TREATMENT OF COPPER FOIL

Abstract: A method of treating copper foil for use in the production of printed circuit boards includes the step of depositing onto the copper foil a layer containing nodular or dendritic zinc in such a manner as to increase the bond strength of the foil with a base material compared with untreated foil.

Description: THEA-EHE-IT OP GQPPΞH FOIL TECHNICAL PI-ΞLD The present invention relates to a method of treating copper foil, which may be rolled or, prefer¬ ably, electroplated, for use in the production of printed circuit boards to improve the foir's adhesion to a base material, and to laminates formed as a result of said treatment. BACgGEOUIT-D A-BT Copper foil used in the manufacture of printed circuit boards is usually bonded to a dielectric base material and various proposals have been made for improving the strength of the bond between the copper oil and the base material. For example it is now well-established practice to apply copper dendrites to the matt surface of the foil, said dendrites often being reinforced or partially encapsulated with a continuous, coherent layer of copper. Several subsequent electrcdeposited layers may then follow to achieve a bond of desired strength and one which will be largely unaffected by soldering. Metallic layers of, for example, zinc, brass or nickel may be present to separate the copper dendrites from the reactive resins in the dielectric base material. It is often difficult to control the size and distribution of the dendrites in the primary ' copper layer and it has therefore been proposed to add OMPI various agents to the copper of said layer in an attempt to effect such control, a preferred additive being arsenic. In such a case, a nodular or dendritic layer of metal comprising an alloy of oxidised copper with arsenic is electrodeposited-onto_- _ a matt surface of the copper foil from a bath contain¬ ing copper sulphate and arsenic values. However reliable control in the growth of the dendrites, particularly the larger dendrites, is still not completely satisfactory. DISCLOSURE OP THE I3-TVΕ-ITI0IT According to the present invention there is provided a method of treating copper foil including " the step of depositing onto the foil at least one nodular or dendritic layer containing zinc whereby the bond strength of the foil with an associated base material is improved. The or each nodular or dendritic layer may be of zinc or, alternatively, brass. The method may include the steps of depositing a nodular or dendritic layer of zinc or brass directly onto a matt surface of the foil, and encapsulating said layer with one or more layers of metal such as copper, arsenic, bismuth, brass, bronze, nickel, cobalt or zinc, or alloys / co-deposits of said metals. The presence of such encapsulating layers, which are of continuous, coherent metal as opposed to a powdery or - U OM £ PIA microcrystalline dendritic deposit, can result in an increased peel strength when bonded to glass-epoxy boards compared with foil without such encapsulation of the dendrites, and furthermore ensures that, under conditions of strong resin flow during lamination - such as high pressure and temperature conditions which can cause certain epoxy resins tc flow prior to setting hard - no zinc or brass dendrites are detached from the primary layer to become embedded in the resin mass. The method may further include the step of applying over the, or the outermost encapsulating layer a further layer containing zinc. Said further layer is preferably nodular or dendritic and. is preferably an alloy of zinc and nickel, although an alloy of zinc and cobalt could be used. The amount of nickel in the alloy is conveniently within the range ~ to 15% and is preferably of the order of 10%. The provision of such a further layer increases the bond strength of the foil with a dielectric base material compared with a foil without said outer layer. Such layers are normally thinner than those applied as the innermost layer next to the basic copper " foil matt. In one method according to the invention, the treatment comprises the steps of depositing onto a att surface of the foil a first nodular or dendritic - -. - layer of zinc, depositing a protective layer of • cyanide copper over said first nodular or dendritic layer, depositing an encapsulating layer of pyrophosphate copper over said protective layer of cyanide copper, and depositing a further nodular or dendritic layer of zinc nickel alloy over said encapsulating layer. Under some laminating conditions, when the temperature may be in excess of 165° C,there can be pronounced interdiJTfusion between the outermost nodular or dendritic layer of zinc or zinc alloy and the underlying copper layer which produces a canary yellow brass layer. This phenomenon is often accompanied by considerable loss of peel strength. Accordingly, in a further method there is provided a diffusion-inhibiting layer of metal under¬ lying said further layer containing zinc. The diffus¬ ion-inhibiting layer may replace the encapsulating layer of pyrophosphate copper or may be deposited over said encapsulating layer. The diffusion-inhibit¬ ing layer may comprise, for example, an alloy of copper with arsenic or bismuth or, alternatively, a weak deposit of pure arsenic or bismuth. Thus there is provided a treatment which results in the formation of zinc or brass dendrites as the primary adhesion-promoting layer of a copper., foil to OMPI be laminated to a dielectric substrate, control in the size asd distribution of the dendrites being relatively easy to achieve. The preferred zinc dendritic layers can readily be produced from a number of electrolytes of which perhaps the most economical and simple is a relatively dilute solution . of sodium zincate. Brass dendrites can conveniently be produced from solution containing sodium zincate and copper salt. In general, the size'of the dendrite increases proportionately with increase in zinc concentration,, time of plating and current density and inversely with the temperature of the solution. In methods in which a further, outermost nodular or dendritic layer is provided, there can, under certain circumstances, be problems if pure zinc is used for this layer, particularly if washing between the stages of the process is not perfect. In the case of pure zinc deposited from an alkali bath, there can be difficulties in effecting complete washing of the alkali solution, and any retained solution can interfere with the polymeris¬ ation of the epoxy layer and cause a consequent reduction in peel strength. -Further, and as mentioned above, if the outer zinc layer is deposited over a copper-based encapsulating layer, there is a tendency for the pure zinc to react with the copper whereby the zinc is converted into brass, which can result in a variability in peel strength dependent upon the nature of the conversion. The provision of an outermost nodular or dendritic layer which is an alloy of zinc with nickel tends to obviate these problems, in that the presence of the ' nickel in said layer slows down or prevents the diffusion of the zinc into the underlying copper and ' also prevents undercut during etching (the metal of this layer being less soluble than zinc in acid). A further method of inhibiting the conversion of the zinc of the outer layer into brass, which method may be used instead of or in addition to the incorporation of nickel into the outer layer, is, again as mentioned above, to provide a layer of or containing arsenic underlying said outer layer, although bismuth could be used in place of arsenic. Conveniently such a conversion-inhibiting layer replaces the encapsulating layer of pyrophosphate copper, although it may be included in addition to ' said encapsulating copper layer between said encapsulating copper layer and the outermost nodular or dendritic layer of zinc or zinc nickel alloy. The presence of such an arsenic-based layer enhances OMPI the method by making it less sensitive to contamin¬ ation which is particularly useful in industrial processes where 100% washing is rarely achieved. BEST MOSES OP CARRYING OUT THE DTVΞffTION ■' - EXAMPLE 1 The following electrodeposited layers were applied to the matt side of an electroformed copper foil 55 microns thick, all formulae being expressed as concentrations in water: 1st Layer A bath was made up as follows: Zno 20 g/1 ffaOH 90 g/1 Current density 10 amps/sq.dm. Time 30 sees. Temperature 2 C. Anode zinc, or insoluble such as nickel or stainless steel. This process produces a fine layer of zinc dendrites which covers the matt structure of the foil, producing the greatest concentration of dendrites on the peaks of the matt. 2nd Layer Before these dendrites are encapsulated by copper plating, a thin layer of cyanide-type copper plating is applied to prevent the formation of precipitated copper by the chemical replacement reaction between zinc metal and copper ion. Such a cyanide-type copper layer is conveniently deposited using the following bath: CuCiT 15 g/1 aC 23 g/1 Na 2 C0 3 15 0 sΛ Temperature 40° C. Current density 3 amps/sq.dm. Anode Copper Time 10 seconds 3rd^Lay ' -r _ A continuous, coherent layer of copper is then plated on the foil using the following bath: sq.dm A pi k coloured encapsulating plating layer of sound pyrophosphate copper is deposited under the above conditions. 4th Layer An outer layer of nodular or dendritic zinc/nickel alloy is then formed using the following bath: !TiS0 6H 2 0 263 gA ZnS0 7H 2 0 287 g PH 5 ' Current density 2-8 amps/sq.dm. Anode Lead or-zinc Temperature 40 C. Time 10 sees. These conditions produce a slightly dendritic final covering layer of zinc/nickel which gives the matt O PI surface a clean dark grey colour. The foil is well washed by means of wa er sprays between each treatment stage. The overall process is carried out by passing copper foil in serpentine fashion, using both submersed and free standing rollers, .through a series of plating tanks and rinsing stations. The current in each plating tank is seperately controlled and the fime of plating is determined by the speed of the copper web and the length of the anode used in the particular cell. Preferably the plated foil is passed through a dilute warm solution of chromic acid which renders the shiny side of the foil passive and provides a shelf life of some months before staining of the foil occurs. This chromate treatment also stabilises the treatment surface and gives a somewhat enhanced peel strength. After treatment the foil is washed, air dried and then reeled up.. Copper foil treated in the above manner, when laminated to epoxy resin impregnated glass cloth sheets under heat and pressure, provides a laminate the peel strength of which is of the order of 5 to 5.5 kp/in. both before and after solder float. The time taken to blister when floated on solder is in excess of 5 mins. Etching with ferric chloride or persulphate type etches gives no undercut. The O PI - treatment is firm and produces no "tape transfer" and, even with very reactive resins, there is no evidence of any discolouration showing that the treatment is inert. It is noteworthy that surprisingly good peel strength is already available after the first layer of zinc dendrite is deposited onto the copper matt. Such a single stage, treatment (taking the 1st layer only and passing it through dilute warm chromic acid solution) produces a copper foil having a peel strength of 4-4.2 kp/in. on epoxy glass laminate. It is to be noted that the provision of an outer- layer of zinc nickel alloy substantially -inhibits the conversion of said layer into brass. Said layer conveniently incorporates 10% nickel, although this percentage can be adjusted by varying the constituents of the bath. It is however preferred to keep the percentage of nickel above 3 - otherwise the degree of inhibition of conversion reaction is slight, and below 15, otherwise the resultant alloy is not etc able with commercially available etchants. Example 2 A further means of controlling the diffusion of the zinc of the outermost layer into the underlying copper is to provide a layer containing arsenic between the zinc-based and copper-based layers. The first two steps of such a process are identical to those of Example 1, but the third layer is plated from a bath containing copper sulphate solution as follows: Cu (as metal) 3 to 30 g/1 As 100 to 1000 mg/1 H 2 S0 50 g/1 Temperature 20*-° C. Current density 1-10 a ps/sq dm. Anode Lead Time 10 seconds Such a bath produces a diffusion-inhibiting layer, of copper, copper oxide and arsenic, and, if an outermost layer of zinc nickel alloy is electro- deposited as in Example 1, a peel strength of- 4.S to 5.2 kp/inch is achieved with no conversion of the zinc in said outermost layer to yellow brass. Further, a s is the case in Example 1, the presence of the nickel in the outermost layer prevents any undercutting on etching, even with aggressive etchants such as ferric chloride. Clearly the third layer could be plated from a solution similar to that above but without copper to give a weak layer of pure arsenic, while the outermost layer, if desired, could be plated from a conventional zinc sulphate bath or any alternative zinc - containing electrolyte to give a layer of zinc. Example 3 In cases where lamination to certain glass epoxy materials is to be eff.ected at temperatures which are somewhat lower than 165° C. and the time for poly- erisation is to be relatively short, it may be desirable to utilise a copper foil which has undergone a three-stage treatment as follows: the first layer is zincate zinc produced as the first layer of Example 1; the second layer is of cyanide-type copper, as the second layer of Example 1 but the time of plating being 2 seconds at- 1 amp/sq. dm; the third layer is of pure zinc deposited from a zincate solution for 8 seconds at 1 amp/sq. dm. Under these conditions, the degree of inter- diffusion between the outermost zinc dendrites and the underlying copper layer is limited and the resultant peel strength is in the region of 5- kp/inch. The colour of the treatment after lamination is yellow/ brown and the undercut a er etching is only very slight ' . O- PT

Claims: CLAIMS 1. A method of treating copper foil characterised by the step of depositing onto the foil at least one nodular or dendritic layer containing zinc whereby the bond strength of the foil with an associated' base material is improved. 2. A method of treating copper foil as claimed in claim 1 in which the or each nodular or dendritic layer is of zinc. 3. A method of treating copper foil as claimed in claim 2 including the steps of depositing a nodular or dendritic layer of zinc directly onto a matt surface of the foil and encapsulating said layer with one or more layers of metal. 4. A method of treating copper foil as claimed ia claim 3 in which the metal of the encapsulating layer is one or more of the group of copper, arsenic, bismuth, brass, bronze, nickel, cobalt or zinc, or alloys or co-deposits of said metals. 5- A method of treating copper foil as claimed in claim 3 or claim 4 and further including the step of applying over the, or the outermost encapsulating layer a further layer containing zinc. 6. A method of treating copper foil as claimed in claim 5 in which said further layer containing zinc is nodular or dendritic. "5U £^ 7. A method of treating copper foil as claimed in claim 6 in which the further nodular or dendritic layer is an alloy of zinc and nickel. 8. A method of treating copper foil as claimed in claim 7 in which the amount of nickel in the alloy of the further layer falls within the range 3 to 15% • 9. A method of treating copper foil as claimed in claim 8 and comprising the steps of depositing onto a matt surface of the copper foil a rst nodular or dendritic layer of zinc, depositing a protective ' layer of cyanide- copper over said first nodular or dendritic layer, depositing an encapsulating f layer of pyrophos- phate copper over said protective layer of cyanide copper and depositing a further nodular or dendritic layer of zinc nickel alloy over said encapsulating laye . 10. A method of treating copper foil as claimed in claim 5 and including the step of -depositing a diffusion-inhibiting layer of metal'underlying said further layer containing zinc. 11. A method of treating copper foil as claimed in claim 10 in which the diffusion-inhibiting layer contai s arsenic. 12. A method of treating copper foil as claimed in claim 11 in which the diffusion-inhibiting layer comprises an alloy of copper with arsenic. 13. A method of treating copper foil as claimed in claim 12 and comprising the steps of depositing onto a matt surface of the copper foil a first nodular or dendritic layer of zinc, depositing a protective layer of cyanide copper over said first nodular or dendritic layer, depositing a diffusion-inhibiting layer of copper arsenic alloy over said protective layer of cyanide copper and depositing a further nodular or dendritic layer of zinc nickel alloy over said diffusion-inhibiting layer. 14. A method of treating copper foil as claimed in claim 6 and comprising the steps of depositing onto a matt surface of the copper foil a first nodular or dendritic layer of zinc, depositing a protective layer of cyanide copper over said first nodular or dendritic layer of zinc,and depositing a further nodular or dendritic layer of zinc over said protective layer of cyanide copper. 15. Copper foil treated according to the method of claim 1. 16. A laminate comprising a dielectric substrate to which is bonded copper foil treated according to the method of claim 1. "gUREA OMPI

Inventors: BUSH MICHAEL EDWARD, TORDAY JOHN
Applicants: BUSH MICHAEL EDWARD, ELECTROFOILS LTD, TORDAY JOHN
Application year: 1982
Main classification: H05K3
IPC codes: B32B15, B32B37, B32B38, C25D3, C25D5, H05K3