PROTECTIVE COATINGS

Abstract: A printed circuit board having applied thereto a protective coating of a thermally depolymerisable polycarbonate.

Description: PROTECTIVE COATINGS The present invention relates to protective coatings and, in particular, to the use of a thermally depolymerisable polymer as a protective coating for printed circuit boards (PCB's) . The copper tracks on printed circuit boards are frequently coated with a thin layer of an organic material, to reduce oxidation in storage and handling, prior to fluxing (by spray, foam and so on) and subsequent soldering operations. Traditionally, these coatings have been based on the use of low viscosity rosin or shellac lacquers (more recently, similar synthetic materials have been used) in a volatile solvent. This composition is roll-coated onto the printed circuit board, and dried to give a layer typically 0.5 μm thick. Modern formulations have the disadvantage that they have low solubility in the new low-solids fluxes and these fluxes are therefore required to be very active (and therefore potentially corrosive) , in order to allow good wetting, by liquid solder. Thermally depolymerisable polycarbonates have been described by F.M. Houlihan, F. Bouchard, J.M.J. Frechet and G.G. Wilson in Macromolecules 1986, 19, 13-19. The materials which are described are copolycarbonates containing a tertiary diol structure which was prepared by using a solid-liquid phase-transfer-catalyzed polycondensation of the bis(carbonyli idazolide) of 2,5-dimethyl-2,5- hexanediol with various other diols in the presence of powdered potassium carbonate. These copolycarbonates decompose cleanly upon heating to about 200°C with liberation of volatile materials, leaving no solid residue. We have now surprisingly found that thermally depolymerisable polycarbonates can be used as very effective protective coatings for printed circuit boards. This is, essentially a low/no residue approach to the problems encountered with wave soldering to PCB's. Furthermore, as the polycarbonates depolymerise rather than pyrolyse, they do not require the presence of oxygen and can be processed in an inert atmosphere such as nitrogen. Accordingly, the present invention provides a printed circuit board having applied thereto a protective coating of a thermally depolymerisable polycarbonate. The thermally depolymerisable polycarbonates used in the present invention may be produced by the condensation reaction between a diol, or a mixture of diols, and phosgene or a phosgene analogue such as l,l'-carbonyldiimidazole. The resulting polymers generally will contain the repeating unit ( R l_0- -0-R 2 -0-C-0) where R^ and R 2 are the same or different and each independently represents a hydrocarbon group containing from 4 to 30 carbon atoms, the said group having a tertiary carbon atom, an allylic, propargylic or benzylic group attached directly to at least one oxygen atom. The hydrocarbon group may be saturated or unsaturated containing one or more double or triple bonds, or an aromatic group. The polycarbonates owe their thermal lability to the structure of the diols which are used in their production. The diols generally possess tertiary benzylic, allylic or propargylic groups bonded to the hydroxyl functional groups in the diols. Examples of diols which may be used in the preparation of the polycarbonates used in the present invention include m-benzenedimethanol, 2,5-dimethyl-2,5-hexanediol, 2-cyclohexen-l,4-diol and 2-butyne-l,4-diol, or a mixture of two or more thereof. The thermally depolymerisable polycarbonates are used, in accordance with the present invention, as protective coatings for printed circuit boards. They generally decompose at a temperature in the range of from 200° to 250°C and thus, when heated to a temperature in this range leave no residues. It will be appreciated by those skilled in the art that it may be advantageous to use a mixture of thermally depolymerisable polycarbonates in the protective coating used in the invention. The use of such a mixture will provide the ability to vary the depolymerisation temperature, the depolymerisation rate and the coating properties of a solution formed therefrom. The protective coating used in the present invention preferably includes an activator therein, such as dibromostyrene, dibromobutanediol, benzotriazole or mixtures thereof, which are aids to soldering to the protected printed circuit board. Other activators may also be used as will be apparent to those skilled in the art. The coating of a board with activator alone would result in a premature activation of the board which would hinder the subsequent soldering process. However, by encapsulating the activator in a polycarbonate coating, activation of the board is delayed. Thus the activator is dispersed over the board, but does not come into contact with it until the board is heated and the polycarbonate begins to thermally decompose. By this means, adequate activation of the board is successfully achieved at the right time to allow good solder joints to be formed. One great advantage from incorporation of the activator in this manner is that the number of steps in the soldering cycle can be decreased. Passage of the board through an activating flux or preparation fluid is no longer required, and if a low residue level polycarbonate and volatile activator are used, then the cleaning step may also be left out. Chemical protection of a board may be effected by coating the board with an inhibitor such as imidazole. However, such coatings are not robust, and afford little mechanical protection. Incorporation of the inhibitor in a coating of polycarbonate results in an improvement in both mechanical and chemical protection. The present invention also provides a method of applying a protective coating of a thermally depolymerisable polycarbonate to a printed circuit board which comprises coating the printed circuit board with a solution of the polycarbonate in an organic solvent and removing the solvent to form a coating of the polycarbonate on the printed circuit board. The protective coating may be applied to the printed circuit board by any suitable method, for example by dip coating, spray coating or roller coating. In carrying out the method of the invention the solvent is generally removed from the coating by heating to a temperature below the thermal degradation temperature of the polycarbonate. The protective coating of thermally depolymerisable polycarbonate applied to the printed circuit boards will generally have a thickness of l.o micrometres or below, preferably 0.50 micrometres or below, more preferably about 0.25 micrometres. An additional advantage of the present invention which could not have been foreseen is that the decomposition products of the polycarbonates themselves, i.e. CO2 etc, help to maintain an inert atmosphere in the vicinity of the board which assists in good soldering performance. The present invention also includes within its scope the use of a thermally depolymerisable polycarbonate as a protective coating for printed circuit boards. The protective coating of the thermally depolymerisable polycarbonate protects the copper surfaces on the printed circuit boards from atmospheric oxidation and mechanical damage. The protective coatings, particularly those containing activators, enhance the wetting of the copper surfaces during subsequent soldering operations. Furthermore, the protective coatings either disappear before or during soldering, thus leaving little or no residues. The present invention will be further described with reference to the following non-limiting Examples in which parts and percentages are by weight unless otherwise stated. EXAMPLE 1 A polycarbonate was prepared from 2,5-dimethyl- 0,0'bis(1-imidazolylcarbonyl)-2,5-hexanediol and a mixture of 80% 1,4- and 20% 1,3-benzenedimethanol. This polycarbonate is hereinafter designated polycarbonate-I. A 0.5wt% solution of polycarbonate-I in dichloromethane (CH2CI2) was applied by dip coating to one half of a precleaned single sided copper clad printed circuit board (PCB) . A thin continuous glossy coating of the polycarbonate was formed on removal of the solvent. The board was aged in an air circulated oven, at 100°C for half an hour. On removing the board it was evident that substantial protection had been given to the coated region. The board was then heated to 220°C for 5 minutes in an air circulated oven. The board developed a substantially matt appearance from which it was deduced that no polycarbonate remained. EXAMPLE 2 A 0.5wt% solution of polycarbonate-I in dichloromethane was applied, by dip coating, to one half of a precleaned copper substrate. The dried film thickness was not discernable by a micrometer and was estimated at approximately 0.25 / _m thick. The copper substrate was precleaned in ammonium persulphate solution and then water, washed with isopropyl alcohol and dried. The copper substrate was placed in an oven at 160°C and left for 15 hours after which time it could be seen that the substrate was clearly less oxidised where it had been coated. EXAMPLE 3 A 0.5wt% solution of polycarbonate-I in dichloromethane (CH2CI2) was prepared with 0.36wt% of dibromostyrene as an activator to aid in soldering. This activator liberates HBr on heating to soldering temperatures. Two PCB's were partially dip coated in this solution and allowed to dry at ambient temperature. The two boards were then heated up to and held at 80°C/80%RH for 4 hours and 72 hours respectively. The boards were then dipped in to a Pb 2%Sb solder bath at 260°C for 15 seconds. On removing the boards it was evident that areas previously coated with polycarbonate and activator were substantially solder tinned, whilst uncoated regions remained unwetted by the solder. EXAMPLE 4 Printed circuit boards were cleaned prior to coating using the following procedure: (i) wash in ammonium persulphate (20% w/w) solution for 1 minute (ii) rinse thoroughly with tap water iii) wash in sulphuric acid (5% w/w) for 1 minute (iv) rinse thoroughly with tap water and then deionised water (v) rinse with isopropylalcohol and allow to dry The solution used to coat the PCBs had the following composition: Polycarbonate I 2.5% w/w Dibromobutanediol (DBBD) 0.25 w/w gamma-Valerolactone 48.525% w/w p-Xylene 48.625% w/w One week after coating, the coated boards were soldered. A significant improvement in solderability was seen when the boards were soldered under N2 rather than in air. Components on boards coated with this solution all soldered well, with sound joints which were satisfactory. Furthermore there were no problems with bridging between leads on components placed closely together on the board. Uncoated boards cleaned in the manner as described above showed very poor solderability, with very little solder attaching thereto.

Claims: CLAIMS : 1. A printed circuit board having applied thereto a protective coating of a thermally depolymerisable polycarbonate. 2. A printed circuit board as claimed in claim 1 wherein the thermally depolymerisable polycarbonate contains the repeating units where R 1 and R 2 are the same or different and each independently represents a hydrocarbon group containing from 4 to 30 carbon atoms, the said group having a tertiary carbon atom, an allylic, propargylic or benzylic group attached directly to at least one oxygen atom. 3. A printed circuit board as claimed in claim 2 wherein the thermally depolymerisable polycarbonate is prepared by the condensation reaction between a diol, or mixture of diols, and phosgene or a phosgene analogue. 4. A printed circuit board as claimed in claim 3 wherein the diol used in the preparation of the thermally depolymerisable polycarbonate is a diol containing a tertiary carbon atom, an allylic, propargylic or benzylic group, or a mixture of two or more such diols. 5. A printed circuit board as claimed in any one of the preceding claims wherein a mixture of thermally depolymerisable polycarbonates is used. 6. A printed circuit board as claimed in any one of the preceding claims wherein the protective coating additionally includes one or more activators therein. 7. A printed circuit board as claimed in any one of the preceding claims wherein the protective coating additionally includes an inhibitor therein. 8. A method of applying a protective coating of a thermally depolymerisable polycarbonate to a printed circuit board which comprises coating the printed circuit board with a solution of the polycarbonate in an organic solvent and removing the solvent to form a coating of the polycarbonate on the printed circuit board. 9. A method as claimed in claim 8 wherein the thermally depolymerisable polycarbonate is as defined in claim 2. 10. A method as claimed in claim 8 or claim 9 wherein a mixture of thermally depolymerisable polycarbonates is used. 11. A method as claimed in any one of claims 8 to 10 wherein the protective coating additionally includes one or more activators therein. 12. A method as claimed in any one of claims 8 to 11 wherein the protective coating is applied by dip coating, spray coating or roller coating. 13. A method as claimed in any one of claims 8 to 12 wherein the solvent is removed from the coating by heating to a temperature below the thermal degradation temperature of the polycarbonate. 14. A method as claimed in any one of claims 8 to 13 wherein the polycarbonate coating has a thickness of 1.0 micrometres or below. 15. A printed circuit board as claimed in any one of claims 1 to 7 which has one or more components soldered thereto. 16. The use of a thermally depolymerisable polycarbonate as a protective coating for printed circuit boards.

Inventors: CAMPBELL JOHN STEWART, KING CHARLES EDMUND, MACKIE ANDREW CHRISTOPHER, PANG PAK KEUNG, CAMPBELL, JOHN, STEWART, KING, CHARLES, EDMUND, MACKIE, ANDREW, CHRISTOPHER, PANG, PAK, KEUNG
Applicants: COOKSON GROUP PLC
Application year: 1992
Main classification: H05K3
IPC codes: C08G64, C09D169, H05K1, H05K3