Printed circuit boards, backplanes, midplanes, printed wiring boards, flex circuits, rigid flex-circuits, multi-chip modules (MCM), interposers and the like are herein referred to collectively as “PCBs”.
A via structure typically provides a conductive path between conductive layers in the z-axis direction (orthogonal to the x-y plane of a PCB). Via holes are formed by a variety of techniques including but not limited to laser drilling, mechanical drilling, and techniques based on photo definition. Via holes are subsequently partially or wholly filled or coated with a conductive material, usually metal. Such via structures may be blind, buried, through-hole and may or may not include pads on the conductive layers, as is well known to those skilled in the art of PCB design.
Specifically, a blind via hole is an interconnect structure that provides a conductive path between two or more conductive layers in a PCB. One of the two or more conductive layers is an external conductive layer of the PCB. The other conductive layers of the two or more conductive layers are internal layers within the PCB. In other words, a blind via does not extend through all the layers of the PCB.
However, one of the disadvantages of the above approach is that the depth of the via hole that can be formed in the PCB is limited by the aspect ratio of the blind via hole. The aspect ratio of the blind via hole is the ratio of the depth of the blind via hole to the diameter of the blind via hole before any conductive material is deposited in the via hole. The limitation of the aspect raito is due the current approaches of depositing conductive material in the blind via hole in order to make the blind via a conductive interconnect structure between conductive layers of the PCB. For aspect ratios that are greater than 2:1 for small blind vias, currebt deposition approaches are unable to guarantee that a functionally adequate conductive layer will be deposited on the walls of the via hole. This limitation is due to an increased incidence of chemical contamination as the aspect ratio of the blind via hole increases. Further, under conventional plating methods, there is an increased difficulty in gaining adequate thickness in the deposited conductive layer in the via hole due to hole shielding.
Another disadvantage of current approaches of depositing conductive material in blind via holes is that the blind via hole must be accurately drilled down to the desired conductive layer in the PCB to which the blind via is required to connect. In order to ensure that that the blind via hole terminates at the desired conductive layer that the via hole is required to contact, the thickness of dielectric layers above and below the desired conductive layer must be at least 5 mils.
Thus, in view of the foregoing, a plated through hole of a blind via with a high aspect ratio that is greater than 8:1 and that allows for using dielectric layers that have thicknesses less than 5 mils is needed.
In certain exemplary embodiments, a high aspect ratio plated through hole (PTH) or blind via hole in a PCB stackup is made by building a sub-composite structure that includes an external conductive layer and an inner conductive layer in the PCB stackup. The inner conductive layer of the PCB is the conductive layer to which the PTH or blind via hole is required to connect. A via hole is drilled through the sub-composite structure such that the via hole is open at both ends and extending from the external conductive layer through the inner conductive layer.
The walls of the via hole are plated with a conductive material. The plated via hole is then filled with a conductive medium. The sub-composite structure proceeds through the remainder of the processing that is necessary to manufacture the printed circuit board up to the completion of the solder mask step. The conductive medium of the via hole is then drilled out to achieve a hole size that is of the desired diameter as required by the printed circuit board design.
These and other embodiments and other features disclosed herein will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.
At block 502, a sub-composite structure comprising several layers, such as sub-composite structure 209 of
At block 504, a via hole of a desired aspect ratio is drilled through the sub-composite structure. For example, a via hole with an aspect ratio greater than 8:1 is drilled through the sub-composite 209 by indexing from the surface of the sub-composite structure and drilling down to the conductive layer 112 of
At block 506, the drilled holes are cleaned and desmeared. For example, a chemical process by which the coating of resin that is produced by the heat of drilling is removed from the drilled hole walls and edges of the drilled hole. Additionally, metal burrs and other debris caused by the drilling can be removed and cleaned from the drilled hole.
At block 508, the drilled hole is catalyzed in preparation for deposition of an activation layer. As a non-limiting example, a thin coating of electroless copper is chemically deposited on the surface of the sub-composite structure and on the walls of the drilled hole. Such an activation layer creates a metallic base for subsequent electroplating operations.
At block 510, an image of a desired circuit is deposited on the inner conductive layer, such as conductive layer 212 of
At block 512, a layer of conductive material is deposited on the exposed areas of the imaged inner conductive layer, the surface of the external conductive layer and walls of the drilled hole. For example, additional copper is electrically plated through an electroplating process onto the exposed electroless copper surfaces of the sub-composite structure including the walls of the drilled hole.
At block 514, a protective metal is deposited on the exposed electroplated areas of the sub-composite structure. For example, solder or tin-lead can be plated onto the copper plated surfaces.
At block 516, the resist coat described at block 510 is removed from the patterned inner layer of the sub-composite structure. For example, the plating resist can be chemically removed from the patterned inner layer.
At block 518, any unwanted base conductive material is etched away from the patterned inner layer at areas that are not protected by the solder or tin-lead protective layer.
At block 520, the protective metal layer (solder or tin-lead) is removed. For example, the solder or tin-lead is chemically stripped from all the surfaces.
At block 536, the plated via hole, such as hole 308 is filled with a conductive medium, such as conductive medium 318 of
At block 538, one or more additional cores that make up the PCB stackup, such as core 316 of
At block 540, normal PCB manufacturing steps are performed until after the process of depositing a layer of soldermask, such as layer 420 of
At block 542, a hole, such as hole 426 of
As an alternate process, according to certain embodiments, after the process of block 508, a conductive layer is deposited on all exposed surfaces of the sub-composite structure at block 522. Next at block 524, an image of a desired circuit is deposited on the inner conductive layer, such as conductive layer 212 of
The process as described with reference to
In the foregoing specification, embodiments of the invention have been described with reference to numerous specific details that may vary from implementation to implementation. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.
This application claims the benefit of U.S. Ser. No. 60/654,591 entitled, “Systems and Methods For A Blind Via In A Printed Circuit Board” by inventors, Suzanne Knight and Douglas Thomas, filed Feb. 17, 2005, incorporated herein by reference in its entirety.
Number | Date | Country | |
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60654591 | Feb 2005 | US |