GB2133783A - Oxidising compositions - Google Patents

Abstract

Simple compositions for oxidising the surface of the copper tracks on a printed circuit board contain: from 37.3 to 89.5gpl (measured as chlorite, C102) alkali metal chlorite; from 4.5 to 8.5gpl (measured as hydroxide, OH) of alkali metal hydroxide; and from 12.5 to 2.5gpl (measured as phosphate, PO4) of alkali metal phosphate; the total alkalinity of the composition being equivalent to from 15 to 25gpl NaOH (6.5 to 10.5gpl hydroxide, OH).

Description

SPECIFICATION Oxidising compositions This invention concerns oxidising compositions, and relates in particular to aqueous oxidising compositions suitable for use in the oxidising of copper to form a copper oxide film on the surface thereof.

In the early days of electronics the relevant circuit components-resistances and valves, for exampic were mounted upon a chassis (often of an electrically conducting material), and interconnected by what is known as point-to-point wiring (individual wires from one component to the next). Later, and still used today, the chassis was replaced by an electrically insulating sheet or board, and the individual, separate, wires were replaced by conductive strips (tracks) secured to the board surface.These tracks were commonly formed starting with a sheet of conductive material covering the whole board and etching away those areas not required and since this selective etching was effected by printing onto the material a protective pattern defining the areas to be etched and the areas to be left, the finished boards were (and are) known as "Printed Circuit Boards" (PCB's).

In more recent times the need for higher and higher component densities, and more and more complex circuits, has led to the use of multilayer PCB's. These are in effect a whole set of PCB's laminated (sandwiched) together; accordingly, they commonly consist of a series of sheets of the electrically insulating substrate material, typically a glass fibre reinforced epoxy resin, in face-toface contact, these having disposed therebetween (and on the outside) thin strips (tracks) of the electrically conducting material, typically copper.The copper tracks define the main conducting elements of the circuit, the insulating sheets (boards) are the substrate carrying the circuit, and depending upon the nature of the circuit various components-resistors, capacitors, transistors, for instancc will be mounted upon the exterior surfaces of the assembled boards in suitable electrical connection with the tracks (and thus with each other).

Multilayer PCB's having a plurality of circuit and substrate layers-for example, three circuit layers and two substrate layers, disposed circuit/substrate/circuit/substrate/circuit-are now quite common in complex electronic equipment.

PCB's of the multilayer variety may conveniently be prepared by making individual boards (one substrate layer with tracks on one or both sides) and bonding a number of these together, face-to-face, with-if appropriate--an intermediate layer of substrate (or substrate-compatible) material. However, some difficulty has been experienced in the past in ensuring that the thusassembled individual boards do in fact stay bonded one to the next. In an effort to reduce the chances of delamination of multilayer boards using copper tracks it is present-day practice to form, prior to the bonding stage, a thin oxide film on the originally-open surface of the tracks.

The idea, born out by extensive practice, is that a microscopically rough copper oxide layer which adheres tightly to the underlying copper should provide a good mechanical key to the overlying substrate layer that is to be laminated thereto, and thus the risk of de-lamination should be very considerably reduced. Current practice suggests that the oxide layer be formed by oxidation of the base copper, and numerous oxidising compositions have been suggested for this purpose. One successful class of such compositions is that comprising an aqueous, alkaline buffered, chlorite solution, and it is with this class that the present invention is concerned.

For some years it has been usual to employ, as an oxidising composition for forming a copper oxide layer on the open surface of the copper tracks of PCB's (prior to their lamination into multilayer PCB's), an aqueous alkaline buffered chlorite solution. In general the chlorite is an alkali metal chlorite (conveniently sodium chlorite) at about 30gpl, the solution's alkalinity is achieved using an alkali metal hydroxide (conveniently sodium hydroxide) at about 1 Ogpl, and the buffering is effected with an alkali metal phosphate (conveniently sodium phosphate) at about Sgpl. The total alkalinity of such a buffered solution is equivalent to about 11gpl NaOH, and in use the solution is contacted with the copper at roughly 1 00 C.

This type of simple solution has its drawbacks, however. Thus: it is rather unstable, with a short operating life; in use, it is difficult to control analytically; and the need for a very high use temperature (coupled with the rather corrosive nature of the solution ingredients) makes it unpleasant to handle. Accordingly, in an effort to overcome some or all of these defects various proprietary compositions of the same general type (chlorite/hydroxide/phosphate buffer) are available.One of these (A) employs 50gpl NaClO2, about 20gpl NaOH and 70gpl Na 3P04.12H20), has a total alkalinity equivalent to about 25 to 35gpl NaOH, and is said to be useable at 55 to 65 C. Another (B) has the same ingredients but contains 1 65gpl NaClO2, has a total alkalinity equivalent to about 1 7gpl NaOH, and is also said to be useable at 55 to 65 C.

Unfortunately, though these (like the simple solution mentioned first above) give good quality oxide coatings that do indeed enhance lamination, also like the simpler solution they suffer from various drawbacks. Thus:- the A solution is easily fatigued (though it may recover in time), and is not very temperature stable (it crystallises out when cold, which is inconvenient in a production process); the B solution is unstable, with a short working life, and its use is difficult to control analytically.

It is the purpose of the present invention to provide a novel chlorite-based copper-track oxidising solution which does not suffer to any significant extent from the disadvantages of those presently available.

In one aspect, therefore, the invention provides an aqueous alkaline buffered oxidising composition containing: from 37.3 to 89.5gpl (measured as chlorite, Coo2) alkali metal chlorite; from 4.5 to 8.5gpl (measured as hydroxide, OH) of alkali metal hydroxide; and from 12.5 to 2.5gpl (measured as phosphate, P04) of alkali metal phosphate; the total alkalinity of the composition being equivalent to from 15 to 25gpl NaOH (6.5 to 10.5gpl hydroxide, QH).

Depending upon the exact make-up (see below), the inventive compositions are suitable for use at a temperature of from 50 to 85 G, and in the case of the preferred compositions advantageously 65 to 75 C, especially about 70 C.

The alkali metal chlorite is conveniently sodium chlorite (NaClO2). it is employed in amounts of from 37.3 to 89.5gpl chlorite (roughly 50 to 1 20gpl of NaClO2), for less results in a very "slow", easily fatigued, composition, while more causes the formation of undesirable deposits on the treated surfaces; advantageously the compositions contain from 4 & to 56gpl chlorite (roughly 65 to 75gpl Na2Cl02), and in general the higher the chlorite level the lower the use temperature needs to be. A particularly preferred chlorite level is 51 gpl (68gpl NaClO2).

The alkali metal hydroxide is advantageously sodium hydroxide (NaOH). It is conveniently employed at about 6.4gpl hydroxide (roughly 1 5gpl NaOH).

The alkali metal phosphate is preferably sodium phosphate (Na3PO4), conveniently employed as the dodecahydrate (NA3PO4. 1 2H20). It is in amounts from 12.5 to 2.5gpl (50 to 1 Ogpl of the preferred hydrated sodium salt), and in genral the more hydroxide the less phosphate. More phosphate results in problems with temperature stability, while less means the buffering ability may deteriorate rapidly.

The inventive compositions must have a total alkalinity-made up of the actual hydroxide and the effect of the strong base/weak acid phosphate buffer cquivalent to 15 to 25gpl NaOH (roughly 6.5 to 10.5gpl hydroxide). Compositions with lower alkalinity cause undesirable deposits to be thrown down during use. The preferred total alkalinity is equivalent to about 21gpl NaOH (about 9gpl hydroxide).

Although it is fully expected that each component of the inventive composition may individually be employed in amounts up to the limits of the given ranges, there is a possibility that if all the components are at or near one or other limit the composition may not behave in entirely the desired fashion. It is recommended, therefore, that compositions in which all the ingredients are at the extremes of their ranges be avoided.

The compositions of the invention, which in general have a pH of about 1 2#5, may be made up in any conventional manner commonly used with the known compositions, and no specialprecautions need to be taken. For example, they may be prepared simply by dissolving, with some heating, first the hydroxide and then the phosphate in distilled water, and then adding thechlorite (conveniently as an aqueous concentrate) and finally making the volume- up to the use level with distilled water.

Similarly, in use each composition may be replenished in the normal way-thus, by dissolving sufficient hydroxide in the operating bath to correct any deficiency in its alkalinity, and then adding as much chlorite as is required to bring the oxidant content back up to the chosen start level. Errors resulting in an overabundance of any component are best corrected by dilution.

The use of the compositions of the invention may also be according to any of the conventional methods presently employed or suggested. Thus: The copper surface to be oxidised is first cleaned with- a slightly acidic copper cleaner (such as Oxytron 14 from Oxymetal Industries) and then mildly etched with, for example, an acid solution of ammonium persulphate. The cleaned copper surface is then brought into contact with the inventive oxidising solution at the chosen working temperature (i.e., about 65 C), and after sufficient time to produce a dark (but not jet black) layer of oxide coating-typically about 7 minutec the oxidised surface is removed, rinsed in cold tap water, and then given a-final rinse with deionised water to remove the last traces of inorganic salts from the coating. The coating is then dried in warm air so as quickly to remove any remaining water, thus preventing any undesirable corrosion reactions.

The invention naturally extends to a process, using a composition of the invention, for oxidising a copper surface (such as the tracks of a printed circuit board), and to such an article with a surface whenever so oxidised.

Various aspects of the invention are now described, though by way of illustration only, with reference to the following Examples.

Example 1: Preparation of three different compositions according to the invention The following compositions were formulated in the described manner: Composition 1 2 3 Ingredient g gpl g gpl g gpl NaOH 20 10 36 18 30 15 Na3PO4. 1 2H20 90 45 32 16 60 30 NaClO2 110 55 220 110 136 68 (338ml (676ml) (41 8ml) Alkalinity 16.8 22 20.4 (as gpl NaOH) Use Temp ("C) 78 i 2 55 i 2 70 i 2 The sodium hydroxide, in the form of standard pellets, was dissolved in 1 litre of distilled water in a three litrebeaker, and the sodium phosphate added to the formed solution with moderate heating.The sodium chlorite was then added (in the form of a previously-prepared aqueous concentrate containing 325gpl), and the total volume was made up to 2 litres with distilled water.

The alkalinity of each bath was then measured using the method outlined below, and found to be acceptable.

Example 2: Use of the compositions of Example 1 Each of the compositions of Example 1 was used to oxidise the surface of a number of typical copper-bearing articles (there were printed circuit boards having a glass fibre/epoxy resin substrate and bearing copper tracks on both major surfaces, cut into 4-by-4 inch (roughly lO-by- 10cm) test panels). The process used was as follows: (A) Cleaning pre-treatment Each test panel was immersed at room temperature in a 40% v/v solution of Oxytron 14 for 3 minutes, then rinsed in tap water for 1 minute. The panel was then immersed in an acid solution of ammonium persulphate (a mild etchant) for 2 minutes, followed by a 1 minute tapwater rinse. Finally, the panel was immersed in 20% v/v sulphuric acid for 30 seconds, followed by a 2 minute tap water rinse.

(B) Oxidative treatment Each panel was immersed in the chosen inventive oxidant composition (1, 2 or 3) at the relevant remperature for 7 minutes (with gentle agitation throughout), whereupon it was removed and inspected. If the copper detail was completely coated with a very dark brown oxide the panel was passed to the next stage, otherwise it was returned to the oxidising solution for a further three to five minutes (this was only occasionally necessary, and was probably related to poor initial cleanliness).

The satisfactorily oxidised panel was rinsed in tap water for 1 minute, and then again in demineralised water for 1 minute, and dried carefully in an oven at 70 C for 10 minutes. Care was taken to ensure that the coated surfaces did not contact the sides of the oven or other panels. When dry, the oxidised panels were laid between tissue paper for protection prior to lamination.

(C) Bath replenishment At periodic intervals the bath was tested for the correct concentration of oxidant and for the desired total alkalinity. The tests (which are standard) were as follows: (1) Alkalinity The alkalinity was determined by titrating a Sml sample with 0.1 ml HCI using phenolphthalein indicator. Any deficiency was remedied by adding sodium hydroxide.

(2) Oxidant Strength This was determined by means of an iodine titration using sodium thiosulphate as the reagent. Sodium chlorite was added to remedy any deficiency.

Test Results Panels treated using the methods of Example 2 were compared with similar panels treated using two commercially-available oxidising compositions (A and B referred to herein before; both were employed according to the manufacturer's instructions). All the compositions gave good quality coating (both visually and as determined by standard peel tests), but only those of the invention were satisfactory as regards fatiguing (a reduction of efficiency, often only temporary, during hard use), stability (as measured by crystallisation when left for a long period), useful life (the length of time the bath, with replenishment, continues to provide acceptable coatings) and response to process control (the ease with which the ingredients could be replenished following simple tests during use).

The Test Results are given in the Table below.

Table Composition 1 2 3 A B Test Test Temperature 53 78 70 59 59 Observed Fatigue None None None Yes None Overnight Crystallisation Temperature ( C) 16 14 15 28 20 Useful life (Lab scale (days) 8-11 12-14 12-16 12-14 7-10 Easeof control good good excellent good poor Composition No. 3 was employed on a production scale and was still operating satisfactorily after a month.

These results clearly demonstrate the advantages of the inventive compositions as regards stability and ease of control.

Claims (7)

1. An aqueous alkaline buffered oxidising composition containing: from 37.3 to 89.5gpl (measured as chlorite, ClO2) of alkali metal chlorite; from 4.5 to 8.5gpl (measured as hydroxide, OH) of alkali metal hydroxide; and from 12.5 to 2.5gpl (measured as phosphate, PO4) of alkali metal phosphate; the total alkalinity of the composition being equivalent to from 15 to 25gpl NaOH (6.5 to 10.5gpl hydroxide, OH).

2. A composition as claimed in claim 1, which contains from 48 to 56gpl chlorite.

3. A composition as claimed in either of the preceding claims, wherein the alkali metal chlorite is sodium chlorite (NaCIO2).

4. A composition as claimed in any of the preceding claims, wherein the alkali metal hydroxide is sodium hydroxide (NaOH).

5. A composition as claimed in any of the preceding claims, wherein the alkali metal phosphate is sodium phosphate dodecahydrate (NA3PO4. 1 2H20).

6. An aqueous alkaline buffered oxidising composition as claimed in any of the preceding claims and substantially as described hereinbefore.

7. A process for oxidising a copper surface, using an aqueous alkaline buffered oxidising composition as claimed in any of the preceding claims, which process is substantially as described hereinbefore.