Method of positioning and soldering of SMD components

Abstract: The invention relates to a method of positioning and soldering of electronic components on a printed circuit board. To this end, first the soldering material is heated to a suitable adhesion temperature, after which the component is put in the liquid soldering material with its connections and the soldering material is allowed to cool down. Soldering is preferably carried out in an inert or slightly reducing atmosphere. Heating of the soldering material may take place in a single stage or in two stages.

Description: The invention relates to a method of positioning and soldering of electronic components on a printed circuit board, in which method soldering material is locally applied on the printed circuit board and a positioned component is fastened to the printed circuit board by melting and subsequent cooling down of said soldering material. Such a method is disclosed in US Patent 4,312,692. As described in this Patent, SMD components (Surface Mounted Devices) are put on the printed circuit board in contact with soldering paste, upon which the assembly is heated to a high temperature to soften or melt the soldering paste to a sufficient degree for obtaining a good adhesion between the components and the soldering material. The assembly is subsequently allowed to cool down again. As this publication itself notes, however, it is a disadvantage of this method that components can easily become displaced during soldering, so that no positioning accuracy can be guaranteed, while also the formation of an even solder layer is difficult. To solve these problems, the said US Patent 4,312,692 proposes to fix the components first on the printed circuit board with an adhesive, which is cured by irradiation. The assembly thus obtained of printed circuit board and components glued to it may then be brought into contact with molten soldering material in order to achieve the soldered joint. A disadvantage of this method is that glueing of the components on the printed circuit board involves an additional process step, which considerably increases both the manufacturing cost and the time required for manufacture. From the document US-A-4 735 354 there is known a method for soldering components comprising the steps of placing the components on a circuit board, melting the solder while holding the component, and air cooling. The invention has for its object to provide a method by whose application the disadvantages described above can be avoided. According to the invention, this can be achieved in that first the soldering material is heated to a suitable melting temperature, then the component is positioned in the liquid solder with its connections, and finally the soldering material is cooled down again. To carry out the method described above, soldering material will be applied in the relevant spots of the printed circuit board. The soldering material may consist of a solder layer applied by thermal means or by electroplating, but the use of molten soldering paste is also possible. The printed circuit board provided with soldering material in this way is heated to a temperature close to or above the melting temperature of the soldering material. Subsequently the components to be applied may be pressed into the molten soldering material, which is provided on the printed circuit board in an adapted pattern, by means of suitable tools in accordance with the pattern in which they are to be provided on the printed circuit board, upon which the said soldering material is allowed to cool down. Soldering takes place preferably in an inert or slightly reducing atmosphere to prevent oxidation of the molten soldering material, of the components and of the connection surfaces of the printed circuit board. The reproducibility of the soldering process can be well controlled in a preferred embodiment of the method according to the invention in that the heating of the soldering material takes place in two stages. To heat the soldering material, the printed circuit board may be heated in its entirety, for example by means of infrared radiation or with a hot plate. Another possibility is to heat the soldering material on the printed circuit board locally. Infrared irradiation and hot air are very suitable heating means for this. The required thermal energy may be supplied in a very short period, in a concentrated form and accurately aimed by the use of laser irradiation for heating of the soldering material. Another possibility is the use of heated grippers for positioning the components, in particular ceramic components such as resistors and capacitors. A combination of several of the above measures is obviously possible, for example, heating of the soldering material in two stages both by heating of the entire printed circuit board and by local heating. Thus, for example, the printed circuit board may be heated from above by infrared irradiation up to a temperature well below the melting temperature of the soldering material. The soldering material is then locally heated further in the soldering spots to well above the melting temperature of the soldering material, upon which the components are positioned. The soldering material will harden when the heat sources used for the second heating step are switched off, and the components will become fastened. The invention will be described in more detail with reference to the drawing which shows the principle of an equipment, suitable for applying the method of the invention. The equipment 1 comprises an x-y table 3, two IR-lamps 5, a halogen lamp 7, an IR-thermometry system 9 and a component placement system 11, all controlled by a computer 13. The process of placing components in liquid solder will now be described. A PCB 15 is applied with solder in a known way and is fed to the x-y table 3. The IR-lamps 5 are being switched on and the PCB 15 is heated to a temperature of about 150°C. The PCB 15 is moved by the x-y table 3 to a position whereby the placement position 17 which need to be heated is under the halogen lamp 7. The halogen lamp 7 is switched on and if the thermometry system 9 measures the right local temperature (± 250°C), the component is placed by the component placement machine 11 in the liquid solder. The halogen lamp 7 is then switched off and the PCB 15 is moved by the x-y table 3 to a next position. The IR-lamp 5 is a 1000 Watt IR-radiator, fitted with a parabolic reflector to produce a good parallel beam. Output from the lamps 5,7 is controlled both by input voltage level and by a switch, controlled by the computer. The component placement can be done by an automated component placement system. It will be clear that, when the method according to the invention is applied, components to be applied on a printed circuit board can also be simultaneously positioned correctly relative to the circuit board in one operation and can be soldered to it. Compared with methods which have been used up to now, the number of operations to which the printed circuit board and the components are to be subjected for achieving the soldered joints can be considerably reduced by this. In addition, the use of glue becomes redundant. It is also possible in the method according to the invention to position the components closer together than in the case of the known method since the components, once positioned, will not or only very slightly move relative to one another or relative to the substrate. This also has a favourable influence on the freedom of design of the circuit configuration. Positioning and soldering of the components can take place while the printed circuit board is securely supported and a thinner printed circuit board can be used, for example a printed circuit board of 0,8 mm thickness as compared with board thicknesses of 1,6 mm which were usual up to now, because the printed circuit board need not be subjected to further treatments, such as, for example, dipping in a solder bath or the like, after positioning and soldering. An additional saving in material as well as a lighter design of the printed circuit board can be achieved in this way as well. Moreover, a better heat supply through the bottom of the printed circuit board takes place. Care must be taken to avoid degradation, delamination or warping of the PCB due to excessive heating, especially by heating the entire PCB to the temperature whereby the solder is molten. To heat the solder only locally by means of the halogen lamp has the disadvantage that the time to reach the right temperature is relatively long. Therefore the best method is the combination of total heating of the PCB to a temperature below the temperature at which the solder is molten and local heating for heating the solder spots where a component must be placed to a temperature above the temperature at which the solder is molten. The local heating can be done by the halogen lamp as mentioned above. Because solder oxides so readily, fluxing determines whether or not wetting takes place. Wetting before heating is the easiest method: the disadvantage is that the flux loses its activity during the process. This implies that the last solder surfaces and components will not be cleaned, and oxides present will prevent wetting. Fluxing at the instant of placing the component is similarly unfavourable. The surface is cleaned, but evaporation of flux compound removes a lot of heat, so wetting takes longer. Fluxing before local heating is probably the best method. No local heat is removed, the flux cleans while the surface is being heated and wetting proceeds rapidly. When the correct temperature and fluxing are chosen, soldering times of 1 second per component are achievable and proper joints are made. The time between wetting the printed circuit board (PCB) and the placement of the component on the board must be as short as possible to avoid that the flux loses its activity. The emissivity of the PCB and the solder spots where the component must be placed plays an important role in the measurement of a correct contactless temperature. For the PCB a factor of 0.9 was derived, the solder has an emission factor of 0.2.

Claims: A method of positioning and soldering of electronic components on a printed circuit board, in which method soldering material is locally applied on the printed circuit board and a positioned component is fastened to the printed circuit board by melting and subsequent cooling down of said soldering material, characterized in that first the soldering material is heated to a suitable melting temperature, then the component is positioned in the liquid solder with its connections, and finally the soldering material is cooled down again. A method as claimed in Claim 1, characterized in that soldering takes place in an inert or slightly reducing atmosphere. A method as claimed in Claim 1 or 2, characterized in that heating of the soldering material takes place in two stages. A method as claimed in Claim 1, 2 or 3, characterized in that the printed circuit board is heated in its entirety for heating the soldering material. A method as claimed in Claim 1, 2 or 3, characterized in that the soldering material on the printed circuit board is heated locally. A method as claimed in any one of the preceding Claims, characterized in that the soldering material is heated by infrared irradiation. A method as claimed in any one of the Claims 1 to 5, characterized in that the soldering material is heated with hot air. A method as claimed in any one of the Claims 1 to 5, characterized in that the soldering material is heated by means of laser irradiation. A method as claimed in anyone of the preceding Claims, characterized in that the component is positioned by means of heated grippers.

Inventors: MOLLEN HUBERTUS THEODORUS, VERGULD MARTINUS MARIA FRANCIS, MOLLEN, HUBERTUS THEODORUS, VERGULD, MARTINUS MARIA FRANCISCUS
Applicants: PHILIPS NV, N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1990
Main classification: H05K3
IPC codes: H05K3