Method and composition for etching tri-metal layers to form electronic circuits

Abstract: An etchant useful for chemically milling aluminium from tri-metal layers of copper-aluminium-copper being copper circuit patterns present on opposing surfaces of an aluminium foil, one of the copper patterns being laminated onto a substrate, the etchant comprising 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) 70 to 500 g/l of a nitrate salt.

Description: The present invention relates to an etchant and a method for chemically milling circuit boards (rigid/flexible or 3-dimensional circuitry) comprising copper on aluminium foil laminated to a substrate using an alkaline aqueous solution containing nitrate salts to selectively etch out the aluminium. Reference is made to related US patent application Serial No. 08/976007 entitled "ETCHING OF AL-CU LAYERS TO FORM ELECTRONIC CIRCUITS USING BASIC SOLUTIONS INCLUDING NITRITES, BORATES OR BROMATES". 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 utilise 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 layer of dissimilar metals. That is, copper patterned circuit images are carried on both sides of an aluminium foil. This copper-aluminium-copper tri-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 aluminium 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 aluminium and form an electronic circuit board. Alternately, copper patterns could be selected electroplated on both sides of an aluminium foil, with or without a seed layer, and then after lamination of one of the copper patterns on a substrate, the aluminium 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 crossovers of Cu-tracks. In addition, this new technique is seen capable of providing better heat dissipation through the aluminium interlayer. In order to obtain the desired circuit pattern of such a Cu-Al-Cu construction, the aluminium needs to be selectively etched away. To provide durable circuits, it is necessary to control the etching of the aluminium carefully to have an optimum undercut. Sodium hydroxide solutions have been long used to etch aluminium. Attempts at etching the tri-metal Cu-Al-Cu described above with a conventional sodium hydroxide solution was found by the inventors, however, to result in an unacceptably large degree of undercutting of the aluminium as shown in Fig. 1. This is believed by the inventors to be based on the difference in electrochemical potential between aluminium and copper which unfavourably enhances aluminium etching rates in areas adjacent to the copper in this solution. The undercut of the aluminium was found to be so high, that the copper pads are lifted. We have now unexpectedly found a way to overcome the high undercut problems associated with such chemical etching of aluminium for tri-metal Cu-Al-Cu laminates which manufactures durable chemically milled circuits. This process involves, according to one embodiment, the selective etching of the tri-metal with a particularly defined alkaline etchant solution of sodium or potassium hydroxide which includes defined amounts of nitrates at a given temperature. Several patents were identified after invention which disclose etchants for preparing aluminium surfaces. US patents 3,957,553 discloses an alkaline etching bath for aluminium to improve its appearance and prepare it for subsequent treatment. The bath comprises an alkali metal hydroxide (15-200g/l), a chelating agent (0.5-15 g/l) like sorbitol for suppressing alumina precipitation during etching, and an oxidising agent (0.0001-1 g/l) such as nitrates or nitrites, US patent 5,091,046 discloses a process for caustic etching of aluminium to obtain a matte finish with a solution containing free sodium hydroxide and dissolved aluminium in a 0.6-2.1 g/l ratio and an etch equalising agent at a temperature above about 70°C. The latter patent discloses total sodium hydroxide concentrations of 40-120 g/l, nitrate up to 50 g/l and nitrite up to 20 g/l. These etching solutions have the drawbacks that they would have an undesirable low etching rate with the Cu-Al-Cu tri-metals when the temperature is below 55°C and would attack plastic at temperatures above 65°C. United States Patent 4,417,949 (D1) is directed to increasing the etch rate of aluminium while increasing the tank life of the etching composition by adding sodium nitrate to the composition containing sodium hydroxide. In the invention now to be described (see claim 1), the etchant used is distinct from that of D1 in at least two significant aspects, the concentration of the nitrate and the etchant temperature. The etchant in D1 contains up to 70 g/l of nitrate while the invention utilises an etchant containing from 70 to 500 g/l of nitrate salt. There is an isolated reference (the last Example in Table No 1) to a higher nitrate content of 110 g/l but this causes a reduced etch rate compared with the etch rate achieved in the invention method. Indeed D1 teaches away from using nitrate concentrations above 70 g/l at col 4, lines 14-16, where it is taught that "additional amounts of NaNO 3 are counterproductive and do not enhance etch rates or solution life". Further, the only etchant temperature disclosed is 93-107°C (which would at least soften a polymeric substrate) whereas the method of the invention utlises a lower temperature. BE691281 describes etching compositions having a NaNO 3 content of 40 g/l. In DD233592 the described etching composition is 8-13% NaOH, 7-11: NaNO 3 , 5-9% NaNO 2 and 4-8 Na 3 PO 4 . In the composition described in DE3405437, the NaNO 3 concentration is 0.2-3.5%. It would be desirable to have an etching method for etching the aluminium from copper-aluminium laminates which would selectively etch out a portion of the aluminium while leaving sufficient aluminium to provide a structurally sound circuits, such as 3-dimensional circuits. And the etching should take place at a commercially acceptable rate. The present invention provides such a method of etching out the aluminium. According to the invention we provide a method for chemically milling aluminium from copper circuit patterned on a surface of an aluminium foil, the aluminium surface free of copper being laminated on a substrate, the method comprising the steps of: contacting the laminate with an etchant comprising an aqueous etching solution of: (1) 60 to 500 g/l base selected from (a) sodium hydroxide, (b) potassium hydroxide, and (c) their mixture; and (2) 70 to 500 g/l of a nitrate salt at a temperature of 25 to 95°C for a time sufficient to remove a desired amount of the aluminium layer and form electronic circuitry including a conductive copper circuit layer. Advantageously, the method of the invention method can be used for chemically milling aluminium from copper-aluminium-copper tri-metal layers being copper circuit patterns present on opposing surfaces of an aluminium foil, one of the copper patterns being laminated on a substrate. The etchant solution used in the invention method 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 copper 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. 2. Further it is advantageously found that the etchant solutions used in the present invention leave the copper surfaces clean, i.e., they are not oxidised or corroded, thus eliminating the need for a follow-up cleaning process. In addition, preferred etchant solutions disclosed herein containing sodium hydroxide and sodium nitrate 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 technology, which has been termed Etched Tri-Metal (ETM), is considered to optimal for use in the Super Integration of interior automotive subsystems. The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic of a comparative example etching of a tri-metal of copper patterned on two sides of aluminium foil and adhered to a substrate etched in an aqueous NaOH solution; Fig. 2 is a schematic of an example of etching a tri-metal according to embodiments of the present invention using an aqueous alkaline solution of nitrate salts; The inventors have unexpectedly found a novel and commercially desirable way to manufacture multiple layer circuits adhered to a substrate. As disclosed above, the method is carried out by selectively etching aluminium from a tri-metal of copper-aluminium-copper (or a bi-metal) with an etchant solution which avoids causing any serious undercut problems. In the present invention ETM method, one embodiment comprises etching the aluminium from an aluminium foil where copper circuits have been provided on opposing surfaces of the aluminium foil. As discussed above, we found that etching aluminium when it adjoins a metal of different potential, like copper, can be problematic. That is, using a conventional sodium hydroxide etching solution causes serious undercut problems when too much of the aluminium is removed underneath the copper circuits. We have found that by adding nitrate salts to the aqueous alkaline solution in specified amounts and using the etchant solution as a chosen temperature, these undercut problems are significantly minimised while providing faster etching. We believe that this problem of undercutting is tied to the evolution of NH 3 gas at the copper/aluminium interface during etching. We theorise that the agitation produced at the interface by the gas evolution results in undesirable undercut. We found that by adding nitrate salts to the bath in specific amounts and having the etching solution at a given temperature, the gas evolution is reduced and the aluminium undercutting is significantly lessened. That is the aluminium etching laterally is lessened while increasing the etching of the aluminium in the vertical direction. Neither the validity nor the understanding of this theory, however, is necessary for the practice of the present invention method. The copper-aluminium-copper layers being chemically milled according to one preferred embodiment of the present invention are made of an aluminium foil populated with copper patterns on the both sides of the aluminium foil, one of the copper patterns being laminated on a substrate. As discussed above, however, this method and etchant can be used for milling aluminium from copper-aluminium layers laminated to a substrate. The way in which this tri-metal or bi-metal is made is not critical to the present invention. One way to make this copper-aluminium-copper tri-metal is by first depositing and developing a photo-resist on both sides of a copper-aluminium-copper foil as could be made by cladding, lamination, or plating. This then could be followed by selectively etching the copper to form copper patterns on both sides of the aluminium foil. Thereafter, before etching out the aluminium to form the three-dimensional circuits, one of the copper patterns is laminated on substrate as by using an adhesive. Another method to make this tri-metal of copper-aluminium-copper using a photo-resist image on both sides of an aluminium foil, with or without a seed layer of other metal, followed by selectively electroplating copper patterns on both sides of the aluminium foil. Then the photo-resist would be removed and one of the copper patterns laminated on a substrate to form a tri-metals of copper-aluminium-copper for aluminium etching. The embodiment of bi-metal copper patterned on one side of aluminium foil could be similarly made as would be apparent to one skilled in the art in view of the present disclosure. The substrate can be, for example, a polymeric material like plastic, FR-4 (fibre reinforced resin), or a metal with a coating of a plastic. Neither the particular design of the circuitry on the aluminium foil nor the method by which it is produced, nor the particular substrate to which it is laminated, however, is critical to the practice of the invention etching method. The solution used herein comprises an aqueous etching 10 solution containing 60 to 500 g/l base selected from (a) sodium hydroxide, (b) potassium hydroxide, and (c) their mixture and nitrates. Preferably, the base concentration is 150 to 250g/l. The base is preferably sodium hydroxide. The optimal concentration of base and nitrate in the invention etchant depends on several factors including desired etching rate and undercut. For example, the etching rate is increased when the concentration of sodium hydroxide is lower than 150 g/i or more than 250 g/l. The solution also contains a nitrate salt or mixture of nitrate salts. The nitrate salt is included in 70 to 500 g/l, more preferably being 70 to 150g/l. The nitrate salt when used in these amounts was unexpectedly found to allow the uniform etching of the aluminium from the tri-metal to maintain the integrity of the final electronic circuitry as explained above. The nitrate salt dissolved in the etching solution may be any nitrate salt including, but not limited to, such salts of metals like sodium, potassium, iron (e.g., ferric nitrate), manganese, zinc, lithium, silver, calcium, ammonium, cobalt (e.g., cobaltous nitrate), copper (e.g., cupric nitrate), aluminium, mercury (e.g., mercuric nitrate), thorium, barium, guanidine, lead, cerium, bismuth, zirconium, strontium, cadmium, lanthanum, nickel (e.g., nicklous nitrate), and mixtures of any of them. The particularly preferred nitrates are those of sodium and potassium because both of them have lower cost relative other nitrates. The aqueous etchant solution used in the present invention can be used to etch aluminium by any convenient method, including dip etching and spray etching. The etching rate is almost the same using either dip or spray etching. To form the etchant solution, the nitrate salt generally would be added into the alkaline aqueous solution at room temperature or at an elevated temperature which facilitates dissolution. Tri-metal layers of copper-aluminium-copper (or copper-aluminium layers, as disclosed herein, are then contacted with the etchant solution for a time sufficient to etch out the aluminium in the desired amount. The etchant solution is at a temperature of 25 to 95°C, more preferably being 40 to 65°C, most preferably being 45 to 55C. Selection of optimal base, nitrates and their concentration, as well as etchant temperature and etch time, would be within the skill of one in the art in view of the present disclosure. Factors to be taken in consideration in choosing the etchant temperature include, e.g., desired etching rate, desired undercut, and the propensity of the etchant to attack the adhesive or substrate. The aluminium concentration in the etchant solution can be varied to optimise etching rate and undercut as would be apparent to one skilled in the art in view of the present disclosure. Table 1 shows that the change of the aluminium undercut with the concentration of sodium nitrate and sodium hydroxide in the alkaline aqueous solution of sodium nitrate. Aluminium undercut is high and unacceptable at low (2.5%) nitrate concentration. Examples 1-8 and 14-20 show the etching rate (time to remove 152.4 µm (6 mils) thick aluminium) with concentration of sodium nitrate at different concentrations of sodium hydroxide. It can be learned from these examples that the etching rate is low when sodium nitrate concentration is low (2.5%) or high (20-40%). Examples 9-13 show the etching rate (time to remove 152 µm (6 mils) aluminium) with concentration of sodium hydroxide at 7.5 wt% sodium nitrate. These examples suggest that etching rate is low when sodium hydroxide concentration is low (10%). Examples 21-27 show the etching rate in the different alkaline nitrate salt solutions indicating fast etching rates with these solutions also. Solution Temp.(°C) Average undercut x 25.4 µm (mil) **2.5 wt% NaNO3 + 15% wt NaOH 55 5.7 **5.0 wt% NaNO3 + 15% wt NaOH 55 4.8 7.5 wt% NaNO3 + 15% wt NaOH 55 4.9 10 wt% NaNO3 + 15% wt NaOH 55 5.3 7.5 wt% NaNO3 + 10% wt NaOH 55 6.8 7.0 wt% NaNO3 + 20% wt NaOH 55 5.5 7.5 wt% NaNO3 + 25% wt NaOH 55 5.0 7.5 wt% NaNO3 + 30% wt NaOH 55 5.0 Examples 1-4 Example Etchant Temp.(°C) Time **1 2.5 wt% NaNO3 + 15% wt NaOH 55 13.5 **2 5.0 wt% NaNO3 + 15% wt NaOH 55 11.5 3 7.5 wt% NaNO3 + 15% wt NaOH 55 10.5 4 10 wt% NaNO3 + 15% wt NaOH 55 10.5 Examples 5-8 Example Etchant Temp.(°C) Time* **5 5.0 wt% NaNO3 + 20% wt NaOH 55 11.0 6 7.5 wt% NaNO3 + 20% wt NaOH 55 10.5 7 10 wt% NaNO3 + 20% wt NaOH 55 10.5 8 15 wt% NaNO3 + 20% wt NaOH 55 11.5 Examples 9-13 Example Etchant Temp. (°C) Time* 9 7.5 wt% NaNO3 + 10% wt NaOH 55 14.5 10 7.5 wt% NaNO3 + 15% wt NaOH 55 10.5 11 7.5 wt% NaNO3 + 20% wt NaOH 55 10.5 12 7.5 wt% NaNO3 + 25% wt NaOH 55 10.5 13 7.5 wt% NaNO3 + 30% wt NaOH 55 12.0 Examples 14-20 Example Etchant Temp. (°C) Time* 14 20 wt% NaNO3 + 8% wt NaOH 55 20 15 30 wt% NaNO3 + 8% wt NaOH 55 25 16 35 wt% NaNO3 + 8% wt NaOH 55 21 17 40 wt% NaNO3 + 8% wt NaOH 55 24 18 40 wt% NaNO3 + 10% wt NaOH 55 24 19 40 wt% NaNO3 + 15% wt NaOH 55 21 20 40 wt% NaNO3 + 20% wt NaOH 55 22 Examples 21-22 Example Etchant Temp.(°C) Time* 21 7.5 wt% NaNO3 + 15% wt KOH 55 12 22 7.5 wt% NaNO3 + 0.5% KNO3 + 55 12 15% wt KOH Examples 23-27 Example Etchant Temp.(°C) Time* **23 5.0 wt% Fe(NO 3 ) 3 + 15% wt NaOH 55 11.5 **24 5.0 wt% Mg(NO 3 ) 2 + 15% wt NaOH 55 10.5 **25 5.0 wt% Cr(NO 3 ) 3 + 15% wt NaOH 55 15.0 26 7.5 wt% KNO3 + 15% wt NaOH 55 15.5 27 7.5 wt% CoNO3 + 15% wt NaOH 55 14.5 Note: In table 1 and in the above examples all examples marked ** fall outside the scope of the invention.

Claims: A method of chemically milling aluminium from copper circuit patterned on a surface of an aluminium foil, the aluminium surface free of copper being laminated on a substrate, the method comprising the steps of: contacting the laminate with an etchant comprising an aqueous etching solution of: (1) 60 to 500 g/l base selected from (a) sodium hydroxide, (b) potassium hydroxide, and (c) their mixture; and (2) 70 to 500 g/l of a nitrate salt at a temperature of 25 to 95°C for a time sufficient to remove a desired amount of the aluminium layer and form electronic circuitry including a conductive copper circuit layer. A method according to claim 1, wherein said etchant temperature is 25 to 65°C. A method according to claim 2, wherein said etchant is at a temperature of 40-65°C. A method as claimed in any one of the preceding claims wherein the base comprises 150 to 250 g/l. A method according to any preceding claim, wherein said nitrate is a salt of a metal selected from the group consisting of sodium, potassium, iron, manganese, zinc, lithium, silver, calcium, ammonium, cobalt, copper, aluminium, mercury, thorium, barium, guanidine, lead, cerium, bismuth, zirconium, strontium, cadmium, lanthanum, nickel, and mixtures of any of them. A method of chemically milling aluminium from copper-aluminium tri-metal layers being copper circuit patterns present on opposing surfaces of an aluminium foil, one of the copper patterns being laminated on a substrate, the method comprising the steps: contacting the tri-metal laminate with an etchant comprising 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) 70 to 500 g/l of a nitrate salt at a temperature of 25 and 95°C for a time sufficient to selectively remove a desired amount of the aluminium layer and form electronic circuitry which contains multiple conductive copper circuit layers.

Inventors: ACHARI ACHYUTA, LI DELIN, ACHARI, ACHYUTA, LI, DELIN
Applicants: FORD MOTOR CO, FORD MOTOR COMPANY
Application year: 1998
Main classification: C23F1
IPC codes: C23F1, H05K3