on and off during the drilling process. In this latter case the water will flush out the abrasive between the drilling pulses. It will be noted that if the board is moved during the drilling, it is not necessary to use a circular hole. In some circumstances noncircular holes are a distinct benefit.

Two particular benefits were obtained from this stacking process.

Firstly although the running cost of a high pressure water drill is similar to that of a laser drill, a laser is unable to operate successfully with stacked substrates. The cost per hole when using the high pressure water drill is therefore reduced by the number of layers in the stack. Since circuits, or more particularly a panel containing several circuits, with in excess of 3000 holes are not uncommon, this saving can be very significant. Secondly laser drilling leaves a layer of ceramic on the surface of the hole that melted during the drilling process.

This recrystallized material has several undesirable properties and the various conductor materials often have difficulty adhering to it. The high pressure water drilling leaves no such residue and good metal coating characteristics have been observed.

Compared to an ultrasonic drill, the high pressure water drill cuts just one hole at a time. The ultrasonic drill can cut a large number. However expensive mechanical tooling is required for the ultrasonic drill and this can take quite a long time to manufacture. The water drill on the other hand can be programmed by computer to drill the required pattern. The tooling time is therefore very short compared to the time taken to machine the fixed tooling for the ultrasonic drill. Modifications to the program can be made in a matter of minutes. Single hole drilling with the ultrasonic drill is possible but very slow.

It will be noted that the accuracy with which the holes can be placed is limited only by the means used to adjust the relative position of the ceramic sheets under the orifice of the drill and this can, if required, be a few microns. Thus quite large ceramic panels may be drilled with high accuracy. This is a distinct advantage when compared to the process of stamping the holes in the ceramic before the ceramic is sintered (the so called "green state process). Typical accuracies of this latter method are limited to +/- 1% of the linear dimensions of the sheet and this is often an order of magnitude too much for high precision circuit boards.

It will be noted that the invention is in no way limited by the composition of the ceramic or its thickness. Typical ceramics include steatite, berylia, alumina in all it grades, silica in all its grades, aluminium nitride and any other glass, ceramic or glass-ceramic used for the fabrication of electrical circuit boards.

Similarly the choice of conductor deposition technique is not relevant to the invention and could be a thin film coating, a thick film coating, a plated coating using either electroless- or electro- plated metal, adhesive bonding, direct bonding or any other technique or combination of techniques. The choice of metal used for the conductor could be any appropriate material such as gold, silver, copper, nickel, aluminium, tin, tin-lead, or any appropriate combination of these either as individual metals, in multi-metal unalloyed combinations with these or any other metals, or alloyed with these or any other metals.

It will also be appreciated that the method has certain benefits when used with organic insulators. In particular there is no drill smear when polyimide or epoxy circuit boards are drilled.

Such drill smear can be a major problem for the subsequent coating of conductor on the surfaces of the holes. Another area of benefit occurs when PTFE boards are drilled. Certain types of PTFE board have ceramic impregnated into them for thermal compensation reasons. When these boards are drilled, they wear out the drill very rapidly. It is believed that the water drill provides a lower cost hole for this type of board. Finally there is also the possibility of using water without any abrasive to drill epoxy based boards. This may well also be lower cost than conventional drilling and give rise to very small holes.

Claims

What is claimed is

1A method for the manufacture of a printed wiring board that includes the following steps a A sheet of electrically insulating material is drilled at the locations where electrical connections from one side of the sheet to the other are required. The method of drilling consists of firing a quantity of liquid at high pressure, which may optionally be mixed with an abrasive material, through an orifice at the surface of the insulator, the pressure of the liquid and diameter of the orifice being such as to drill a hole through the entire thickness of the insulator.

b Coating the surface of the drilled holes and the surfaces of the insulating sheet with an electrically conducting material.

c Patterning the conductor on the surfaces of the insulator to form the desired electrical interconnection pattern including electrical connection from one surface of the insulating sheet to the other surface using the conductor deposited on the surface of the drilled hole.

2 A method for the manufacture of a printed wiring board that includes the following steps a A sheet of electrically insulating material clad or coated on both sides with an electrically conducting material is drilled at the locations where electrical connections from one side of the sheet to the other are required. The method of drilling consists of firing a quantity of liquid at high pressure, which may optionally be mixed with an abrasive material, through an orifice at the surface of the insulator, the pressure of the liquid and diameter of the orifice being such as to drill the hole through the entire thickness of the insulator.

b Coating the surface of the drilled holes with an electrically conducting material, such electrically conducting material to make contact with and optionally coat the electrically conducting material clad to the surface of the insulating sheet.

c Patterning the conductor on the surfaces of the insulator to form the desired electrical interconnection pattern including electrical connection from one surface of the insulating sheet to the other using the conductor deposited on the surface of the drilled hole.

3 A method for the manufacture of a printed wiring board that includes the following steps a A sheet of electrical insulating material is drilled at the locations where electrical connections from one side of the sheet to the other are required. The method of drilling consists of firing a quantity of liquid at high pressure, which may optionally be mixed with an abrasive material, through an orifice at the surface of the insulator, the pressure of the liquid and diameter of the orifice being such as to drill the hole through the entire thickness of the insulator.

b The surface of the drilled holes and the surfaces of the insulating sheet are coated with an electrically conducting material. The electrically conducting material is deposited on the surfaces of the sheet in the pattern required for the electrical interconnection topography.

4 A method for the manufacture of a printed wiring board that includes the following steps a A sheet of electrically insulating material is made up of laminated layers of conducting material and insulating material and is drilled at the locations where electrical connections from one side of the sheet to the other are required and optionally where connections to one of the inner layers is also required.

The method of drilling consists of firing a quantity of liquid high pressure, which may optionally be mixed with an abrasive material, through an orifice at the surface of the insulator, the pressure of the liquid and diameter of the orifice being such as to drill the hole through the entire thickness of the insulator.

b Coating the surface of the drilled holes and, optionally if the surfaces of the insulator are not already coated with an electrical conductor, the surfaces of the insulating sheet with an electrically conducting material such electrically conducting material to also make contact with any of the inner layers that are exposed by the drilling.

c Patterning the conductor on the surfaces of the insulator to form the desired electrical interconnection pattern including connection from one surface of the insulating sheet to the other using the conductor deposited on the surface of the drilled hole.

5 A method according to any of the previous claims in which the thickness of the insulator is less than 5 millimeters.

6 A method according to any of claims 1 to 4 in which the thickness of the insulator is in the range 0.1 millimeters to 2.5 millimeters.

7 A method according to any of the previous claims in which the time taken to drill the hole is less than 30 seconds.

8 A method according to any of the preceding claims in which the time taken for the drilling is less than 5 seconds.

9 A method according to any of the preceding claims in which the time taken for the drilling is less than 1 second.

11 A method according to any of the previous claims in which a stack of several insulating sheets is drilled, the drilling being characterised by each hole passing through the entire thickness of the stack.

12 A method according to any of claims 1 to 10 in which a stack of several insulating sheets is drilled, the stack being characterised by being temporarily bonded together to form a solid structure and the drilling being characterised by passing through the entire thickness of the stack.

13 A method according to any of the previous claims in which the burst of high pressure liquid mixed with an abrasive is characterised by the flow of the abrasive is pulsed on and off during the drilling process.

14 A method according to any of the previous claims in which the board is moved in an oscillating motion during the drilling process in order to provide a path for the used abrasive to escape from the hole as the drilling progresses.

15 A method according to any of the preceding claims in which the conductor is copper, an alloy thereof or is at least in part made of copper.

16 A method according to any of the claims 1 to 14 in which the conductor is gold, an alloy thereof or is at least in part made of gold.

17 A method according to any of the claims 1 to 14 in which the conductor is silver, an alloy thereof or is at least in part made of silver.

18 A method according to any of the claims 1 to 14 in which the conductor is nickel, an alloy thereof or is at least in part made of nickel.

19 A method according to any of the claims 1 to 14 in which the conductor is tin, an alloy thereof or is at least in part made of tin.

20 A method according to any of the claims 1 to 14 in which the conductor is lead, an alloy thereof or is at least in part made of lead.

21 A method according to any of the claims 1 to 14 in which the conductor is aluminium, an alloy thereof or is at least in part made of aluminium.

22 A method according to any of the preceding claims in which the insulator is an organic material.

23 A method according to any of the claims 1 to 21 in which the insulator is an inorganic material.

24 A method according to any of the claims 1 to 21 in which the insulator material is aluminium oxide ceramic with a purity exceeding 90t.

25 A method according to any of the claims 1 to 21 in which the insulator material is aluminium nitride.

26 A method according to any of the claims 1 to 21 in which the insulator material is a glass ceramic.

27 A method according to any of the claims 1 to 21 in which the insulator material is silica in any of its forms.

28 A method according to any of the claims 1 to 21 in which the insulator material is crystalline aluminium oxide.

29 A method according to any of the claims 1 to 21 in which the insulator material is epoxy resin.

30 A method according to any of the claims 1 to 21 in which the insulator material is polyimide.

31 A method according to any of the claims 1 to 21 in which the insulator material is PTFE.

32 A method according to any of the claims 29 to 31 in which the insulator material is reinforced with glass fibre, organic fibre or ceramic.

33 A method according to any of the previous claims in which the the liquid used for drilling is water.

34 A method according to any of the claims 1 to 32 in which the liquid is water and is mixed with an abrasive material.

35 A method of manufacturing an electrical circuit board of any description that includes the steps of drilling a hole by means of firing a pulse of liquid, optionally mixed with an abrasive material, at high pressure at the surface of the insulator and subsequently coating the surface of the drilled hole with an electrical conductor.

36 An electrical circuit or wiring board of any description manufactured substantially as described herein with reference to the accompanying drawings.