GB2285002A - High volume electronic circuit production using foam slab support - Google Patents

Abstract

An electronic circuit production process involves application of epoxy deposits followed by component placement (2) on one side of a circuit board. Circuit production and quality data may be coded on to the panel border using a grid reference. Polystyrene slabs (55) are used for support of the circuits during production and these receive legs of components after the circuit has been flipped over with the surface mount components held in position by the epoxy being on the underside. At later stages, the polystyrene slabs (55) are of use for support of the border, even after soldering. During curing (3) of the epoxy, the circuit is heated to ensure components such as ceramic components are not damaged. Functional testing is carried out with circuits being mounted in trays which have a primary lead for each tray and a set of secondary leads in each tray provide the necessary connections. <IMAGE>

Description

"Hiah Volume Electronic Circuit Production" The invention relates to the production of electronic circuits in high volumes. Such a production requirement is typical for electronic circuits for such things as hand-held transmitters for remote control groups, or for domestic appliances.

In general, there is increasing competition worldwide in production of such electronic circuits. It is therefore absolutely essential to keep production costs to a minimum. In many instances, this has led to production being shifted from so-called "developed" countries where wage costs are relatively high to low-labour cost countries. Accordingly, for production in countries such as Ireland, it is not only important to ensure that quality is of the highest standard, but also to ensure that production efficiency is very high. It is known that the minimisation of handling of circuits is one way of improving efficiency and also quality and this is one of the objectives of the invention.

At present, many highly specialised machines have been developed for carrying out various operations in electronic circuit production. For example, Universal Instruments Corporation produce machines for placement of components and also for deposit of solder paste for surface mounting of components. Examples of the prior art in this area include EP-A2-0 345 061, US 5 029 383, US-A-4 327 483 and US-A- 3 808 662 (all of Universal Instruments Corporation). European Patent Specification No. EP-A2-0415527 (Hewlett-Packard) describes a fixture for supporting a double-sided surface mount printed circuit board. The fixture is a foam composition which is shaped to conform to the underside of the circuit board so that surface mount components can be installed on the other side. While this is undoubtedly of benefit in handling of the circuit board during production, it does involve the production of specific fixtures for different types of board, and this may be expensive and timeconsuming where there are frequent design changes, and indeed where there are many batch changes for production of different types of circuit.

The invention is directed towards providing an improved electronic circuit production process where there are high volumes in which handling of the circuits during the production stages is minimised, and in which quality of the product is maintained.

According to the invention, there is provided a method for high volume production of electronic circuits, the method comprising the steps of : producing a printed circuit board in a panel having an outer border for support of the circuits during production, the border being unmasked and fiducial marks being applied to the border; applying epoxy deposits to the circuit board using an epoxy applicator head, the applicator head being controlled to automatically apply a test deposit for each nozzle onto the border of the panel before applying deposits on to the circuit, the side of the border to which the test deposits are applied being associated with a particular epoxy application machine for machine identification; placing surface mount components on the epoxy deposits on the circuit; curing the circuit in an infra-red oven, in which the temperature reaches a peak in the region of 130 to 1600C for a time period in the range of 170 to 190 seconds; turning over the circuit to a position where the surface mount components are facing downwardly being held in position by the cured epoxy deposits, and placing the circuit on a slab of foam material having a high surface friction and being capable of being penetrated by component legs, the density of the foam material being in the range of 12 to 18 kg/m3; placing components having legs on the circuit by passing the legs through the appropriate through-holes in the circuit and embedding the legs in the foam slab supporting the circuit from underneath; carrying out visual inspection of the circuit while mounted on the foam slab; removing the circuit from the foam slab and soldering the circuit in a high-turbulence wave soldering machine in an orientation whereby the surface mount components are facing downwardly, the soldering machine soldering leads of both the surface mount components facing downwardly and the throughhole components facing upwardly; carrying out a visual inspection of the soldered circuit; carrying out in-circuit testing; and carrying out functional testing of a group of circuits together when interconnected by test leads when mounted in a test jig.

Preferably, the surface mount components are placed under control of a program file generated by a computer-aided design apparatus, the program file including fiducial information, panel origin and component orientation data, said data being retrieved from a component database connected to the computer aided design apparatus.

In one embodiment, the method comprises the further steps of applying a set of epoxy deposits onto the panel to form a traceability code.

Ideally, the traceability code is applied by applying epoxy deposits onto a grid imprinted on the panel border.

In one embodiment, the grid is formed by copper screen printed onto the panel.

Preferably, curing of the circuit involves pre-heating the circuit over a period in the range of 170 to 190 seconds at a substantially constant pre-heating rate.

In another embodiment, the panel is heated during the preheating stage to a temperature in the range of 1500C to 155"C over a time period in the range of 170 to 190 seconds, temperature being maintained in the region of 1500C to 1550C for a period in the range of 170 to 190"C.

In a still further embodiment, the foam slab is of polystyrene material.

Ideally, the test jig used for functional testing includes a plurality of trays, each of which is slidably mounted on a support.

In one embodiment, the support comprises a primary test stimuli lead for each tray, and wherein the support is wheel-mounted for manoeuvrability.

The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only, with reference to the accompanying drawings in which : Fig. 1 is a diagrammatic plan view showing a production process of the invention; Fig. 2 is a perspective view from above shown an epoxy application stage; Fig. 3 is a detail view showing portion of a panel border on which a code has been marked; Fig. 4 is a perspective view from above showing a different panel after application of epoxy deposits; Fig. 5(a) is a diagrammatic side view showing a panel after placement of components, and Fig. 5(b) is a graph showing temperature plots for epoxy curing; Fig. 6 is a perspective view from above showing the manner in which the panel is handled after placement of components; Fig. 7 is a diagrammatic cross-sectional view showing initial insertion of large components in a circuit; Fig. 8 is a diagrammatic side view showing the panel after soldering; Fig. 9 is a side view showing the panel being supported after the soldering operation; Figs. 10 and 11 are perspective views showing the manner in which circuits are stacked after soldering; and Figs. 12 and 13 are perspective views showing the manner in which the circuits are handled for functional testing.

Referring to the drawings, and initially to Fig. 1, a production process of the invention is described. The process 1 is carried out by a production plant shown in Fig. 1 and indicated generally by the reference numeral 1.

The plant 1 comprises a pair of parallel epoxy and component placement machines 2, each of which is connected to an infra-red oven 3 for curing of the epoxy.

Components which have been placed using these machines are surface mount components and larger components are installed at manual assembly stations 4, after which there is a pair of quality control and solder preparation stations 5. A solder machine 6 is then provided and for initial quality control of the output of the solder machine 6, there is a quality control station 7 at which both visual inspection and in-circuit testing is carried out. Functional testing is carried out at a burn-in station 8 and test stations 16. The plant 1 includes work stations 17 for production and packaging of ancillary devices such as lamps, and final inspection and packaging is carried out at a set of work stations 18 where the product is packed before dispatch at a dispatch area 19.

Within each placement machine 2 there is an epoxy placement station 20 at which nozzles 21 place epoxy deposits onto a panel 21. In the specification, the term "panel" means a single production unit which may be one or more circuit boards, or as shown in Fig. 2 a set of six circuit boards which have been produced together and may be easily separated. A panel 21 is shown in Fig. 2 and this is mounted on rails 22 at the epoxy placement station 20. The panel 21 comprises six boards 23 and a border 24.

The panels 21 are produced by applying the various conductors to the circuit boards 23 and subsequently masking all parts to which solder is not required to be adhered using a masking film having a glossy surface.

Fiducial marks provided by copper deposits on the panel 21 are applied in the border section 24, which section has not been masked and thus has a matt surface in which there is little light reflection. The fiducial marks are of diamond shape and are indicated by the numeral 28 on the panel 21. It has been found that the fiducial marks may be very easily recognised by the optical sensing system of the machine 20 when they are applied in this manner.

The epoxy deposits 25 are also shown in Fig. 2 and these are applied where surface mount components are to be placed. Before applying the epoxy deposits on the circuits 23, the nozzles 21 apply test epoxy deposits 27 on one side of the border 24 of the panel 21. There is a large test deposit 27 and a small test deposit 26 being applied by the relevant nozzles. This operation has two purposes. One purpose is to help ensure that the epoxy dispensing at the start of each panel 21 is consistent by starting flow through the relevant nozzle. Another purpose is that at a later stage the size of the epoxy deposit may be easily checked visually as a trace of how the nozzles 21 were operating for that particular panel 21. At such a later stage, the epoxy deposits on the circuits 23 will have been covered over by the surface mount components.

Another important step of the invention which helps provide traceability in an extremely efficient manner with little circuit handling is illustrated in Fig. 3. In this diagram, there is shown a grid 40 which is formed by copper conductors applied to part of the border 24. The grid 40 defines a frame of reference for application of epoxy deposits 40 within the grid, the deposits 41 in the grid 40 providing codes giving information about the particular panel 21. The information given by this coding system includes the date, machine number, batch number and any other desired information about the particular panel and its production. By providing this information using the nozzles 21, separate handling of the panel is not required and further, it is almost impossible for these indicia to be removed, thus providing for reliable traceability of a circuit. It is envisaged that the grid 40 may be formed by conventional printing techniques or indeed it may not exist at all - an overlay being used for reading. Further, there may be a grid on each individual circuit 23.

As shown in Fig. 4, different machines 4 are programmed to apply test epoxy deposits on different sides of the border 24 so that the machine may be identified at a later stage. Thus, as shown in Fig. 4, the other epoxy application station applies test epoxy deposits 31 and 32 on the opposite side of the border 24 on a panel 30.

Surface mount components 50 are then placed on the deposits 25 as shown in Fig. 5(a) and the epoxy is then cured in an infra-red oven 3. Regarding component placement before curing in the ovens 3, it has been found that efficiency has been considerably improved by generation of circuit designs on a computerised design apparatus and generating a program file compatible with controlling software for the placement machine 4. The file which is generated includes fiducial information, panel origin and component orientation information in addition to component information retrieved from databases. The file is transmitted to the placement machine and the feeders and the panel datum are set up for an initial trial run.

Reference is now made to Fig. 5(b) which illustrates the manner in which the epoxy is cured. The temperature profile during the curing processes are shown in these diagrams and curing is very carefully controlled to ensure that damage does not occur to components such as ceramic components, while at the same time ensuring that the epoxy is adequately cured. It has been found that by gradually raising the temperature to 1500C - 15 SaC over a time period of approximately 180 seconds and subsequently maintaining this temperature level for a further 180 seconds approximately, a consistently high standard of curing is achieved without damage to the circuit or its components.

Both of these time durations are in general preferably in the range 170 - 190 seconds.

It will be appreciated that the combination of features described for epoxy and component placement and oven curing provide for high quality production in an extremely efficient manner.

After curing of the epoxy, the components 50 are secure in their positions on the panel 21 and each panel 21 is then flipped over with the components 50 facing downwardly and resting on a polystyrene slab 55 shown in Fig. 6. The polystyrene slab 55 has flat high-friction surfaces and in this embodiment has a density of 15 kg/m3, the density being preferably in the range of 12 to 18 kg/m3. Because the top side of the panel 21 as viewed in Fig. 5(a) only has surface mount components (which are relatively small) the panel 21 rests evenly on the slab 50. The slab 50 then provides another extremely important function whereby at the assembly stations 4 large components such as those indicated by the numeral 60 in Fig. 7 may be placed on the side facing upwardly (which has no components) by inserting legs or leads of the components 60 through holes in the panel 21, the leads 61 becoming embedded in the foam slab 55. Thus, the slab 55 provides a fixture for support of the circuit 21 during production and also helps to ensure that the large components 61 remain in position.

While this is an extremely simple step, it has enormous advantages in the production process. One advantage is that the material is inexpensive and production of special jigs or fixtures is not required. Another advantage is that the slab 55 provides support for the circuit 21 and avoids the need for an operator to hold the board 21 directly. Finally, another major advantage is the manner in which the components 60 are held securely in place, with a very low chance of them falling out of position.

As shown in Fig. 8, the panel 21 is then soldered at the soldering machine 6 after any required further assembly and quality checks have been carried out at the stations 5. High efficiency is achieved because soldering is carried out in one step only using a high-turbulence wave soldering machine capable of soldering the surface mount components 50 and the larger components 60. The circuit passes through the machine with the surface mount components facing downwardly, these being heat resistant.

The soldering station 7 comprises a preheating unit having a power consumption of 7 KW. The soldering unit has a 180 kg solder pot with a regular wave which has a maximum height of 10 mm. The speed of conveying of the circuit through the soldering unit is approximately 0.3 m/min.

After soldering, the panel 21 may be simply placed on top of a polystyrene slab 55 as shown in Fig. 9, and thus the slab 55 is of use after the soldering operation as a support for the circuit. This is apparent in Figs. 10 and 11 in which various stacking arrangements are shown for circuits, and in one example, a crate 70 is used for stacking of circuits having heavy components such as transformers, the circuits being stacked on the slabs 55 in the crate 70. The stacking arrangements shown in Figs.

10 and 11 are used for visual inspection after soldering and also for in-circuit testing which is carried out in a conventional manner. The circuits are removed from a stack or a crate 70 and are replaced in a testing jig shown in Figs. 12 and 13 after visual and in-circuit testing.

Efficiency of functional testing at the burn-in station 8 and test stations 16 has been achieved with very little handling of the circuits using a testing jig 80 shown in Figs. 12 and 13. The jig 80 comprises a support frame 81 mounted on casters 82 and which supports a number of sliding trays 83, each of which is constructed to support a number of circuits 23. A controller 85 provides test signal primary sockets 86 connected to leads 87, there being one for each tray 83. Secondary leads 88 connect the primary lead 87 to each of the circuits 23 on the tray 83.

Again, this arrangement is extremely simple but it has major advantages. One advantage is that a large number of circuits may be tested together whereby the same test stimuli are transmitted to each circuit. Another advantage is that the connections are made in an extremely simple manner with their being a primary lead for each tray and secondary leads connecting each circuit. The circuits are housed within the frame 81 and are thus protected and are thus very unlikely to be accidentally damaged. Where inspection of one or more circuits is required, it is only a matter of the operator pulling out the relevant tray 83 and inspecting the circuit or making the connections, whichever is required. Further, the jig 80 may be easily moved from one test station to another by simply rolling the frame 80 on the casters 82. What has been achieved by this arrangement is functional testing of the circuits in an efficient manner with relatively little handling of the circuits.

After functional testing, various specific tests may be carried out relating to the particular type of circuit.

Final assembly and packaging are then carried out at the stations 17 and 8 before dispatch from the despatch area 19.

It will be appreciated that the invention provides a combination of process steps which together provide for high volume production of electronic circuits in a manner in which relatively little handling is required and whereby a high quality standard may be easily maintained.

The invention'also provides for very good traceability of the circuits and feedback for both production and quality analysis.

The invention is not limited to the embodiments hereinbefore described, and may be varied in construction and detail.

Claims (12)

1. A method for high volume production of electronic circuits, the method comprising the steps of : producing a printed circuit board in a panel having an outer border for support of the circuits during production, the border being unmasked and fiducial marks being applied to the border; applying epoxy deposits to the circuit board using an epoxy applicator head, the applicator head being controlled to automatically apply a test deposit for each nozzle onto the border of the panel before applying deposits on to the circuit, the side of the border to which the test deposits are applied being associated with a particular epoxy application machine for machine identification; placing surface mount components on the epoxy deposits on the circuit; curing the circuit in an infra-red oven, in which the temperature reaches a peak in the region of 130 to 1600C for a time period in the range of 170 to 190 seconds; turning over the circuit to a position where the surface mount components are facing downwardly being held in position by the cured epoxy deposits, and placing the circuit on a slab of foam material having a high surface friction and being capable of being penetrated by component legs, the density of the foam material being in the range of 12 to 18 kg/m3; placing components having legs on the circuit by passing the legs through the appropriate through-holes in the circuit and embedding the legs in the foam slab supporting the circuit from underneath; carrying out visual inspection of the circuit while mounted on the foam slab; removing the circuit from the foam slab and soldering the circuit in a high-turbulence wave soldering machine in an orientation whereby the surface mount components are facing downwardly, the soldering machine soldering leads of both the surface mount components facing downwardly and the through hole components facing upwardly; carrying out a visual inspection of the soldered circuit; carrying out in-circuit testing; and carrying out functional testing of a group of circuits together when interconnected by test leads when mounted in a test jig.

2. A method as claimed in claim 1 wherein the surface mount components are placed under control of a program file generated by a computer-aided design apparatus, the program file including fiducial information, panel origin and component orientation data, said data being retrieved from a component database connected to the computer aided design apparatus.

3. A method as claimed in claims 1 or 2, comprising the further steps of applying a set of epoxy deposits onto the panel to form a traceability code.

4. A method as claimed in claim 3, wherein the traceability code is applied by applying epoxy deposits onto a grid imprinted on the panel border.

5. A method as claimed in claim 4, wherein the grid is formed by copper screen printed onto the panel.

6. A method as claimed in any preceding claim, wherein curing of the circuit involves pre-heating the circuit over a period in the range of 170 to 190 seconds at a substantially constant pre heating rate.

7. A method as claimed in claim 6, wherein the panel is heated during the pre-heating stage to a temperature in the range of 1500C to 155"C over a time period in the range of 170 to 190 seconds, temperature being maintained in the region of 1500C to 1550C for a period in the range of 170 to 1900C.

8. A method as claimed in any preceding claim, wherein the foam slab is of polystyrene material.

9. A method as claimed in any preceding claim, wherein the test jig used for functional testing includes a plurality of trays, each of which is slidably mounted on a support.

10. A method as claimed in claim 9, wherein the support comprises a primary test stimuli lead for each tray, and wherein the support is wheel mounted for manoeuvrability.

11. A method substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.

12. A circuit whenever produced by a method as claimed in any preceding claim.