Etching a substrate: processes – Forming or treating electrical conductor article
Utility Patent
1997-12-22
2001-01-02
Bueker, Richard (Department: 1763)
Etching a substrate: processes
Forming or treating electrical conductor article
C216S102000, C216S105000, C252S079500
Utility Patent
active
06168725
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an etchant and a method for chemically milling electronic circuitry (rigid/flex or 3-dimensional circuitry) comprising copper on aluminum foil laminated to a substrate using an alkaline aqueous solution containing a nitrite, borate or bromate salt to selectively etch out the aluminum.
BACKGROUND OF THE INVENTION
It has been suggested that the complexity and weight of electronic component designs in automotive vehicles can be improved by assembling the electronic components on substrates, e.g., on the underside of vehicle dashboards. This type of system has been termed Instrument Panel (IP) Super-Integration. In this system, it is intended that individual electronic circuit boards be first assembled on substrates, with electronic components attached thereupon. Currently used conventional printed circuit board technologies utilize either additively or subtractively defined copper, a dielectric laminate and plated through hole for interconnection. One of the drawbacks of this method is that it is expensive.
Another way is being explored by the inventors to provide three-dimensional circuits for use as part of Super-Integration Systems. It involves forming interconnected electronic substrates through the preferential chemical etching of a tri-metal layers of dissimilar metals. That is, copper patterned circuit images are carried on both sides of an aluminum foil. This copper-aluminum-copper tri-layer metal system could be made by cladding, lamination, or plating of copper followed by selective etch of copper to form the copper patterns on both sides of the aluminum foil. After the tri-metal is laminated on a polymeric substrate as by means of an adhesive , it needs to be selectively etched to remove the aluminum and form an electronic circuit board. Alternately, copper patterns could be selectively electroplated on both sides of an aluminum foil, with or without a seed layer, and then after lamination of one of the copper patterns on a substrate, the aluminum would be selectively etched away to form an electronic circuit board. One of the advantages of this Cu—Al—Cu construction is the ability to have higher density circuit patterns by providing cross-overs of Cu-tracks. In addition, this new technique is seen capable of providing better heat dissipation through the aluminum interlayer.
In order to obtain the desired circuit pattern of such a Cu—Al—Cu construction, the aluminum needs to be selectively etched away. To provide durable circuits, it is necessary to control the etching of the aluminum carefully to have optimum undercut. Sodium hydroxide solutions have been long used to etch aluminum. Attempts at etching the tri-metal Cu—Al—Cu described above with a conventional sodium hydroxide solution was found by the inventor, however, to result in an unacceptably large degree of undercutting of the aluminum as shown in FIG.
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. This is believed by the inventor to be based on the difference in electrochemical potential between aluminum and copper which unfavorably enhances aluminum etching rates in areas adjacent to the copper in this solution. The undercut of the aluminum was found to be so high, that the copper pads are lifted. I have now unexpectedly found a way to overcome the undercut problems associated with such chemical etching of aluminum for tri-metal Cu—Al—Cu laminates which manufactures durable chemically milled circuits. This process involves the selective etching of the tri-metal with a particularly defined alkaline etchant solution of sodium or potassium hydroxide which includes defined amounts of nitrites, borates (perborates or tetraborates), bromates at a given temperature.
Several patents were identified after invention which disclose etchants for preparing aluminum surfaces. U.S. Pat. No. 3,957,553 discloses an alkaline etching bath for aluminum to improve its appearance and prepare it for subsequent treatment. The bath comprises an alkali metal hydroxide (15-200 g/l), a chelating agent (0.5-15 g/l) like sorbitol for suppressing alumina precipitation during etching, and an oxidizing agent (0.0001-1 g/l) such as borates or bromates. U.S. Pat. No. 5,091,046 discloses a process for caustic etching of aluminum to obtain a matte finish with a solution containing free sodium hydroxide and dissolved aluminum in a 0.6-2.1 ratio and an etch equalizing agent at a temperature above about 70° C. The latter patent discloses total sodium hydroxide concentrations of 40-120 g/l and includes equalizing (oxidizing) agents such as sodium nitrate (up to 50 g/l), sodium nitrite (up to 20 g/l), sodium sulfide. These etching solutions have the drawbacks that they would have an undesirable low etching rate and high Aluminum undercut with the Cu—Al—Cu tri-metals and would attack plastic at temperatures above 70° C. It would be desirable to have an etching solution and method for etching the tri-metal which would selectively etch out a portion of the aluminum while leaving sufficient aluminum with low undercut to provide a structurally sound electronic circuitry, e.g., 3-dimentional circuits. And the etching should take place at a commercially acceptable rate. The present invention provides such an etchant and method of etching out the aluminum.
DISCLOSURE OF THE INVENTION
The invention is an etchant and method useful for selectively etching aluminum from, according to one embodiment, copper-aluminum-copper tri-metal layers which comprise copper circuit patterns present on opposing surfaces of an aluminum foil, one of the copper patterns being laminated onto a substrate. The etchant comprises an aqueous solution of: (1) 60 to 500 g/l base selected from (a) sodium hydroxide, (b) potassium hydroxide, and (c) their mixture; and (2) 30 to 500 g/l of a nitrite salt, a borate salt, a bromate salt, or a mixture of any of them. According to one embodiment of the method, the tri-metal laminate is contacted with the etchant disclosed above at a temperature of 25 and 95° C. and for a time sufficient to selectively remove a desired amount of the aluminum layer and form electronic circuitry which contains multiple conductive copper circuit layers. While this invention is particularly useful for tri-metal etching as described above, this invention etchant can also be used to etch out aluminum from tri-metals not involved with electronic circuitry or any other surface finishing applications such as for weight reduction of aluminum components in the aerospace industry. In addition, the present invention etchant and method can more generally be used to etch out aluminum from a copper-aluminum configuration laminate, i.e., where the a copper pattern is provided on an aluminum layer, the aluminum layer being laminated to a substrate, to form electronic circuitry.
Advantageously, the present invention etchant and method provides for fast and uniform etching of the multiple layer Cu—Al—Cu or Al—Cu electronic circuitry disclosed above. That is the present invention etchant solution has an etch rate almost twice as fast as conventional sodium hydroxide etchants which results in lower manufacturing costs, while at the same time avoids causing the undesirable undercutting of the Aluminum layer as was the case with conventional sodium hydroxide etchants. The optimal ratio of the downward cut to horizontal cut during etching is about 1:1 which generally takes place in the present invention method as shown in FIG.
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Further we have advantageously found that the etchant solutions of the present invention leave the copper surfaces clean, i.e., they are not oxidized or corroded, thus eliminating the need for a follow-up cleaning process. In addition, preferred invention etchant solutions disclosed herein containing sodium hydroxide, sodium nitrite, sodium borate (perborate or tetraborate) or sodium bromate are relatively inexpensive, readily available, and easily treated and disposed of which enhances the advantage of the present invention method using this new etchant. This new invention makes manufacturing of Etchted Tri-layer Metal (ETM)circuitry more comm
Achari Achyuta
Paruchuri Mohan R.
Bueker Richard
Hodges Leslie C.
Powell Alva C.
Visteon Global Technologies Inc.
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