GB2206541A

GB2206541A - Manufacturing printed circuit boards

Info

Publication number: GB2206541A
Application number: GB08815275A
Authority: GB
Inventors: Daniel J Barnett; Paulo A Diehl; Norwell J Nelson
Original assignee: Psi Star; PSI Star Inc
Current assignee: Psi Star; PSI Star Inc
Priority date: 1987-07-02
Filing date: 1988-06-27
Publication date: 1989-01-11
Also published as: FR2617667A1; FR2617667B1; JPS6489588A; DE3822403A1; GB8815275D0

Description

J 2 1r12 0 6 5 4 11 METHOD OF MANUFACTURING PRINTED CIRCUIT BOARDS This

invention pertains generally to the manufacture of printed circuit boards, and more particularly to a method of manufacturing printed circuit boards in which nickel is employed as an etch resist in the formation of electrically conductive areas on the boards.

In the manufacture of printed circuit boards, a conductive layer of copper is laminated to an insulative substrate such as a glass-epoxy board, and portions of the copper are etched away to leave the desired circuit patterns. The areas to be left are generally defined by a mask of etch resistant material which is applied to the board prior to etching. The circuit components are installed on the etched board and soldered to the conductive patterns, typically in a wave soldering machine. In order to prevent an excessive amount of solder from adhering to the board, an organic solder mask is generally applied to the foil side of the board except in the limited areas where the actual connections are to be made.

A number of materials, including both organic materials and metals, can be utilized as etch resist. One material which is commonly utilized for this purpose is an alloy of lead and tin similar to the eutectic alloy which is employed in the wave soldering operation. This alloy has the advantage of being highly compatible with the solder, but it has the disadvantage of melting and shifting beneath the solder mask during the wave soldering operation. This can result in undesirable wrinkling or other damage to the solder mask.

U.S. Patent 4,605,471 discloses one method of utilizing nickel as an etch resist to overcome some of the disadvantages of a lead-tin resist.. Because of its relatively high melting point, the nickel can be left on the board without wrinkling or otherwise damaging the solder mask during the wave soldering -10 operation. Nickel also has the advantage of being a particularly good underplating material for gold which is sometimes plated onto the contact areas of the copper patterns.

Nickel also has certain limitations and disadvantages as an etch resist. It does not solder well, and it is generally necessary to remove the nickel or plate over it in the areas where solder joints are to be made. The nickel which is comrAonly used as an etch resist is a bright nickel which is not suitable for use with etching chemistries such as the nitric acid process disclosed in U.S. Patent 4, 497,687. The bright nickel reacts with the nitric acid at almost the same rate as the copper, with the result that the nickel is removed almost as fast as the copper. Consequently, the use of nickel as an etch resist in this process is marginal at best.

We have now developed a new and improved method of making printed cicuit boards using nickel as an etch resist which overcomes the limitations and disadvantages heretofore encountered when nickel is employed as an etch resist, and in which a nitric acid etching solution is employed.

Accordingly, the present invention provides a method 1 fl of fabricating a printed circuit board in which an etch resist consisting essentially of a tin-nickel alloy, semi-bright nickel, or a combination of these two metals is applied to the portions.of the copper to be retained, and the remaining copper is removed with a nitric acid etchant. The tin-nickel alloy is particularly suitable for use in applications where the etch resist does not have to be removed, and the semi-bright nickel and the combination of the tin-nickel alloy and the semi-bright nickel are preferred in applications where the resist must be removed since they are more readily removed with the strippers which are currently available.

The etching process employed in the present invention can be similar to the process which is disclosed in detail in U.S. Patents 4,497,687 and 4,545,850. In that process, nitric acid reacts with copper according to the relationship 3Cu + 8HN03 = 3Cu(N03)2 + 2NO + 4H20 with the nitric acid serving both as an oxidant and as an anion source for the dissolved copper. The dissolved copper is removed by adding sulfuric acid to precipitate the copper according to the relationship 3CUM3)2 + 3H2S04 = 3CUS04 + 6HNO3. To protect the etch resist and the substrate board from attack by the nitric acid, either copper nitrate or sulfuric acid is included in the etching solution. When sulfuric acid is employed, the consumption of nitric acid is reduced, and the reaction proceeds according to the relationship 3Cu + 2HNO 3 + 3H 2 so 4 = 3CuSO 4 + 2NO + 4H 2 0.

Copper is removed as the sulfate pentahydrate according to the relationship CUS0 4 + 5H2 0 - CUSO 4 5H 2 0. The copper nitrate or sulfuric acid increases 'the reactivity of nitric acid toward copper such that reasonable reaction rates can be achieved with much lower concentrations of nitric acid than would otherwise be required. Thus, the amount of nitric acid can be kept to levels which do not react with the substrates or organic resists. Certain metal resists can also be utilized with these etchants, including tin, nickel and alloys thereof. If a small amount of phosphoric acid is added to the solution! tin-lead alloys can be utilized as etch resists.

A polymer is included in the etching solution to control the surface reaction rate and to enhance the anisotropy of the etch. one suitable polymer is a polyacrylamide which is solub-1e in water and has a high molecular weight. The amount of polymer employed is on the order of 0. 05% (volume) of the solution. Suitable polyacrylamides include Dow Separan CP-7HS (a high molecular weight cationic polymer), Hercules Reten 520 (a. high molecular weight neutral polyacrylamide), and Dow Separan NP10 (a high molecular weight slightly anionic polyacrylamide).

A surf actant is also included in the etching solution to reduce surface tension and facilitate the movement of nitric oxide bubbles from the surface of the copper. The nitric oxide gas is generated during the etching process, and the surf actants serve to keep the bubble size small and prevent the bubbles from bridging between resist lines and blocking etching between the lines. Only a very small amount of surfactant is required, e.g. 0.1 to 0.2% (volume) of the etching solution. Suitable surfactants include fluorocarbon surfactants such as 3M1s FC-100 (an amphoteric surfactant) and FC-135 (a cationic surfactant).

1 -1 As discussed in U.S. Patent 4,545,850, the etching solution can be regenerated by adding a controlled amount of sulfuric acid to the solution. The nitric acid dissolves the copper to form a nitrate of the copper. and the sulfuric acid reacts with the coppernitrate to form nitric acid and a precipitate of the copper.

As noted above, the nitric acid etching solution is reactive toward nickel as well as copper, and an example is even given in U.S. Patent 4, 40,687 for etching nickel. An example is given for etching nickel with a solution of 50Occ of 50% nickel nitrate (Ni(NO 3Y1 125cc of 70% nitric acid (HNO 3) and 7% Separan CP-7HS (Dow Chemical poly(acrylamide)). As disclosed in U.S. Patent No. 4,556,449, the nitric acid etching solution can be made even more reactive toward nickel by adding a halogen additive such as periodic acid (H 5 10 6).

Notwithstanding the reactivity of the copper nitrate etching solution toward bright nickel, it has now been found that semi-bright nickel can be used satisfactorily in some applications a s an etch resist with the nitric acid chemistry disclosed in U.S. Patent No. 4,497,687. This is believed to be due to the fact that bright nickel plating solutions contain levelers and brighteners, such as saccharin, which contain sulfur. The sulfur is incorporated into the plated nickel. The plating solutions utilized for semi-bright nickel do not contain any sulfur or sulfurcontaining compounds, and there is no sulfur in the semi-bright nickel. The rest potential of semi-bright nickel is over 100 mv more noble than the rest potential of the sulfur-containing bright nickel. As a result, the semi-bright nickel is less reactive than the bright nickel, and it can be utilized as an etch resist with a nitric acid etching solution in some applications.

Even though semi-bright nickel is not as reactive as bright nickel, it will react with a nitric acid etching solution to some extent. However, this reaction is not as fast as the reaction of the nitric acid solution with copper, and the semi-bright nickel can be used as an etch resist in certain situations. e.g. where the amount of copper to be removed is relatively small, as with thinner (e.g. 9 micron) copper foils. The semibright nickel is readily removed from the copper patterns with commercially available stripping agents such as Shipley Niposit 428, Circuit Services Nic-Strip or CP Chemicals Tru Strip.

It has also been found that plated tin-nickel alloy (65% tin, 35% nickel) is an excellent etch resist for use with a nitric acid etching solution in that it does not dissolve perceptibly in nitric acid. Some difficulty has been- encountered, however, in removing the tin-nickel alloy resist following the etching operation. Even though the tin-nickel alloy does not dissolve perceptibly in nitric acid, it must be removed with a stripping solution which contains nitric acid. one suitable solution for this purpose is Ardron Tin-Nickel Stripper No. 1800. solution also contains a sulfamic acid etch inhibitor which is intended to protect the underlying copper from attack by the nitric acid in tin-nickel stripper. Unfortunately, the inhibitor is gradually exhausted with use, particularly in any crevices which may exist between the tin-nickel layer and the underlying copper layer, leading to attack of the underlying copper. The reaction between the nitric acid and the tin produces nitrous acid which catalyses the attack on copper unless removed by the etch inhibitor (sulfamic acid). This leads to localized pitting of the copper, commonly known as Umousey bites". When a zinc adhesion layer is employed between the copper layer and the substrate, the stripper may attack the zinc even more rapidly than the copper, and in severe cases, this can result in a separation of the copper lines from the substrate. Accordingly, with the stripper which is currently available, the tin- nickel alloy is best used in applications where it does not have to be removed from the copper.

The best results have been obtained with an etch resist consisting of a combination of semi-bright nickel and a tin-nickel alloy. This combination has been found to provide a stable etch resist which is not attacked by the nitric acid and which can be removed by a commercially available stripper without damaging the copper foil or its bond with the substrate. The combination also permits the etch resist to be plated as a thinner layer, which is beneficial in etching.

In this combination, a layer of semi-bright nickel is plated onto the copper, and a layer of tin-nickel is plated over the semi-bright nickel. The semi-bright nickel layer should have a thickness of at least 0.2 mil to provide adequate separation between the copper and the tin-nickel, and it is important that the nickel layer be free of all sulfur. Suitable plating solutions for the semi-bright nickel layer include M&T Chemical Sulfamate Nickel without additives, Lea Ronal Nikal PC-3 without additives, Harshaw Novatronix Nickel without additives and CP Chemicals Nickel Sulfainate without additives.

The tin-nickel layer can have any desired thickness, as long as it is thick enough to entirely cover the underlayer of semi-bright nickel. Layers ranging in thickness between 0.1 and 0.3 nil are adequate. Suitable tin-nickel plating baths include Lea Ronal Nicostan and Electrochemicals Electro-Brite Tin/Nickel.

With this combination, the tin-nickel layer is not etched by the nitric acid etching solution, and it protects the semi-bright nickel from etching. Both the semi-bright nickel and the tin-nickel can be removed with a semi-bright nickel stripper which etches. away the semi-bright nickel beneath the tin-nickel layer by undercutting. Once the semi-bright nickel has been removed, the tin-nickel layer falls off the board. Suitable strippers for this purpose include Shipley Niposit 428, Circuit Services Nic-Strip and CP Chemicals Tru Strip.

S d

Claims

CLAIMS:

1. A method of fabricating a printed circuit board by the selective removal of copper from an insulative board, comprising the steps of:-. applying a layer of semi-bright nickel to portions of the copper to be retained, forming a layer of tin-nickel alloy over the semi-bright nickel, and exposing the board to a nitric acid etching solution to remove the portions of the copper which are not covered by the semi-bright nickel and the tin-nickel alloy.

2. A method as claimed.in claim 1 wherein the semi-bright nickel layer has a thickness of at least 0.2 mil.

3. A method as claimed in claim 1 or claim 2 wherein the nitric acid etching solution comprises an aqueous solution of nitric acid, copper nitrate or sulfuric acid, a polymer,undercutting inhibitor and a surfactant.

4. A method as claimed in any one of the preceding claims which includes the step of removing the semi-bright nickel and the tin-nickel alloy with a semi-bright nickel stripper after the uncovered portions of the copper have been removed.

5. A method of fabricating a printed circuit board by the selective removal of copper from an insulative board, comprising the steps of: applying an etch resist consisting essentially of a tin-nickel alloy to portions of the copper to be retained, and exposing the board to a nitric acid etching solution to remove the portions of the copper which are not covered by the tin-nickel alloy etch resist.

6. A method as claimed in claim 5 wherein the nitric acid etching solution comprises an aqueous solution of nitric acid, copper nitrate or sulfuric acid, a polymer undercutting inhibitor and.a surfactant.

7. A method as claimed in claim 5 or claim 6 including the step of removing the etch resist after the uncovered portions of the copper have been removed.

8. A method of fabricating a printed circuit board by the selective removal of copper from an insulative board, comprising the steps of: applying an etch resist consisting essentially of a semi-bright nickel to portions of the copper to be retained, and exposing the board to a nitric acid etching solution to remove the portions of the copper which are not covered by the semi-bright nickel etch resist.

9. A method as claimed in claim 8 wherein the nitric acid etching solution comprises an aqueous solution of nitric acid, copper nitrate or sulfuric acid, a polymer undercutting inhibitor and a surfactant.

10. A method as claimed in claim 8 or claim 9 including the step of removing the etch resist after the uncovered portions of the copper have been removed.

11. A method of preparing a printed circuit board as claimed in any one of claims 1,5 or 8 1 - 1 1 - t substantially as hereinbefore described.

12. A printed circuit board whenever prepared by a method as claimed in any one of the preceding 5 claims.

is Published 19813 at The Patent Wipe. State House. 66'71 High Ho' lborn, London WC1R 4TP. FurUier copies may be obtained from The Patent Office, Sales Brairch, St Mary Cray. Orpington, Zerit. BM 3RD- Frinted by Multiplex cechjaTies ltd, St Mary Cray, Kent. Con. 1'87.

baies z5rax2u&i, a ui.j -y. --- - --- --- 1 - -1 - - -