Selective partitioning of via structures in printed circuit boards

Abstract

The embodiments herein relate to a method for selective partitioning of a via in a printed circuit board as to produce an electrically isolating portion between two electrically conducting portions in said via. The method involves the step of prior to drilling the hole for the via, laminating plating resist layers to the printed circuit board at a distance from each other corresponding to a desired length of the electrically isolated portion of the via. After drilling, copper is added to selected portions of the interior of the via in two different processing steps followed by a step of removing undesired copper as to produce the electrically isolating portion.

Claims

1. A method of partitioning a via in a multilayer printed circuit board as to produce an electrically isolating portion between two electrically conducting portions in said via, the method comprising of: placing at least one island of a first plating resist layer on a first layered structure comprising a first conductive layer and a first dielectric layer, and placing at least one island of a second plating resist layer on a second layered structure comprising a second conductive layer and a second dielectric layer; laminating the first and second layered structures with a third intermediate layered structure comprising at least one third dielectric layer adapted so that the islands of the first and second plating resist layers become embedded in the at least one third dielectric layer; drilling a first hole in the printed circuit board so that the first hole passes through the islands of the first and the second plating resist layer; placing said circuit board in a copper seed catalyzing bath so that a first copper layer is placed on an interior of the first hole except for portions with the plating resist layer; placing the printed circuit board in an electrolytic copper plating bath where the first copper layer placed on the at least one third dielectric layer portion of the first hole is electrically isolated from the first and second conductive layers so that additional copper is placed on the interior of the first hole except for portions of the first and second plating resist layers and except for portion of the at least one third dielectric layer; removing the first copper layer placed on the at least one third dielectric layer portion of the first hole after the electrolytic copper plating bath.

2. The method of claim 1, wherein the third processing step of removing copper from the at least one third dielectric layer portion of the first hole is done by microetching.

3. The method of claim 1, wherein the at least one third dielectric layer is made of an impregnated fiber weave adapted to embed the islands of the first and second plating resist layers.

4. The method of claim 1, wherein the first hole is partly penetrating the printed circuit board.

5. The method of claim 1, wherein the first hole is a through hole through the printed circuit board.

6. The method of claim 5, wherein the step of drilling the first hole comprises the additional step of drilling with a larger drill from the opposite side of the printed circuit board as to produce two portions of the via with different diameters and wherein the two via portions meet each other in a position between the first and second plating resist layers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A and 1B are block diagrams illustrating partitioned via structures.

(2) FIGS. 2A, 2B and 3 are block diagrams illustrating a PCB and the method steps for producing an improved via structure within said PCB.

(3) FIGS. 4 and 5 are block diagrams illustrating other embodiments of improved via structures.

(4) FIG. 6 is a flow chart illustrating the method steps for producing the improved via structures.

DETAILED DESCRIPTION

(5) An embodiment of a method for producing an improved via structure is illustrated in FIGS. 2A, 2B, 3 and 6. FIG. 2A illustrates four steps of the method applied on one and the same PCB 200. The PCB 200 has a plurality of conductive layers (normally copper layers) 201-208 and sandwiched between the copper layers 201-208 are dielectric layers as prepreg layers 211-214 and laminate layers 221-223 respectively. Prepreg which is an abbreviation for pre impregnated is a fiber weave impregnated with a resin bonding agent.

(6) Before lamination, islands of at least two plating resist layers 231-234 are added at predetermined places on the copper layers 203-206 on the laminates 221-223 in step 1 (as shown in FIG. 6).

(7) The plating resist layer can also be added directly on the laminate (not shown in FIG. 2A). In the lamination process, the islands of the plating resist layers are embedded in the prepreg layers 212 and 213 as seen in step 2 (as shown in FIG. 6). In step 3 (as shown in FIG. 6) a through hole 240 is drilled in the PCB 200 through the copper layers 201-208 and the plating resist layers 231-234. In step 4 (as shown in FIG. 6) and before plating, a thin layer 251 of chemical copper, is added to the inside of through hole 240 by placing the PCB 200 is a seed catalyzing bath. This thin layer 251 adheres to all inner parts of the through hole 240 except for the plating resist layers 231-234 as seen in positions 252 and 253 in FIG. 2B. In step 5 (as shown in FIGS. 3 and 6) the PCB is placed in an electrolytic copper plating bath. As the portion 254 of the thin layer 251 of chemical copper located between the two plating resist layers 231 and 232 and the portion 255 located between the two plating resist layers 233 and 234 are electrically isolated from the other conductive layers no copper is plated on these portions during the electrolytic plating process. After copper plating of the through hole 240, the thin copper layer that remains on portions 254 and 255 is removed by using micro etch (or an equivalent post processing operation). The resulting via structure is seen in step 6 (as shown in FIG. 6) with three conductive portions 301-303 and two non-conductive portions 254 and 255 where the non-conductive portions 254 and 255 have a significant larger isolation distance than when using one thin plating resist layer only.

(8) In the embodiment illustrated in FIGS. 2A, 2B and 3 the plating resist layers 231 and 232 are embedded in the same prepreg layer 212. The improved method is not limited to this configuration. FIG. 4 illustrates an embodiment of a PCB 400 having a plurality of copper layers 401-407 and dielectric layers 411-413 and 421-423. In the PCB 400 the plating resist layers 431 and 432 are embedded in different prepreg layers 412 and 413 apart from each other and where the non-conductive portion 451 becomes larger.

(9) The improved method is not limited to producing through hole vias only but can also be applied to blind vias or vias with different diameters within the same via structure. An example of the latter is illustrated in FIG. 5. In FIG. 5, a via structure in a PCB 500 is divided into two via portions 530, 535 with different diameters. The narrower via portion 530 is electrically isolated from the broader via portion 535 by using two plating resist layers 541,542 in the same manner as described above. The broader via portion 535 is produced by an additional step of back drilling with a larger drill before the seed catalyzing bath in step 4. The resulting plated portion 546 of the broader via portion 535 can for example be used for mounting components to the PCB 500. As the plated portion 545 of the narrower via portion 530 is isolated from the plated portion 546 of the broader via portion 535 it can be used for conducting current between other conductive layers in the PCB 500.

(10) The embodiments can be implemented in a apparatus that further includes at least one microprocessor, a computer-readable medium including computer-readable instructions, when executed by the at least one microprocessor, are configured to control fabrication equipment to perform the methods described herein. Embodiments can also be implemented in digital electronic circuitry, in computer hardware, firmware, software, or in combinations thereof. Storage device suitable for embodying the computer program instructions include signals capable of programming a data processing system, all forms of non-volatile memory including, but not limited to: semiconductor memory devices such as EPROM, EEPROM, and flash devices; magnetic disks (fixed, floppy, and removable); other magnetic media such as tape; optical media such as CD-ROM, DVD-ROM, and Blu-ray disks; and magneto-optic devices. Any of the foregoing may be supplemented by, or incorporated in, specially-designed application-specific integrated circuits (ASIC) or suitably programmed field programmable gate arrays (FPGAs).