It is a known technique in the assembly of printed circuit boards to first deposit solder paste where connections are to be made between components and the circuit board, prior to placing the components, and then heating the entire assembly to reflow the paste and complete the connections.

The reflow step is generally carried out by passing the printed circuit board and component assembly through an oven where the temperature of the entire assembly is raised to the level required to melt the solder, cause it to reflow and complete the connection between the component leads and the circuit board. Several types of oven are in general use; one uses a series of infra-red heating elements to directly heat the assembly while another passes the assembly through a bath containing, hot, inert vapour. Because the entire printed circuit board assembly is heated, it is necessary to control the rate of heating and cooling of the assembly so that the electronic components are not damaged by thermal shock. In addition some components, such as ball grid array devices have many connections sandwiched between the component and the board where it is difficult for the heat to penetrate. To achieve this ovens are typically 6 to 10 meters long with a critically controlled temperature profile along their length. This in turn requires a lengthy period of running for the profile to stabilise prior to use and frequently ovens are kept running 24 hours per day. Also it is essential that high levels of planned maintenance are carried out as a breakdown during operation can result

in the destruction of the product in the oven and loss of production while the oven is allowed to cool sufficiently for repairs to be carried out and then returned to its required profile.

It will be apparent that the current reflow methods present a number of problems for the electronic manufacturer in that they require a large amount of factory space, require high levels of maintenance, consume a large amount of power, electronic components are subjected to high temperatures for prolonged periods and no record can be kept of the actual temperature experienced by the components.

Simple printed circuit boards are manufactured by taking an insulating base material referred to herein as a printed circuit board substrate such as synthetic resin bonded fibre, that has a layer of copper deposited on one surface, this copper is then chemically etched to form conductor tracks and contact areas for the attachment of components and connectors, optionally a protective coating and/or a printed legend may be added. As circuit complexity increases, copper conductor tracks can be formed on both sides with connections between the two sides formed by plating the inner bore of holes through the board. Modern high density circuits are formed by creating a sandwich of several such boards and laminating them by applying heat and pressure to form a single multi-layer circuit board.

According to a first aspect of the invention there is provided a printed circuit board assembly comprising a stack of printed circuit board substrates and a heating element, the heating element being provided internally of the stack of substrates, the arrangement being such that when power is supplied to the heating element, heat is generated which acts to heat a region of the printed circuit board assembly.

Preferably the heating element is operative to heat a portion of solder material provided in or on the printed circuit board assembly. In use, the temperature of the portion of solder material is raised sufficiently to cause reflow of said solder material.

Preferably the heating element is spaced in the direction of the height of the stack from the solder material.

Preferably the heating element is provided as a layer of the assembly.

Preferably the heating element is provided on an intermediate surface of the stack of substrates, said intermediate surface being a surface which faces an adjacent substrate of the stack.

Preferably the heating element is so shaped and positioned to be substantially in register with the region which requires to be heated.

Advantageously since the heating element is localised to those areas of the board where heating is required, unnecessary heating of the board and components is reduced and the amount of power required is minimised.

The assembly preferably comprises externally accessible electrical contact means and connection means connecting said contact means to the heating element such that external power supply means may connect with the contact means to supply power to said heating element.

The electrical contact means is desirably provided on an outermost substrate of the stack.

Preferably where the heating element is divided into a number of portions or the assembly comprises a plurality of heating elements, electrical circuitry may be provided which enables a selected heating element portion or a selected heating element or a selected group of inter- connected heating element portions or heating elements to be powered individually. This will enable the removal and replacement of a particular defective component by melting the solder for that defective component only.

According to a second aspect of the invention there is provided a method of heating a region of a printed circuit board assembly, the assembly comprising a stack of printed circuit board substrates, the method comprising supplying power to a heating element provided internally of the stack of substrates.

Preferably the method is a method of reflow soldering a portion of solder material provided in or on the printed circuit board assembly.

Preferably power supply means is applied to electrical contact means, the electrical contact means being provided on the assembly.

Preferably the method comprises measuring the temperature of a particular region or regions of the assembly. The temperature measurements may be used to control the power supply means.

Preferably the power supply means supplies power in pulse form. This will facilitate reflow of the solder without heating a respective electronic component or damaging the component by way of thermal shock.

The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a sectional view of a conventional infra-red reflow oven; Figure 2 is a cut away view of part of a multi-layer printed circuit board; Figure 3 is a cut away view of part of a first multi-layer printed circuit board assembly in accordance with the invention; Figure 4 is a partially exploded view of a second multi-layer printed circuit board assembly in accordance with the invention; and Figure 5 is a schematic side view of an apparatus which is operative to carry out the reflow method in accordance with to the invention.

Figure 1 shows a typical infra-red reflow oven 100 as currently used which comprises a conveyor belt 110, usually made from stainless steel mesh, for transporting the circuit board assemblies 130 through the oven.

In use the circuit board assemblies 130 are placed at regular intervals on the input end of conveyor 110 and are transported slowly through the oven 100 where a series infra-red heating units 120 located both above and below the conveyor heat raise the temperature to the level required to reflow the solder, typically 220 to 250 degree Celsius. By individually controlling the power to each of the heating units 120, the time/temperature profile of the circuit board assemblies 130 passing through the oven can be controlled.

Figure 2 shows a cut away view of part of a multi-layer printed circuit board assembly 150 of known format. In this example an upper board substrate 210 and a lower board substrate 220 have been laminated together to form a three layer circuit board. The three layers being the

conductor tracks 215 on the upper surface of substrate 210, conductor tracks 225 on an intermediate surface, the intermediate surface in question being the upper surface of substrate 220. Further conductor tracks, not shown, are provided on the lower surface of the substrate 220. Interconnections are made between the various conductor layers by plated through holes 235 in which the inner bore of holes drilled through the board are plated with a conducting material, a sectional view of such a plated through hole is shown as in Figure 2. Contact pads 240 for the connector leads 255 of components 250 are also formed on the upper surface of the board.

During the known board assembly process, solder paste is screen printed on each of the contact pads 240 before placement of component 250, then the assembly is heated in the oven 100 to cause the solder paste to melt and form a permanent connection between the component lead 255 and the contact pad 240.

Figure 3 shows a cut away view of part of a multi-layer printed circuit board assembly 200 which is similar to assembly 150. The assembly 200 comprises a stack of two board substrates 210'and 221'. Conductor tracks 215'are provided on an upper surface 211'of substrate 210'and conductor tracks 225'are provided on an upper surface 221'of substrate 200', upper surface 221'being an intermediate surface of the stack.

Heating elements 310'are also provided on the surface 221'and can thus be considered as an inner layer of the stack. The heating elements 310' comprise a strip of thin foil of nickel/chrome alloy. The heating elements 310'are located directly below contact pads 340'for component leads 255'and as is evident from Figure 3 the heating elements are so shaped and positioned to be substantially in register with the location of the solder material which will be provided on the pads 340'. The

assembly 200 further comprises two electrical contact pads 315'which, via respective plated through holes 317'are connected to the heating elements 310'. Thus when power is supplied to the pads 315'heat is conducted through the substrate 210'and to the pads 240'so as to heat and cause reflow of the solder material (not illustrated) provided thereon.

The heating elements 310 can be provided as a dedicated heating element layer within a multi-layer board structure or as part of an existing layer as shown in this example. The heating elements 310'can be formed of any resistive material so that an electrical current passed through it will generate heat, this could be in the form of a thin foil of a nickel/chrome or nickel/chrome/iron alloy, or could be in the form of a printed film of a resistive polymer ink.

Figure 4 shows an exploded view of a second embodiment of another example of a multi-layer printed circuit board assembly in accordance with the invention. The assembly 300 comprises a stack of three printed circuit board substrates 210", 220"and 230", electronic components 252", 254"and 256"and heating elements 310". The electronic components 252", 254"and 256"are provided on the substrate 210".

The heating elements 310"are provided on upper surface 221"of the substrate 220", the heating elements 310"are again so shaped and positioned to be substantially in register with the location of the solder material (not illustrated) which is deposited on the pads (not illustrated) for the leads of the components 252", 252"and 256". Electrical contacts 315"are provided on the surface 211"and are connected to plated through holes 320"provided in the substrate 220". The plated through holes 320"connect to the heating elements 310", the heating elements 310"being connected in three electrical paths to the plated through holes by connecting tracks 251"which are provided on the surface 221"of the substrate 220". One path covers the contacts for the

three Dual-in-line packages 252", a second path covers the six chip capacitors 254"and a third path is dedicated to the contacts of the Quad- in-line package 256". Advantageously this tri-path arrangement enables different amounts of power to be supplied for each component type and also enables re-heating of a selected region of the board for component replacement.

Referring to Figure 5 an apparatus 400 is shown which is adapted to supply power to the board assembly 300. In this example the board assembly 300 is placed on a conveyor belt 410 and transported into the apparatus 400. When the board assembly 300 is in the required position in the apparatus 300 lifting mechanism 430 raises support table 420 and support pillars 425 to lift the board assembly 300 off of the conveyor belt and into contact with a series of spring loaded probes 440 and 445.

Power supply probes 440 make contact with the contact pads to activate the heating elements 310". Temperature measuring probes 445 monitor the temperature at selected locations on the board assembly 300 and/or selected components. The probes 440 and 445 are connected via cables 455 to a controller unit 450 which is operative to monitor the temperature detected by the sensors and adjust the power and duration of the supply to the individual electrical paths in response to the measured temperatures. When reflow has been completed the mechanism 430 lowers the board assembly 300 down onto the conveyor belt 410 which transfers the assembly out of the unit.

It should be noted that other methods of board handling and probe arrangements could be used either in a dedicated machine or as part of another machine in the production line such as the placement machine or a board testing machine.

In all of the above descriptions the heating elements according to the invention have been used to reflow solder, however it will be apparent that they could be used for any process that requires heat to a selected region of the printed circuit board, such as the curing of heat cured adhesives.

It will be appreciated that although in the above examples the heating elements are arranged to heat only particular localised regions of the printed circuit board assembly, the region which requires to be heated may comprise substantially the entire surface area of a substrate. In that situation a heating element or elements may be arranged in a tortuous zigzag path on an intermediate surface so that substantially all of the surface area of the substrate is sufficiently heated.