PROCESS FOR PRODUCING PRINTED CIRCUITS AND PRINTED CIRCUITS THUS OBTAINED

Abstract: There is described a process for producing printed circuits comprising a laminar support (1), at least one electrically conductive track (7) on the said laminar support, and at least one auxiliary conductive element (21) soldered to the said conductive track (7); there is provision to apply the said auxiliary conductive element by means of an apparatus for applying SMD components.

Description: "PROCESS FOR PRODUCING PRINTED CIRCUITS AND PRINTED CIRCUITS THUS OBTAINED" DESCRIPTION Field of the art The present invention relates to a process for producing printed circuits, of the type comprising a laminar support or board, on which is produced a plurality of electrically conductive tracks for connecting a plurality of electronic components . More particularly the process according to the invention relates to a novel technique for applying auxiliary conductive elements to the board, having the objective of increasing the conductivity of the tracks at specified points in the circuit, where the power components are located. The invention also relates to a printed circuit obtained by this novel process as well as to a loader of auxiliary conductive elements which is produced so as to be used with a process according to the invention. State of the art For producing electronic circuits, the use is known of laminar supports or boards, produced from "vetronite" or equivalent materials, on one or both faces of which tracks of conductive material, typically copper, are produced. These are obtained by depositing a lamina of copper or equivalent material on the face of the laminar support and then carrying out a printing and etching procedure. The components of the electronic circuit, which are typically power components with high consumption and logic components with low consumption, are applied to the tracks thus produced. The cross-section of the various conductive tracks varies according to the power consumed by the individual electronic components, so as to offer sufficiently limited resistance and avoid excessive heating. The cross-section can be increased by increasing the thickness of the copper lamina applied to the laminar support, or by increasing the width of the track. Both dimensions of the transverse cross- section of a track cannot be increased at will for several reasons. As far as the thickness is concerned, since all the tracks are produced from the same conductive lamina, the latter must not be too thick so as not to increase the cost of the circuit beyond certain limits. Consequently it is important not to overdimension the thicknesses of the tracks dedicated to the logic components in order to have sufficient conductivity in the tracks dedicated to the power components. Moreover, it is necessary to avoid the effects of the underetching which occurs where the conductive lamina from which the conductive tracks of the circuit are made is of great thickness. As far as the width of the track is concerned, this is limited by circuit layout and spatial requirements . To obviate these drawbacks, it is known to apply to the tracks, where this is made necessary by the high consumption of the components connected thereto, auxiliary conductive elements consisting of copper slugs which are soldered to the tracks. The application of these auxiliary conductive elements currently requires an appreciable input of manual labour, since each conductive element is produced in a manner dedicated to the specific application and soldered manually to the points at which an increase in the cross-section of the conductive track is required on account of the presence of power elements characterized by high consumption. Objectives and summary of the invention The objective of the present invention is the provision of a process for mounting auxiliary conductive elements on printed circuits, which is simpler, more economical and faster than the conventional systems. Within the scope of this general objective, a particular objective of the present invention is the provision of a process which can be carried out using currently known machines and apparatus for mounting electronic components on printed boards . A further objective of the present invention is the provision of a process which allows simple and economical production of printed circuits of different types without the necessity of dimensioning the auxiliary conductive elements in a specific and dedicated manner for each individual circuit. A further objective of the present invention is the provision of a printed circuit with auxiliary conductive elements which is cheaper and simpler than conventional circuits. These and further objectives and advantages, which will become clearly apparent to those skilled in the art from reading the following text, are achieved in substance by providing for the auxiliary conductive elements to be applied to the laminar support by means of an apparatus for applying SMD (Surface Mounted Device) technology components. The invention is therefore based on the idea of manipulating the auxiliary conductive elements as if they were normal SMD technology electronic components. This allows mounting to be enormously simplified since the auxiliary conductive elements are also applied by the automatic machines in the same phase of application of the SMD components. In practice, the process according to the invention can exhibit the following phases: - producing at least one conductive track on the laminar support; - applying a so-called "solder-resist" protective layer to the laminar support, leaving on the conductive track at least one zone substantially devoid of the said solder-resist layer; - applying to the conductive track. in the region of the said zone an adhesive for adhesively bonding the auxiliary conductive element; - adhesively bonding the auxiliary conductive element and possible further SMD circuit components to the laminar support; - soldering the auxiliary conductive element and possible further SMD circuit components to the laminar support . Electronic components can be mounted on the laminar support either on one face or on two faces, there normally being provision for the auxiliary conductive elements to be mounted . on a single face together with the other SMD technology circuit components . In a manner similar to what has been done hitherto in SMD technology, when components are mounted on both faces of the same laminar support, SMD technology circuit components, including the auxiliary conductive elements, will firstly be applied to one face by adhesive bonding. Once application has been done by adhesive, the drying of the adhesive will be carried out in an appropriate oven and there will be provision for the rotation of the support so as to allow mounting of the circuit components on the opposite face, by inserting the legs into the appropriate holes normally provided in the laminar support . Final soldering of the electronic components and of the auxiliary conductive elements, which completes the production cycle, will be carried out by a soldering procedure of known type, for example wave soldering with a tin or similar alloy. The auxiliary conductive elements are preferably mutually identical so as to achieve the maximum standardization of the process. However, the use also of different dimensions, or at least of a few different dimensional standards, on the same circuit, is not excluded. These may consist of metal pads of substantially rectangular outline of limited height, and may be produced from copper or some other highly conductive metal or metal alloy and may possibly be coated with a suitable metallic layer which facilitates the adhesive bonding and subsequent soldering. For example, the auxiliary conductive elements may have a forge tinning treatment or the like, with the objective also of preventing oxidation thereof during storage. To allow automation of the procedure for mounting the auxiliary conductive elements, the invention advantageously provides for them to be loaded in strips of flexible material (for example plastic, cardboard or the like) furnished with a plurality of mutually identical recesses in which the auxiliary conductive elements are housed. These strips have the same structure as the normal loaders of SMD technology components used for mounting systems. It is thus possible to manipulate the auxiliary conductive elements as normal SMD technology components . Further advantageous characteristics of the process according to the invention, of the printed circuit obtained therewith and of the strips for loading the auxiliary conductive elements are indicated in the appended claims . Brief description of the drawings The invention will be better understood by following the description and the accompanying drawing, which shows a practical non-limiting illustration of the invention. In the drawing are shown: Fig. 1 a plan view of a portion of a laminar support with a conductive track prepared for the application of auxiliary conductive elements according to the invention; Figs. 2 to 5 show, in a greatly enlarged transverse cross-section on II-II of Fig. 1, of the laminar support, successive phases of the mounting procedure according to the invention; Figs . 6 and 7 show a longitudinal section through and a plan view of a loading strip for the auxiliary conductive elements; Figs. 8 to 10 show alternative forms of adhesively bonding the auxiliary conductive element; and Fig. 11 shows an embodiment with reflow rather than wave soldering. Detailed description of an embodiment of the invention Fig. 1 shows, purely as a simplified example, a portion of laminar support (board) for making printed circuits by the process according to the invention. Fig. 2 shows a greatly magnified transverse cross-section on II-II of Fig. 1 of the portion of laminar support of Fig . 1. The arrangement of the conductive tracks, of the holes and of the lands for applying the SMD components is purely random and merely has the objective of facilitating the understanding of the process according to the invention. In substance the laminar support 1 is made in a manner known per se from a lamina of "vetronite", or equivalent material, which on a first face (the one visible in Fig. 1) exhibits a series of tracks and lands for applying SMD technology circuit components indicated by 3 and 5. A conductive track 7 of larger cross-section for connecting power components is also provided on the same face of the laminar support 1. As is known to those skilled in the art, the tracks 3 and 7 and the lands 5 are obtained by printing and etching a copper lamina applied to the laminar support 1 and hence cut to a predetermined design. A layer of protective varnish 9 (see Fig. 2) referred to as "solder-resist" is applied to those zones of the tracks 3 and 7 to which electronic components or auxiliary conductive elements are not to be soldered. In the example depicted the conductive track 7 , along which are indicated merely by way of illustration a few holes 11, 13 for mounting electronic components, is coated with a layer of solder-resist 9 over the whole of its surface with the exception of a series of zones with substantially rectangular outline indicated by 15. The auxiliary conductive elements will be applied, in the manner described below, in the region of these zones by the process of the present invention. Each uncovered zone 15 of the track 7 has inside it two limited areas of rectangular outline indicated by 17, coated with "solder-resist". The mounting of the auxiliary conductive elements is undertaken in the following way. Spots of adhesive 19 (Fig. 2) are applied to the areas 17 as provided for when mounting SMD technology circuit components. The application of the adhesive and the subsequent application of the auxiliary conductive elements can be effected in the same manufacturing phase wherein the SMD circuit components are adhesively bonded in the region of the lands 5. Having applied the adhesive 19 to the various areas 17 of the zones 15, an auxiliary conductive element 21, consisting of a metallic pad of substantially rectangular outline and of generally (but not necessarily) smaller dimensions than the dimension of the zone 15, is applied to each zone 15 using a normal SMD technology components mounting system (Fig. 3) . The mounting of the auxiliary conductive elements 21 on the respective face of the laminar support 1 is effected in the same phase of application of all the SMD technology circuit components to the same face. Having completed this operation, the laminar support is placed in an oven, wherein (as is known in the prior art) the adhesive 19 is dried so as to stabilize the SMD components and the auxiliary conductive elements 21 on the respective face of the laminar support. At this point the laminar support is inverted so as to carry out the mounting of the circuit components on the opposite face. Merely by way of illustration, Fig. 4 shows the mounting of a component 23 with respective legs 25 which are inserted into corresponding holes (not visible in Fig. 4) made in the laminar support 1 so as to be soldered together with the components located on the opposite face, where the auxiliary conductive elements 21 are sited. Conductive tracks indicated by 27, suitably covered where necessary with a layer of solder-resist 29, are also made on the face where the components 23 are mounted. The laminar support thus furnished with electronic components on both faces and with the auxiliary conductive elements 21 is subjected to a series of operations of a type known per se so as to carry out the soldering of the components and of the conductive elements 21. This soldering can be carried out, with prior fluxing if necessary, by means of a wave soldering technique using tin alloy or the like. The final result is shown diagrammatically in Fig. 5: in addition to the normal soldering of the SMD components to the lower face (in this figure) and of the components 23 furnished with their respective legs 25 to the upper face, the soldering via a layer of tin 31 of the auxiliary conductive elements 21 is also obtained. The tin coats each auxiliary conductive element 21 and also penetrates under it between the surface of the zone 15 of the conductive track 7 and that surface of the conductive element 21 which faces this track. In substance the auxiliary conductive element 21 is "encapsulated" in a coating of tin alloy or some other equivalent solder alloy, which guarantees electrical contact in addition to the adhesion of the element 21 to the printed circuit. Shown diagrammatically in Figs . 6 and 7 is a configuration of a loader of auxiliary conductive elements 21 for feeding a machine for mounting SMD technology components. The loader consists of a flexible strip 31 made of plastic or the like, wherein are made recesses 33 in which are seated auxiliary conductive elements 21, in the form of metal pads. The strip 32 is fed to the mounting machine and manipulated by the latter as a normal strip for loading SMD components. Shown in Figs. 8 to 10 are three different alternatives for the adhesive bonding of the elements 21 to the laminar support 1. In Fig. 8 the bonding is effected by two spots of adhesive 19 placed in the region of the perimeter of the auxiliary conductive element 21. In Fig. 9 bonding is effected by a spot of adhesive 19 in the central position, applied directly to the insulating layer 1, having previously removed the conductive lamina. Shown in Fig. 10 is a modified form of the method of application of Fig. 9, with a central hole in the auxiliary element 21, into which the adhesive 19 penetrates. In the foregoing reference was made to wave soldering the auxiliary conductive elements 21 and the possible electronic components. However, soldering by different techniques, for example by reflow, in particular when SMD components only are present on the circuit (on one face only or on both) , is not excluded. In Fig. 11 (where identical or corresponding parts are indicated with the same reference numbers) , the conductive element 21 is applied to the circuit in the following manner: a cream solder 34 is applied to the soldering lands 15, preferably by a silk- screen printing method; the auxiliary conductive element 21 is then applied with adhesion to the cream solder which in this case acts as a cement in substitution for the cement 19; the circuit is placed in an infrared or equivalent oven, so as to cause reflow of the cream solder 34. Other soldering procedures are not excluded. It is understood that the drawing shows only an illustration given merely as a practical demonstration of the invention, it being possible for this invention to vary in its forms and arrangements without however departing from the scope of the concept underlying the invention. The possible presence of reference numbers in the attached claims has the objective of facilitating the reading of the claims with reference to the description and to the drawing, and does not limit the scope of the protection represented by the claims.

Claims: CLAIMS 1. Process for producing printed circuits comprising a laminar support (1) , at least one electrically conductive track (7) on the said laminar support, and at least one auxiliary conductive element (21) soldered to the said conductive track (7) , characterized in that the. said auxiliary conductive element is applied by means of an apparatus for applying SMD components . 2. Process according to Claim 1, characterized by the phases of : - producing the said at least one conductive track (7) on the said laminar support (1) ; - applying a protective layer (9) to the laminar support (1) , leaving on the said conductive track (7) at least one zone (15) substantially devoid of the said protective layer (9) ; - applying to the said conductive track, in the region of the said zone (15), an adhesive (19; 34) for adhesively bonding the said auxiliary conductive element (21) ; - adhesively bonding the said auxiliary conductive element and possible further circuit components (23) to the said laminar support (1) ; - soldering the said auxiliary conductive element (21) and the said possible further circuit components to the said laminar support. 3. Process according to Claim 2, characterized in that inside the said zone (15) substantially devoid of the said protective layer (9) , at least one area (17) coated with the said layer is left, to which the said adhesive (19) is applied. 4. Process according to one or more of the preceding claims, characterized in that the soldering of the said auxiliary conductive element (21) is effected by a wave soldering procedure. 5. Process according to one or more of Claims 1 to 3, characterized in that the soldering of the said auxiliary conductive element (21) is effected by means of a reflow soldering procedure . 6. Process according to Claim 5, characterized in that an adhesive consisting of a cream solder (34) is applied to the said zone (15) substantially devoid of protective layer (9) and in that after the application of the auxiliary conductive elements (21) and of the said possible electronic components the laminar support is placed in an oven so as to remelt the cream solder (34) . 7. Process according to one or more of the preceding claims, characterized in that before the soldering, further circuit components (23) are applied to both faces of the said laminar support (1) , the circuit components applied to that face to which the said at least one auxiliary conductive element (21) is applied being SMD components. 8. Process according to one or more of the preceding claims, characterized in that a plurality of mutually identical said auxiliary conductive elements is applied. 9. Process according to one or more of the preceding claims, characterized in that the said auxiliary conductive elements are in the form of metal pads . 10. A printed circuit comprising a laminar support (1) , on at least one face of the said laminar support at least one conductive track (7) , onto which is soldered at least one auxiliary conductive element (21) , characterized in that the said auxiliary conductive element is applied by means of an apparatus for applying SMD components . 11. Circuit according to Claim 10, characterized in that the said at least one auxiliary conductive element is applied to the said laminar support by means of an adhesive and soldered thereto by a wave soldering procedure . 12. Circuit according to Claim 10, characterized in that the said at least one auxiliary conductive element is applied to the said laminar support by means of a cream solder and a reflow soldering procedure . 13. Circuit according to Claim 10, 11 or 12, characterized in that it exhibits on one face a plurality of auxiliary conductive elements (21) and a plurality of SMD electronic components and on the other face a plurality of electronic components furnished with legs which pass through the thickness of the said laminar support. 14. Circuit according to Claim 13, characterized in that the said auxiliary conductive elements are mutually identical. 15. Circuit according to one or more of Claims 10 to 14, characterized in that the said auxiliary conductive elements are in the form of metal pads. 16. Circuit according to one or more of Claims 10 to 15, characterized in that the said at least one auxiliary conductive element is produced from a metal with high electrical conductivity coated with a metallic layer with high wettability and with a melting temperature higher than the melting temperature of the solder alloy. 17. A strip of flexible material (32) comprising a plurality of adjacent recesses (33), in each of which is housed a respective auxiliary conductive element (21) for carrying out the process according to one or more of Claims 1 to 7. 18. Strip according to Claim 17, characterized in that the said auxiliary conductive elements (21) are in the form of metal pads. 19. Strip according to Claim 17 or 18, characterized in that each auxiliary conductive element is produced from a metal with high electrical conductivity coated with a metallic layer with high wettability and with a melting temperature higher than the temperature of the solder alloy.

Inventors: CANOVA ANTONIO, CANOVA, ANTONIO
Applicants: CANOVA ANTONIO, MAGNETEK SPA, CANOVA, ANTONIO, MAGNETEK S.P.A.
Application year: 1998
Main classification: H05K3
IPC codes: H05K1, H05K3