MULTI-LAYER CIRCUIT BOARD

Abstract: The invention relates to a process for manufacturing a structure (100) for a circuit board which comprises a plurality of supporting layers (10, 30, 60, 90), all of different materials, which supporting layers (10, 30, 60, 90) support electrically conducting patterns (20, 50, 70, 120). According to the process, the material for a first supporting layer (10) is that material which has the highest melting point of the different materials for supporting layers, subsequent to which, beginning from the first supporting layer (10), new supporting layers (30, 60, 90) are arranged successively. As material for each new supporting layer (30, 60, 90), a material is chosen with lower melting point than the supporting layer which is closest in the direction of the first supporting layer (10). Each new supporting layer (30, 60, 90) is attached to the structure (100) by exposing the structure (100) to a temperature which exceeds the melting point of the new supporting layer (30, 60, 90) but is lower than the melting point for that supporting layer which is the closest in the direction of the first supporting layer (10). Furthermore, on at least one side of each new supporting layer (30, 60, 90) electrically conducting patterns (50, 70, 120) are arranged, and the electrically conducting patterns (20, 50, 70, 120) on at least two supporting layers (10, 30, 60, 90) are connected to each other. The invention also relates to a circuit board manufactured according to this process.

Description: TITLE: Multi-layer circuit board. TECHNICAL FIELD: The present invention relates to a process for manufacturing a multi-layer circuit board and a multi-layer circuit board manufactured according to this process. PRIOR ART: A conventional technique for obtaining circuit boards with small interspaces between the components which are mounted on the surface of the circuit board, in other words circuit boards having a high so-called packing density, is to produce the circuit board as a multi-layer board. A multilayer circuit board comprises a plurality of supporting layers of non-conducting material on which a conducting pattern is arranged. The conducting patterns on the various supporting layers are connected to one another in desired fashion through the creation of holes, so called "via- holes", between the various supporting layers at the locations in which connection is required. The via-holes are made electrically conducting by being wholly or partially filled with electrically conducting material. A known technique for joining together the various supporting layers in a multi-layer circuit board is by so- called "fusion bonding". This means that the conducting patterns are arranged on the various supporting layers, and the various layers are arranged one on top of the other in desired fashion, subsequent to which the structure which is thus formed is exposed to a temperature in excess of the melting point for the material in the supporting structure. A problem which arises in this process is that when the supporting layers reach or exceed their melting point, the forms of the conducting patterns can alter due to movements in the supporting layers. When the supporting layer then stiffens, the conducting patterns might therefore not have precisely the same form which they had earlier. Because of the above-described problem, the conducting patterns often have to be made wider than required in order to ensure that the desired region is covered by conducting patterns. This adds to the difficulty of producing circuit boards with small conducting patterns and small intervals between the conducting patterns. An alternatively previously known technique in fusion bonding is to place between the supporting layers materials which have a lower melting point than the supporting layers. When heated, the structure will thus be joined together without the structure of the conducting patterns being affected by dimensional changes in the supporting layers. This technique leads however to increased manufacturing costs, and results in the height of the structure becoming greater than desired. Japanese patent document JP 63 047127 describes a method for manufacturing circuit boards which is based upon a number of basically identically structured laminates being joined together one on top of the other. Each laminate comprises a core which, on its upper and lower side, is surrounded by an outer material with lower melting point than the core. The laminate is jheated to a temperature which exceeds the melting point of the outer material but is lower than the melting point of the core. The various strata of the laminate are thus made to fasten together. A drawback with this method seems to be that each supporting stratum has to be surrounded by two other strata, whose only real function is that of adhesive layers. Japanese patent document JP 01 025597 shows a method in which supporting layers are surrounded by various types of paste with different melting points. This method appears to lead to higher material costs and multiple stages in the production of the circuit boards. Document WO 96/08945 shows a method which is typical of the method using various strata whose only function is that of adhesive layers. SUMMARY OF THE INVENTION: The problem which is solved by the present invention is thus to achieve a method for manufacturing multi-layer circuit boards, which method is cheap and simple to use, and allows the various supporting layers in the board to be fastened in place without adhesive strata or other intermediate strata. The invention solves this problem through a process for manufacturing a multi-layer circuit board comprising a plurality of supporting layers for conducting patterns, in which the supporting layers are all of different materials. As material for a first supporting layer for conducting patterns, that material is chosen which has the highest melting point of the different materials for supporting layers for conducting patterns. On this first supporting layer there is arranged a first conducting pattern of desired shape. Starting from the first supporting layer, new supporting layers are arranged successively, in which, as material for each new supporting layer, a material is chosen with lower melting point than the supporting layer which is closest in the direction of the first supporting layer. Each new supporting layer is attached to the structure by exposing the structure to a temperature which exceeds the melting point of the new supporting layer but is lower than the melting point for the supporting layer which is closest in the direction of the first supporting layer. In this way, the conducting patterns present in the structure will not be affected by movements in their respective supporting layers . Once the structure, after each heating, has regained a temperature which allows the new uppermost supporting layer to be worked, a conducting pattern of desired shape is created on at least one side of the new uppermost supporting layer. Electrically conducting patterns in at least two supporting layers are connected to each other. These connections are preferably constituted by via-holes . The process is repeated until the desired number of layers are present in the structure. On the uppermost layer, conducting patterns and/or electronics components can be arranged. DESCRIPTION OF THE DRAWINGS: The invention will be described in greater detail below with the aid of an example of an embodiment, and with reference to the appended drawing, in which Figs, la-lg show diagrammatically in cross-section the manufacture of a circuit board according to the present invention. Fig. 2 shows a flow chart of a process according to the invention. PREFERRED EMBODIMENT: Fig. la illustrates in diagrammatic cross-section a supporting layer 10 bearing a conducting pattern 20. The supporting layer 10 will constitute a first layer in a structure 100 for a circuit board. The structure 100 will comprise a plurality of supporting layers for conducting patterns, which supporting layers are all of different materials. A suitable material for the first layer 10 is a PTFE material (polytetrafluoroethylene) with a relatively large content of glass, thereby producing good dimensional stability, for example TACONIC TLC 32 ®. In Fig. lb, the structure 100 from Fig. la is enlarged by a second supporting layer 30. As material for the second supporting layer 30, a material is chosen with lower melting point than the first supporting layer 10. The second supporting layer 30 is attached to the structure 100 by exposing the structure 100, inclusive of the second supporting layer 30, to a temperature which exceeds the melting point for the second supporting layer 30 but is lower than the melting point for the first supporting layer 10. In this way, the conducting pattern 20 which is present in the structure 100 will not be affected by movements in the supporting layer 10 which bear-s the conducting pattern 20. When the second supporting layer 30 has thus been attached to the structure 100 and has subsequently assumed a temperature which is lower than its melting point, a second conducting pattern 50 of desired shape is arranged on top of the second supporting layer 30, thereby producing the structure 100 which is shown in Fig. lc. The conducting patterns 20, 50 which are now present on the supporting layers 10, 30 in the structure 100 are connected at desired locations by means of via-holes 40. The via- holes 40 are preferably created by holes being made in that part of the structure which separates the conducting patterns, which holes are filled wholly or partially with electrically conducting material. The principle of the invention should now be clear, but for the sake of completeness, the creation of a further two layers will also be described. In Fig. Id, the structure has been enlarged by a third supporting layer 60. As material for the third supporting layer 60, a material is chosen with lower melting point than the second supporting layer 30. In a preferred embodiment of the invention, a suitable material for the third supporting layer 60 is PFA, perfluoroalkoxyl. The third supporting layer 60 has been attached to the structure 100 in a way which is analogous with the second supporting layer 30, in other words by exposing the structure inclusive of the third supporting layer 60 to a temperature which exceeds the melting point for the third supporting layer 60 but is lower than the melting point for the second supporting layer 30. In Fig. le, a third conducting pattern 70 of desired shape has been arranged on the third supporting layer 60. In the same way as described above, this third conducting pattern 70 has been created when the third supporting layer 60, after heating of the structure 100, has assumed a temperature which is lower than its melting point. The third conducting pattern 70 is connected at desired locations to the second conducting pattern 50 by means of via-holes 80. In Fig. If, on top of the third conducting pattern 70 there is shown a fourth supporting layer 90, which has been attached to the structure 100 in a manner which is analogous with what has been described above for other layers 10, 30, 60 in the structure 100. In a preferred embodiment of the invention, a suitable material for this fourth supporting layer 90 is FEP, fluorinated ethylene- propylene. The fourth supporting layer 90 is intended to be the uppermost layer in the structure 100, for which reason not only conducting samples 120 of desired shape but also components 130, suitably electronics components, have been arranged on this fourth layer 90 in Fig. lg. Conducting patterns 120 of desired shape have been created and components 130 have been applied to the fourth supporting layer 90 once the fourth supporting layer 90, after heating of the structure 100, has assumed a temperature which is lower than its melting point. By means of via-holes 110 and the conducting patterns 120 on the fourth supporting layer 90, the components 130 are connected to other conducting patterns in the desired manner. The differences between the melting points of the various supporting layers 10, 30, 60, 90 are suitably within the range 1-1,000°C, preferably within the range 10-300°C, suitably within the range 20-200°C. In a preferred embodiment, those conducting patterns which are arranged on the supporting strata are arranged by arranging a layer of conducting material (not shown in the figures), for example copper, which substantially covers the whole of the supporting stratum, on the supporting layers. After a supporting stratum has been attached to the structure in the above-stated manner, the desired pattern is then created in the layer of conducting material. Fig. 2 shows a flow chart of a process according to the invention. The conducting patterns on the supporting layers can be created in a large variety of ways. Three different such methods will be described below. These three methods are subtractive, semi-additive and additive methods. A subtractive method means that essentially the whole of the surface on which it is wished to have an electrically conducting stratum is covered with electrically conducting material. Over the locations in which it is wished, at the end of the process, to have electrically conducting material, so-called "photoresist" is arranged. The surface is thereafter exposed to etching, whereby the electrically conducting stratum is removed, apart from the parts covered by the photoresist. After this, the photoresist can be removed by, for example, washing, whereby the remaining surfaces of electrically conducting material are laid bare. In other words, the surface now has electrically conducting material only at the locations in which this was desired. A semi-additive method means that, by using, for example, a chemical method, the whole of the surface on which it is wished to have electrically conducting strata is covered with an extremely thin first stratum of electrically conducting material. Photoresist is thereafter arranged at those locations on this extremely thin stratum of electrically conducting material in which it is not wished, at the end of the process, to have electrically conducting strata. By galvanic means, for example, a somewhat thicker, second stratum of electrically conducting material, in which the surface is not covered by photoresist, is subsequently applied. The photoresist is thereafter removed, for example by washing. As a final stage, electrically conducting material is removed, for example by etching, to a depth corresponding to the thickness of the first stratum of electrically conducting material. Electrically conducting material is therefore left in place only where the second stratum of electrically conducting material has been applied. An additive method means that the surface on which it is wished to have electrically conducting material is covered with photoresist wherever electrically conducting material is not wanted. At other locations on the surface, electrically conducting material is arranged. The photoresist is thereafter washed away, after which electrically conducting material remains on the surface, only at the desired locations. The invention is not limited to the above-described illustrative embodiment, but can be freely varied within the scope of the following patent claims. As material for the supporting layers, any arbitrary suitable materials with different melting points can in principle be chosen according to the principle described. Examples of other materials for supporting layers are polyamide and polyester. The number of supporting layers, for example, can also be freely varied and is obviously not limited to the number shown above. The structure can be further extended not only on the one side of the first supporting layer; using the method according to the invention, new supporting layers can be arranged on both sides of the first supporting layer. Between the supporting layers bearing conducting patterns, it is conceivable to arrange other materials with a function other than to bear conducting patterns. These layers can in this case have melting points which deviate from the principle defined above according to the invention. When a new layer is attached to the structure by heating, the new layer has to be fixed against the structure during at least part of the heating. This can be done in a large number of ways and is not fundamental to the invention, for which reason it has not been described here. It is also conceivable to arrange conducting patterns on more than one side of the supporting layers.

Claims: PATENT CLAIMS: 1. Process for manufacturing a structure (100) for a circuit board comprising a plurality of supporting layers (10, 30, 60, 90), all of different materials, which supporting layers (10, 30, 60, 90) bear electrically conducting patterns (20, 50, 70, 120), c h a r a c t e r i z e d i n that it comprises the following stages: - as material for the first supporting layer (10) that material is chosen which has the highest melting point of the different materials for supporting layers, - beginning from the first supporting layer (10), new supporting layers (30, 60, 90) are arranged successively, - as material for each new supporting layer (30, 60, 90), a material is chosen with lower melting point than the supporting layer which is closest in the direction of the first supporting layer (10), - each new supporting layer (30, 60, 90) is attached to the structure (100) by exposing the structure (100) to a temperature which exceeds the melting point of the new supporting layer (30, 60, 90), but is lower than the melting point for the supporting layer which is closest in the direction of the first supporting layer (10), - on at least one side of each supporting layer (30, 60, 90) there is arranged a desired electrically conducting pattern (50, 70, 120), - the electrically conducting patterns (20, 50, 70, 120) on at least two supporting layers (10, 30, 60, 90) are connected to each other. 2. Process according to claim 1, c h a r a c t e r i z e d i n that the side of the new supporting layers (30, 60, 90) on which a desired electrically conducting pattern (50, 70, 120) is arranged is the side facing away from a supporting layer with higher melting point. 3. Process according to either of claims 1 or 2, c h a r a c t e r i z e d i n that the temperature difference between the melting points of two adjacent supporting layers lies within the range 1-1, 000°C, preferably within the range 10-300°C, suitably within the range 20-200°C. 4. Process according to any one of claims 1 - 3, c h a r a c t e r i z e d i n that the electrically conducting patterns (20, 50, 70, 120) in at least two supporting layers (10, 30, 60, 90) are connected to each other through the creation of holes (40, 80, 110) in that part of the structure (100) which separates the patterns (20, 50, 70, 120), which holes (40, 80, 110) are filled wholly or partially with electrically conducting material. 5. Process according to any one of claims 1 - 4, c h a r a c t e r i z e d i n that at least one of the desired electrically conducting patterns (20, 50, 70, 120) is created with a subtractive method. 6. Process according to any one of claims 1 - 4, c h a r a c t e r i z e d i n that at least one of the desired electrically conducting patterns (20, 50, 70, 120) is created with a semi-additive method. 7. Process according to any one of claims 1 - 6, c h a r a c t e r i z e d i n that at least one of the desired electrically conducting patterns (20, 50, 70, 120) is created with an additive method. 8. Circuit board (100) comprising a plurality of supporting layers (10, 30, 60, 90), all of different materials, which supporting layers (10, 30, 60, 90) bear electrically conducting patterns (20, 50, 70, 120), c h a r a c t e r i z e d i n that - the circuit board (100) contains a first supporting layer (10), whose material has the highest melting point of all supporting layers (10, 30, 60, 90), - beginning from a supporting layer (30) which is arranged in connection with the first supporting layer (10), all supporting layers (10, 30, 60, 90) are arranged in connection with a supporting layer with higher melting point, - on at least one side of each supporting layer (10, 30, 60, 90) there are disposed electrically conducting patterns (20, 50, 70, 120) of desired shape, - the electrically conducting patterns (20, 50, 70, 120) on at least two of the supporting layers (10, 30, 60, 90) are connected to each other. 9. Circuit board (100) according to claim 8, c h a r a c t e r i z e d i n that, beginning from a • supporting layer (30) which is arranged in connection with the first supporting layer (10), all supporting layers (30, 60, 90) are arranged above a supporting layer having a higher melting point. 10. Circuit board (100) according to either of claims 8 or 9, c h a r a c t e r i z e d i n that the side of the supporting layers (10, 30, 60, 90) on which electrically conducting patterns (20, 50, 70, 120) are arranged is the side facing away from a supporting layer with higher melting point. 11. Circuit board (100) according to any one of claims 8 - 10, c h a r a c t e r i z e d i n that the difference in melting point between two closely situated supporting layers lies within the range 1-1, 000°C, preferably within the range 10-300°C, suitably within the range 20-200°C. 12. Circuit board (100) according to any one of claims 8 - 11, c h a r a c t e r i z e d i n that the electrically conducting patterns (20, 50, 70, 120) on the supporting layers (10, 30, 60, 90) are connected to one another by holes (40, 80, 110) in that part of the structure (100) which separates the patterns (20, 50, 70, 120), which holes (40, 80, 110) are filled wholly or partially with electrically conducting material.

Inventors: BERGSTEDT LEIF, BERGSTEDT, LEIF
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON
Application year: 1998
Main classification: H05K3
IPC codes: H05K1, H05K3