PASTE, INK OR CREAM FORMULATIONS FOR USE IN THE ELECTRONICS INDUSTRY

Abstract: A paste, ink or cream for use in the electronics industry which comprises: i) a particulate filler material, and ii) a vehicle which is an emulsion of a) a polar organic solvent, or a mixture thereof, and b) an aliphatic hydrocarbon, or a mixture thereof, the component forming either the disperse or continuous phase of the emulsion including therein at least one thermally depolymerisable polymer which decomposes to volatile or gaseous products at a temperature of below 350 DEG C and leaves a residue level of less than 5 % based on the weight of the polymer.

Description: PASTE. INK OR CREAM FORMULATIONS FOR USE IN THE ELECTRONICS INDUSTRY The present invention relates to paste, ink or cream formulations for use in the electronics industry and, in particular, to solder paste formulations which are reflowable in air with low residues. Vehicles which are commonly used in the preparation of pastes for the electronics industry contain a volatile solvent, a thickener/ binder, a rheological modifier to enhance thixotropy or pseudoplasticity, a surfactant and, optionally, an activating or reducing agent. In particular, reducing or fluxing agents may be used where surface preparation or fluxing is necessary. The level of non-volatile organic solids to provide the desired viscosity is normally from 40 to 75wt%. The materials used are either polymeric materials, such as ethyl cellulose or derivatives thereof, polyamides or castor oil derivatives. There is a trend in the electronics industry towards the automated manufacture of printed circuit boards (pcb's) , where residues after a soldering operation are required to be minimal, and a subsequent cleaning operation is obviated. The solids content of the vehicle/flux for conventional solder pastes used in pcb manufacture is typically 40 to 80wt% and much of this will remain after solder reflow, requiring a cleaning operation. Furthermore some solder paste vehicles with a high solids content have poor hot slump characteristics; this can lead to solder powder being washed away from the printed circuit pad during reflow. Discrete solder balls are then produced and these may lead to short circuits. Accordingly, the present invention provides a paste, ink or cream for use in the electronics industry which comprises i) a particulate filler material, and ii) a vehicle which is an emulsion of a) a polar organic solvent, or a mixture thereof, and b) an aliphatic hydrocarbon, or a mixture thereof, the component forming either the disperse phase or continuous phase of the emulsion including therein at least one thermally depolymerisable polymer which decomposes to volatile or gaseous products at a temperature of below 350°C and leaves a residue level of less than 5% based on the weight of the polymer. Preferably the thermally depolymerisable polymer is incorporated into the disperse phase of the emulsion vehicle in the form of a gel. This gelation may further improve the stability of the emulsion vehicle by preventing coalescence of disperse phase droplets. Alternatively, although not preferred, the thermally depolymerisable polymer may be dispersed throughout the disperse phase in the form of finely divided submicron particles. The vehicle which is used in the pastes of the present invention is a nonaqueous dual or multiphase emulsion. The disperse phase will generally comprise the major volume fraction of the emulsion. Because the thermally deploy erisable polymer is preferably held within the disperse phase, it makes only a minor contribution to the overall vehicle rheology. The disperse phase may be the polar organic solvent or the aliphatic hydrocarbon and a thermally depolymerisable polymer will be chosen which either gels in the disperse phase or can be dispersed in finely divided form therein. The continuous phase of the emulsion will substantially govern the rheological behaviour of the emulsion as a a whole and thus, if the continuous phase has a high viscosity, then this characteristic will be imparted to the emulsion vehicle. The thermally depolymerisable polymers which are included in the paste formulations of the present invention may be the thermally depolymerisable polycarbonates which are described in our European Patent Application No. 91305298.1, i.e. the thermally depolymerisable polymer may be a thermally depolymerisable polycarbonate containing the repeating units 0 0 (R I -0-C-0-R"-0-C-0) where R 1 and R n 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 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 1,1*-carbonyldiimidazole. 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, 2-butyne-l,4- diol, p-bis(l-hydroxyethyl)benzene and α,α,α f ,α r - tetramethyl-l,4-benzenedimethanol, or a mixture of two or more thereof. Other thermally depolymerisable polymers which may be used in the present invention are the polyoxalates and polymalonates which are disclosed in our British Patent Application No. 9119247.6. The polyoxalates and polymalonates as described therein are polymers or copolymers containing repeating groups of the formula wherein R 1 and R 2 are each independently a hydrocarbon group containing from 3 to 30 carbon atoms which has a tertiary carbon atom attached to at least one of the bridging oxygen atoms, R 3 , R 3 • and R 4 are each independently a hydrogen atom, an alkyl group containing from 1 to 12 carbon atoms, an aryl group, a heterocyclic group or an acyl group containing from 1 to 12 carbon atoms, and n and n 1 are 0 to 1. When n and n' are 0 the polymers are oxalates and when n and n* are 1 the polymers are malonates. When n and ' are 1 preferably R 3 , R 3 ' and R 4 , R 4 ' are both hydrogen atoms, or one of R 3 , R 3 ' and R 4 , R 4 ' is a hydrogen atom arid the other is an • acyl group containing from 1 to 12 carbon atoms. Copolymers which comprise groups of the formula where R 1 , R 2 , R 3 , R 3 ' , R 4 , R 4 • n and n • are each as defined above and groups of the formula (R5-0-C-0-R 6 -0-C-0) where R 5 and R 6 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 atoms, i.e. copolymers of oxalate/carbonate or malonate/ carbonate, may also be used. The oxalate and malonate polymers used in the present invention may be produced by the condensation reaction between a diol, or a mixture of diols and an oxalyl and/or malonyl moiety, such as an acid chloride, acid or ester. The diols which may be used in the preparation of the polymers have the general formula: HO-R^—OH or H0-R 2 -0H where R 1 and R 2 are as defined above. The polyoxalates and polymalonates used in the present invention owe their thermal lability to the structure of the diols which are used in their preparation, as a result of the diols possessing a tertiary carbon atom bonded to at least one of the hydroxyl functional groups in the diol. Examples of diols which may be used are 2,5- dimethylhexane-2,5-diol, 2-5-dimethylhex-3-yne-2,5- diol and α,α,α' ,α'-tetramethyl-l,4- benzenedimethanol, or a mixture of two or more thereof. Further examples of thermally depolymerisable polymers which may be used in the present invention are: i) poly(methylmethacrylate) , the decomposition of which is essentially a reverse of its polymerisation leading to a monomer in 100% yield, ii) poly(α-methylstyrene) , the decomposition of which begins at 250°C and proceeds rapidly at 300°C to essentially 100% monomer. iii) poly(oxymethylene) , which depolymerises between 100° and 180°C to form formaldehyde. The uncapped polymer is unstable in most solvents but end-capped poly(oxymethylene) is stable, the end-capped polymers containing thermally removable groups such as tertiary butyl being thermally depolymerisable. iv) acid depolymerisable polymers, such as polyacetals, polyketals, polyorthoesters, polyamidoacetals, polyenolethers and poly-N- acylaminocarboxylic acids, which undergo clevage under the influence of acid catalysis. Materials of this type have been described for lithographic use in US Patents Nos. 4101323, 4427611, 4248957 and 4250247. For these materials to be useful in the solder paste formulations of the present invention an acid, which will also act as an activator, must be released at the soldering temperature. Such an acid ' can be released thermally from suitable materials. v) poly(phthaldehydes) as described in Polymer Engineering and Science, 1983, j23_, 1012 as materials for dry film resists. The polymers have ceiling temperatures well below room temperature and must therefore be prepared at cryogenic temperatures. If the polymers are end-capped by acylation or alkylation prior to warming, they are stable to about 180°C and will decompose totally by 230°C. vi) polyesters which incorporate a tertiary, benzylic, allylic or propargylic group adjacent to the ester oxygen in the main chain will depolymerise on heating to a temperature in the range of from 100°C to 350°C. These polyesters are derived from dicarboxylic acids, such as isophthalic or terephthalic acids, with appropriately substituted diols containing the tertiary, benzylic, allylic or propargylic groups. The synthesis and use of these materials as self-developing imaging systems, are described in Poly. Mater. Sci. Eng. , 1989, 60. 170 and J. Photopolym. Sci. Technol., 1990, 3_, 235. It should be noted that the temperatures of decomposition of this family of polymers can be reduced if a catalytic amount of a strong acid is present. Such an acid can be released thermally from suitable materials, vii) polyethers which incorporate a tertiary, benzylic, allylic or progargylic group adjacent to the ether oxygen will decompose in the temperature range 150°C to 300°C, if a catalytic amount of strong acid is present. This acid can be released thermally from suitable materials. The synthesis and use of these polymers as self-developing imaging systems are described in Poly. Mater. Sci. Eng. , 1989, 60. 170. One of the immiscible organic liquids used to form the emulsion is a polar organic solvent, such as a glycol, which may be used in an amount of up to 95vol%. The other of the immiscible organic liquids is an aliphatic hydrocarbon, such as a mineral oil, which may be used in an amount of up to 95vol%. Preferred ratios of the two immiscible organic liquids are in the range of 60:40 to 80:20 parts by volume. Mixtures of polar organic solvents or aliphatic hydrocarbons may be used, as appropriate. Preferably both the polar organic solvent, or mixture thereof, and the aliphatic hydrocarbon, or mixture thereof, will have a lower boiling point greater than 100°C. When the thermally depolymerisable polymer is a polycarbonate, the disperse phase will generally be the polar phase. At a temperature below its boiling point a polycarbonate of the type as defined above will generally be solvated by the polar solvent, such as a glycol, and form a gel which is then dispersed throughout the disperse phase. An emulsifier is generally used to form the emulsion from the immiscible liquids and generally will be present in an amount of from 0.2 to 15wt%, more preferably 0.5-10wt%, based on the total weight of the vehicle. Suitable emulsifiers are poly(oxyalkylene)alkyl ethers, amine derivatives of oligomeric polyesters, polymeric surfactants or similar compounds. The vehicle may additionally contain a co-solvent, which is included in order to improve surface tension parameters, in an amount of up to 20vol%. A suitable co-solvent is a C^-C^, chain length, aliphatic alcohol. The vehicle may additionally include a thickener, which should be chosen so as to be compatible with either the continuous phase or the disperse phase, in an amount of up to 60wt%. Suitable thickeners include polyacrylic acid or polyamides or resins for the polar fraction and hydrogenated castor oil, aliphatic and non polar resins for the aliphatic fraction. The thermally depolymerisable polymer, if suitable, may be used to thicken either phase. The viscosity of the continuous phase is increased by the inclusion of a thickener and this enables the rheological characteristics of the vehicle to be adapted as required. It may be necessary for further components to be added to the vehicle to provide, for instance, fluxing activi: > • for solder reflow. Typical fluxing additives, familiar to those skilled in the art, may be used and may also aid the emulsion stability. Activators used in the preparation of solder paste will be included in amounts up to 7wt%. The vehicle as described will form a paste when blended with suitably sized particulate material and may be conveyed to a substrate by a variety of processing techniques used in hybrid microelectronics and pcb manufacture, for example by screen or stencil printing, dispensing or dipping. The particular application technique will depend upon the nature of the article being produced. The nature of the particulate material, which is preferably an inorganic particulate material incorporated into the paste will depend upon the intended application of the paste. For example, solder powders which are alloys of tin or lead or bismuth or silver or antimony or indium or a combination thereof may be used in the present invention. Examples of such alloys are SnPb such as 63Sn:37Pb, SnPbBi, SnBi, Pbln; SnPbln; SnAg; SnPbAg; SnSb or Snln of a particle size less than 80 micrometres, may be used to form solder pastes. Preferably the solder powder will have a particle size in the range of from 20 to 45 micrometres. A paste for manufacturing multilayer electronic devices, such as capacitors, may comprise a palladium/silver alloy powder as the filler. As the polymers used in the present invention are thermally depolymerisable, their addition to the emulsions does not contribute substantially to any residues formed. Furthermore, since the polymers decompose on heating to form small volatile molecules such as carbon dioxide and other species, it has been found possible to take advantage of this property and form an air-reflowable solder paste in which the carbon dioxide and other volatile molecules act as a blanket to prevent air oxidation. Preferably, the thermally depolymerisable polymer is added in an amount of from 10 to 30% by weight, based on the weight of the disperse phase in order to obtain an air-reflowable solder paste with low residues, e.g. preferably less than 3.0% by weight. The air reflowable solder pastes known in the art give much higher residues of greater than 4.2wt% and generally more than 4.5wt%. It will be appreciated by those skilled in the art that it may be advantageous to use a mixture of thermally depolymerisable polymers in the electronic paste formulations of the invention. The use of such a mixture will provide the ability to vary the depolymerisation temperature, the depolymerisation rate, etc. The ratio of the emulsion vehicle to the inorganic particulate material will depend upon various factors including the particle size/ morphology of the particulate material and the intended use of the paste. Generally, however, a greater quantity of the emulsion vehicle will be required for the production of pastes containing fine particulate material having an average particle size of up to about 5 micrometres. Present day, solder pastes, however, are usually prepared from particulate material having an average particle size in excess of 10 micrometres. The pastes of the present invention contain substantially lower levels of residual organic solids after solder reflow than those systems conventionally used. Solder pastes made with an emulsion vehicle as described herein, are reflowable using e.g. vapour phase systems or infra-red reflow. Dependent on the amount of and nature of thermally depolymerisable polymer included in the emulsion, the systems are reflowable in either air or nitrogen atmospheres, and the residues may be left on the printed circuit board. The present invention will be further described with reference to the following non-limiting Example. EXAMPLE A flux for use in the manufacture of a solder paste for stencil printing was prepared by blending an aliphatic liquid phase with a polar liquid phase in a ratio of 29wt% : 71wt% wherein the aliphatic phase is comprised of: Polyisobutylene 0.26 wt% Mineral spirits 38.0 wt% Hercures C resin 39.0 wt% Hypermer KD3 surfactant 22.74 wt% and polar phase is comprised of: Dipropylene glycol 53.0 wt% Propylene glycol 43.0 wt% Adipic acid 4.0 wt% An amount of 25%wt of a thermally depolymerisable polycarbonate prepared from 2,5-dimethyl-0,0'bis(l- imidazolylcarboxyl)-2,5-hexanediol and 1,3-benezedi- methanol was further added to the glycol phase at a temperature of below 140°C. The polar and aliphatic phases were blended together in the ratios given above to form an emulsion solder paste vehicle. This emulsion was used to prepare a solder paste by admixing it with a solder powder (63:37 tin:lead w:w) having a particle size in the range of from 20 to 45 μm in a weight ratio of 9:91 vehicle to solder. A screen printable solder paste was produced. The solder paste was printed through a brass stencil on to an alumina substrate. The substrate passed through an air, infrared solder reflow oven with a temperature profile typical of that used by the industry for this solder alloy. A 2.2wt%, clear and hard residue was found, and a single bright solder bead was produced.

Claims: CLAIMS : 1. A paste, ink or cream for use in the electronics industry which comprises i) a particulate filler material, and ii) a vehicle which is an emulsion of a) a polar organic solvent, or a mixture thereof, and b) an aliphatic hydrocarbon, or a mixture thereof, the component forming either the disperse or continuous phase of the emulsion including therein at least one thermally depolymerisable polymer which decomposes to volatile or gaseous products at a temperature of below 350°C and leaves a residue level of less than 5% based on the weight of the polymer. 2. A paste, cream or ink as claimed in claim 1 wherein the thermally depolymerisable polymer is a thermally depolymerisable polycarbonate containing the repeating units (R - -0O--CC a--OO--RR"--OO--CC--0O): where R and »R~. " 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 paste, cream or ink as claimed in claim 2 wherein the groups R 1 and R π in the thermally depolymerisable polycarbonate each independently represent a hydrocarbon group containing from 4 to 12 carbon atoms. 4. A paste, cream or ink as claimed in claim 2 or claim 3 wherein the thermally depolymerisable polycarbonate is prepared by the condensation reaction between a diol, or mixture of diols, and phosgene or a phosgene analogue. 5. A paste, cream or ink as claimed in claim 1 wherein the thermally depolymerisable polymer is a polymer or copolymer containing repeating groups of the formula: -0—4- wherein R 1 and R 2 are each independently a hydrocarbon group containing from 6 to 30 carbon atoms which has a tertiary carbon atom attached to at least one of the bridging oxygen atoms, R 3 , R 3 ' and R 4 , R 4 ' are each independently a hydrocarbon atom, an alkyl group containing from 1 to 12 carbon atoms, an aryl group, a heterocylic group or an acyl group containing from 1 to 12 carbon atoms, and n and n' are 0 to 1. 6. A paste, cream or ink as claimed in claim 5 wherein n and n• are 1 and R 3 , R 3 ' and R , R 4 ' are both hydrogen atoms, or one of R 3 and R 4 , or R 3 ' and R ' , is a hydrogen atom and the other is an acyl group containing from 1 to 12 carbon atoms. 7. A paste, cream or ink as claimed in claims 5 to 6 wherein one of n and n' is 0 and the other of n and n* is l. 8. A paste, cream or ink as claimed in claims 5 to 7 wherein the polymer or copolymer is prepared by the condensation of a diol of the general formula: Ho-Rl-OH and/or H0-R 2 -0H wherein R 1 and R 2 are as defined in claim 5 with an oxalyl and/or malonyl moiety. 9. A paste, cream or ink as claimed in any one of the preceding claims wherein the polar organic solvent is a glycol. 10. A paste, cream or ink as claimed in any one of the preceding claims wherein the aliphatic hydrocarbon is a mineral oil. 11. A paste, cream or ink as claimed in any one of the preceding claims wherein the polar solvent, or mixture thereof, and the aliphatic hydrocarbon, or mixture thereof, each has a lower boiling point of >100°C. 12. A paste, cream or ink as claimed in any one of the the preceding claims wherein the vehicle contains.0.2 to I5wt%> based on the total weight thereof, of an emulsifier. 13. A paste, cream or ink as claimed in claim 13 wherein the emulsifier is a poly(oxyalkylene)alkyl ether, an amine derivative of an oligomeric polyester, or a polymeric surfactant. 14. A paste, cream or ink as claimed in any one of the preceding claims wherein the vehicle includes a co-solvent in an amount of up to 20vol%. 15. A paste, cream or ink as claimed in any one of the preceding claims wherein the vehicle includes a thickening agent in an amount of up to 60wt%. 16. A paste, cream or ink as claimed in any one of the preceding claims wherein the thermally depolymerisable polymer is included in an amount of from 10 to 30% by weight based upon the weight of the disperse phase. 17. A paste, cream or ink as claimed in any one of the preceding claims which is a solder paste in which the particulate material is solder powder. 18. A paste, cream or ink as claimed in claim 17 wherein the solder powder is an alloy of tin or lead or bismuth or silver or an antimony or bismuth combination thereof. 19. A paste, cream or ink as claimed in claim 17 or claim 18 which includes an activator in an amount of up to 7wt%. 20. A solder paste, cream or ink as claimed in any one of claims 17 to 19 which is reflowable in air or nitrogen or any inert atmosphere and which has a residue after reflow soldering at a temperature of up to 270°C of no more than 3.0wt% based on the weight of the solder- paste. 21. A paste, ink or cream as claimed in any one of claims 1 to 16 for manufacturing multilayer electronic devices in which the particulate material is a palladium/silver alloy powder.

Inventors: KING CHARLES EDMUND, MACKIE ANDREW CHRISTOPHER, PATERSON GEOFFREY ANDREW, WAGLAND ALISON MARY, KING, CHARLES, EDMUND, MACKIE, ANDREW, CHRISTOPHER, PATERSON, GEOFFREY, ANDREW, WAGLAND, ALISON, MARY
Applicants: COOKSON GROUP PLC
Application year: 1992
Main classification: B23K35
IPC codes: B23K35, H05K1, H05K3