Process and Apparatus for Etching Copper Masked by a Nickel-Gold Mask

This invention pertains generally to the manufacture of printed circuit boards and more particularly to a process and apparatus for anisotropically etching copper through a nickel-gold mask on a circuit board.

In one method of manufacturing corrosion resistant circuit boards, a photoresist pattern is formed on the outer surface of the copper layer, with the portions of the copper to be preserved being exposed through the resist. A layer of gold is formed on the exposed portions of the copper, with a layer of nickel being interposed between the copper and the gold to prevent diffusion of the gold into the copper. The photoresist is removed, and the newly exposed portions of the copper are etched away, with the gold serving as a mask during the etching process.

As the exposed portions of the copper are etched away, an undesired undercutting or removal of the copper beneath the nickel-gold mask occurs with every prior etching technique known to applicant. Somewhat surprisingly, it has been found that this undercutting can be almost entirely eliminated by connecting the copper electrically to the reactor wall or to any other electrical conductor which is contacted by the etchant.

It is in general an object of the invention to provide a new and improved process for etching copper through a nickel-gold mask on a circuit board.

Another object of the invention is to provide a process and apparatus of the above character which produce an anisotropic etch with substantially no undercutting of the copper beneath the nickel-gold mask.

These and other objects are achieved in accordance with the invention by making an electrical contact between the copper on the circuit board and the reactor wall or other electrical conductor contacted by the etching solution. With this connection, the copper being removed and the other conductor are maintained at substantially the same electrical potential, and it has been found, somewhat surprisingly, that the etching reaction becomes anisotropic without significant undercutting of the copper beneath the nickel-gold mask.

FIG. 1 is a fragmentary sectional view of a circuit board with a nickel-gold mask to be etched in accordance with the invention.

FIG. 2 is a fragmentary sectional view, somewhat schematic, of one embodiment of apparatus for etching copper in accordance with the invention.

In the drawings, the invention is illustrated in connection with a printed circuit board 11 having an insulative substrate 12 and a layer of copper 13 on one side of the substrate. A nickel-gold mask comprising a layer of nickel 14 and a layer of gold 17 is formed on the outer surface of the copper over the portions of the copper to be retained. The mask can be formed by any suitable technique such as by coating the entire surface of the copper with a layer of photoresist, removing the photoresist where the copper is to be retained, plating the board with nickel and gold which adhere only where the photoresist has been removed, then removing the remainder of the photoresist.

The exposed copper is removed by exposing the board to an etching solution. Any suitable etching solution can be employed, and one preferred solution is described in, now U.S. Pat. No. 4,497,687. This solution comprises an aqueous solution of NO.sub.2 or HNO.sub.3, a dissolved copper salt such as Cu(NO.sub.3).sub.2, and a polymer additive which serves as a surfactant and as an inhibitor of lateral etching or undercutting. This etching solution and its use are described in greater detail in the aforementioned patent.

The etching reaction can be carried out in any suitable reactor, and in the embodiment illustrated in FIG. 2 the circuit board 11 is illustrated as being etched in a reactor 21 having a flow channel 22 in which the board is placed. The etching solution flows through the channel, and the flow rate can be adjusted to optimize the removal of copper. A reactor of this type, is described in detail in, now U.S. Pat. No. 4,482,425.

An electrically conductive wire 26 is connected between the copper on the circuit board and the electrically conductive metal wall 27 of the reactor to maintain the copper and the wall at substantially the same electrical potential. A similar connection is made between the board and any other electrical conductor (not shown) which may be contacted by the etching solution with the copper. Somewhat surprisingly, this connection has been found to be very effective in eliminating lateral etching or undercutting of the copper beneath the nickel-gold mask. The reason for this result is not fully understood, but it is believed to be due at least in part to the elimination of a voltaic effect between the copper and the other conductor.

When the copper is etched, it enters the etching solution as Cu.sup.++. If the solution deviates very far from electrical neutrality, very large electrostatic forces are created and bring the solution back to neutrality. In order for this to occur, anions must be present to neutralize the positive charge of the copper cations. The copper cations and anions diffuse away from the circuit board when they are in high concentrations, and if they do not have the same diffusion coefficient, there will be sufficient charge separation to set up an electrical field of sufficient magnitude and direction to make the negative and positive charges move together. A voltage then exists between the board and the interior of the etching solution, with no net current. A similar effect takes place at the reactor wall or other electrical conductor, and a difference in voltage is created between the board and the wall or other conductor. When this occurs, undercutting of the copper occurs.

When an external electrical connection is made between the copper board and the reactor wall or other conductor, an electron current can flow between the two, and there can be no electrical potential difference. When the Cu.sup.++ goes into solution, it is neutralized by a shifting of the whole negative charge distribution and the appearance at the reactor wall or other conductor of two electrons which have flowed from the copper board through the wire. It may also be that the Cu.sup.++ is neutralized by a negative surfactant which then attaches itself to the wall of the cut.

It is apparent from the foregoing that a new and improved process and apparatus for etching copper have been provided. While only certain presently preferred embodiments have been described in detail, as will be apparent to those familiar with the art, certain changes and modifications can be made without departing from the scope of the invention as defined by the following claims.