Patent GB-1484604-A (GB)
Title: Methods and apparatus for vapour heating of articles
IPC: H05K3/34
Filed: 1974-09-04
Abstract: 1484604 Heating chambers WESTERN ELECTRIC CO Inc 4 Sept 1974 [7 Sept 1973 5 June 1974] 38584/74 Heading F4U [Also in Division B3] In heating an article to perform an operation thereon, a liquid is boiled to generate a saturated vapour, and the article is immersed in the vapour. A vessel 1, Fig. 1, is in an embodiment provided with electric heating coils 2 at the bottom and at the top cooling coils 3, e.g. of copper receiving a cooling medium, e.g. water. The vessel may be also located on a hot plate for additional heating. The liquid 4 is boiled in the vessel and the vapour therefrom fills the vessel, the coils 3 condensing the vapour and the condensed vapour draining back. A printed circuit board 6 having components mounted thereon for soldering thereto by solder preforms or plating is suspended in the vapour in the vessel, whereupon the vapour condenses on the board giving up latent heat of vaporization and heating the board to the temperature of the vapour which is at least equal to and is prepreferably above the melting point of the solder. The condensed vapour on the board carries away grease or dirt on the board and may eliminate the need for fluxing the board. A batch of boards may be simultaneously soldered. The liquid employed is preferably a fluorocarbon, e.g. a fluorinated polyoxypropylene which is chemically stable, inert, non-toxic and non- inflammable and the apparatus described may be used for brazing or fusing a coating of solder on a board. In another embodiment a vessel 8, Fig. 2, with inlet and outlet conduits 9, 10 at opposite ends and cooling coils 11, 12 in the conduits has a conveyer 13, for articles to be soldered, passing in to the vessel through the coil 11 and out through the coil 12. Articles are brought into the vapour in the vessel on the conveyer, are heated by condensation thereon of the vapour and then are conveyed out of the vessel. In a modification a wall 15, Fig. 3, is provided in the apparatus of Fig. 2 and a wave or fountain 16 of solder is generated within the wall, the molten solder of the wave being at a temperature above the boiling point of the liquid 4 which is above the melting point of the solder. Articles are conveyed into the vessel each being preheated by the vapour condensing thereon to a temperature above the melting point of a protective coating of solder thereon the preheated article then enters the wave of solder which provides solder for the article and the article leaving the wave carries molten solder which flows due to surface tension effects. The articles finally leave through the cooling coil 12. The liquid 4 may have a boiling point below the melting point of the solder. In a further embodiment a vessel 1a, Fig. 5 has a heating coil 2a, a first set of cooling coils 53 within the vessel and a second set of cooling coils 54 adjacent the upper end of the vessel and operating at a lower temperature than the first set of coils. A trough 55 receives condensate dripping from the coils 53 and a trough 57 receives condensate dripping from the coils 54. The condensate from trough 57 is discharged to trough 55 which discharges into the lower part of the vessel and a make up tank 73 is connected to the trough 55. A mixture of two liquids, one having a boiling point at atmospheric pressure at least equal to the melting point of the solder and a vapour which is denser than air at atmospheric pressure and the other having a boiling point at atmospheric pressure lower than that of the first liquid and a vapour which is less dense than that of the first liquid, is boiled in the vessel. The second liquid boils off first and then the first liquid boils and its denser vapour pushes the vapour of the second liquid above it to form a vapour blanket. The denser vapour extends to about the line 70 a mixture of lighter and denser vapour exists between lines 70 and 71 and lighter vapour exists up to line 72. A printed circuit board 77 is lowered into the vessel and held in the denser vapour where this condenses to heat the board to the melting point of its solder. The first liquid may be a fluorocarbon, e.g. fluorinated polyoxypropylene and the second may be a halogenated hydrocarbon, e.g. trichloro-trifluoroethane. Modifications of the apparatus shown in Figs. 2 and 3 are included in which the single cooling coils in the inlet and outlet conduits are replaced by the dual system described with reference to Fig. 5 using a mixture of liquids in the vessel.