APPARATUS FOR SOLDERING PRINTED CIRCUIT BOARDS

Abstract: A vapor phase soldering apparatus for soldering printed circuit boards (16) having electric components (18) mounted thereon by solder preforms (19), wherein the articles to be soldered are heated with hot vapors of a heat transfer liquid (14) so that the solder provided between the printed circuit boards (16) and the electric components (18) is melted to effect the soldering. The apparatus includes a vessel (11) for containing the heat transfer liquid (14), a heater (12) for vaporizing the heat transfer liquid (14), inlet (23a) and outlet (23b) port means provided in the vessel (11) to allow the introduction and removal of the articles to be soldered, conveying means (17a) to convey the articles into and out of the soldering zone of the vessel (11) via the inlet and outlet port means (23a,23b), cooling means provided in the closure means (21a,21b) for condensing vapors of the heat transfer liquid (14), and control means (24) to control the volume of the vapors of the heat transfer liquid which ascend into the soldering zone.

Description: This invention relates to an apparatus for soldering printed circuit boards having electric components mounted thereon by solder preforms, provided between the circuit boards and the components, by heating the assembly with hot vapors of heat transfer liquid to melt the solder. There is known a vapor phase soldering method in which the objects to be soldered are heated to a temperature appropriate for the soldering operation by condensing thereon hot vapors of a heat transfer liquid. Fig. 4 is a vertical cross-sectional view illustrating an apparatus for carrying out the vapor-phase soldering method as described in US-A-3,866,307, in which indicated at 1 is a vessel, at 2 a heating coil, at 3 a cooling coil and at 4 a heat transfer liquid such as a fluorinated polyoxypropyrene. The heat transfer liquid 4 is present in the vessel 1 up to the level shown, and heated to and maintained at a temperature above its boiling point by the heating coil 2. Hot vapors 4a of the heat transfer liquid 4 fill the space above the liquid level up to the level of the cooling coil 3, which is indicated by the chain line 5, and condensed at that level and returned to the body of the heat transfer liquid in the lower portion of the vessel 1. Denoted at 6 is a printed circuit board, at 7 a supporting member for mounting and holding the printed circuit board 6 thereon, at 8 a chip part such as an IC component to be bonded to the printed circuit board 6, and at 9 a bonding agent which is in this case soldering paste having powdery solder mixed with a paste and which is applied on the upper side of the printed circuit board 6 and on the lower side of the chip part 8 beforehand. The heat transfer liquid has a boiling point not lower than the melting point of the soldering paste. With this arrangement, the heat transfer liquid 4 is boiled to generate vapors and to melt the soldering paste 9 by using the latent heat of the vapors. Then the holder member 7 is lifted up to cool and solidify the solder in the air with the aid of the cooling coil 3, thereby bonding the chip part 8 to the printed circuit board 6. The transfer of the printed circuit boards into and out of the vessel may be also effected with the use of a belt conveyor as disclosed in the above U.S. patent, if desired. One problem associated with the conventional vapor phase soldering apparatus is that it consumes much heat energy. Since the body of hot, saturated vapors of the heat transfer liquid is required to be formed up to a level sufficiently higher than the level at which the printed circuit board is positioned and subjected to soldering operation, a greater part of the vapors of the heat transfer liquid are condensed on the cooling means without being utilized for heating the printed circuit board especially when the printed circuit board is small in size. In the vapor-phase soldering method as described in US-A-3 904 102 a blanket of secondary vapor, having a density intermediate that of the primary vapor and the atmosphere, is floated on the body of primary vapor to reduce losses thereof to the atmosphere. The present invention has been made with a view to overcoming this problem of the conventional vapor phase soldering apparatus. More particularly, the present invention provides an apparatus for soldering printed circuit boards having electric components mounted thereon by solder preforms, comprising: a vessel for containing a heat transfer liquid such that a space is defined above the liquid level of said heat transfer liquid, said heat transfer liquid having a boiling point higher than the melting point of said solder preforms; heating means provided in said vessel for heating said heat transfer liquid within said vessel to a temperature higher than the boiling point of said heat transfer liquid so that vapors of said heat transfer liquid fill said space in said vessel; a cooler provided in an upper portion of said space for condensing vapors of said heat transfer liquid; entrance and exit port means provided in said vessel for the introduction into said space and the removal from said space, respectively, of the printed circuit boards; means for conveying the printed circuit boards along a predetermined path into and out of said space via said entrance and exit port means; and control means having a pair of plates which are locatable between said path and said liquid level to define an aperture therebetween through which vapors of the heat transfer liquid ascend to said path and which are moveable to permit the aperture therebetween to be adjusted. The present invention will now be described in detail with reference to the accompanying drawings, in which: Fig. 1 is a vertical cross-sectional view diagrammatically showing one embodiment of fuse-bonding apparatus according to the present invention; Fig. 2 is a cross-sectional view taken on line II-II in Fig. 1. Fig. 3 is a vertical cross-sectional view, similar to Fig. 1, diagrammatically showing a further embodiment according to the present invention; and Fig. 4 is a vertical cross-sectional view, similar to Fig. 1, showing a conventional soldering apparatus. Referring first to Fig. 1, the reference numeral 11 designates a closed vessel for containing a heat transfer liquid 14 such as a fluorocarbon generally having a boiling point of at least 200°C, e.g. Fluorinate (trade mark of Sumitomo 3M Inc.). A heater 12 is disposed in the lower portion of the vessel 11 for heating and vaporizing the heat transfer liquid 14 so that hot vapors 14a of the heat transfer liquid 14 can fill the upper space above the liquid level of the heat transfer liquid 14. Provided in an upper portion of the vessel 11 is a cooler 13 for cooling and condensing the vapors of the heat transfer liquid 14. Thus, there are formed a heating zone in the lower portion of the upper space above the heat transfer liquid 14 and a cooling zone in the upper portion of the upper space. The vessel 11 has its intermediate portion 20 formed of a heat insulating material such as a ceramic or a heat resisting plastic material. The heat insulating portion 20 block the thermal conduction therethrough from the lower portion of the vessel adjacent to the heating zone to the upper portion of the vessel adjacent to the cooling zone, so that the heating and cooling efficiencies in the heating and cooling zones, respectively, may be improved. Designated as 17a is conveying means for conveying a printed circuit board 16 having chip part 18 previously temporarily bonded thereto with a solder paste or preform 19. The conveying means 17a is in the form of a conveying belt which is moved horizontally through the heating zone of the vessel 11 through a pair of opposite inlet and outlet ports 23a and 23b for effecting the soldering. That is, the solder paste 19 which has a melting point equal to or lower than the boiling point of the heat transfer liquid 14 is melted by receiving the heat energy from the vapors 14a. The fused solder is solidified when the printed circuit board 16 is withdrawn from the heating zone by the operation of the conveying belt 17a. Designated as 13a are coolers for condensing the heat transfer liquid. Adjacent to the inlet and outlet ports 23a and 23b are provided hollow closure plate members 21a and 21b, respectively. The plate members 21a and 21b, which are continuously supplied with a cooling medium, are swung to open the ports 23a and 23b (as shown by the phantom line in Fig. 1) to allow the passage of articles to be soldered carried by the conveyor belt 17a. During the soldering operation, the plate members 21a and 21b are closed (as shown by the phantom line in Fig. 2) to prevent the leakage of the vapors of the heat transfer liquid 14 therethrough. The conveying means 17a and the closure means 21a, 21b may be operated by any suitable known activation means such as including an electric motor, cams, latch, etc. Means 24 are provided for controlling the height of the body of hot vapors of the heat transfer liquid so as to minimize the leakage of the vapors of the heat transfer liquid and the consumption of heat energy. The control means 24 has a pair of horizontal plates 24a and 24b locatable between the liquid level of the heat transfer liquid in the vessel 11 and the path along which the articles to be bonded travel and capable of extending into the inside of the vessel 11 and of retracting therefrom. In the fully extended state, the plates 24a and 24b are abutted to each other so that the vapors of the heat transfer liquid 14 cannot pass therethrough and cannot reach the path for contact with the articles to be bonded. By controlling the aperture between the plates 24a and 24b correspondingly to the size of the articles to be bonded, it becomes possible to decrease the amount of heat supplied by the heater 12 to the heat transfer liquid while maintaining the body of hot, saturated vapors of the heat transfer liquid at a height suitable to effect the bonding operation. Fig. 3 illustrates a variant of the bonding apparatus shown in Fig. 1, in which similar component parts have been designated by the same reference numerals. The control means in this variant includes a pair of hinged plate members 24a and 24b swingably mounted about shafts 27a and 27b, respectively. By adjusting the angle or orientation of the plate members 24a and 24b by any suitable drive means, the area of the aperture therebetween can be varied. Though this variant is not shown provided with port closure means as shown in Fig. 1, the provision thereof is preferred. The control means as shown in Fig. 1 or 3 may, if desired, be also provided in the apparatus shown in Fig. 1 of the parent Application EP-A-0218391 from which the present Application has been divided.

Claims: An apparatus for soldering printed circuit boards (16) having electric components (18) mounted thereon by solder preforms (19), comprising: a vessel (11) for containing a heat transfer liquid (14) such that a space is defined above the liquid level of said heat transfer liquid, said heat transfer liquid having a boiling point higher than the melting point of said solder preforms; heating means (12) provided in said vessel for heating said heat transfer liquid within said vessel to a temperature higher than the boiling point of said heat transfer liquid so that vapors of said heat transfer liquid fill said space in said vessel; a cooler (13) provided in an upper portion of said space for condensing vapors of said heat transfer liquid; entrance and exit port means (23a, 23b) provided in said vessel for the introduction into said space and the removal from said space, respectively of the printed circuit boards; means (17a) for conveying the printed circuit boards along a predetermined path into and out of said space via said entrance and exit port means; and control means (24a, 24b) having a pair of plates which are locatable between said path and said liquid level to define an aperture therebetween through which vapors of the heat transfer liquid ascend to said path and which are moveable to permit the aperture therebetween to be adjusted. An apparatus as claimed in Claim 1, wherein said vessel (11) is a closed vessel, said entrance and exit port means includes a pair of opposite openings (23a, 23b) provided in the side wall of said vessel and said vessel and said conveying means includes a belt conveyor (17a) capable of supporting said articles to be soldered together thereon and having a portion horizontally running through said space. An apparatus as claimed in Claim 1 or Claim 2, wherein said pair of plates are slidably mounted plates. An apparatus as claimed in Claim 1 or Claim 2, wherein said pair of plates are swingably mounted plates. An apparatus as claimed in any preceding claim, wherein said vessel has a middle portion (20) formed of a heat insulating material so that heat transfer between portions of said vessel across said middle portion is minimized.

Inventors: KONDO, KENSHI
Applicants: NIHON DEN-NETSU KEIKI CO., LTD.
Application year: 1986
Main classification: H05K3
IPC codes: B23K1