DE4421996A1 - Method and device for combined wave and reflow soldering of printed circuit boards and the like - Google Patents

Abstract

The invention concerns a method of soldering circuit boards and similar articles, the method calling for the circuit boards or other articles to be passed through a basically enclosed, tunnel-like installation in which they are heated, soldered and cooled at least sufficiently for the solder to solidify. The invention is characterized in that the soldering technique used is determined in advance for one or, preferably, several circuit boards, the principal techniques used being wave, reflow and, in particular, combined wave/reflow soldering or a special soldering technique. Depending on the soldering technique selected, the heating and conveyor elements in particular are also adjusted to heat and convey the circuit boards in a suitable way through the installation, and the associated soldering bath brought to the appropriate operating conditions. Following suitable adjustment of at least these parameters, each of the circuit boards can be processed.

Description

The invention relates to a method for soldering printed circuit boards and the like, in which the circuit boards through a largely closed, tunnel-like through running system in which heating, soldering and cooling of the printed circuit boards take place at least until solder solidification. The same applies to Invention a corresponding continuous soldering system.

The use of electronic components in the form of printed circuit boards with pluggable, glue-on or surface-mountable components in some other way elements that are finally to be soldered are constantly experienced Growth. To manufacture these components, it is known to use components either initially without placing any solder on the circuit boards or in advance or during the assembly process itself solder in the form of solder paste, solder preform or other solder deposits adjacent to the areas to be soldered.

This assembly process is followed by the soldering process, which in the case of those without solder deposits assembled circuit boards is usually carried out today in wave soldering systems, which are also often under protective gas or low pressure. In such wave solder systems, liquid solder is touched by one or more PCBs from below Transferring solder waves to the solder joint, the solder joint must be exposed or open  must otherwise be accessible for a solder wave (see, for example, DE-OS 37 44 917 or DE-OS 42 25 378).

On the other hand, in the case of component parts provided with solder preforms or other solder deposits, PCB combinations just a re-melting of the respective, already Lot portions located in place by appropriate heating of the Effect unit. This is done in a very common manner in accordance with training deten, d. H. in particular heatable continuous systems with suitable transport units directions and often several chambers (see for example DE-OS 40 16 366 or again DE-OS 42 25 378 or trade magazine "ELEKTRONIK PRODUKTION & PRÜFTECHNIK - April 1989 - pages 37 to 39).

Today there is an increasingly common situation that mixed assemblies are also used Printed circuit boards occur, d. that is, for example, part of the components to be soldered is already provided and placed with a sufficient amount of solder for soldering, while another part of the components without any solder stock only in intended Openings are inserted or glued on.

In such a case, a unit is usually soldered to next a reflow soldering process and then a wave soldering process. To usually come two corresponding, known per se, one behind the other switched soldering systems, namely a reflow soldering system and a wave soldering system for Commitment. This means that in a corresponding production at least two Soldering systems must be available to perform any soldering tasks that may arise to be able to perform, which is a not inconsiderable investment requirement for represents such fabrications. Other negative aspects in this together The space requirements, the maintenance effort and the machine also make a difference utilization.

Against this background, the applicant has set itself the task of soldering and to specify a soldering system which allows the execution of both, in one process Can perform basic soldering processes and which also the execution of a allows every single basic procedure.  

This task is carried out in conjunction with a suitably designed continuous system procedurally solved in that - in advance - the soldering principle to be used for if possible, a plurality of printed circuit boards is determined and fixed, the Wave, reflow or a combined wave and reflow soldering to the bottom approved options include and that according to this definition, the heating and transport elements of the system in be suitable for heating and forward movement of the printed circuit boards the and the associated solder bath of the system in the corresponding operating state is moved and then the processing of the circuit boards - or other, comparison bare components - done.

This means that at least three elements of a continuous soldering system, namely Hei tion, transport and solder bath, must be available in a suitable way to achieve the fiction enable moderate (soldering) processes. Specifically, this means that according to the invention a transport speed adapted to the soldering principle to be carried out in each case must be executable that a suitable heating by the heating elements the circuit boards must be producible, especially in the case of the reflow soldering principle a peak area with regard to the circuit board temperature must be able to be formed, and that the solder wave generation and possibly together with the solder bath heating must be switched off. The solder transfer from the solder wave (s) to a circuit board can - in addition to switching off the solder wave - in principle also by changing the Transport path of the circuit board can be prevented. In the end this comes a lot Shutdown equal.

On the basis described it will be possible to use different soldering principles unite in one process and in one plant. Depending on the basic circuit to be determined The different soldering processes result from one system. More on this units:

If the goal is to carry out a reflow soldering process, this is a comparative one slow transport (approx. 1 m / min) with a relatively high heating output of the respective Heating devices, which are usually available in the usual wave soldering systems goes beyond, in addition, the solder wave generation remains out of operation; in the case of wave soldering there are higher transport speeds and set lower heating powers and also the solder wave generation switch on. The mixed operation essentially works with one with the pure one  Reflow soldering comparable setting of heating and transportation, however in addition, the solder wave generation is in operation.

The various processes can be characterized, for example, on the basis of the temperature values of the printed circuit boards as they pass through a corresponding system. Such temperature diagrams are shown as Fig. 2. It can be seen there that - depending on the basic setting - there is a different temperature profile, with R + M being a reflow soldering or a wave and reflow mixed operation, and W being a pure wave soldering process. In principle, it should be noted that with the method according to the invention, even with mixed printed circuit boards, there is only a one-time thermal load on the components and not, as previously, a double, as occurs in the successive execution of the two soldering processes required.

The application of the invention is particularly advantageous in the case of soldering which is carried out under Shielding gas, run under low pressure or under plasma. The one here Setting plants are expensive capital goods, so that the invention This entails special cost advantages.

The most far-reaching advantages arise in particular with plasma soldering, i. H. in the case of soldering, this is in the low pressure and under the plasma of a supplied process gases takes place, since it is possible to work without flux and therefore otherwise subsequent cleaning processes can be omitted.

Further advantageous refinements of the procedure according to the invention are shown in further subclaims specified.

A continuous soldering system according to the invention, which is basically constructed like a tunnel and largely sealed off from the environment by means of a housing, is characterized in particular by
that heating elements ( 35 ) are arranged above the solder bath, which can be switched on and off and regulated independently,
and that at least on a section in the inlet tunnel to the soldering area below and / or above the transport device, heating elements which can be variably adjusted, switched on and off are also arranged,
and that finally the transport device is also variably adjustable at least in relation to the transport speed.

The basic features just listed, namely heating elements above the solder bath as well as below or above the transport device in the inlet area to the soldering station and also the variably adjustable circuit board transport mechanism and the solder wave that can be switched on and off are the essential characteristics of an invention appropriate continuous soldering system.

Further, advantageous configurations of such a system are shown in FIGS specified in the subclaims.

The invention is described below with the aid of the attached schematic drawings exemplified in more detail. Show it:

Figure 1 is a soldering system according to the invention, working under plasma.

Fig. 2 shows a heating curve for the printed circuit boards when passing through a soldering system according to the invention measured on the top of the printed circuit board - curve W: wave soldering - curve R + M: reflow soldering and mixed operation.

Fig. 1 shows a continuous soldering system according to the invention, which works under low pressure and with plasma formation. This system initially has an inlet lock 3 , a main or system chamber 4 with an inlet tunnel 26 , a soldering station 34 and an outlet tunnel 51, and an outlet lock 7 , which is only partially visible. A transport device 44 , for example a circulating chain transport with holding fingers, is arranged in the system chamber 4 , which is traversed in the longitudinal direction with an incline of approximately 3 to 10 °.

With regard to the longitudinal extent of the system chamber 4, approximately in its central part, a solder bath 48 is connected in an airtight manner from below, a device 42 for forming a solder wave 41 being present therein. Alternatively, a double solder wave would be possible and advantageous in certain applications (e.g. when soldering under normal pressure and under protective gas). In front of the solder bath 48 and practically over the entire length of the inlet tunnel 26 , a heating device 28 , for example an electrical resistance heating conductor, is provided within the system tunnel and below the circuit board transport 44 . Above the transport device for the Lei terplatten a transmitter antenna 21 for emitting elec tromagnetic waves for plasma excitation of the process gas is arranged in the same system area (z. B. a high-frequency transmitter with about 40 kHz and at least about 500 watts and preferably up to 3000 watts of power). Also in the inlet part 26 of the system chamber 4 , a feed line 27 for process gas coming from a gas bottle 29 is placed in the example shown. In operative connection with this, a suction line 52 is provided for "venting" the system chamber 4 in the outlet part 51 of the system. Due to this arrangement of gas supply and discharge, a gas flow from the chamber inlet to the chamber outlet is effected in the main chamber 4 of the system shown.

Above the solder bath 48 and the transport device 44 and over a length that extends somewhat beyond the width of the solder bath, a further heating device 35 having several modules is provided (e.g. several infrared radiator units). In addition, a temperature measuring device for the top of the printed circuit board is arranged just behind the solder wave 41 in the transport direction.

For the rest, the gas lock, upstream of the system chamber 4 , upstream 3 with its lock chamber 15 , its lock doors 11 and 12 , its means of transport 13 and its evacuation line 14 is to be listed in detail, as is the outlet lock 7 connected on the aisle side, which has exactly corresponding elements and which is therefore only partially shown.

The functional sequence of this system is now as follows:

1. Wave soldering

In the case of printed circuit boards with component assembly without the addition of solder, only a wave soldering process must be carried out. In the "wave soldering" mode, the heating device 35 located above the transport 44 is generally switched off completely, while the solder wave generation 42 is in operation and likewise the heating device 28 serving in this case a suitable preheating. The transport speed for the printed circuit boards is to be set to a specific value for wave soldering of 1.5 to 2.2 m / min, while for the rest, conditions such as those specified in DE-OS 42 25 378 are to be established for plasma formation.

In operation, the circuit boards designated 10 in the figure are then moved in groups into the lock 3 , the outer door 11 is closed and subsequently the evacuation of the lock chamber 15 is initiated via the discharge line 14 , with a pressure level of approximately 0.1 first in the lock chamber up to 1 mbar is the goal and finally a pressure equalization with the soldering system chamber 4 and the atmosphere there is established, which is preferably at a pressure level of 1 to 5 mbar.

After pressure equalization has taken place - for example by opening a connection between the lock chamber 15 and main chamber 4 - the inner lock door 12 of the entry lock 3 opens, whereupon the circuit boards located in the lock are successively transferred to the circulating chain conveyor 44 in the main chamber 4 . This transport device transports the circuit boards further through the inlet area 26 in the main chamber, the bottom-side heating device 28 causing the circuit boards to be heated to an advantageous level for the actual wave soldering process of approximately 90 to 130 ° C. In the area above the transport device 44 and also in the feed area of the printed circuit boards to the solder waves, there is the high-frequency transmission antenna 21 , which works in a suitable mode of operation with approximately 40 kHz and 1000 watts of power. With this transmission power, the process gas supplied via the feed line 27 is preferably made of a mixture containing oxygen and hydrogen

0.5 to 10 vol.% Oxygen,
80 to 20 vol.% Hydrogen and
20 to 80 vol.% Of a fluorinated

Hydrocarbon gas (CF₄, C₂F₆,...) Or SF₆ is, optionally additionally contains an inert gas, a plasma is formed, with attention being paid to a spread and action of the plasma in almost the entire inner main chamber 4 . The effects of the plasma consist in the fact that under its action an excellent flowable, good gap-permeable and finely wetting solder, in particular also from solder waves, is obtained and, on the other hand, the surfaces to be soldered that are still exposed before the solder application are favorable preparation (grease cleaning and Activate by removing the oxides) for the soldering process. Overall, a soldering process under plasma - for example already under an air, nitrogen or noble gas plasma, but in particular under a plasma gas composed as described above - results in an extremely favorable and, above all, potentially flux-free soldering process.

However, it should be noted that with regard to the one in question here Invention, namely different soldering processes in one process and in one system  to unite, also take place a low-pressure soldering process without exposure to plasma can, just like performing soldering under protective gas This approach is advantageously possible in principle.

In the present example, however, soldering under plasma is to be described in more detail. In the case shown, a plasma is formed in the feed tunnel 26 from the process gas introduced via the feed line 27 and flowing in the direction of the discharge 52 , where the circuit boards are already under the influence of plasma on the feed path to the solder wave 41 . Under this, the printed circuit boards remain even during solder transfer through the solder wave 41 , and even in the outlet zone 51 there is still a plasma effect in the system shown. In this way, the above-mentioned, advantageous effects for the solder and the joining partners result in pronounced form. In the outlet tunnel there is also a first cooling from the temperature level of wave soldering, so that the printed circuit boards arrive at the exit lock 7 at a significantly lower temperature (see also temperature curve W in FIG. 2 - the temperature at the top of a board depends on this) whose position is shown in the system).

Finally, the discharge process is to be carried out, which in the example shown consists in the fact that a group of printed circuit boards which has arrived on the independent transport tunnel 50 located in the tunnel 51 is transferred with the slide 71 open into the chamber 75 of the exit lock 7 and immediately then - after closing the gate valve 71 - the chamber 75 is flooded with air or nitrogen, for example. After producing the full constant pressure equalization with the environment, the circuit board group in question is quickly conveyed out of the lock 7 and a new venting of the lock chamber 75 is carried out quickly, so that after a short time the next removal process can be initiated.

The circuit boards soldered under plasma with waveforms as described do not require any Post-cleaning, which eliminates the use of environmentally harmful detergents. The Manufacturing of the printed circuit boards processed in this way is therefore largely after soldering completed, while with the other soldering methods mentioned above the usual, final washing processes would still have to be connected.

2. Mixed wave and reflow soldering

If printed circuit boards with mixed assembly are to be processed, that is to say printed circuit boards which have both components with and without solder addition, then "mixed operation" is to be provided according to the invention. This means that in the example shown, both the heating device 28 and the ceiling heating device 35 are to be put into operation with the corresponding power, that the solder wave generation 42 is to be switched on and that the transport device is also to be set to a forward movement which meets the requirements of reflow soldering . The requirements of reflow soldering - for example the flux activation from the inside of solder deposits that can be achieved when entering the soldering area - limit the feed rate. This is usually below 1.5 m / min, but it can be increased up to the speeds of wave soldering with flux-free solder deposits only.

In principle, the transport processes in mixed operation are to be carried out as in the above-described, pure wave soldering, but due to the lower transport speed and due to the operation 'of the heater 35, higher circuit board temperatures are achieved when passing through the inlet tunnel 26 than with pure wave soldering (see Fig. 2 - Curve R + M). With solder wave 41 , top temperatures of the circuit boards of approx. 190 ° C have already been reached in this operating mode, which, however, as has been established, does not have any negative effects on the wave soldering process taking place on the underside of the circuit boards (on the contrary, rather than when climbing through the board) Solder through the printed circuit board, the solder remains particularly thin due to the "top heat" present). After the solder wave 41 , the circuit boards to be processed run into the area of influence of the last module 35 d of the heating device 35 . The peak or peak temperature of the circuit board is generated in this area. In the case of the usual lead / tin solders, this must be in the range from 200 to 220 ° C. to ensure that the existing solder deposits are sufficiently melted and liquefied. For this purpose, the power setting of the heating module 35 d must be made accordingly. After leaving the peak zone, both the solder applied by the solder waves and the solder of the solder pots solidify after a short time and form particularly high-quality solder connections, in particular when the entire process is carried out under plasma. The plasma generation is - as already described above for wave soldering - to be carried out.

In a particularly advantageous variant of the invention, the power setting of the heating module 35 d is configured in a feedback manner. This can be done in such a way that a temperature sensor 36 is arranged shortly after the solder wave contact point and the actual temperature of the circuit board passing through is measured there. This actual temperature is compared in a corresponding, programmable control unit with a predetermined target value and starting from there Then the fine adjustment of the heating output of the heating module 35 d is determined and carried out, the heating output of the heating module 35 d being suitably increased or decreased, depending on whether the circuit board temperature was measured too low or too high.

In the example shown, provision is thus made for something slightly behind the solder wave Peak zone, the remelting of the solder deposits that were also included in the assembly effected either with constant or with feedback heating power. As well the arrangement of the peak area exactly above that would also be suitable Solder wave, for which to place the heating devices changed only slightly would have to be set. In any case, however, with one as described led mixed operation achieved high quality soldering results, again, above all Processes carried out under plasma are particularly advantageous. With the described Processes must also be ensured that there is no overheating of existing components is caused, therefore comes with sensitive components feedback method variant of particular importance.

3. Reflow soldering

Are printed circuit boards to be processed that consistently contain solder deposits or solder paste Components have a pure reflow soldering process is required. This is subject to with regard to the heating and the transport speed of the circuit boards in the essentially the same conditions as mixed operation, and therefore it is comparable settings to make in mixed operation. Operation of the However, solder wave is not necessary in this case. The soldering result corresponds again high quality requirements, especially in the case of a soldering process under plasma.

With transport speeds that are 1 to 2.5 m / min, heating outputs that even voluminous PCB units at over 200 ° C during the said transport can heat up speeds and solder that can be switched on and off independently Wave devices are the essential characteristics of an inventive Continuous soldering system called. Such a system increases the flexibility of one Printed circuit board production with relatively low investment costs, with every common solder quality and even the highest quality soldering can be achieved.

Claims (14)

1. A method for soldering printed circuit boards and the like, in which the printed circuit boards or other components are moved through a largely closed, tunnel-like continuous system in which heating, soldering and cooling of the printed circuit boards take place, at least until the solder is solidified ,
that the soldering principle to be applied for one or - preferably - several circuit boards is determined and defined, the wave, reflow and a combined wave and reflow soldering being one of the basic setting options, and the heating and transport devices according to the determination made System in terms of heating and forward movement of the circuit boards are set and the associated solder bath of the system is set to the appropriate operating state and then the processing takes place. 2. The method according to claim 1, characterized in that the soldering under a protective or process gas is carried out. 3. The method according to claim 1 or 2, characterized in that the soldering is carried out under low pressure. 4. The method according to one or more of claims 1 to 3, characterized characterized in that the soldering was carried out under low pressure and plasma formation becomes. 5. The method according to one or more of claims 1 to 4, characterized records that, if necessary, several different types of printed circuit boards in advance Sort similarity and when starting the processing of a new circuit board group the setting of the Continuous soldering system is changed accordingly.   6. The method according to one or more of claims 1 to 5, characterized records that the heating temperature of the circuit boards at one or more points is measured during the run and based on this measured value (s) the on position of one or possibly several heating elements is made. 7. The method according to one or more of claims 2 to 6, characterized characterized in that a containing oxygen and hydrogen at the same time Process gas with a clearly predominant hydrogen content is used. 8. The method according to one or more of claims 2 to 7, characterized in that a process gas mixture
0.5 to 10 vol.% Oxygen,
80 to 20 vol.% Hydrogen and
20 to 80 vol.% Of a fluorinated hydrocarbon gas or SF₆
is used. 9. The method according to claim 8, characterized in that the process gas mixture additionally contains an inert or inert gas. 10. Continuous soldering system with a substantially tunnel-like and largely enclosing housing with access and exit devices ( 3 , 7 ), with a separately switchable soldering station ( 34 ) arranged in the area of the main chamber ( 4 ) with solder bath ( 48 ) and Associated devices ( 42 ) for generating solder wave as well as tunnel areas ( 26 , 51 ) leading to and from this soldering station and with a transport device ( 44 ) crossing the system tunnel, characterized in that above the solder bath ( 48 ) via transport device ( 44 ) heating elements ( 35 ) are arranged, which can be independently switched on and off and regulated, and that heating elements ( 28 ) which can also be variably set and switched at least on a section in the inlet tunnel ( 26 ) to the soldering station below and / or above the transport device ( 44 ) are arranged, and that finally the transport device ( 44 ) with respect to the transport speed - and optionally also with regard to their arrangement in relation to the solder wave - is variably adjustable. 11. System according to claim 10, characterized in that the, the soldering station ( 34 ), the inlet tunnel ( 26 ) and the outlet tunnel ( 51 ) comprehensive main chamber ( 4 ) is sealed gas-tight against the environment and at least one inlet and an outlet ( 27 , 52 ) for a protective or process gas. 12. Plant according to claim 10 or 11, characterized in that the main chamber is connected to means for generating a low pressure in the range of 0.1 to 100 mbar, in particular has a suction line ( 52 ) and a suction pump connected thereto. 13. Plant according to claim 12, characterized in that in the main chamber ( 4 ) means ( 21 ) for generating a plasma from low pressure gas are present, for. B. a radio frequency transmission antenna. 14. Plant according to one or more of claims 10 to 13, characterized in that in the system chamber with respect to the transport path of the circuit boards shortly before and / or shortly after the or the solder waves ( 41 ) sensors ( 36 ) for measuring the temperature of the circuit board top and / or the underside are arranged, and these sensors are connected to a display and / or processing unit which indicates a corresponding change in the setting of the heating and / or transport elements or which independently detects the heating and / or or transport elements.