Process for providing bond enhancement and an etch resist in the fabrication of printed circuit boards

Applicants claim priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 60/130,720 filed Apr. 22, 1999, the disclosure of which is incorporated by reference herein.

N/A

In the fabrication of printed circuit boards, a thin coating of tin, typically 0.1 microns in thickness, is applied to a copper or copper alloy substrate. A silane coupling agent is applied over the tin coating. This process is primarily used as a bonding enhancement to improve the subsequent adhesion of an epoxy prepreg to the metal of a multilayer printed circuit board. This tin coating produces only marginal improvement as a bonding intermediate for the adhesion of copper to no-flow and low-flow epoxy prepregs. Also, the tin coating is typically too thin and porous to be used as an etch resist for the manufacture of printed circuit boards.

Examples of tin coating processes for circuit boards may be found in U.S. Pat. Nos. 5,073,456, 5,196,053, 5,211,831, and 5,217,751.

The present invention provides the use of multiple tin coatings that have superior chemical resistance and enhance adhesion in fabricating printed circuit boards. A first coating of tin provides a white tin layer that serves as an etch resist at the primary or secondary etching operation during the fabrication of the printed circuit traces. A second coating of tin provides a layer that serves as a bonding agent or bonding enhancement layer that allows the lamination of no-flow and low-flow prepreg materials to a copper surface with resulting adhesion between the polymer and the metal exceeding four pounds per inch.

The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a flow chart of steps in the process of the present invention;

FIG. 2 is a scanning electron image of a circuit board after application of a first immersion tin coating (heavy tin); and

FIG. 3 is a scanning electron image of a circuit board after application of a second immersion tin coating (thin tin).

The process begins with a sheet or film of copper, typically ranging from 0.5 mils to 3.0 mils in thickness. The sheet may also be a lamination of a dielectric of, for example, an epoxy material or KAPTON® sandwiched between two layers of copper. Referring to FIG. 1, the sheet is sheared and cleaned to prepare for the etching of a circuit pattern thereon. Then a dry film is laminated to the sheet, and the sheet is exposed, developed, and etched to produce the circuit pattern in the copper. The etch resist is stripped, and the sheet is cleaned and inspected. Any necessary holes are punched in the sheet.

At this point, the sheet is immersed in an immersion tin solution, preferably by a conveyorized liquid flood regime. The sheet could alternatively be dipped in a bath or reservoir of the immersion tin solution. Other coating techniques could be used, as would be known in the art. The immersion tin solution suitably comprises 10 to 30 g/l of total tin metal (stannous and/or stannic ions) and a complexing agent, such as thiourea in a concentration of 20 to 80 g/l, in an acid medium, such as 20 to 80 g/l of methane sulfonic acid. In the presence of the acid medium, the copper is displaced by tin ions. The displaced copper is complexed with the complexing agent for subsequent removal, as by filtering, from the process. Other compounds, such as various reducing agents, are typically included in the immersion tin solution, as would be known in the art. Preferably, the solution is stored in a tank or reservoir at a temperature range of 120 to 175° F. and is circulated through the process to the sheet. The tank may be replenished with the tin metal, the completing agent, or other compounds as needed.

The sheet is immersed in the tin for a time sufficient to deposit a layer of tin that is able to serve as an etch resist in subsequent processing. Typically, 1 minute is a sufficient time. The thickness of the first tin layer may range from 0.1 microns to 1.5 microns. This tin layer is referred to as a heavy tin or high build immersion tin layer.

The sheet is conveyed from the first immersion tin solution and rinsed with water. Typically, deionized water is used in this rinse step.

Next, the sheet is immersed in a second immersion tin solution. The composition of the second immersion tin solution suitably comprises 6 to 15 g/l of a stannous ion, 10 to 25 g/l of total tin metal, 40 to 80 g/l of a complexing agent such as thiourea, and 0 to 3.0 g/l of copper. The acid medium may be, for example, 8 to 12% by weight of sulfuric acid. The temperature may range from 80 to 120° F. in the second immersion step, a thin tin or low build immersion tin layer is deposited over the first heavy tin layer. In this step, there appears to be a selective chemical mechanism that modifies the initial heavy tin deposit through dissolution of discrete areas, creating cavities and pinholes in the heavy tin deposit. The resulting surface has a rough texture with multiple craters or pores. This rough surface provides a greater surface area, which provides more binding sites for the epoxies and organic polymers of subsequent processing steps.

The sheet is held in the second immersion tin solution for a time sufficient to create a sufficiently rough texture, but not so long that the underlying heavy tin deposit is etched through to the copper layer beneath. The heavy tin deposit must remain sufficiently thick to serve as an etch resist in subsequent processing. A time of 30 seconds has been found to be sufficient for the second immersion tin step. The thickness of the resulting second tin coating at selected sites is thin, approximately 0.05 microns to 0.1 microns.

After removal of the sheet from the second immersion tin solution, a coupling agent, such as a silane coating, is applied to prevent oxidation of the tin and serve as a coupling agent to enhance bonding to the subsequent epoxy or other organic polymer layers, as is known in the art. The sheet then proceeds on to further processing as known in the art.

FIG. 2 is a scanning electron image of a copper sheet that has been immersed in a first high build immersion tin solution as described above. FIG. 3 is a scanning electron image of a copper sheet that has been immersed in a second low build immersion tin solution as described above. FIG. 3 shows the rough surface with greater surface area that results after the second immersion tin solution and the selective dissolution of discrete areas of the heavy tin deposit. The resulting cavities and pinholes provide mechanical adhesion sites into which polymer resins flow and anchor during the subsequent lamination step in the construction of multilayer printed circuit boards. These mechanical adhesion sites in conjunction with a silane treatment with epoxy prepregs have been shown to provide an improvement of two to four times in the adhesion of the polymer to the copper/tin/tin/silane inner layer over the prior art process of a single tin layer and silane treatment.

As noted above, the heavy tin deposit of the present invention is useful as an etch resist. For example, in flexible printed circuit boards having bend areas, it is known to protect the copper circuitry in the exposed bend area with a polyimide or epoxy layer intrinsic with the design of the board. This requires additional inner layer components, which in turn create thicker and more costly multilayer constructions. According to the present invention, the outer layer copper in the bend area may be protected from etching by the dual heavy tin and thin tin layers as discussed above.

The invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims.