Via in a printed circuit board

The miniaturization of the electronics industry has put pressure in the printed circuit board (PCB) industry to create features of fine circuitry. The print and etch processes often used to create PCB's and PCB cores are not precise enough for fine features down to one mil lines and spaces and below. Instead, additive processes using catalytic laminates allows copper (Cu) plating to be performed selectively in photolithographically defined channels and vias using plating resist.

The structure of a multilayer board can be created in many different ways. One way is that no-catalytic cores are made by print and etch to create the circuitry on both sides. The cores are stuck up and laminated followed by drilling and circuitization of the outer layers and the holes.

FIG. 1 shows a simplified diagram illustrated vias a printed circuit board in accordance with an implementation.

FIG. 2 , FIG. 3 , FIG. 4 , FIG. 5 , FIG. 6 and FIG. 7 illustrate steps in a process where vias are formed in a printed circuit board in accordance with an implementation.

FIG. 8 is a flowchart summarizing a process where vias are formed in a printed circuit board in accordance with an implementation.

FIG. 9 is a flowchart summarizing a process where vias are formed in a printed circuit board in accordance with an alternative implementation.

In the fabrication of printed circuit boards (PCBs), where routing of metal is difficult because of high density requirements and where outer layers can be formed at the end of the fabrication processes, then a catalytic adhesive coating can be applied on both sides of the PCB core. The coating can be made using the same or similar material used to make the laminate of the PCB core. This allows for good adhesion of metal traces. Fabrication of the outer layers of the PCB can be done using lasers to create blind vias and a photoimageable mask to create traces.

FIG. 1 illustrates vias in a portion of a printed circuit board (PCB) 9 . Dielectric laminate material 10 functions as a PCB substrate (two-sided laminate core) and is composed of, for example, glass or non-glass reinforcement and a resin such as, for example, epoxy, polyimide, Teflon or any other type of resin suitable for inclusion in a PCB substrate. Dielectric laminate material 10 is, for example, approximately 0.028″ thick and is sandwiched between a top adhesive layer 11 and a bottom adhesive layer 12 . Top adhesive layer 11 and bottom adhesive layer 12 are, for example, each approximately 25 um (1 mil) thick. Top adhesive layer 11 and bottom adhesive layer 12 are composed of, for example, a dielectric adhesive such as an epoxy, polyimide, cyanate ester or another suitable dielectric adhesive. The dielectric adhesive includes, for example, both non-catalytic and catalytic filler particles. The catalytic filler particles are composed of, for example, a metal such as palladium (Pd), iron (Fe) and/or other catalytic particles used for copper plating where electroless copper (Cu) is reduced from its Cu ++ to Cu. For example, the catalytic particles can be made of inorganic filler with metal coated over the inorganic filler. For example, the inorganic filler can be silicon dioxide, kaolin, or some other inorganic filler with suitable properties for the particular application. Vias 13 , for example, composed of copper, extend through top adhesive layer 11 , dielectric laminate material 10 and bottom adhesive layer 12 allowing electrical connection between circuitry on different faces of PCB 9 .

A process for forming the vias is illustrated in FIGS. 2 through 7 . Dielectric laminate material 10 is shown in FIG. 2 .

As illustrated by FIG. 3 , for each via, a hole 14 is formed in dielectric laminate material 10 . Hole 14 is, for example, approximately 8 mil in diameter. For example, hole 14 is formed by drilling or another know method of forming a hole. The diameter of via holes will vary depending on application, available manufacturing processes and so on.

Both sides of dielectric laminate material 10 are coated with a catalytic adhesive to form top adhesive layer 11 and bottom adhesive layer 12 . Hole 14 is also filled with catalytic adhesive material 25 . For example, the catalytic adhesive material is composed of a dielectric adhesive such as an epoxy, polyimide, cyanate ester or another suitable dielectric adhesive. The reology (viscosity) of the adhesive is adjusted and is based on the type of method used to coat and fill the holes at the same time. The dielectric material contains catalytic particles that, for example, have a particle size in the range of 2 to 12 micrometer (um). Alternatively, other particle sizes can be used. For example, smaller particle sizes are better as bigger particle size may affect uniformity and roughness of copper plating placed on top adhesive layer 11 and bottom adhesive layer 12 . For example, by weight the particles are between six and fifteen percent of the total weight of the catalytic adhesive material 25 . This percentage is only an example as for various applications the weight of the particles may be some other percentage of the total weight of the catalytic adhesive material 25 . The catalytic adhesive material is deposited using, for example, screen printing, stenciling, or squeegee coating using a coating machine such as those available from the ITC, Intercircuit, N.A., or another coating device able to perform or one or more of the known processes and techniques in the industry used to deposit material on a PCB substrate.

FIG. 5 illustrates a hole 26 formed through catalytic adhesive material 25 . For example, hole 26 is formed by drilling or another know method of forming a hole. For example, hole 26 is 6 mil in diameter, leaving a layer 15 of catalytic adhesive material around the diameter of hole 26 . The diameter of hole 26 will vary depending on application, available manufacturing processes and so on.

A layer of resist is applied over top adhesive layer 11 and bottom adhesive layer 12 . The layer of resist is exposed to produce a resist pattern 17 over top adhesive layer 11 and a resist pattern 18 over bottom adhesive layer 12 . The result is shown in FIG. 6 .

Full electroless copper plating is deposited leaving a copper patterned layer 16 over top adhesive layer 11 and a copper patterned layer 19 over bottom adhesive layer 12 where there is no resist. Copper regions 20 are also formed within hole 26 . For example, the thickness of copper patterned layer 19 and copper regions 20 is between 0.5-1.4 mils. The resist is stripped away, as shown in FIG. 7 . The catalytic adhesive material around the diameter of hole 26 assures good adhesion of copper regions 20 within hole 26 .

Copper patterned layer 16 and a copper patterned layer 19 function as traces for the PCB. After the traces are formed, exposed portions of top adhesive layer 11 and bottom adhesive layer 12 (i.e., those portions of top adhesive layer 11 and bottom adhesive layer 12 not covered by the traces) can (optionally) be removed, for example, by using plasma etch, laser ablation or some other process suitable to removing the adhesive layers without damaging copper plating.

One advantage of the above-described implementation is that there is no copper between the traces to etch away. For example, if instead of the described method above, a copper clad laminate and print and etch techniques are used to form the traces, this become problematic when trace width and space in the PCB is below 1 mil as copper particles left imbedded in the laminate surface can create near shorts. In the above-described implementation, any metal particles can be removed simply merely by removing the portions of top adhesive layer 11 and bottom adhesive layer 12 not covered by copper traces.

In addition, top adhesive layer 11 and bottom adhesive layer 12 aid in forming straight walls for copper patterned layer 16 and copper patterned layer 19 . This is because use of adhesive layers allows copper patterned layer 16 and copper patterned layer 19 to be defined by resist pattern 17 and resist pattern 18 . Using a resist pattern to form copper plating allows for better defined traces (i.e. traces with straighter wall formation) which helps in better trace electrical characteristics such as impedance and line signal loss. When copper traces are formed, for example, using a subtractive print and etch process, the cross section of the traces looks like a trapezoid rather than a square or rectangle as they appear when formed using resist.

Once the two-sided laminate core is circuitized, multilayer constructions can be made using known techniques such as applying additional catalytic adhesive over the circuitized layers and forming vias by laser or plasma to build additional layer(s).

FIG. 8 summarizes the above-described implementation. In a block 41 , a hole is formed in dielectric laminate material. For example, the hole is formed by drilling or another know method of forming a hole. In a block 42 , both faces of the dielectric laminate material are coated with a catalytic adhesive. This coating includes filling the hole in the catalytic adhesive.

In a block 43 , a second hole is formed through the catalytic adhesive where the catalytic adhesive fills the first hole. For example, the second hole is formed by drilling or another know method of forming a hole. The second hole has a smaller diameter than the first hole so that a layer of catalytic adhesive remains at a diameter of the second hole.

In a block 44 , a patterned metal layer is formed over the catalytic adhesive material on both faces of the dielectric laminate material. This includes placing the metal layer over the layer of catalytic adhesive that remains at the diameter of the second hole.

In an optional block 45 , exposed portions of the catalytic adhesive material that are not covered by the patterned metal layer are removed.

FIG. 9 summarizes an alternative implementation. In a block 51 , a hole is formed in dielectric laminate material. In a block 52 , both faces of the dielectric laminate material are coated with a catalytic adhesive. This coating does not fill the hole. For example, electrostatic spraying, spraying or some other coating process is performed, for example, using standard coating equipment in the industry, to coat the dielectric laminate material with the catalytic adhesive so that the walls of the hole is coated at the same time both faces of the dielectric laminate material are coated.

In a block 53 , a patterned metal layer is formed over the catalytic adhesive material on both faces of the dielectric laminate material. This includes placing the metal layer over the layer of catalytic adhesive that is coated on the hole.

In an optional block 54 , exposed portions of the catalytic adhesive material that are not covered by the patterned metal layer are removed.

The foregoing discussion discloses and describes merely exemplary methods and embodiments. As will be understood by those familiar with the art, the disclosed subject matter may be embodied in other specific forms without departing from the spirit or characteristics thereof. For example, after block 43 or block 52 a thin layer of copper (e.g., 1-2 um) can be plated in an electroless copper bath followed by block 44 or block 53 , respectively. After the features are formed the thin copper layer on the substrate surface can be removed by chemical means. In addition, one way stack vias can be performed by filling a plated via with a dielectric material and then going through standard electroless copper plating followed by resist that exposes only the plugged via area and plating up more copper. Many of the steps can be eliminated by adding catalytic powder to the dielectric material filling the plated via with and then plating up the copper over the via using a fully additive electroless copper bath. Accordingly, the present disclosure is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.