WO2001097577A1

WO2001097577A1 - Printed circuit board

Info

Publication number: WO2001097577A1
Application number: PCT/GB2001/002733
Authority: WO
Inventors: Diane Buxton
Original assignee: Lucas Industries Limited
Priority date: 2000-06-16
Filing date: 2001-06-14
Publication date: 2001-12-20
Also published as: GB0014635D0

Abstract

A multi-layer printed circuit board is disclosed which comprises at least two layers (1) of substrate sandwiching a conductive track. A terminal receiving through hole (2) extends through the layers to receive a conductive terminal, such as the leg of a resistor or lead frame. A conductive link (3) is provided adjacent the terminal receiving hole (2) and extends from at least one surface of the board to the track (1). A conductive path (4) provided on the surface provides an electrical connection between the terminal (2) and the link (3), eliminating the need for electrical connection between the terminal (2) and the track (1) at an internal layer.

Description

PRINTED CIRCUIT BOARD

This invention relates to improvements in printed circuit boards, and in particular to multi-layer circuit boards.

Printed circuit boards are used extensively whenever there is a need to mass produce electrical circuits that connect together a number of electrical components. The printed circuit board comprises at least one substrate layer, for example a sheet of insulating material, onto which electrical tracks of conductive material, for example copper, are deposited. Terminals of the electrical components are connected together by the conductive tracks . This is usually by soldering pins extending from the components to the track.

For complicated circuit designs it is commonplace to use a multi-layer printed circuit board of the kind having at least two layers of substrate defining an upper surface and a lower surface of the board and at least one internal surface. Conductive tracks can then be formed on any or all of the surfaces, allowing the tracks to overlap one another spatially. In a simple arrangement two single layer boards which each comprise a substrate and a set of tracks are bonded together to form a sandwich like structure. Conductive pins or terminals extend through holes provided in the layers to permit electrical contact to be made with a conductive track at any layer in the structure.

The connection of the conductive terminals to the tracks is usually achieved by first placing a terminal into a plated through hole (or via) in the board and then applying molten solder to the terminal. The solder is drawn down into the hole around the terminal under capillary action and connects the terminal to conductive tracks that may adjoin the hole at any layer. A connection may be made from a single terminal to conductive tracks at more than one layer in this way and to tracks either on the outer surfaces of the board or within the board.

A problem with this construction arises when it becomes necessary to remove and/or replace a terminal. This may need to be performed to replace a defective component attached to the terminal, or simply to change a component to one of a different rating.

To ensure reliable connection of the terminals to the tracks it is customary to through plate the holes with a conductive material before inserting the terminal. The through plating is difficult to achieve as it must be of the highest quality and electrically connects to the tracks. The terminal need only then connect to the through plating and not to each track itself.

It has been found, for example in the construction of an electrical circuit for an electrical power assisted steering system, that the conductive terminals connect a leadframe of the system to an associated printed circuit board often need to be removed and replaced many times during testing. It will, of course, be understood that by leadframe we mean one or more conductive frets encapsulated within an insulating medium, usually arranged to carry higher currents than can be handled by conventional printed circuit board design. In the case of an electric power assisted steering system the leadframe supports high power switches and carries large currents that drive an electric motor. The printed circuit board carries lower current control signals that operate the switches of the leadframe.

To remove the printed circuit board from the terminals it is first necessary to heat the solder to extract the terminal. This may damage the through plating material where it meets the conductive tracks. In a worst case almost all of the plating may be striped away during removal of the terminals. On replacing the terminal it is often difficult if not impossible to check the integrity of the connection to the internal tracks that lie hidden between layers of the board.

An object of the present invention is to overcome the problems associated with removal and reconnection of connecting terminals in circuit boards of the kind set forth.

In accordance with a first aspect the invention provides a multi-layer printed circuit board comprising at least two layers of substrate defining an upper and a lower outer surface, and at least one conductive track located between the two substrate layers, a terminal-receiving through hole extending through the layers of the board that receives a conductive terminal, a conductive link that is provided adjacent the terminal- receiving through hole and extends from at least one outer surface of the circuit board to electrically connect with the track, and a conductive path provided on the outer surface that provides an electrical connection between the conductive terminal and the conductive link.

Thus, an electrical connection between the terminal and the track can be provided by the conductive link disposed adjacent the terminal with the terminal being connected to the link through a conductive path on the outer surface of the board. The need for an electrical connection between the terminal and the track at an internal layer, as in the prior art, is eliminated.

The conductive path that connects the terminal to the conductive link may at least partially comprise a conductive pad provided on the outer surface of the board. The terminal may be electrically connected to this track by a soldered joint. This solder can be easily removed when removing the terminal. It can then be reapplied whenever the terminal is replaced. As it is on the outer surface the integrity of the joint can be easily checked. The conductive pad may be formed by a conductive material that has been printed onto the surface.

The conductive link may comprise a plated through hole (a via) that extends through the board from the outer surface to the conductive track. Alternatively the conductive link may comprise a through hole that is plugged with a conductive material such as solder.

The plated through-hole preferably extends completely through the board to connect both outer surfaces together. In that case, a conductive path in the form of a portion of track may be provided on both outer surfaces of the board.

The plated through-hole may be wholly or partially filled with a conductive material, i.e. solder. The hole is preferably of such a diameter that solder is drawn into the hole by capillary action.

The internal conductive track may at least partially surround the plated through hole but should preferably totally encircle the hole.

Most preferably a portion of the conductive track comprises a conductive pad that surrounds both the terminal receiving hole and the conductive link. The pad may completely surround the terminal receiving through hole and its adjacent conductive link. It may have a substantially figure of eight outline. It is most preferred that the pads are formed before the holes which may be drilled through the board. The holes may then subsequently be filled with solder that at least partially adheres to the pads. The printed circuit board may be adapted to receive a plurality of terminals. For each terminal to be connected to the printed circuit board an adjacent conductive link may be provided. The terminals may then be connected to the tracks through a respective link. For example, a group of terminal receiving holes may be provided with a group of conductive links provided adjacent to it, the holes being arranged in pairs with conductive tracks or pads connecting together the pairs. Such a row may receive the terminals of a leadframe.

Alternatively, the board may include a number of conductive links adjacent a single terminal receiving hole. Of course, the links may be spaced a considerable distance from the associated terminal receiving hole if that is more convenient.

The conductive tracks may be applied to the substrate either before or after the holes are formed in the substrate. The holes may be formed by drilling.

Where the conductive link extends completely through the board to connect both outer surfaces a separate conductive path may be provided on each outer surface of the board for connection to the terminal.

The walls of the terminal-receiving through hole may be through-plated with a conductive material leaving a sufficient diameter bore to receive the terminal after plating.

Printed circuit boards may be provided having any number of layers of substrate, for example two, three or perhaps more layers. A conductive track may be disposed between any of the layers of the board which may be connected to the conductive link and hence to the terminal. The conductive tracks may be arranged to interconnect two or more conductive links, thus providing an electrical path between different components.

It is envisaged that the terminal may comprise a terminal of a wide range of electrical components or devices. Examples include capacitors, resistors, transistors and encapsulated integrated circuits. Alternatively the terminal may comprise a terminal of a leadframe for the passage of electrical signals from the leadframe to the tracks of the printed circuit board or vice versa.

Each of the layers of substrate of the multi-layer board may be separated by a layer of di-electric material. The through holes will therefore also pass through the layers of di-electric material.

There will now be described, by way of example only, one embodiment of the present invention with reference to the accompanying drawings of which:

Figure 1 is a plan view of a single layer of substrate of a board in accordance with the present invention during an early stage of production;

Figure 2 is a plan view of the substrate of Figure 1 illustrating the location of a plurality of through-holes relative to the conductive pads on the substrate;

Figure 3 is a cross section of a circuit board in which six layers as shown in Figure 2 are sandwiched together, laminated and subsequently plated to form a multi-layer circuit board; Figure 4 is a cross-sectional view corresponding to that shown in Figure 3 with a terminal connected to the board;

Figure 5 is a cross-sectional view of the board of Figure 4 after the terminal has been removed and the hole for the terminal de- soldered; and

Figure 6 is an illustration similar to that of Figure 4 but showing a terminal re-inserted to a de-soldered hole after a repair.

Figure 1 is a plan view of a layer of substrate, which forms a part of a multiple layer circuit board in accordance with the present invention.

The substrate 1 comprises a planar sheet of electrically insulating material such as fibre glass that has been processed in accordance with standard printed circuit board production techniques. To produce the board of Figure 1 , the insulating material is initially coated with a layer of conductive material, usually copper, and a photoresist layer. This photoresist layer is next covered by a mask in the form of a negative photographic image of the copper pattern required, the clear areas allowing the photo-resist to be cured by ultraviolet light. These cured areas prevent the copper from being etched away from the substrate.

As shown in Figure 1, the mask is selected such that a series of eight copper pads 4, in effect islands of copper, are provided in a row along the surface of the substrate after etching. Although not shown, other copper tracks are also formed on the board which connect at one end to the pads for conduction of electrical signals to and from the pads 4 as required. After the pads are formed eight identical primary holes 2 are drilled by a drilling machine or a laser in a line along the substrate that pass through from one side of the sheet to the other. The holes 2 are located within a respective copper pad 3. These holes will each accommodate a connecting terminal of a leadframe structure in a completed electrical circuit.

Adjacent each primary hole 2 a respective secondary hole 3 is also drilled into the board that is of a smaller diameter. The secondary holes 3 are also provided within respective conductive pads 4 adjacent a primary hole 2.

As shown in Figure 2, after the holes are formed in the substrate each terminal primary hole 2 is left surrounded by a conductive pad 4 that also surrounds a respective secondary hole 3. The resulting pad has a generally figure of eight shape.

Six of the layers 1 shown in Figure 2 are then placed together one above the other to form a laminated structure. Adjacent layers of substrate are separated by a sheet of di-electric material 5 and are bonded together using a suitable adhesive as shown in elevation in Figure 3 of the accompanying drawings. The layers 1,5 are overlaid such that the holes 2,3 in each layer 1 are aligned to form through holes that extend continuously through the whole structure to connect an upper outer surface 7 of the board to a lower outer surface 8 of the board.

The areas of copper pad which are not required for subsequent soldering are covered with a solder resist layer. This protects the otherwise exposed copper from corrosion. In particular a small area around each of the two holes in the pads is left free of solder resist. In the next step, both sets of holes are through plated with solder in a known manner. Due to the sizing of the holes the secondary holes 3 draw in solder under capillary action to completely or almost completely fill the hole. The primary holes 2, being of larger diameter, draw in solder that clings to the walls of the holes under surface tension to plate the walls of the holes. The solder also adheres to the portions of the exposed pads that are free of solder resist.

A further conductive pad 6 is also provided on the lower outer surface 8 of the board to electrically connect the plating of the primary holes to the plating in the respective secondary through holes 3.

In use, each primary hole 2 of the multi-layer board receives a terminal 9 of a leadframe. A single terminal 9 is shown in Figure 4 of the accompanying drawings.

The terminal 9 is soldered in position within the primary hole 2 so that an electrical pathway is formed between the terminal 9 and the solder pads 4 of each layer due to solder that is drawn into the terminal holes under capillary action. A second electrical connection to each pad 4 is also provided through the pads 4,6 on the upper and lower outer faces of the board and the plating in the secondary through holes 3.

Figure 5 of the accompanying drawings shows how the board typically appears after the terminal 9 has been removed, for example to permit a repair to be performed or to insert a different terminal. The action of drawing the solder from the primary hole to remove the terminal can often, as shown, also remove the surface plating from the hole 2.

Figure 6 illustrates a terminal that has been re-inserted to the terminal- receiving hole 2 of the board of Figure 5. It can be clearly seen that a reliable electrical connection to all the solder pads can still be achieved by ensuring the terminal 9 is connected to the plating in the conductive through hole 3 through the solder pads 4,6 on the upper and lower outer surfaces 7,8. The integrity of this connection that can be easily checked.

It will of course be readily, understood that the embodiment illustrated is not limiting to the scope of the invention and many variations may be made to the circuit board. Although the terminal in the illustrations is a terminal pin of a leadframe it may be a terminal pin of an electrical component or even the end of a jump lead or similar that needs to connect to a track on the board.

Claims

1. A multi-layer printed circuit board comprising at least two layers of substrate defining an upper and a lower outer surface, and at. least one conductive track located between the two substrate layers, a terminal- receiving through hole extending through the layers of the board that receives a conductive terminal, a conductive link that is provided adjacent the terminal-receiving through hole and extends from at least one outer surface of the circuit board to electrically connect with the track, and a conductive path provided on the outer surface that provides an electrical connection between the conductive terminal and the conductive link.

2. A multi -layer circuit board according to claim 1, in which the conductive path that connects the terminal to the conductive link at least partially comprises a conductive pad provided on the outer surface of the board.

3. A multi -layer circuit board according to claim 2, in . which the terminal is electrically connected to the pad by a soldered joint.

4. A multi-layer circuit board according to claim 2 or claim 3, in which the conductive pad is formed by a conductive material that has been printed onto the surface.

5. A multi-layer circuit board according to any one of claims 1 to 4, in which the conductive link comprises a plated through hole that extends through the board from the outer surface to the conductive track.

6. A multi-layer circuit board according to any one of claims 1 to 4, in which the conductive link comprises a through hole that is plugged with a conductive material such as solder.

7. A multi-layer circuit board according to claim 6, in which the plated through-hole extends completely through the boards to connect both outer surfaces together.

8. A multi-layer circuit board according to any preceding claim, in which a portion of the conductive track comprises a conductive pad that surrounds both the terminal receiving hole and the conductive link.