Patent EA-003263-B1 (EA)
Title: Method for manufacturing a multilayer printed circuit board and composite foil for use therein
Inventor: GALES RAYMOND
IPC: H05K3/38
Filed: 2000-03-23
Abstract: 1. A method for manufacturing a multilayer printed circuit board comprising the following steps: a) providing a core board (20); b) providing a composite foil (10) including a functional copper foil (16) of less than 10 mum mounted on a carrier foil (12), said functional copper foil (16) having a front side facing said carrier foil (12) and a back side coated with a non-reinforced thermosetting resin; c) laminating said composite foil (10) with the resin coated back side on one side of said core board (20); d) removing said carrier foil (12) from said functional copper foil (16), in order to uncover said front side of said functional copper foil (16); e) using a CO2 laser source to drill holes from said uncovered front side of said functional copper foil (16) through said functional copper foil (16) and said resin in order to form microvias. 2. The method as claimed in claim 1, characterised in that said functional copper foil (16) is electrodeposited and has a thickness of about 5 mum. 3. The method as claimed in claim 1 or 2, characterised in that said front side of said functional copper foil (16) has received a surface preparation favouring the absorption of CO2 laser light. 4. The method as claimed in claim 3, characterised in that said surface preparation provides a particular surface profile and roughness favouring the absorption of CO2 laser light. 5. The method as claimed in claim 3 or 4, characterised in that said surface preparation provides a particular surface colour favouring the absorption of CO2 laser light. 6. The method as claimed in claim 3, 4 or 5, characterised in that said surface preparation takes place during manufacturing of said composite foil (10). 7. The method as claimed in any one of claims 1 to 6, characterised in that said front side of said functional copper foil (16) is covered with a black oxide conversion coating prior to laser drilling. 8. The method as claimed in any one of the preceding claims, characterised in that said composite foil (10) further includes a release layer (14) intermediate said carrier foil (12) and said functional copper foil (16); and in that said carrier foil (12) removal of step d) consists in mechanically peeling off the carrier foil (12) and the release layer (14) simultaneously. 9. The method as claimed in any one of claims 1 to 6, characterised in that said composite foil further includes a release layer intermediate said carrier foil and said functional copper foil, said release layer having a particular surface colour favouring the absorption of CO2 laser light; and in that when removing said carrier foil at step d), said release layer remains on said front side of said functional copper foil. 10.The method as claimed in any one of the preceding claims, characterised in that said resin comprises a layer of B-staged resin. 11.The method as claimed in any one of the preceding claims, characterised in that said resin layer comprises a C-staged resin layer applied on the back side of the functional copper foil (16), and a B-staged resin layer applied on said C-staged resin layer. 12.The method as claimed in any one of the preceding claims, characterised in that after step e) copper is deposited by electroless plating over the functional copper foil (16). 13.The method as claimed in claim 12, characterised by a subsequent galvanic reinforcement step, wherein copper is electrodeposited over the functional copper foil (16) in order to increase its thickness. 14.A composite foil for use in a method for manufacturing a multilayer printed circuit board, comprising: a carrier foil (12); a release layer (14) on one side of said carrier foil (12); a functional copper foil (16) deposited on said release layer (14) and having a thickness of less than 10 mum, said functional copper foil (16) having a front side facing said release layer (14) and a back side; characterised in that a non-reinforced thermosetting resin coating (18) is provided on said back side of said functional copper foil (16). 15.The composite foil as claimed in claim 14, characterised in that said front side of said functional copper foil has received a surface preparation favouring the absorption of CO2 laser light. 16.The composite foil as claimed in claim 15, characterised in that said front side has a particular surface profile and roughness favouring the absorption of CO2 laser light. 17.The composite foil as claimed in claim 15 or 16, characterised in that said front side has a particular surface colour favouring the absorption of CO2 laser light. 18.The composite foil as claimed in claim 14, 15 or 16, characterised in that said functional copper foil (16) is electrodeposited on said release layer (14) and has a thickness of about 5 mum. 19.The composite foil as claimed in claim 17, characterised in that said surface colour is obtained by forming a thin layer of dark coloured electrically conductive material between said release layer (14) and said functional copper foil (16). 20.The composite foil as claimed in any one of claims 14 to 19, characterised in that said carrier foil (12) has a thickness between 18 and 150 mum, said release layer (14) has a thickness of less than 1 mum, and said resin coating (18) has a thickness between 5 and 150 mum. 21.The composite foil as claimed in any one of claims 14 to 20, characterised in that said release layer (14) is a chromium based layer. 22.The composite foil as claimed in claim 17, characterised in that said release layer remains attached to said front side of said functional copper foil when said carrier foil is removed; and in that said release layer has a particular surface colour favouring the absorption of CO2 laser light. 23.The composite foil as claimed in claim 22, characterised in that said release layer is a thin layer of dark coloured electrically conductive material. 24.The composite foil as claimed in claim 19 or 23, characterised in that said thin layer of dark coloured electrically conductive material is a layer comprising carbon black and/or graphite. 25.The composite foil as claimed in claim 19 or 23, characterised in that said thin layer of dark coloured electrically conductive material is a layer comprising a dark coloured electrically conductive polymer. 26.The composite foil as claimed in claims 14 to 25, characterised by a bonding layer on said back side of said functional copper foil (16). 27.The composite foil as claimed in claims 14 to 26, characterised by a passivation layer on said back side of said functional copper foil (16), preferably intermediate said bonding layer and said resin coating (18).