APPARATUS FOR PROCESSING PLANAR PRINTED CIRCUIT BOARDS

Abstract: Boards are processed flat in a sequence of treatment chambers (17, 20, 27, 30, 41-44, 51-54, 72-75, 172, 175), arranged to reduce a required workshop floor area by stacking treatment chambers on top of each other and all above a treatment liquid sump (78). The apparatus has at least two horizontal treatment paths (11, 12), each of which passes through a succession of the treatment chambers (17, 20, 27, 30, 41-44, 51-54, 72-75, 172, 175), and each board may advance along one only of the treatment paths (11, 12) or execute an outward journey along one of the paths and a return journey along another of the paths. Each chamber contains conveyor means (94), and those which are wet treatment chambers may contain reciprocatory spray liquid treatment equipment, known per se.

Description: -/- APPARATUS FOR PROCESSING PLANAR PRINTED CIRCUIT BOARDS BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to apparatus for processing planar printed circuit boards, and comprising: a) a first board conveyor, for advancing the boards along a first treatment path in a horizontal disposition (that is, such that the two major surfaces of each board face generally upwards and downwards from the path) ; b) a first treatment chamber which surrounds a first treatment zone on the first treatment path, which zone ' extends from a horizontally-extensive inlet opening in one side wall of the chamber to a horizontally-extensive outlet opening in an opposite side wall of the chamber; c) first liquid spray means within the chamber, for directing a flow of treatment liquid on to at least one of the two major surfaces of each circuit board during advancement of the board through the chamber by the conveyor; and d) a first sump located at a level which is lower than the level of the treatment chamber, for containing the treatment liquid which is directed over the circuit boards. BACKGROUND OF PRIOR ART Apparatus for processing circuit boards, with the boards arranged in a horizontal disposition, is described for example in US-A-3868272. Also known is apparatus for processing circuit boards with the boards arranged vertically, that is, on edge. See, for example, US-A-4371422 and 4506687. One advantage of processing on edge is that there is a tendency in the industry to suppose that the chemical action of treatment fluids on the boards can be controlled more easily with the boards arranged on edge. For example, liquid treatment chemicals on vertical boards will run off the boards at a rate which is considered predictable, whereas a horizontal board disposition raises the possibility that pools of treatment chemical will exist on the top surface of the boards but the length of time for which such pools exist, and how big they are, might vary from board to board. Furthermore, with the boards on edge r chemical treatment agent delivery means can be identical on each of the two vertical faces of the boards whereas, with the boards flat, one delivery means has to emit treatment agent downwards onto boards below whereas the other must eject treatment agent upwards to contact the board face above. A problem faced equally by both the flat and on-edge processing systems, however, is the expense incurred by the heavy demands they made on factory floor area. The processing line typically extends through of the order of 10 processing stations each of which must occupy an irreducible minimum floor space which is fixed by the volume of the boards to be processed. Because the processing steps are sufficiently sensitive to require careful and frequent monitoring, each processing station has to be reasonably accessible for inspection by those conducting the process. As the known equipment has a height of about lm or more, double banking the known equipment would not facilitate the necessary inspection procedures. A further difficulty with double banking lies with the aggressive nature of certain of the treatment agents used as a matter of routine with the boards being processed. The material from which the boards are made is selected, in part, for its chemical inertness. A consequence of this inertness, however, is that the boards are not responsive to chemical processing steps to be applied to the boards during the application of electrical circuit elements to them, unless the chemical treatment agents employed are quite aggressive. Hitherto, . the need to confine these aggressive chemicals safely, so that they are kept from harming the processing equipment or operating personnel, has tended to rule out the use of double banking systems. US-A-4454003 and 4539069 of Fishman et al contain disclosures of systems in which the boards execute an "up, over and return" processing path which is said to minimise floor space requirements but it is clear that the above-identified problems of ease of inspection and confinement of dangerous chemicals are not met. The return path is said to be an "overhead return" and it is clear from the drawings that it is too high for easy inspection. The only teaching how to avoid contamination by drips from the overhead return is to install drip trays below the overhead path but preferably to confine the use of the upper path to dry operations. Thus, the disclosure falls short of providing a practicable double-banked system. BRIEF SUMMARY OF THE INVENTION It is an object of the present invention to ameliorate the afore-mentioned disadvantages of the processing apparatus used hitherto. According to the present invention the processing apparatus is characterized by: e) a second board conveyor, for advancing boards along a second treatment path in a horizontal disposition; f) a second treatment chamber surrounding a treatment zone on the second treatment path, which zone corresponds to the first treatment zone on the first treatment path; and further characterized in that: g) the second treatment chamber is distinct and separate from the first chamber and has its own separate liquid spray means; and h) the first and second treatment chambers are in a mutually stacked disposition, one above the other, and both above the said first sump. In one preferred embodiment such apparatus is provided as a module and a processing line is made up from a plurality (at least two) of these modules linked together so that the first treatment path extends through the first treatment chamber of each of the modules, and likewise all the second treatment chambers lie on the second treatment path. It will often be convenient that the sump in any one module seryes one treatment chamber from that module and one treatment chamber in an adjacent module. Otherwise, the sump might serve both or all the chambers in its own module when, for example, all the treatment paths are performing exactly the same sequence of processing steps in parallel and the same direction of advance of the boards. Conveniently, any two successive treatment chambers in any one treatment path are connected to each other, and spaced from each other, by a link chamber, and it is preferred that there is a fluid-tight connection between each treatment chamber and the or adjacent link chamber, at least below the level of the treatment path, so as to form a fluid-tight trough below the treatment path from one end of it to the other. Conveniently, each of the treatment chambers, and/or each of the link chambers, is made from synthetic polymeric material by a rotational moulding technique. This allows the chambers to be made without seams, with the only essential apertures in the wall surfaces being the "letter-box" slots for entry and exit of boards being processed, and those for inflow and outflow of the required processing fluid. An inspection port would normally also be provided, however. A further attraction of the rotational moulding technique is that it lends itself to the fabrication of the chambers in a way which gives them a cavity wall. An advantage of hollow walls is better thermal insulation of the treatment liquid in the chamber, leading to more accurate control of treatment temperature and, when treatment is at elevated temperature, reduced heating costs and waste heat disposal costs. Already known are means of conveying boards flat through a processing chamber, which comprise a multitude of horizontal shafts arranged transverse to the direction of advance of the boards, and which all rotate together to cause the boards to be advanced through the chamber at the preselected speed, on the upper surfaces of a plurality of the rotating shafts, successively over the shafts. Such means may conveniently be used in the present apparatus. A preferred conveyor construction is described below with reference to the appended drawings. Already known are means for spraying treatment liquid evenly over the upward- and downward-facing major surfaces of the boards as they advance through ' the treatment chambers. In one system, there are several transversely-extending spray bars, each with a multitude of spray orifices directed towards the treatment path and each fed from its own spray liquid supply manifold. Each spray bar is caused to reciprocate end-wise and this reciprocation, transverse to the direction of advance of the boards, together with the provision of a very large number of spray orifices, can achieve the required even distribution of spray chemicals. Nevertheless, satisfactory results require a high volume flow of spray liquids. Each sump will normally be provided with a high capacity electric motor driven pump to feed the spray system and a high capacity filter in the spray liquid flow path to prevent blockage of any of the spray orifices. A range of sensors would normally be provided, to monitor all critical aspects of the processing step in the sump/chamber combination in question, and enable its control, in all probability based on a microprocessor. Between any successive wet processing steps there may be a requirement for .drying. The present treatment chambers are useful not only for wet processing but also for any such dry processing step, simply by installing any required dry processing device instead of wet spraying means. For a better understanding of the present invention, and to show more clearly how the same may be carried into effect, referenc will now be made, by way of example, to the accompanying drawings, in which: FIGURE 1 is a perspective view of a processing module in accordance with the present invention; FIGURE 2 is a side elevation of a processing line which includes four modules; FIGURE 3 is a rear elevation of a module such as is shown in Figrre 2, and part of an adjacent module; and FIGURE 4 is a longitudinal vertical section through one of the j treatment chambers of Figure 3, in the plane of its conveyor drive shaft. Referring to Figure 1, there is shown a processing module 10 which has an upper processing path 11 and lower processing path 12, the start of the first path being associated with a slot 13 in the upstream end wall 14 of a first link chamber 15. Likewise, the start of the lower processing path is associated with a slot 23 in the upstream end wall 24 of a link chamber 25. Each aperture 13, 23 is big enough to accommodate, without excessive clearance, circuit boards to be processed along the first and second treatment paths 11, 12 respectively. At the downstream end 16 of the link chamber 15 there is a liquid-tight seal between the link chamber 15 and a first upper treatment chamber 17. There are matching apertures (not visible) for the boards in the downstream end of the link chamber 15 and the upstream end 18 of the chamber 17. 5 The upper treatment path continues through a second link chamber 19, second upper treatment chamber 20 and third link chamber 21. In precisely the same fashion, the lower treatment path extends through a first lower treatment chamber 27 and further link chambers 29 and 31 and 1.0 treatment chamber 30. Each of the ten chambers visible in the drawing is made by a rotational moulding technique, and so is seamless. This, together with the liquid-tight abutment of successive chambers in each of the treatment paths 11 and 12, minimizes the likelihood of leakage or 15 spillage of chemicals from the chambers. No details are visible in Figure 1 of the arrangements for flowing treatment liquids into, through and out of the treatment chambers. In particular, the large sump and reservoir of liquid treatment agent is not visible behind 20 front 32 and side 33 cladding panels beneath the treatment paths 11 and 12. Connections between the sump and the treatment chambers are channelled behind the chambers, within a volume surrounded by further side 34 and top 35 cladding panels. 25 Referring now to Figure 2, there is shown a double-banked production line wherein boards are treated through four successive treatment chambers 41, 42, 43 and 44 of an upper treatment line 45, or the four corresponding treatment chambers 51-54 of a lower treatment line 55. Work 30 pieces 60 are delivered on a trolley 61 to a peeler 62 and thence to a double buffer 63 which has the task of delivering successive boards 60 to each of the two treatment lines 45 and 55, at the required frequency. At the downstream end 64 of the treatment lines a double 35 unloader 66 picks off processed boards 67 from each of the treatment lines 45 and 55 and stacks them in an upper stack 68 and lower stack 69 respectively. A work transporter system 59 draws off completed stacks from the unloader 66, as required. Turning now to Figure 3, the module 70 has a structural frame 71 which carries an upper treatment chamber 72 flanked by first 73 and second 74 link chambers which are centered on upright parts of the frame 71 and extend outside the frame for abutment to the treatment chamber of an adjacent module. There is a lower treatment chamber 75 identicra-l- with the upper treatment chamber 72 and directly beneath it, and there are likewise first 76 and second 77 lower link chambers below the corresponding upper link chambers 73 and 74_ Beneath all these chambers, at the base of the frame 71 is a large sump 78, the sump vessel 58 being formed by a rotational moulding technique which provides it with cavity walls. The sump is provided with treatment liquid temperature control means, and a variety of sensors to ensure that the treatment liquid has a desired, chemical composition and temperature. A large capacity pump 79 immersed in the sump liquid is driven by a 3 p-ha.se induction motor 80. Treatment liquid is delivered from the pump 79 to spraying devices within the treatment chambers.fed by the pump 79, in a manner known per se. Each of the treatment chambers 72, 75 is divided by ribs 81 into- three zones A, B, and C. Each zone has its own drain aperture 82 and the three apertures 82 from any one chamber are eoraiected by a drain manifold 83. Into the sump 78 treatment liquid from the chamber 75 drains via its drain apertures 82, drain manifold 83 and drainage channel 84. In the arrangement illustrated, the pump 79 serves not only chamber 75 • but also the lower treatment chamber 175 of the adjacent module 170, and so treatment liquid draining from the. chamber 175 flows back to the sump 78 through drainage means 182, 183 and 184 associated with the chamber 175. The drainage channels 84 and 184 empty into the sump 78 via a sump manifold 85 and mesh filter bucket 86. Not shown in the drawing is an inline filter which is provided in the feed channel connecting the pump 79 and the spray means in the chambers 75 and 175. In similar fashion the sump 178 of the adjacent module 170 contains liquid for treatment in the upper treatment chambers 72 and 172, and there are corresponding drain arrangements to return treatment liquid from the upper treatment chambers to the sump 178. In fact, treatment liquid can be fed to any of the treatment chambers from either of the sumps, and drained to either of the sumps from any of the chambers. Now referring to Figure 4, the drawing shows one of the two racks 93 which extend longitudinally and vertically in the chamber 72, one on each side of the circuit board treatment path, thereby defining what looks like a fence on each side of the path. Circuit boards on this treatment path are supported on a group of conveyor shafts 94, each one of which extends between two corresponding recesses 95 in the pair of racks 93. All of the shafts 94 carry, at one end thereof, a pinion 96 which receives a rotational driving force from a worm gear 97 on a drive shaft 98 which extends longitudinally through the treatment chamber 72. Thus, the shaft 98 simultaneously drives all of the shafts 94 in the same rotational sense and at the same speed. (It is convenient to mention at this point that it will normally be necessary to operate a production line of successive modules, such as is shown in Figure 2, in the conventional manner in which, as the work pieces progress downstream, they are advanced through successive treatment stations at least as fast as they advanced through earlier work stations upstream, in order that there should never be any question of work pieces catching up the work pieces which are ahead of them in the processing line) . The drive shaft 98 is driven by a stepping motor (not shown) which provides information on conveyor motion to a microprocessor (not shown) which controls operation of the apparatus. In each of these zones A, B and C of the treatment chamber 72, there is an upper reciprocatory spray bar assembly 99 having a multitude of downwardly directed spray nozzle orifices 100 which deliver treatment liquid to the upward facing surface of circuit boards advancing along the treatment path, and there is also a lower reciprocatory spray bar assembly 101 with a multitude of upwardly directed spray nozzle orifices 102 for subjecting the downward facing- lower surface of the circuit boards to similar chemical treatment. A stepping motor (not shown) drives the spray bar assemblies and generates information for the controlling microprocessor. As shown in the drawing, the drive shaft 98 may extend into adjacent chambers on the treatment line, to drive the conveyor shafts in such adjacent chambers. Because of the likelihood that the required treatment liquids will be chemically highly aggressive, it is very desirable that, within the treatment chambers, moving parts should be of materials which are chemically inert, and of a construction which is as simple as possible. For this reason, the force of gravity alone is relied upon to maintain the pinions 96 and worm gears 97 in meshing engagement. Should there be any malfunction which interferes with the smooth running of any one of the conveyor shafts 94, continued uninterrupted rotation of the drive shaft 98 is possible by a simple upward disengagement of the affected shaft 94, out of engagement with the worm gear 97, and against the biasing force of gravity. Furthermore, the construction of the drive shaft is kept simple and inert by the use of a stack of interengaging modular worm drive elements which mesh one with another, axially along the shaft 98 in the manner of a dog clutch. A thrust bearing serves to mount each opposed end of the drive shaft 98 in the opposite end ' walls of a run of connected treatment and link chambers, with the natural resilience of the synthetic polymeric material of the chambers serving to apply axial compressive stress to the shaft 98 and maintain the interengagement of the plurality of worm drive elements 97 along its length. Each time the drive shaft 98 passes from one chamber to the next adjacent chamber it is journalled in a ball bearing 110 carried within a sealed housing defined by two half shells 111, 112 of synthetic elastomeric material, each of these half shells itself being a snug liquid-tight fit in an aperture in the wall of the associated chamber. A detail in Figure 4 shows this bearing arrangement at larger scale, for the sake of clarity. If ' necessary or desirable, intermediate ' vertically extending wall sections can be installed in the chamber 72 at the ribs 81 to achieve a different chemical environment in any or all of the zones A, B and C of the chamber. Not shown in the drawings is the microprocessor-based electronic equipment which monitors and controls operation of the apparatus, but the provision of such electronic control equipment is now a matter of routine for those involved in developing new process machinery. In the above description, an embodiment with two parallel treatment paths is described. The invention, however, may also be realised in apparatus which features three or more stacked treatment paths, and in apparatus wherein the flow of work pieces includes an outward flow along one treatment path and a return flow along a different treatment path in stacked relationship with said one treatment path. Normally, every sump will be located below all treatment paths, the apparatus conveniently being provided in modular form with each module having one sump only, located below a single column of treatment chambers, each treatment chamber belonging to a different horizontally-extending treatment path.

Claims: CLAIMS 1. Apparatus for processing planar printed circuit boards, and comprising: a) a first board conveyor, for advancing the boards along a first treatment path in a horizontal disposition ' (that is, such that the two major surfaces of each board face generally upwards and downwards from the path); b) a first treatment chamber which surrounds a first treatment zone on the first treatment path, which zone extends from a horizontally-extensive inlet opening in one side wall of the chamber to a horizontally-extensive outlet opening in an opposite side wall of the chamber; c) first liquid spray means within the chamber, for directing a flow of treatment liquid on to at least one of the two major surfaces of each circuit board during advancement of the board through the chamber by the conveyor; and d) a first sump located at a level which is lower than the level of the treatment chamber, for. containing the treatment liquid which is directed over the circuit boards; characterized by: e) a second board conveyor, for advancing boards along a second treatment path in a horizontal disposition; f) a second treatment chamber surrounding a treatment zone on the second treatment path, which zone corresponds to the first treatment zone on the first treatment tiath; and further characterized in that: g) the second treatment chamber is distinct and separate from the first chamber and has its own separate liquid spray means; and h) the first and second treatment chambers are in a mutually stacked disposition, one above the other, and both above the said first sump. 2. Apparatus as claimed in claim 1, which takes the form of a first module adapted to co-operate with at least x one further module, the first treatment path extending through a first further treatment chamber of the further module and the second treatment path extending through a second further treatment chamber of the further module, the 5 further module having a sump which serves one of the treatment chambers of the first module and one of the treatment chambers of the further module, and the sump of the first module serving the remaining two said treatment chambers. ■ 100 32.. Apparatus as claimed in claim 1 or 2, wherein each treatment path extends through a succession (at least two) of spaced treatment chambers, the space between adjacent chambers being filled by a link chamber, the respective board conveyor extending through each of the treatment 15 chambers of the succession and each said link chamber. 4. Apparatus as claimed in any one of claims 1, 2 or 3 including at least one treatment chamber made by a rotational moulding process. 51.. Apparatus as claimed in any one of claims 1, 2 or "3 20 wherein each circuit board conveyor comprises a plurality of. equally spaced conveyor shafts arranged to rotate so as to advance boards overlying the shafts along the respective treatment path, each conveyor shaft being driven from one end thereof by a driven member mounted to the shaft at said 25 end, which rests on a complementary underlying drive member, the shaft atthe said one endlbeing journalled in an open-topped recess whereby the one ' end of the shaft may rise upwardly in its recess to accommodate any failure of co-operation between the drive and driven members. 0 : . 6. Apparatus as claimed in claim 5 wherein the drive member is a helical screw gear and the driven member is a meshing toothed pinion. ./ 7. Apparatus as claimed in 5 wherein the drive member constitutes a part of a common drive shaft which extends perpendicular to the conveyor shafts, said common drive shaft having a drive member for each of the conveyor shafts within the length of the drive shaft. 8. Apparatus as claimed in claim 7 wherein the common drive shaft is formed by the drive members themselves, adjacent drive members in the shaft engaging axially in the manner of a dog clutch. 9. Apparatus as claimed in claim 8 wherein the engagement of adjacent drive members along the length of the shaft is maintained by a reactive force imposed along the axis of the shaft by the wall structure of the treatment chamber within which are the conveyor shafts that are driven by the drive shaft:

Inventors: BASS JAMES, LAIDLER KEVIN, POTTER MICHAEL TREVOR, BASS, JAMES, LAIDLER, KEVIN, POTTER, MICHAEL, TREVOR
Applicants: HIBASS PHOTOMEC, HIBASS PHOTOMEC LIMITED
Application year: 1989
Main classification: H05K3
IPC codes: C23F1, H05K3