GB1602779A - Methods and apparatus for mass soldering of printed circuit boards - Google Patents

Abstract

A fluid stream preferably air or other gases which may be heated, is directed onto a soldered board 20 immediately following deposition of molten solder onto the board. The impinging fluid stream relocates solder on, and/or blasts excess solder from the bottom of the board, and any interconnections, component leads and/or component bodies carried thereon before the solder solidifies as shorts, icicles or bridges. If desired, liquid droplets such as soldering oil may be included in the fluid stream. The fluid is preferably directed by a nozzle 62 and the presence of a board 20 is detected by a photo-electric detection 74,78. <IMAGE>

Description

(54) METHODS AND APPARATUS FOR MASS SOLDERING OF PRINTED CIRCUIT BOARDS (71) We, COOPER INDUSTRIES, INC., a corporation organized under the laws of the U.S.A., of Cleveland, Ohio, U.S.A., do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to methods and apparatus for mass soldering of printed circuit boards, and more specifically to an improved method and apparatus for mass soldering electrical and electronic components, by their leads, to printed circuit boards or the like.

Various soldering systems are well known in the art for mass soldering electrical and electronic components, by their leads, onto printed circuit boards. One technique for mass soldering components to circuit boards is that of dip soldenng. With this technique, the entire side of a circuit board containing the printed wiring, with the leads from the components projecting through apertures in the board, is engaged for a certain period of time with the surface of a bath of molten solder, and then removed. Another technique for mass soldering components onto circuit boards is that of wave soldering. A typical prior art wave soldering system generally comprises a container adapted to hold a supply of molten solder and a sump partially submerged in the molten solder. The sump has an intake orifice below the surface of molten solder, and an elongate horizontal nozzle or slot above the surface of the solder. A positive displacement pump is submerged in the molten solder and is adapted to force molten solder into the sump intake orifice, where the molten solder then flows upward in the sump and out the horizontal nozzle to thereby produce a smoothly rounded standing wave of molten solder above the nozzle.

Other techniques for mass soldering electrical and electronic components onto printed circuit boards are well known in the art and include cascade soldering, jet soldering and drag soldering. So called "leadless" components such as flat packs can also be mass soldered to circuit boards by fixing the components to the bottom of a board, e.g. as by fixturing or with an adhesive, and then engaging the bottom of the board and the components with molten solder.

While known mass soldering systems have provided substantial manufacturing economy to the electronics industry and thus achieved substantial commercial use, the deposition of excess solder on the board circuits, connections and leads has been a continual problem. Deposition of excess solder may result in formation of shorts, icicles and/or bridges, and will increase solder consumption and finished board weight.

The problem of solder shorts, icicling and bridging was believed attributable to the web (sometimes called "backwash") which forms between the printed circuit board and the surface of the solder wave as the board exits the solder wave. Deposition of excess solder also has the technical disadvantages of increasing solder consumption and board weight. Moreover, current trends in the electronics industry to relatively high density electronic assemblies has increased the problem of shorts, icicling and bridging due to backwash.

The prior art has devised various techniques to solve the problems of solder shorts, icicling and bridging. For example, for wave soldering, one technique which has become virtually universally adopted by the industry is to incline the travel path of the circuit boards through the solder wave, i.e. from, the horizontal, to increase the exit angle between a board being soldered and the solder wave. Moreover, the art has also devised various sump and nozzle designs which generate special wave forms for further increasing exit angles and/or changing the point where the circuit board exits the wave.

While increasing the exit angle of the board from the wave has been found to substantially reduce the incidence of solder shorts, bridges and/or icicling, such has not entirely eliminated solder shorts, bridges and icicling, particularly in cases where relatively high density electronic assemblies and/or relatively long lead components are being soldered. Moreover, inclining the conveyor, i.e. so as to increase the exit angle increases the height of the soldering system and also complicates the transport system between the assembly station and the cleaning station which normally employ horizontal conveyors. Modifying the sump and nozzle to increase the exit angle an or changing the point where the circuit board exits the wave complicates wave geometries and control.

Another system for reducing the incidence of solder shorts, icicling and bridging, which has achieved substantial commercial acceptance, is to intimately mix soldering oil in the solder wave in accordance with the teachings of Walker et al U.S. Patent 3058441. While such systems have been found to reduce substantially the incidence of solder shorts, bridging and/or icicling, such systems have not entirely eliminated solder shorts, bridges and icicling, particularly in cases where relatively high density electronic assemblies and/or relatively long lead components are being soldered to circuit boards.

It is thus a primary object of the present invention to provide a mass soldering method and apparatus which overcomes the aforesaid problems of the prior art.

According to one aspect of the present invention there is provided a method of mass soldering printed circuit boards having mounted thereon components with leads protruding downward through apertures in said board, said method comprising the steps of: depositing a quantity of molten solder onto the underside of said board and said downward protruding leads, and, substantially immediately thereafter impinging a stream of heated gas onto the inderside of said board and said protruding leads whereby to displace a portion of the molten solder deposited on said board and leads before said molten solder solidifies thereon.

According to another aspect of the present invention there is provided a method of reducing the incidence of solder shorts, icicling and/or bridging in a mass soldering process in which a quantity of molten solder is deposited onto the underside of a printed circuit board, said method comprising removing a portion of the solder deposited by said soldering process by impinging a stream of heated gas onto said board underside before said deposited solder solidifies thereon as shorts, icicles and/or bridges.

According to a further aspect of the invention there is provided apparatus for mass soldering printed circuit boards having mounted thereon electrical components with leads protruding downward through apertures in said board, and comprising in combination; a mass soldering station adapted to hold a supply of molten solder means for transporting said circuit board through said soldering station whereby a quantity of molten solder may be deposited onto said board underside and said protruding leads, and an excess solder removal station adjacent said mass soldering station having means for directing a stream of heated gas onto the underside of said board and said downwardly protruding leads whereby to remove a portion of said deposited molten solder from said board and/or said protruding leads before said molten solder solidifies thereon.

In carrying the invention into effect as applied to a mass wave soldering method and apparatus the board may be transported horizontally in contact with the upper end of a standing wave of solder and said gas stream directed to impinge on the underside of said board and protruding leads substantially immediately following contact of said board and leads with said solder.

The gas stream may comprise a gas or gas mixture, such as an inert gas or air or a mixture thereof and if desired, may contain liquid droplets such as soldering oil dispersed in the gas.

For a fuller understanding of the present invention, reference should be had to the following detailed description taken in connection with the accompanying drawings in which: Fig. 1 is a side elevational view, diagrammatically illustrating a mass soldering apparatus according to the present invention; Fig. 2 is a side elevational view, partly in section, of the actual soldering portion of the apparatus of Fig. 1; Fig. 3 is a top plan view of the soldering apparatus portion of Fig. 2; tig. 4 is a schematic diagram of the electrical and pneumatic control means of the apparatus; Fig. 5 is an enlarged, side elevational view in section, of a portion of the apparatus of Fig. 2, showing a circuit board assembly at an intermediate stage in the method of the present invention, and showing how excess solder is removed in accordance with the present invention; and Fig. 6 is a view similar to Fig. 5 illustrating a modification of the soldering apparatus of Figures 1 to 5, Fig. 7 is a side elevational view of a second soldering apparatus embodying the present invention, taken along the travel path of a circuit board; Fig. 8 is a side elevational view, partly; 1 section, of the wave soldering portion of the soldering apparatus of Fig. 7 and Fig. 9 is a top plan view of the wave sol dering portion of Fig. 8.

In the following detailed description of the present invention, the term "component" refers to leadless components as well as components having conventional metallic conductors or leads. The term "component lead" refers to that part of the metallic conductor of an electrical or electronic component that is joined to the printed circuit board pattern, i.e. the component leads terminals, lugs, pins, etc. The term "land" as used herein refers to that part of the metallic pattern on the printed circuit board to which a component or component lead is joined by solder. The term "mass soldering" as used herein is intended to refer to any of the several soldering techniques known in the art in which liquid solder is applied to a circuit board from a reservoir of liquid solder, including, by way of example but not limitation: wave soldering, touch or dip soldering, pot soldering, jet soldering, cascade soldering and drag soldiering.

Referring now to Figures 1 to 5 of the accompanying drawings, one preferred embodiment of the present invention will be described in the context of a wave soldering apparatus. Referring first to Fig. 1, a printed circuit board 20 is loaded at an insertion station 22, with a plurality of electrical or electronic components 24 at predetermined positions on the board. The board comprises an insulated wiring board having one or more printed metallic conductors on the board underside, and a plurality of apertures 25 which extend through the board.

Components 24 are loaded onto the top side of the board with their leads 26 protruding downward through the board apertures 25 in juxtaposition to the circuit lands to which they are to be joined. The components may be inserted in the board by any method known in the art which may include manual assembly, semi-automatic, or automatic assembly which may comprise singlestation or multiple-station pantagraph or numerically controlled machines all of which are well known in the art and need not be described further.

The next step involves treating the surfaces to be soldered with a so-called flux at a fluxing station 30. The flux may be any flux well known in the art and may include, for example, a water-wmte rosin flux, an activated rosin flux or a water soluble flux. The flux may be applied in fluxing station 30 by any manner well known in the art, for example, by spraying, foaming, brushing, or from a flux wave.

The board is then passed from fluxing station 30 to a pre-heating station 32 where the board is preheated to mobilize the flux and also drive off the bulk of the flux solvent to form an active flux layer on the board and leads. Preheating station 32 may comprise infrared or convection heaters or a combination of infrared and convention heaters as are well known in the art. Preferably, but not necessarily, preheating station 32 is extended as compared with conventional preheating stations, and/or preheating station 32 may be operated at higher than normal temperatures so that the board 20 is heated to a top side temperature 14-28 C or more higher than normal. Thus board 20 typically will be preheated to a minimum top side temperature of about 66"C, but preferably the board will be preheated to a top side temperature in the range of about 79"C-121"C. The purpose of preheating the board to higher than normal top side temperatures is to increase the time the solder on the board remains molten after the board emerges from the solder wave. The reason for this will become clear from the description following.

The fluxed preheated board is then passed to a mass wave soldering station 36.

Referring also to Figs. 2 and 3 the wave soldering station includes a container of conventional design, indicated generally at 40, for holding a supply of molten solder 42.

Conventional heating means (not shown) may be secured to the bottom andlor side walls of container 40 or immersed in the solder to heat and maintain the supply of solder 42 in molten condition.

A sump and nozzle assembly indicated generally at 44 is disposed interiorly of container 40. The sump and nozzle assembly 44 is of conventional design and typically comprises a rounded bottom wall 46, a pair of substantially vertically opposed end walls 48 and 50, and a pair of inclined side walls 52 and 54. The upper ends of end walls 48 and 50 and side walls 52 and 54 are spaced from one another to form a narrow elongated rectangular nozzle or slot 56 which extends above the molten solder level in container 40 for a suitable distance, e.g. one inch above the molten solder level.

Preferably, the sump also includes a pair of adjustable sluice plates 58A, B spaced from the sump side walls 52 and 54 for controlling solder overflow from the nozzle 56, e.g. in accordance with the teachings of U.S.

Patent No. 3398873 to Kenneth G. Boynton. Completing the soldering station is a variable speed pump (not shown) which communicates through an intake orifice 59 in the lower end of sump and nozzle assembly 44 for pumping solder into the sump where it then rises and overflows the nozzle 56 as a standing solder wave.

An important teature and critical requirement of the present invention is the ability to displace excess solder from the bottom of the circuit board, and/or from any interconnections, component leads and/or component bodies carried thereon before the solder can solidify as shorts, icicles and/or bridges. This is accomplished by treating the soldered circuit board and depending component leads at an excess solder removal station 60. Excess solder removal station 60 follows soldering station 36 immediately in-line and is designed to blow-off excess solder from the board underside before the solder solidifies as shorts, icicles and/or bridges. Solder removal station 60 comprises one or more fluid jets, fluid knives, slots, nozzles or the like indicated generally at 62, from which a gas stream is directed onto the underside of the board. A baffle plate 64 is disposed under the path of travel of boards 20 at an angle of approximately 45C from the horizontal and serves as a deflector for the gas stream of heated gas onto the underside of pre-heated pnor to impinging on the board.

Gas flow rate, gas pressure, and gas temperature and the time elapsed between circuit board emersion from the solder wave and beginning of contact by the gas stream may vary widely depending on the board temp erature, ambient temPerature, melting point of the solder, specific heat of the gas and heat transfer coefficient of the gas to the board, board size and shape, component density, amount of solder deposited and to be removed, conveyor speed, and distance between the soldering station and the excess solder removal station. Preferably nozzles 62 and baffle plate 64 are disposed as close as possible below the path of travel of the boards. Placement of nozzles 62 and baffle plate 64 will of course be limited by the longest lead being soldered. Generally, the gas stream is air at a pressure in the range of from about 35 to about 106 gm./sq/cm., and a flow rate in the range of from about 0.014 to about 0.042 cu. m./sec. per square meter of fluid jet opening. Inert gas may be used as the gas, but preferably the impinging gas comprises air. Preferably the gas is preheated to a temperature in the range of about 93 G260 C. For 63/37 solder alloy, the preferred gas pre-heat temperature is about 182 C.

The gas stream impinging on still molten solder contained on the underside of the circuit board, the interconnections and the component leads and/or bodies blasts excess solder from the underside of the board, interconnections, leads, and bodies, and in doing so also minimizes the possibility of solder bridging or icicling or short formation upon solidification.

Fig. 4 illustrates a preferred form of electrical and pneumatic control means in accordance with the present invention and is particularly adapted to the use of hot air as the gas stream in accordance with the pres ent invention. Referring to Fig. 4, nozzle 62 is connected via supply line 66 to one side of solenoid actuated valve 68. Valve 68 is connected via a line 70 to the outlet of a heater 72 which is adapted to heat air (or other gas) to a desired elevated temperature.

Valve 68 is actuated by a photoelectric cell 74 connected through suitable relay means 76 to provide a blast of heated air when a printed circuit board 20 passes through the solder wave and the board leading edge interrupts a light beam from light source 78 disposed above the path of travel ot tne printed circuit board. The flow of heated air continues until the trailing edge of the printed circuit board passes and permits the beam of light from light source 78 to fall once again uPon the photoelectric cell 74, at which time the valve 68 is closed or nearly closed. Preferably valve 68 is prevented from closing fully, i.e. so as to allow at least a small flow of heated air through the nozzle so that the nozzle will be maintained at the desired elevated temperature and thus eliminate thermal lag.

Completing the soldering system is a circuit board conveyor 80 of conventional construction. The latter comprises a pair of spaced conveyor rails 82 and 84 and suitable drive means (not shown) for carrying a circuit board from the inserting station 22 through the fluxing station 30, wave soldering station 36 and excess solder removal station 60.

Fig. 5 shows a printed circuit board in the excess solder removal station 60 and illustrates how the hot gas flow removes excess molten solder 86 from a circuit board, interconnections and component leads in accordance with the present invention.

One skilled in the art will be able to determine experimentally the preferred operating parameters for achieving icicles bridge- and short-free mass soldering for the particular board being soldered.

For example, to eliminate solder icicles, bridges and shorts from a circuit board which measures 6.35 and 15.24 cm. and includes a net circuit plan area of aDproxi- mately 96.75 sq. cm., 150 plated through apertures of 0.0762 cm. diameter, and including 150 coponent leads of 0.0508 cm.

diameter (50% steel leads, 50% copper leads), solder alloy of 63/37, solder bath temperature 252"C, preheat temperature of about 107"C (top side) conveyor speed of 1.22 m/min., and employing hot air (182"C) at a pressure of about 703 gmisq. cm. and flow rate of about 0.028 cu. m/sec. as the impinging gas, the distance between the point where the circuit board emerges from the solder wave and begins to pass the hot air stream should be not greater than about 12.70 cm. One skilled in the art will recoL- nize that the various aforesaid parameters are interrelated and may be varied to achieve the foregoing objects of the present invention.

Referring now to Figures 7 to 9 these illustrate a second preferred form of mass wave soldering apparatus in accordance with the present invention. As far as possible the same reference numerals have been used as in Figures 1 to 5 to designate corresponding points. The mass wave soldering apparatus illustrated in Figs. 4-6 has many similarities to the mass wave soldering apparatus shown in Figs. 1 to 5. Thus for example, the mass wave soldering apparatus in accordance with Figures 7 to 9 also includes a loading station 22, a fluxing station 30, a preheating station 32 and mass wave soldering station 36. However, in contrast to the system of Figures 1 to 5 the board 22 is carried substantially horizontally through the crest of the solder wave.

Also preferred but not essentially required, is to extend the preheating station 32 as compared with conventional preheating stations, and/or to operate preheating station 32 at higher than normal temperatures, so that the board 22 will be heated to a top side temperature 14-28"C or more higher than normal. Thus, for example, board 22 may be preheated in accordance with the present invention to a minimum top side temperature of about 66"C, preferably a to side temperature in the range of about 79"C to 1210C. The purpose of preheating the board to higher than normal top side temperatures is to increase the time the solder on the board remains molten after the board emerges from the solder wave.

Beyond this operation and arrangement of the system of Figures 7 to 9 is the same as that described above for the system shown in Figures 1 to 5.

The present invention has a number of advantages. For one, contacting the underside of a circuit board with a heated gas stream in accordance with the present invention has been found to level the solder coating on the board conductors. Moreover, eyelets and unloaded plated through holes may be cleared by the fluid stream. Another advantage is that a soldered board emerges from the system somewhat cooler as compared with conventional mass soldering systems. This latter advantage results primarily from the removal of excess solder which is a major heat sink on conventionally soldered boards. Moreover, the gas stream, even when heated, generally is substantially cooler than the solder, and thus further speeds cooling. Cooling the board facilitates handling of the boards subsequent to soldering and also may result in reduced incidence of pad lift in solder-cut-solder systems.

Cooling the board also may result in production of solder joints of finer grain.

Various changes may be made in the above described apparatus. For example, one or more banks of heaters, similar in construction to the preheaters may be incorporated into the excess solder removal station 60 to extend the time the solder remains molten on the board. Also, one or more heater elements may be incorporated integrally with nozzles 62, to supplement and/or in place of heater 72. Moreover, a minor amount, e.g. up to about 20% by weight of liquid droplets may be dispersed in the gas stream, for example, by aspirating the liquid into the gas stream from one or more aspirating nozzles 98 as shown in Fig.

6; when applied to the apparatus of Figures 7 to 9 the board feed will of course be horizontal. Aspirator nozzles 98 are connected through conduit means 100 to a supply 102 of the liquid to be dispersed into the gas stream. Obviously, the liquid droplets could be injected into the gas stream using one or more atomizing nozzles. The liquid should comprise a material which is compatible with the soldering operation. By way of example, the liquid may comprise a conven tional soldering oil or mixture of oils. The liquid droplets increase the fluid flow mass of the gas stream and thus in turn may result in an increased rate of solder removal from the board and leads. Also, employing a soldering oil as the liquid may facilitate postsoldering clean-up and may also produce shinier solder joints. If desired, conventional wetting agents and/or fluxing agents may also be dispersed in the gas stream. Still other changes will be obvious to one skilled in the art. Accordingly, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted in an illustrative sense, the scope of the invention being defined in the

Claims (29)

appended claims. WHAT WE CLAIM IS:

1. A method of mass soldering printed circuit boards having mounted thereon components with leads protruding downward through apertures in said board, said method comprising the steps of: depositing a quantity of molten solder onto the underside of said board and said downward protruding leads, and, substantially immediately thereafter impinging a stream of heated gas onto the underside of said board and said protruding leads whereby to displace a portion of the molten solder deposited on said board and leads before said molten solder solidifies thereon.

2. A method according to Claim 1, wherein said solder is deposited onto said board and leads by passing the underside of said board and said protruding leads at least in part in contact with a wave of molten solder, and including the step of directing said gas stream onto the underside of said board and said protruding leads substantially immediately following contact of said board and said leads with said solder wave.

3. A method according to claim 2, wherein said board is transported substantially horizontally through said wave of molten solder.

4. A method according to any preceding claim wherein said gas stream comprises a stream of heated air.

5. A method according to any preceding claim wherein said gas stream is heated to a temperature in the range of 93"C to 2600C.

6. A method according to any preceding claim further including the step of dispersing liquid droplets in said gas stream, and directing the resultant dispersion onto said board and leads.

7. A method according to claim 6 wherein said liquid droplets comprise soldering oil.

8. A method according to claim 6 wherein said liquid droplets comprise a wettj,g agent.

9. A method according to claim 6 wherein said liquid droplets comprise a flux agent.

10. A method according to any preceding claim wherein leadless components are mounted onto the underside of said board, and including the step of removing a portion of the solder deposited on said leadless comPonents.

if. A method of reducing the incidence of solder shorts, icicling and/or bridging in a mass soldering process in which a quantity of molten solder is deposited onto the underside of a printed circuit board, said method comprising removing a portion of the solder deposited by said soldering pro Cess by impinging a stream of heated gas onto said board underside before said deposited solder solidifies thereon as shorts, icicles and/or bridges.

12. A method according to claim 11 wherein said solder is deposited onto said board underside by passing said board at least in part in contact with a solder wave, and removing said portion of said deposited solder substantially immediately following contact of said board with said solder wave.

13. A method according to claim 11 or 12 wherein said gas stream comprises a stream of heated air.

14. A method according to claim 13 wherein said gas stream is heated to a temperature in the range of 93"C to 2600C.

15. A method according to any of claims 11 to 14 including the step of dispersing liquid droplets in said gas stream, and directing the resultant dispersion onto said board.

16. A method according to claim 15, wherein said liquid droplets comprise soldering oil.

17. A method according to claim 15, wherein said liquid droplets comprise a wetting agent.

18. Apparatus for mass soldering printed circuit boards having mounted thereon electrical components with leads protruding downward through apertures in said board, and comprising in comination: a mass soldering station adapted to hold a supply of molted solder means for transporting said circuit board through said soldering station whereby a quantity of molten solder may be deposited onto said board underside and said protruding leads, and an excess solder removal station adjacent said mass soldering station having means for directing a stream of heated gas onto the underside of said board and said downwardly protruding leads whereby to remove a portion of said deposited molten solder from said board and/or said protruding leads before said molten solder solidifies thereon.

19. Apparatus according to claim 18, wherein said soldering station is a mass wave soldering station adapted to generate a standing wave of molten solder and said transporting means are arranged to transport said circuit board in contact with the upper end of said standing wave.

20. Apparatus according to claim 18 or 19 wherein said transporting means is arranged to transport said circuit board on a substantially horizontal travel path through said soldering station.

21. Apparatus according to any of claims 18 to 20 wherein said gas stream comprises a stream of heated air.

22. Apparatus according to any of claims 18 to 21 wherein said means for directing include a gas jet disposed below the travel path of said board.

23. Apparatus according to claim 22 said excess solder removal station including a source of positive pressurized gas connected to said jet, and a valve for permitting a flow of pressurized gas through said jet for a selected time.

24. Apparatus according to claim 23 wherein said source of pressurized gas comprises a source of pressurized air.

25. Apparatus according to any preceding claim said excess solder removal station further including means for introducing liquid droplets into said gas stream.

26. Apparatus according to claim 25 including a source of said liquid, and wherein said means for introducing comprises at least one aspirator adapted to be connected to said source of said liquid.

27. Apparatus according to claim 25, including a source of said liquid, and wherein said means for introducing comprises at least one atomizing nozzle adapt i to be connected to said source of said liquid.

28. Apparatus according to any of claims 18 to 27 wherein leadless compo nents are mounted onto the underside of said board, and said excess solder removal station is adapted to remove molten solder from said leadless components.

29. A method or apparatus for the mass joining with solder of electrical components on a printed circuit board substantially as hereinbefore described with reference to the accompanying drawings.