Method to anchor foils for green-tape circuits

Abstract: Method to anchor foils for green-tape circuits, whereby the foils of alumina in the raw state or of another like or analogous material, which are personalized, printed and dried, are superimposed on each other in a coordinated and organized manner, at least one green-tape foil (10) being bonded to at least one other previously positioned green-tape foil (10) by means of at least one weld (11-12-13-14-15) made by a source of heat (laser, microwaves, ultrasounds, hot point, etc.).

Description: This invention concerns a method to anchor one foil for green-tape circuits to another foil in the production of multi-layer packs of circuits according to the so-called green-tape technology, and to packs of personalized, printed and dried green-tape foils. To be more exact, the anchorage method of this invention is carried out during the assembly of the green-tape foils before the compression step. The hybrid circuits of the so-called green-tape technology consist of thin foils of alumina in the raw state or of another like or analogous material, which have a printed circuit on one or both of their surfaces. These green-tape foils are superimposed on one another in a number which may even amount to various tens of foils, and are compressed so as to produce a multi-layer element of a slender thickness. The various components of the circuit in the single green-tape foils may be set in communication with each other through suitable connecting holes made in the green-tape foils and filled with a paste of a conductive material. The anchorage operation of the state of the art is performed by applying an adhesive to the edges of each foil forming the green-tape circuit, so that the green-tape foils remain positioned and anchored together before the compression step is effected. In this way any sliding of one foil in relation to another foil in the multi-layer pack of the green-tape circuit is prevented. The above operation of anchorage by means of adhesives is performed by hand and involves considerable times and costs as well as a great number of rejects due to the inaccuracy of the application of the adhesives and to damage caused by manual handling. JP-A-02 241 091 discloses a method wherein printed wiring boards are welded together before being laminated to multilayer structures, in order to prevent positional deviation during lamination. The present applicant has studied, tested and brought about this invention so as to obtain a safe, effective anchorage method together with further advantages. This invention is set forth and characterized in the independent claims 1 and 5, while the dependent claims describe embodiments of the invention. This invention consists in providing a method which enables the single green-tape foils to be anchored to each other to form hybrid circuits according to the so-called green-tape technology. To be more exact, the method of this invention enables each superimposed green-tape foil to be bonded to the previously positioned green-tape foil by at least one weld. According to an embodiment the welding operation is carried out at one and the same time on a pack consisting of a plurality of green-tape foils superimposed on each other, which are all thus anchored to each other at the same time in one single operation. The packs may consist even of various tens of green-tape foils , which are suitably kept firmly in position upon each other until the welding step has been carried out. In this way any sliding of green-tape foils in relation to the adjacent green-tape foils during handling and compression is prevented. The anchorage method of this invention is very accurate, reduces rejects to a minimum and makes possible very short cycle times and an excellent finished result. The welding consists of at least two spot welds or at least two slot welds or at least two line welds. We shall refer in the following description to spot welds but the contents of the description apply also to line welds and slot welds. At least two spot welds can be made at two diametrically opposite corners of the green-tape foil, or at two corners of one and the same side of the green-tape foil, or at two end points of the central axis or else at any other points which fulfil the pre-set task. Where there are three spot welds on each green-tape foil, two of these spot welds can be positioned, for instance, at two corners of one side, while the third spot weld is made at a central position on the opposite side. The spot welds to anchor successive green-tape foils are made advantageously at other positions or at positions which do not coincide with the welding positions previously used, the purpose being to prevent warping of the multi-layer pack forming the green-tape circuit. If a point on a green-tape foil is suitably heated, it is possible to melt the solvents contained in the alumina which constitutes the green-tape foil. Thus, by heating the zones of a green-tape foil to be bonded and by superimposing another green-tape foil, the two green-tape foils adhere to each other by the melting action. The heating can be carried out by precision-casting heating means or with a hot-melting electrode or by a laser or with high frequency systems or with ultrasonic or microwave welding systems or other means suitable for the purpose. After the single green-tape foils have been assembled in a pre-set number and anchored together two by two, or all together simultaneously, with the method of this invention, the green-tape foils are compressed so as to provide a multi-layer electrical circuit of a slender thickness. The attached figures are given as a non-restrictive example and are diagrams of some forms of an anchorage method according to the invention for the production of a green-tape circuit consisting of a pack which comprises a plurality of green-tape foils as follows:- Fig .1 shows a diagram of the method to anchor green-tape foils by spot welds according to an embodiment of the invention; Figs.2 are diagrams of some variants of the method of Fig. 1; Fig.3 is a diagram of a method to anchor three green-tape foils by slot welds according to an embodiment of the invention; Fig.4 is a diagram of a method to anchor green-tape foils by line welds according to an embodiment of the invention. At least two zones of a first green-tape foil 10a are suitable heated by suitable heating means, which are not shown here, so as to obtain two welds, which in this example are spot welds 11. These spot welds 11 are obtained by melting the solvents contained in the alumina which constitutes the green-tape foil 10a. As we said above, the heat which causes the melting can be generated by precision casting heating means, or with a hot casting electrode, or by laser, or by high frequency systems, or with an ultrasonic or microwave welding system, or with ultrasounds or hot point or other means suitable for the purpose, none of which is shown here. In the example of the green-tape foil 10a shown in Fig.1 the two spot welds 11 are positioned near two opposite corners of the green-tape foil 10a. At the same time a second green-tape foil 10b is placed on the first green-tape foil 10a already positioned. A reciprocal anchorage of the first and second green-tape foils 10a-10b is thus achieved. Two spot welds 12 at corners different from the corners of the first spot welds 11 are now made in the same way on the second green-tape foil 10b. A third green-tape foil 10c is then placed on the second green-tape foil 10b so as to accomplish reciprocal anchorage of the three green-tape foils 10a-10b-10c. Thereafter two spot welds 13 are made on the third green-tape foil 10c at positions not coinciding with the positions of the previous spot welds 11-12. A fourth green-tape foil 10d is now superimposed and is thus anchored to the third green-tape foil 10c. When the pack consisting of the green-tape foils 10a-10b-10c-10d has been assembled and these green-tape foils have been anchored together with the method of the invention, the green-tape foils 10a-10b-10c-10d are compressed so as to produce a multi-layer electrical circuit of the so-called green-tape type. According to a variant the green-tape foils 10 are superimposed on each other to form a multi-layer pack and are all welded together thereafter at the same time in one single welding operation according to the invention. Figs.2 show some possible positions of the spot welds 11 made on the green-tape foils 10 as follows: Fig.2a shows green-tape foils 10a, 10b and 10c in which two spot welds 11 are made alternately along one median axis of symmetry and along the other median axis of symmetry of the green-tape foil 10; Fig.2b shows green-tape foils 10a, 10b and 10c in which the spot welds 11 are made alternately along one diagonal and along the other diagonal of the green-tape foil 10; Fig.2c shows green-tape foils 10a, 10b and 10c in which the spot welds 11 are made alternately along one side and along the opposite side of the green-tape foil 10; Fig.2d shows green-tape foils 10a, 10b and 10c in which two spot welds 11 are three in number and are made alternately to form a substantially isosceles triangle with its vertex alternately on one side and on the opposite side. The spot welds 11 can be made also at different positions on the relative green-tape foils 10. In the examples shown only two or three spot welds 11 are provided between adjacent green-tape foils 10, but any desired number of spot welds can be provided. Fig.3 shows green-tape foils 10a, 10b and 10c anchored together by slot welds 14 made along the diagonals of the green-tape foils 10. These slot welds 14 may have different shapes, for instance circular, oval, polygonal, etc. Fig.4 shows green-tape foils 10a, 10b and 10c anchored together by line welds 15 made along the sides of the green-tape foil 10;

Claims: Method to anchor foils for green-tape circuits, whereby the personalised printed and dried green-tape foils are superimposed on each other in a coordinated and organised manner and at least one green-tape foil (10) is bonded with at least one other previously positioned green-tape foil (10) by means of at least one weld (11-12-13-14-15) made by a source of heat, the method being characterised in that the weld connection between the last green-tape foil (10) and the last but one green-tape foil (10) or assembly of green-tape foils takes place at a position between the adjacent surfaces of said foils different from that of the weld connection between the adjacent surfaces of the last but one green-tape foil (10) and the last but two green-tape foil (10) or assembly of green-tape foils. Method as in Claim 1, in which the weld element consists of at least two spot welds (11). Method as in Claim 1, in which the weld element consists of at least two slot welds (14). Method as in Claim 1, in which the weld element consists of at least two line welds (15). Pack of personalised, printed and dried green-tape foils superimposed on each other in a coordinated and organised manner wherein the last upper green-tape foil (10) is welded to the last but one green-tape foil (10) positioned immediately therebelow for reciprocal immovable positioning by means of at least one weld (11-12-13-14-15), characterised in that the weld is positioned between the adjacent surfaces at a position different from that of the preceding weld (11-12-13-14-15) connecting the adjacent surfaces of the last but one green-tape foil (10) to the last but two green-tape foil (10). Pack as in Claim 5, in which the weld between two contiguous green-tape foils consists of at least two spot welds (11). Pack as in Claim 5, in which the weld between two contiguous green-tape foils consists of at least two slot welds (14). Pack as in Claim 5, in which the weld between two contiguous green-tape foils consists of at least two line welds (15).

Inventors: BACCINI GISULFO, BACCINI, GISULFO
Applicants: BACCINI GISULFO, BACCINI, GISULFO
Application year: 1992
Main classification: H05K3
IPC codes: H05K1, H05K3, C04B41