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In a method for manufacturing multilayer PCBs having n circuit layers, a double-sided flexible substrate strip is provided. The strip comprises a number of PCB units, each comprising m segments, wherein m=n/2 if n represents an even integer, and m=(n+1)/2 if n represents an odd integer. Each segment

In a method for manufacturing multilayer PCBs having n circuit layers, a double-sided flexible substrate strip is provided. The strip comprises a number of PCB units, each comprising m segments, wherein m=n/2 if n represents an even integer, and m=(n+1)/2 if n represents an odd integer. Each segment includes two foil portions. In a reel to reel process, the strip is treated to form n−2 foil portions of each PCB unit into traces, further remove one foil portion if n represents an odd integer. The other two foil portions are left untreated. Then the strip is cut to separate the PCB units from each other. The PCB unit is folded in such a manner that the traces are arranged between the other two foil portions. The folded PCB unit is laminated to form a multilayer substrate and traces are formed in the two foil portions.

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1. A method for manufacturing multilayer flexible PCBs, each of the flexible PCBs comprising n circuit layers, n representing an even integer greater than 2, the method comprising: (1) providing a flexible substrate strip wound around two spools, the flexible substrate strip comprising two copper fo

1. A method for manufacturing multilayer flexible PCBs, each of the flexible PCBs comprising n circuit layers, n representing an even integer greater than 2, the method comprising: (1) providing a flexible substrate strip wound around two spools, the flexible substrate strip comprising two copper foils and an insulating layer sandwiched between the two copper foils, the flexible substrate strip including a plurality of PCB units arranged along a length of the flexible substrate strip, each PCB unit comprising n/2 segments connected along a length of the PCB unit, each segment including two foil portions;(2) forming n−2 foil portions of each PCB unit into traces with remaining two foil portions of each PCB unit left untreated, in a reel to reel process;(3) cutting the flexible substrate strip into pieces along border lines between the PCB units to separate the PCB units from each other;(4) folding each PCB unit along border lines between the n/2 segments in such a manner that the traces formed from the n−2 foil portions are folded and sandwiched between the remaining untreated two foil portions;(5) laminating each folded PCB unit to form a plurality of laminated multilayer substrates; and(6) forming traces in the remaining untreated two foil portions. 2. The method of claim 1, wherein the step (2) comprises: unwinding the flexible substrate strip from one spool;treating the unwound portion between the two spools such that the n−2 foil portions of each PCB unit are formed into the traces; andfeeding and winding the treated unwound portion onto the other spool. 3. The method of claim 2, wherein the copper foils are etched in the step of treating the unwound portion. 4. The method of claim 1, wherein the n/2 segments have a same size. 5. The method of claim 4, wherein the border lines between the n/2 segments are substantially parallel to each other and substantially perpendicular to the length of the flexible substrate strip, and the n segments are folded along the border lines therebetween. 6. The method of claim 1, wherein the border lines between the PCB units are substantially parallel to each other and substantially perpendicular to a length of the flexible substrate strip. 7. The method of claim 1, wherein each of the segments are folded in a manner such that the segments are substantially parallel with each other. 8. The method of claim 1, wherein (n−2)/2 adhesive sheets are provided and each adhesive sheet is sandwiched between two adjacent segments in step (5). 9. The method of claim 1, wherein after step (6), two coverlays are formed over the traces of the remaining two foil portions, respectively. 10. A method for manufacturing multilayer flexible PCBs, each of the flexible PCBs comprising n circuit layers, n representing an odd integer greater than 2, the method comprising: (1) providing a flexible substrate strip wound around two spools, the flexible substrate strip comprising two copper foils and an insulating layer sandwiched between the two copper foils, the flexible substrate strip including a plurality of PCB units arranged along a length of the flexible substrate strip, each PCB unit comprising (n+1)/2 segments connected along a length of the PCB unit, each segment including two foil portions;(2) treating each PCB unit to remove one foil portion and to form n−2 foil portions into traces with remaining two foil portions of each PCB unit left untreated, in a reel to reel process;(3) cutting the flexible substrate strip into pieces along border lines between the PCB units to separate the PCB units from each other;(4) folding each PCB unit along border lines between the (n+1)/2 segments in such a manner that the traces formed from the n−2 foil portions are folded and sandwiched between the remaining untreated two foil portions;(5) laminating each folded PCB unit to form a plurality of laminated multilayer substrates; and(6) forming traces in the remaining untreated two foil portions. 11. The method of claim 10, wherein the step (2) comprises: unwinding the flexible substrate strip from one spool;treating the unwound portion between the two spools such that the n−2 foil portions of each PCB unit are formed into the traces; andfeeding and winding the treated unwound portion onto the other spool. 12. The method of claim 11, wherein the copper foils are etched in the step of treating the unwound portion. 13. The method of claim 10, wherein the (n+1)/2 segments have a same size. 14. The method of claim 13, wherein the border lines between the (n+1)/2 segments are substantially parallel to each other and substantially perpendicular to the length of the flexible substrate strip, and the n segments are folded along the border lines therebetween. 15. The method of claim 10, wherein the border lines between the PCB units are substantially parallel to each other and substantially perpendicular to a length of the flexible substrate strip. 16. The method of claim 10, wherein each of the segments are folded in a manner such that the segments are substantially parallel with each other. 17. The method of claim 10, wherein (n−1)/2 adhesive sheets are provided and each adhesive sheet is sandwiched between two adjacent segments in step (5). 18. The method of claim 10, wherein after step (6), two coverlays are formed over the traces of the remaining two segments, respectively.