A METHOD OF MIXING SOLDER PASTE The present invention relates to a method of mixing solder paste such as is used in the formation of printed circuit boards (PCB's).
It is known to use solder paste in the construction of printed circuit boards and such paste normally comprises a major portion of solder beads (possibly in the region of 90%) with such beads currently normally being a lead alloy and the remainder comprising flux and liquid solvents. It is believed that in the future the lead alloy solder will be replaced by other metal alloys to which the present invention is equally applicable. The flux is chemically active in that it has a tendency to react with the lead beads or other solder metal so as to oxidise such with the result that the quality of the solder paste can deteriorate very rapidly and manufacturers of the paste recommend that the users store solder paste, normally contained in known cylindrical reservoir cartridges, in a refrigerator prior to use so as to slow down any deterioration in the quality of the solder paste prior to use. However, even with such careful treatment there is only a limited shelf life of the solder paste and poor quality paste printing and paste wastage can occur if the paste is not used promptly on arrival on site from the paste manufacturer.
It is an object of the present invention to provide a method by means of which the paste user is not under such pressure as hitherto as regards ensuring supply of fresh solder paste from a solder paste manufacturer and keeping such in refrigerated storage and using in time having regard to the date of mixture by the manufacturer.
According to a broad aspect of the present invention there is provided a method of printing solder paste onto a circuit board (PCB) or other electronic component comprises storing the metal solder beads or particles separately from the flux until shortly before needed for printing and then bringing together the metal solder particles and flux shortly prior to printing (and preferably in the same premises as the printing machine) and mixing the flux and solder metal particles in the required proportions and then applying the thus mixed solder paste to a circuit board or electronic component.
Also according to the present invention a method of mixing the ingredients of solder paste comprises applying the flux and any necessary liquid solvent used in solder paste to the interior of a hollow body, rotating said hollow body so as to distribute the flux and any liquid solvent about the interior surface of said hollow body, and, preferably whilst continuing to rotate said body, applying lead alloy beads or other solder metal beads or powder or like used in solder paste to the interior of said rotating body.
The application of lead alloy beads or other metal alloy particles will normally be such that a predetermined ratio of lead alloy or other metal to flux (and any solvent mixture) is reached. Also, a pneumatic delivery system may be provided using a displaceable supply pipe through which the metal solder is blown by compressed air or, preferably by an inert gas such as nitrogen to reduce the oxidation likelihood.
Preferably the rotation will be continued until the necessary mixing has been achieved. Subsequently any gas in a formed void in the centre of the hollow body can be discharged prior to use of the mixed solder paste.
Preferably the body is an elongate cylinder and which is preferably also the cartridge body in which the solder paste is to be used in the paste printing machine and the flux and any solvents (which generally have the consistency of axle grease) are initially loaded into the cylinder which is then rotated so as to distribute the flux and any solvent around the interior surface of the cylinder under centrifugal force and then the lead alloy beads or powder or other solder metal alloy particles or powder is introduced into the interior of the cylinder, preferably by pneumatic inert gas means such as via a pipe or other ducting means along which the lead alloy beads or other metal alloy solder powder material flows, and is carried by a pressurised gas stream so as to discharge at the end of the tube within the cylinder. The outlet of said tube is preferably displaced along the axis of the cylinder so as to distribute the lead alloy beads or other metal alloy solder powder therealong wherein, under the action of centrifugal force, the lead alloy beads (or other metal alloy solder powder) are forced into the layer of the flux and any solvent and such become mixed therewith. The metal alloy solder particles and the flux and any solvent will be provided in the requisite proportions to produce the resultant required known solder paste mixture.
Normally metal alloy solder paste may comprise a mixture of nine parts of weight of lead alloy beads to one part by weight of flux so that, for example, 50 gm. of flux may have mixed therewith 450 gms. of leads to produce a'/2 kg of solder paste mixture. Solder metal alloys other than lead alloy may be used in appropriate mixtures.
It is envisaged that the device as disclosed in my co-pending UK Patent Application 9904818.3 could be used for the rotation of a cylindrical solder paste reservoir cartridge about its longitudinal axis. The compressed air supply or inert gas supply will force the lead alloy beads or other metal alloy solder powder along a distribution tube and into the flux lining the inside of the cylindrical cartridge reservoir.
It will thus be appreciated that it is the end user of the solder paste who can or who will readily effect the mixing of the ingredients of the solder paste just before use and hence any deterioration in the quality or characteristics of the mixed solder paste is avoided or minimised and the shelf life of the resultant product is considerably extended. The on site/in situ mixing of the solder paste also enables requisite amounts of solder paste to be mixed so as to minimise any wastage. Of course, any unused solder paste may be stored in a refrigerator in known manner or may be so stored prior to use of the solder paste.
Also according to the present invention there is provided solder paste whenever mixed according to the method of the present invention.
Also according to the present invention there is provided a printed circuit board or electronic component whenever produced using solder paste whenever mixed in accordance with the method of the present invention.
In a broad aspect, the method of invention comprises applying solder paste via a stencil to a printed circuit board or other electronic component and is characterised by the feature that immediately prior to the application of the solder paste, the ingredients of the solder paste are mixed by the user or are mixed on the premises of the solder paste printing machine. Preferably the mixing is effected by placing the ingredients of the solder paste in a rotary body and rotating such. Alternatively, the ingredients may be mixed by other means such as mechanically stirring or even by means of an ultrasound generator.
The present invention will be described further, by way of example, with reference to the accompanying drawing, which is a schematic cross- sectional illustration of an apparatus for carrying out the method of the present invention.
In the drawing there is illustrated an apparatus 1 for performing the method of the present invention operable on the premises of a solder paste printing machine (not shown) and by the user and comprising the main tubular/cylindrical body 2 of a solder paste reservoir cartridge (known per se) for receiving a paste having a reduced diameter end 3 for normal outlet of solder paste. An end closure 4 is provided in place of the normal cartridge cap (not shown) and a non-rotary supply pipe 5 with side-directed outlet 5'for supplying metal solder alloy powder is slidable along its axis in the closure 4 whilst the body 2 and closure 4 is rotated The body 2 is locatable in a rotary cylindrical housing (not shown) of a rotation device (not shown) and body 2 is also closed by closure 4 which closure also keeps flux 6 in the body 2. Closure 4 also has means (not shown) for connection to said outer rotary cylindrical housing to clamp the peripheral flange 2'of the cartridge body 2 against said housing.
A rotary support bearing (not shown) for said housing is provided and axially aligned with said rotary bearing, is an electric drive motor (not shown) with spindle connectable to end of said housing adjoined end 3 of body 2 so as to enable both to be rotated about their axes. The closure 4 has an opening through which the distribution and supply tube or pipe 5 is insertable into the reservoir cartridge and displaceable axially therealong. The tube 5 has outlet orifice 5'directed towards the cylindrical interior surface of the cartridge body 2 and a pressurised supply (not shown) of inert gas is in communication with said tube 5 provided for forcing lead alloy beads or other solder metal alloy powder 7 out of the orifice 5'and into the flux material 6 which has hitherto been applied to line the interior of the cylindrical body 2 as a result of placing flux material in the cylinder and rotating such.
To perform the method of the invention, a predetermined amount of flux and solvent material 2 is loaded into the body 2 which is then clamped in position in the housing of rotation device (not shown). The smaller diameter end 3 can be left open although the closure 4 is applied to prevent the flux 6 from flowing out the larger opening at the other end. The rotation device is then switched on and rotation in direction of arrow"A"takes place and the effect of such rotation is to cause the flux 6 to spread along and around the body 2 to line the interior of the body 2 but not to exit via outlet opening 3 because of the centrifugal force on the flux causes such to move outwards and the amount of flux in the thus formed cylindrical layer is not of a sufficient thickness to pass out of outlet 3'.
Subsequently, a reciprocating drive device (not shown) for tube 5 is energised to cause reciprocating axial movement as indicated by arrow"B" and the pressurised inert gas supply opened to displace metal solder alloy powder 7 in direction of arrow"C"to discharge a predetermined amount of solder powder 7 out of orifice 5'as the tube 5 is displaced axially and the outer housing and body 2 rotated, so as to force the powder 7 into flux 6 so as to cause an effective mixing to form the requisite solder paste. Inert gas is able to vent out of outlet 3'.
The resultant paste mixture lines the inner wall of body 2 and will have a central void e. g. of approximately 20 mm diameter, which will contain inert gas and this void can be filled or removed by moving the body 2 from the rotation device and inserting a punch or piston (not shown) into the large diameter opening in body 2 and then disposing the body 2 in a generally vertical disposition with end 3 uppermost, and then displacing the piston into the body 2 for a sufficient amount so as to cause the void to collapse from the lower end and drive the inert gas out of opening 3'. The body 2 may be reorientated and the piston removed and the normal closure cap (not shown) for the solder paste cartridge with body 2 is then applied to the wide opening end and the cartridge comprising body 2 and closure cap is then ready for use in a solder paste printing machine.
