IE911691A1

IE911691A1 - Method and device of dipsoldering circuit boards

Info

Publication number: IE911691A1
Application number: IE169191A
Authority: IE
Original assignee: Dieter Friedrich Dr Ing; Gitta Friedrich
Priority date: 1990-05-18
Filing date: 1991-05-17
Publication date: 1991-11-20
Also published as: HU9203537D0; WO1991019414A3; JPH06503681A; CS145991A3; BG97083A; DE4016107A1; PL293482A1; BR9106468A; FI925201A; CA2083135A1; EP0457346A1; WO1991019414A2; PT97706A; HUT63293A; FI925201A0

Abstract

The invention relates to a process for producing printed circuit boards with solder pads which are to be provided with a solder deposit and which are initially surrounded by a boundary layer of defined thickness which prevents soldering; the printed circuit board is then introduced into a solder bath in which the regions to be soldered are covered with a sealing element; when the printed circuit board is removed from the bath, the covering achieves a separation of the enclosed volumes of solder from the liquid solder in the dipping bath or in the soldering chamber, appropriate volumes of solder being defined in the cavities above the solder pads. Compared with the prior art, this has the advantage that the covering can be carried out by simple means and the process of forming the solder pads can be carried out separately from the actual dip soldering operation.

Description

DESCRIPTION The invention relates to a method of dipsoldering circuit boards and a device of dipsoldering circuit boards with a limiting layer laterally defining the solder deposition.

From the European patent application EP-OS 0 336 232 a method of dipsoldering circuit boards is known in which substantially the following steps are performed: a) producing a circuit board having electrically conducting areas (solder pads) to be deposited with solder; b) circumscribing these areas by a limiting layer with a defined layer thickness preventing solder deposition, which thickness is substantially corresponding to the thickness of solder deposition to be produced; c) dipping the circuit board thus prepared into a solder bath with a suitable solder alloy; d) covering the areas to be solderd with a covering element using a defined pressure; e) withdrawing the covered circuit board from the solder bath and allowing the temperature to fall I below the hardening temperature of the solder alloy which is in a covered state, whereby the solder depositions are formed? f) finally, covering element and the limiting layer are removed.

The covering and forming processes of the known method substantially run simultaneously and lead in principle to excellent processing results, but, in respect of a continuously good covering in connection with a continuously good surface development of the solder pads, they are not without problems. Furthermore, difficulties are arising with the devices when the covering device, being at least partially in the solder bath, performs at the same time the solder pads surface forming process so that covering has to be also maintained during the whole cooling down phase.

It is therefore the object of the invention to further develop the method of the state of the art such that simpler means can be used for covering the areas to be solderd and the forming pocess of the solder pads can be performed without connection to the dipsoldering process itself.

This object is achieved in that the covering process is maintained only until the covered area is removed from the solder bath and thus a connection of the enclosed defined solder volumes with the liquid solder remaining in the solder bath does no longer exist.

Surprisingly it proved to be also possible, to uncover the solder volumes when still not hardened, then they grow hard in an intermediate phase in a form dependent on the spatial position of the board and the surface tension. This means that small round hunches are formed having a thickness slightly bigger than that of the coating around them. The additional effect obtained is especially desirable, namely that crimped solder material in the sealing area is drawn bak to the solder volume so that forming of bridges between the solder pads, which could negatively effect the circuit, is prevented.

The improved method has further the advantage that special material can be used for levelling the solder volumes which differ from the sealing material itself. Die forming materials need not directly be dipped into the solder bath and thus, are not continiously subjected to very high temperatures. The repeated heating of the hardened solder volumes, necessary for levelling the solder volumes, may be performed purposefully and very cautiously, so that the forming process may be adapted to different requirements.

According to claim 2 it may also be desirable to subject the circuit board after the first hardening of the solder volumes to a cleaning procedure. Therewith fluxes formed in the coating area, which may also contribute to the forming of bridges, may be rinsed out.

The covering element only serves the purpose to produce a possibly true definition of the solder volumes, it need not remain for a longer period on the solder pads. As a result, very simply formed covering elements may be selected to carry out the process, which, for example, are only touching the surface of the circuit board or the surface to be coated, thus performing the necessary, short disconnection between defined solder volume and the solder remaining in the solder bath.

As a covering element it is also possible to use a simple roll which is rollingly applied to the surface of the circuit board or the surface to be coated. As soon as the defined solder volume came out from the solder bath surface or was taken out from the solder bath in another way (laterally through something like a sealing lock) the covered area may quasi be opened again, the first solder is hardening without disturbances whereby hunches are formed as a result of surface tension.

It may also be advantageously., to subject the defined solder volumes when in the opened state to high-short heating beyond the melting point, by this procedure hardened solder remainders projecting over the cavity area may safely be contracted back into the cavity area or be removed from the coating surface through suitable measures.

The covering elements, on the one hand, may be arranged in the area of the solder bath level, on the other hand, is it also possible to remove the circuit boards from the solder bath below the solder bath level through a solder bath side wall with covering elements.

If the circuit board surface provided with cavity areas is positioned horizontally, the hardening solder hunches are formed symmetrically,because of the gravitational force which is desirable for the following forming procedure.

The forming element which is provided separately from the covering element may be a forming roll, but also oseillizing forming roll segments, a forming band or, for example, forming dies may be used which, in the case of continuous production, follow together with the boards and carry out a forming action on a certain section of the production line.

The forming process may positively be influenced in that the forming elements are at least partially or timely heated. If the forming element is made as a conveyor, this fact offers to provide different temperature zones in the transport area. If, however, forming dies are used for forming, they may be heated beyond theta 8« for a short time and purposefully cooled below theta Er by cooling agents.

Die device according to one of claims 16 cf. is characterized in that the covering device covers the areas of the circuit board determined to be soldered only during the withdrawing procedure and that for forming the solder surfaces a special forming device is provided which may advantageously be spatially separated from the covering device.

It is most desirable to provide the device as a production line having the following production stations: - a board introduction device, where the the boards are put on a conveyor and feeded to the conveyor line, - a solder container into which the prepared circuit boards are dipped for soldering and withdrawn in a covered state by sealing elements, and - a forming device subsequently arranged in the transport direction, at the end of which a board output station is arranged in the transport direction.

The individual stations may subsequently be arranged on one level, a cleaning station may additionally be provided carrying out a cleaning procedure of the boards between solder volume definition and forming procedure.

Further advantageous features may be taken from the following subclaims. •Ε 911691 The present invention will now be explained in greater detail according to embodiments of the invention by means of figures. The figures show: Fig. 1 block diagram of the method according to the invention; Fig. 2 diagramatic sectional view of a device for carrying out the method with the boards being withdrawn from the solder bath in a perpendicular position and then formed; Fig. 3 another representation according to Fig. 2 with the boards being perpendicularly withdrawn from the solder bath and formed In a horizontal position; Fig. 4 diagramatic sectional view of a modified device which may be used in devices of a continuous production; Fig. 5 section of a circuit board prior to being dipped into the solder bath after the temporary solder stop mask has been applied and exposed as well as releasing the SMD solder, surfaces; Fig. 6 diagramatic sectional view of a circuit board dipped into a solder bath; Fig. 7 diagramatic sectional view of a circuit board dipped into a solder bath with the cavities being filled with liquid material and closed by a closing element; Fig. 8 diagramatic sectional view with hardened unformed solder volumes; Fig. 9 diagramatic sectional view of a circuit board after the forming process; and Fig. 10 diagramatic sectional view of a circuit board showing the final state when the temporary solder stop mask was removed; Fig. 11 diagramatic sectional view of a modified device with a closed solder chamber; Fig. 12 diagramatic sectional view of a modified device whith the solder chamber being formed as tuyere stock.

According to Fig. 1 the block diagram of the method of the invention 15 shows individual process steps.

As a first process step the circuit board is coated in the usual manner and exposed to light in the second step, whereupon in the third step the SMD solder pads are freed which is also a known process step in the production of circuit boards. in the fourth process step the circuit board thus prepared is dipped into a solder bath which may be done through the solder bath level, but it is also possible to dip the circuit boards through a leading-in lock which is arranged in a side wall of the solder container below the solder bath level and has corresponding lock seals and solder pickup elements when liguid solder has flown out through the lock.

The cavities above the SMD lotpads are filled with liguid solder in the solder bath. If this will be necessary, means for producing turbulences may be used in order to secure that cavities above solder pads with a small surface are also completly filled.

The next step consists in sealing the solder volumes of the cavities against the liguid solder of the solder bath which is carried out by a sealing element of a suitable form. According to process step seven the circuit board is then withdrawn from the solder bath, but it should be provided that the cavities just being withdrawn from the solder bath are sealed. It is not necessary to seal the whole circuit board, in this method it is sufficient to secure sealing of the cavities just being withdrawn from the solder bath in order that a solder volume definition is performed within the cavities. As soon as the separation between the defined solder volume and solder bath was carried out, the seal may be opened; because of the surface tension the defined solder volume remains on the bottom of the cavity, that is, sticks to the SMD solder pads and according to process step eight, because of the surface tension and the spatial position of the board, forms a hunch when the seal has been removed, which is levelled or flattened by special formin elements according to process step twelve, so that the solder volume adapts to the form of the cavity. After cooling down the forming elements may be removed, further, the coating may be removed according to process step 14; the circuit board is ready soldered.

As process step nine it is optionally possible to firstly permit the solder volumes to harden in the form of a hunch between the withdrawing procedure and the removing of the sealing. If this is the case, prior to levelling the solder volumes according to process step eleven, the circuit boards must be heated again according to process step twelve.

Optionally it is further possible to clean the circuit board according to process step ten between process steps nine and eleven.

The device for carrying out the method according to Fig. 2 substantially consists of a solder container 1 filled with liquid solder 2. Further, an only briefly indicated transport device 3 with a driving motor 4 is provided, by means of which the circuit board 5 may be dipped into the solder 2. Reference number 6 indicates a covering device (element) suitable to cover limited areas (SMD solder pads 22) of the circuit board 5 circumscribed by a limiting layer 23 provided to be soldered below the solder level 7. The covering device is made such that the areas of the circuit board 5 to be soldered are covered only, when they pass the solder bath level 7 ? a special forming device 8 is provided for forming the surfaces by means of which process steps twelve and thirteen or, if necessary, also step eleven according to Fig. 1 may be performed.

It es important that the covering device 6 and the forming device 8 are separate elements which may also be arranged at a distance from each other.

The covering device of Fig. 2 and 3 consists of sealing rolls 9 the axises thereof are arranged approximately at the level of the solder level 7. When the cavities pass the solder level 7, because of the elasticity of the sealing rolls 9 a complete separation between the defined solder volumes 25 and the solder 2 in the solder container 1 is obtained, so that the cavities can again be opened after leaving the pair of sealing rolls, whereby the solder volume in the cavities does not change.

As may be seen in Fig. 4, the sealing rolls 9 may also be arranged in a side wall 10 of a solder chamber 11; between the side wall sections and the sealing rolls 9 further sealing elements 12 may be arranged preventing that too much solder is flowing out in the area of the lock such formed. However, since an outflow of liquid solder 2 from the solder chamber 11 must be expected when the circuit boards 5 are passing through, a heated collector 13 is provided for collecting the outflowing solder 14/ maintain911691 ing it in a liquid condition and pumping it back into the solder chamber 11 via a solder pump arranged in a solder line 16. The embodiment of Fig. 4 shows the passage of the circuit board through the solder level 7, but it is also possible, to provide the side wall 10’ opposite of the side wall 10 with a lock through which die circuit board 5 is introduced into the solder 2 below the solder level 7. A leading-in lock is provided substantially like the covering device 6.

After the covering device 6 follows the forming device in the transporting direction 17 which may be produced _ according to the forming devices 8 of Fig. 2 and 3.

The device according to Fig. 3 differs from the device of Fig. 2 only in that the circuit board 5 after having been withdrawn from the solder container is positioned horizontally and is transported to undergo the forming process in the horizontal position.

The forming devices 8 are provided as a conveyor arrangement and it is possible that between the band rolls 18, 18’ different temperature zones are produced, since heating elements 19 are provided in the leading-in area of the forming device 8 which are not described in detail and which are followed by cooling areas 20 in the transport direction 17.

Solder turbulence jets 21 may further be provided within the solder 2 before the covering device 6, which are responsible for a turbulent movement in the solder area before the covering device 6, for which reason they are connected with solder pumps 15 which are not shown.

The diagramstic sectional views of a circuit board 5 shown /E 911691 in Fig. 5-10 are described with reference to the block diagram of Fig. 1.

The circuit board 5 of Fig. 5 shows freed SMD solder pads 22 which are encompassed by a coating 23, the hight 24 of which is substantially corresponding to the hight of the solder tc be deposited. For more detailed information it is expressly referred to the European patent application EP-OS 0 336 232. The correspondingly formed circuit board 5 is the result of the process steps one to three according tc Fig. 1.

According to Fig. 6 the circuit board 5 such prepared is now dipped into the solder 2 and, if necessary, by help of the solder turbulence jets 21 the solder 2 will completly fill the cavities above the SMD solder pads 22. The representation according to Fig. 6 is in principle corresponding to the process steps four and five of Fig. 1.

According to Fig. 7 the dipped circuit board 5 is covered by the covering device 6, the solder volumes 25 are defined in the cavities which are separated from the solder 2 remaining in the solder container 1 by the covering device 6. In this state the circuit board 5 is taken out from the solder bath. The diagramatic representation according to Fig. 7 is in principle corresponding to the process steps six and seven of Fig. 1.

After removing the covering device 6, the solder volumes 25 will obtain the form shown in Fig. 8 which is in accordance with the diagramatic representation of Fig. 8 and thus substantially to the process steps eight and nine of Fig. 1. It is indicated that the hardening of the solder volumes 25 need not necessarily be performed before the levelling of the solder volumes 25 according to process step twelve of Fig. 1, it is also possible to level the still liquid hunch-shaped solder volumes 25 with the forming device 8, wich may also consist of a covering element of a suitable form.

In Fig. 9 the levelling process according to process step twelve of Fig. 1 and the cooling process according to process step thirteen of Fig. 1 are completed.

Fi. 10 shows the final state of the circuit board 5 as obtained after performing the process step fourteen (when the coating 23 according to process step fifteen of Fig. 1 is removed), Fig. 11 shows a diagramatic sectional view of a modified device with a closed solder chamber. This device of Fig. 11 substantially corresponds to the device of Fig. 4, the circuit board is however moved into a closed solder chamber by a pair of sealing rolls in which either the whole surface or sections thereof are wetted on both sides by liquid solder and again moved out on the other side by the pair of rolls 9 ’. Thus dipping into the liquid solder is performed below the solder level.

Fig. 12 shows a device similar to Fig. 11 in which the solder chamber is however formed as a tuyere stock positioned between the pair of leading-ing rolls and the sealing rolls. Liquid solder is applied on both sides of the circuit board which is passed under pressure in the tuyfere stock, the solder is squeezed off by the pair of sealing rolls 99 * in order that only the given cavities remain over the solder pads.

The form of the tuyere stock is selected such that solder flows prevailingly into the gore of the pairs of rolls, so /E 911691 that especially good results are obtained.

Fig. 11 and 12 have in common that superflous solder from the pairs of rolls falls down into a collector 13 which is heated. Liquid solder is pressed into the solder chamhgr or the tuydre stock via line 16 and pump 15 where it is held under presure and is available for the soldering process.

Claims

1.CLAIMS 1. Method for soldering circuit boards comprising the following steps: a) procuding a circuit board with electricallyconducting areas (solder pads) to which solder has to be applied; b) providing a coating of a defined layer thickness circumscribing these areas preventing a soldering; c) dipping or introducing the prepared circuit board in sections in a solder bath or a solder chamber with a suitable solder alloy; d) covering the cavities above the areas to be soldered filled with liquid solder in the dipping bath or in the solder chamber such that the covering when being withdrawn or taken, out performs a separation of the enclosed solder volumes from the liquid solder in the solder bath or in the solder chamber, whereby corresponding solder volumes are defined in the cavities ? e) withdrawing or taking out the circuit board which is at least partially covered, removing the sealing from the areas only, when the defined solder volumes can no more come in contact with the liquid solder remaining in the dipping bath;. f) hardening the solder volumes, if necessary, in a form dependent on the surface tension, the spatial position of the circuit board and the size of the solder volume; g) repeated heating of the hardened solder volumes, if necessary; h) levelling the solder volumes by applying a forming element; i) cooling down and hardening the solder by means of the forming element; j) removing the limiting layer.

2. Method according to claim 1, characterized in that the circuit board ie subjected to a cleaning procedure after the first hardening of the solder volumes according to process step f).

3. Method according to claim 1 or 2, characterized in that the forming element is formed as a special sealing element.

4. Method according to claims 1 to 3, characterized in that the covering element is touchingly applied to the circuit board surface or the coating surface.

5. Method according to one of the preceding claims, characterized in that the covering element is rollingly applied to the circuit board surface or the coating surface.

6. Method according to one of the preceding claims, characterized in that the solder volumes after the end of the covering procedure are heated above the melting point when in an uncovered state whereby hardened solder remainIE 911691 ders projecting over the cavity area are contracted back into the cavity area.

7. Method according to claim 6 characterized in that the temporary heating is carried out by radiation heating.

8. Method according to one of the preceding claims characterized in that withdrawing of the circuit board from the solder bath is performed by use of covering elements arranged in the area of the solder bath level.

9. Method according to one of the preceding claims characterized in that withdrawing of the circuit board from the solder bath is performed through the side wall below the solder bath level by use of covering/sealing elements.

10. Method according to one of the preceding claims characterized in that withdrawing of the circuit board from the solder bath is performed such that the circuit board surface provided with cavities is beneath.

11. Method according to one of the preceding claims characterized in that the forming element is formed as an elastical forming roll, if necessary.

12. Method according to one of the preceding claims characterized in that the forming element is formed as an oscillating 15. forming roll segment. Method according to one of the preceding claims characterized in that the forming element is formed as a forming band (conveyor). Method according to one of the preceding claims characterized in that the forming element is at least partially cooled. Method according to one of the preceding claims characterized in that when the forming element is produced as a transport ing element, first at least a heating zone and then at least a cooling zone is provided. Device for dipsoldering circuit boards with a limiting layer laterally defining the solder deposition comprising - a solder container (1) or a solder chamber (11) with liquid solder (2); a transporting device (3) by use of which the circuit board (5) can be dipped or introduced in the solder container (1) and - a covering device (6) by use of which areas (SMD solder pads 22) circumscribed by the limiting layer (23) to be soldered below the solder level (7) can be sealed characterized in that the covering devide (6) is produced such that the areas (SMD solder pads 22) of the circuit board (5) to be soldered are covered only during the withdraw ing procedure or the taking-out procedure and that special forming device (8) is provided for forming the solder surfaces. 16. 17. Device according to claim 16, characterized in that the forming device (8) is arranged spatially separated from the covering device (6). 17. 18. Device according to one of the preceding claims 16 or 17, characterized in that the covering device (6) has at least one sealing roll {9}. 18. 19. Device according to one of the preceding claims, characterized ia that at least one sealing roll (9) is arranged in the area of the solder level (7). 19. 20. Device according to one of the preceding claims, characterized in that at least one sealing roll (9) is arranged together with further sealing elements (12) in a side wall (10) of the solder container (1) or the solder chamber (11) below the solder level (7). 20. 21. Device according to one of the preceding claims, characterized in that the forming elements (8) are arranged directly after the sealing device in the transport direction (17). 21. 22. Device according to one of the preceding claims, characterized in that the solder container (1) or the solder chamber below the solder level (7) is provided with first sealing elements suitable to guide the board on a first side and with further sealing elements suitable to guide the board on an opposite side the latter comprising or forming the covering device (6). 22. 23. Device according to one of the preceding claims, characterized in that a solder collector (13) is provided below the sealing elements (12). 23. 24. Device according to one of the preceding claims, characterized in that the forming elements are provided as conveyor devices which are substantially horizontally arranged. 24. 25. Device according to one of the preceding claims, characterized by a production line having the following production stations: a) board leading-in device where the boards are put on a conveyor and feeded to the conveyor line, b) at least one solder container into which the prepared circuit boards are introduced for soldering and put out being sealed by means of sealing elements and c) a forming device subsequently arranged in the transport direction at the end of which a board output station is arranged in the transport direction. 25. 26. Device according to claim 25 characterized in that a cleaning device is arranged between the covering device (6) and the forming device (8) for cleaning 26. 27. 27. 27. 28. 28. 29. 29. 30. 30. 31. 31. 32. 28. 29. 30. 31. 32. the board having unformed solder depositions. Device according to one of the preceding claims, characterized in that a heating device (radiation heating, hot air line or the like) is arranged between the covering device (6) of the solder container (1) and the forming elements. Device according to one of the preceding claims, characterized in that solder turbulence jets (2Ϊ) are arranged in the solder container (1) in the transport direction (17) before the covering device (6) . circuit board characterized in that it is produced according to one of the claims 1 to 15. Method for soldering circuit boards substantially as described herein with reference to the accompanying drawings. A circuit board whenever produced by a method as. claimed in claim 30. Device for dip soldering circuit boards substantially as described herein with reference to the accompanying drawings.