Device and method for wave soldering of workpieces with solder recirculation

The present invention relates to a method and to a device for wave soldering. Prior art

Wave soldering is, together with reflow soldering, one of the most frequently used connection methods in the production of electronic components. In wave soldering, a workpiece, for example a circuit board and components to be soldered on the board, is passed across a pumped wave or waterfall of liquid solder by a conveyor. The solder wave is generated via a solder nozzle comprising one or more solder outlet openings from which liquid solder emerges. The solder flows along the top of the solder nozzle and wets the bottom of the workpiece and the pin or lead of the component with solder. At specific locations, wettable by solder and corresponding to solder joints, the solder provides the desired connections.

A critical parameter in wave soldering is oxygen present in the system. Oxygen causes oxidation of the solder and of the corresponding surfaces of the workpiece whereby the quality of the resulting solder joints and/or the reliability of the soldering process may be impaired. By exposure to oxygen, dross is formed which is to be disposed of or regenerated, thus generating additional costs. A major focus in the development of wave soldering processes is therefore the reduction of harmful oxygen effects.

From DE 195 41 445 A1 and from DE 10 2004 048 474 A1 , devices for wave soldering of workpieces are known in which specific areas can be supplied with a protective gas atmosphere, resulting in an almost complete exclusion of oxygen.

However, devices and procedures are desirable which are operable without or with reduced protective gas flow and nevertheless provide satisfying results, or which, leaving the amount of protective gas unchanged, provide improved results. Disclosure of the invention

In view of the above, according to the invention a device for wave soldering of workpieces and a corresponding method with the features of the respective independent claims is provided. Preferred embodiments of the invention are subject of the corresponding subclaims and of the description below.

An inventive device for wave soldering of workpieces comprises a solder reservoir for liquid solder and at least one solder nozzle. The at least one solder nozzle comprises at least one solder outlet opening arranged above a solder level of the solder in the solder reservoir. At least one solder collection and recirculation device is provided to collect the solder emerging from the at least one solder outlet opening and not adhering to the workpieces, and for stepwise recirculating or guiding the collected solder back into the solder reservoir. The solder collection and recirculation device is thus adapted to collect the solder above the solder level and to guide the collected solder back into the solder reservoir while reducing harmful oxgen influences.

By means of the device according to the invention, the effective drop height of the solder from the solder outlet opening(s) to the solder level is reduced significantly. The higher a solder wave, the more air, and therefore oxygen, can be introduced by the solder wave into the solder reservoir. As explained below with reference to Figure 1 , conventional solder nozzles in wave soldering devices comprise nozzle outlet openings located at a considerable height above the solder level. The solder exiting the nozzle, as mentioned, is contacting a workpiece to be soldered, for example a circuit board. The surface or upper side of the solder nozzle must therefore be positioned close to the workpiece, and the distance to the solder level can not be arbitrarily reduced due to mandatory constructive requirements and, in some cases, heat sensitivity of the workpieces. The cascading effect of the solder wave flow in conventional devices effects an intensive mixing of the solder with air or oxygen, particularly if large drop heights are present. Thus dross formation and oxidation is increased. Via the inventively suggested solder collection and recirculation device, the effective drop height, in contrast, is significantly reduced.

For this purpose, a solder collection and recirculation device advantageously comprises an open solder collection tray adapted to catch the solder and, fluidly connected to the open solder collection tray, a solder conduct ending at or below the solder level. The solder does thus not fall over the entire drop height into the solder reservoir but is caught above the solder level in the open solder collection tray of the solder collection and recirculation device. This results in a stepwise solder recirculation. Turbulences in the solder are reduced because the inventive solder collection and recirculation device ensures a very gentle, low-turbulence solder guidance. The solder can be isolated in much of its flow path of ambient oxygen in the proposed solder collection and recirculatioh device which can comprise a closed solder duct. The solder collection and recirculation device thus provides a beneficial effect as, on the one hand, it reduces turbulences and, on the other hand, it reduces the contact areas between solder flow and the ambient atmosphere.

An advantageous device comprises a solder collection and recirculation device provided for collecting solder emerging from several solder nozzles. Thus, wave soldering devices may be constructed significantly easier. For example, a solder nozzle may be surrounded by an annular solder collection tray.

The advantages of the invention can especially be realized if, additionally to the solder collection and recirculation device, at least one protective gas device is provided. By an appropriate protective gas device, for example via corresponding shielding gas nozzles, protective gas can be supplied specifically to areas of the solder which is collected in an appropriate solder collection and recirculation device. Especially in such critical areas, oxidation can be reduced significantly. As not the overall device, however, is to be supplied with protective gas, protective gas consumption is significantly reduced as compared to conventional devices.

The solder collection and recirculation device can be benificial in particular in wave soldering devices which comprise at least one solder pump. A solder pump can be tuned in its capacity to the solder stream provided by the solder collection and recirculation device.

Devices of the invention advantageously comprise at least one heating device which is provided for continuous liquefaction of the solder. A heater can be arranged in close proximity to the solder collection and recirculation device to ensure a targeted reheating of the solder flowing back into a defined area. An overheating of the overall solder batch can thus be reduced.

As to the method according to the invention, reference is explicitly made to the above explanations regarding the device features and the respective embodiments and advantages.

Brief description of the drawings The invention and further details and advantageous embodiments of the invention are hereinafter explained with reference to the drawings in which schematically illustrated embodiments are shown.

Figure 1 shows an apparatus for wave soldering according to the prior art.

Figure 2 shows an apparatus for wave soldering according to a particularly preferred embodiment of the invention.

In the figures, corresponding elements are indicated by identical reference numerals. A repeated explanation thereof is omitted for clarity.

For wave soldering according to the prior art, devices as shown in Figure 1 are used. Figure 1 shows an apparatus 10 for wave soldering of workpieces 2. The device comprises a solder reservoir 3 in which liquid solder 4 (shaded) is contained. The solder 4, in the solder reservoir, is contained up to a certain height, variable to some extent, and a solder level 4' is formed. A solder nozzle 5 is provided which has at least one solder outlet opening 5'. The solder outlet opening 5' is located above the solder level 4'. The workpiece 2 is moved via a conveyor, not shown, in processing direction 2' obliquely upward, illustrated by a dashed arrow, across the solder nozzle 5 or the solder outlet opening 5' and gets in contact with solder 4 emerging from solder nozzle 5. Workpiece 2 is wetted in specific areas (solder joints) by the solder 4. Solder not adhering to the workpiece 2 flows back into the solder reservoir 3 via solder streams 4". The flow of the solder 4 is illustrated by solid arrows. The solder 4 in the solder reservoir 3 is kept liquid by means of a heating device 9 and is pumped by means of a soldering pump 8 through solder nozzle 5. Figure 2 shows a device according to a particularly preferred embodiment of the invention. The device is generally designated with 1 and comprises the elements of device 100 previously explained in relation to figure 1 .

In the apparatus 1 , a solder collection and recirculation device 6 for the solder 4 is provided, which comprises, in the exemplary illustration of figure 2, a solder collection tray 6' and a solder duct 6". The solder collection tray 6' is arranged above the solder level 4' and thereby ensures a greatly reduced drop height of the solder 4. In addition, a solder guiding means 7 can be provided, for example in the form of accordingly bent metal sheets, and ensures a particularly gentle transfer of the solder 4 into the tray 6' of the collection and recirculation device 6. The collection and recirculation device 6 may surround the solder nozzle 5 in an annular manner. Additionally, but not shown in Figure 2, at least one protective gas device may be provided by which specific areas of the tray 6' or solder 4 collected therein can be supplied with an inert protective gas.

List of Reference Numerals

1 , 10 device for wave soldering

2 workpiece

2' processing direction

3 solder reservoir

4 solder

4' solder level

5 solder nozzle

5' solder nozzle outlet opening

6 collection and recirculation device

6' collection tray

6" solder duct

7 solder guiding means

8 solder pump

9 heater