The invention relates to a method for populating and soldering a circuit board, to a reflow oven for soldering the circuit board and to a circuit board for said method. Especially, the invention relates to such circuit boards, which are populated with a wired electrical component having at least one connection wire or pin and a housing or casing thermally critical for conventional automatic soldering processes.
A basic consideration is that one strives currently to perform the populating, respectively soldering, of circuit boards as much as possible by machine, in order to optimize manufacturing costs and effort.
The currently best-known machine soldering methods for the soldering of electrical and electronic components on a circuit board are the so-called wave-soldering method and the so-called reflow soldering method. These two methods are described in detail in comparison with other conventional methods of soldering in the article of Dr.-Ing. Hans Bell entitled “Gibt es einen Paradigmenwechsel in der Löttechnik (Is There a Paradigm-Shift in Soldering Technology?”, in the journal “VTE—AUFBAU-UND VERBINDUNGSTECHNIK IN DER ELEKTRONIK”, No. 6/December 1999, Pages 297 to 301. The author describes therein the procedures and components to be used in the various solder processes for populating a circuit board and the details of performing the soldering.
In most currently available reflow ovens, a more or less diffuse, hot gas stream of pure hot air or a heated special gas is supplied perpendicularly onto the circuit board surface to be soldered. The circuit boards are heated upon entering into such a reflow oven and then transported into the actual working, i.e. soldering, area. Usual temperatures in the area of the circuit board surface to be soldered rise to 220° C. at a residence time of up to 30 s.
A great problem with soldering in a reflow oven is presented currently, however, by those components that cannot withstand the thermal conditions in usual reflow ovens and which become deformed or even destroyed under the conditions existing therein. Thus, for example, plug connectors, flex connectors, DIP-switches and other components, also semiconductor components, conventionally supplied with a plastic housing, are not suited for the usual reflow ovens.
Moreover, there are still other components which are used on circuit boards and which are not suited for soldering in reflow ovens, because they include non-heat-resistant parts, adhesives and/or coatings.
Components, which cannot withstand the temperatures existing in reflow ovens during the soldering process, cannot participate in the cost-favorable, machine populating and soldering in reflow overs, but, instead, require additional, labor-intensive and, consequently, cost-intensive, individual-, respectively special, populating in plural, special process steps.
Some of these components are available in special embodiments resistant to high temperatures, but they are markedly more expensive than the usual components. Thus, their use is frequently uneconomic, since they negate the cost savings which would otherwise be achieved by a purely machine populating and soldering procedure.
It is, therefore, an object of the present invention is provide a method for populating and soldering a circuit board, a reflow oven and a circuit board for such a method, permitting also those components, which are not resistant to the temperatures existing in reflow ovens during soldering, to be used in a machine soldering procedure, without requiring complicated and cost-intensive, individual populating and/or manual, individual soldering.
This object is achieved according to the invention by a first variant of a method for populating and soldering a circuit board having a first side and a second side and at least one wired, electrical component (THT-component) with at least one connection wire or connection pin and a housing or casing thermally critical for conventional, automatic soldering technology, which method includes the following method steps:
This object is also achieved according to the invention by a second variant of a method for populating and soldering a circuit board having a first side and a second side and at least one wired, electrical component (THT-component) with at least one connection wire or connection pin and a housing or casing thermally critical for conventional, automatic soldering technology, which method includes the following method steps:
In a preferred form of embodiment of the method of the invention, for populating the second side of the circuit board with at least one SMD-component, solder paste is applied to solder contact surfaces provided therefor, and, following populating of the second side of the circuit board with the SMD-component, such, together with the connection wire of the THT-component, are soldered in a process step in the reflow oven.
In another preferred embodiment of the method of the invention, also the first side of the circuit board is populated with at least one SMD-component.
A further preferred form of embodiment of the method of the invention includes the following process steps:
Further forms of embodiments of the method of the invention concern a dressing of connection wires of the THT-components before the printing of the solder paste onto the second side of the circuit board.
Still other forms of embodiments of the method of the invention concern a securement of THT-components on the circuit board.
Yet another preferred form of embodiment of the method of the invention includes the following method steps:
Yet another form of embodiment of the method of the invention concerns a populating of the circuit board with at least one pin-in-hole component (PIH-component).
In a further, preferred form of embodiment of the method of the invention, the first side of the circuit board populated with one or more THT-components is shielded, respectively thermally separated, essentially by the circuit board itself from the heat or energy feed onto the second side for the soldering.
Yet another, preferred form of embodiment of the method of the invention concerns a horizontal arrangement of the circuit board during the traversing of the reflow oven, with the one or more thermally critical THT-components to be soldered being located beneath the circuit board.
Still another preferred form of embodiment of the method of the invention concerns cooling the first side of the circuit board in the reflow oven during the soldering of the second side.
In another preferred form of embodiment of the method of the invention, in the reflow oven, those areas of the circuit board inclined to an above-average uptake of heat energy due to circuit board layout, are covered by a covering preventing or delaying the uptake of heat energy.
In a further, preferred form of embodiment of the method of the invention, in the reflow over, those areas of the circuit board where an above-average uptake of heat energy is desired, are covered by a covering which improves uptake of heat energy.
The above-mentioned object is, furthermore, achieved, according to the invention, by a first variant of a reflow oven for soldering a circuit board having a first and a second side and at least one wired electrical component (“THT-component”) having at least one connection wire or connection pin and a housing or casing thermally critical for conventional automatic soldering technology, wherein the first side of the circuit board populated with the THT-component is shielded during the soldering of the second side of the circuit board, in the area of a contact surface printed with a solder paste and containing a fed-out connection wire of the THT-component, from a heat or energy feed effecting the soldering.
The above-mentioned object is, moreover, achieved, according to the invention, by a second variant of a reflow oven for soldering a circuit board having a first side and a second side and at least one wired electrical component (“THT-component”) having at least one connection wire or connection pin and a housing or casing thermally critical for conventional automatic soldering technology, wherein the first side of the circuit board populated with the THT-component is separated, during the soldering of the second side of the circuit board in the area of a contact surface printed with a solder paste and containing an emerging connection wire of the THT-component, from a heat or energy feed effecting the soldering and wherein a temperature difference between the first and second sides of at least 28° C. can be set by suitable means.
In a preferred form of embodiment of a reflow oven of the invention, the side of the circuit board populated with the one or more THT-components is shielded, respectively thermally separated, essentially by the circuit board itself from the heat or energy feed effecting the soldering.
In another form of embodiment of the reflow oven of the invention, a cooling apparatus is provided therein, by means of which the side of the circuit board populated with the one or more THT-components is cooled during the soldering process.
Yet another form of embodiment of the reflow oven of the invention has at least one infrared radiation source, which delivers energy effecting the soldering.
The above-mentioned object is also achieved by a circuit board for one of the above-described methods of the invention, wherein the circuit board is so designed or embodied that it makes possible locally pre-determinable areas of above-average heat energy uptake in the case of heat energy acting externally on the circuit board.
A preferred form of embodiment of the circuit board of the invention concerns an inner layer of the circuit board, which is so designed, respectively embodied, that in the areas where above-average heat energy absorption is desired, there is always a large-area, metallic and/or electrically conducting part.
The invention involves the idea that thermally sensitive components be so located during the passage through the reflow oven that they are essentially shielded from the heat or energy feed onto the surface of the circuit board to be soldered.
In a preferred form of embodiment, the shielding is most simply achieved by way of the circuit board itself, with this effect being supported in further, preferred forms of embodiment of the invention by supplemental coverings and/or temperature-sinking measures. In another form of embodiment of the invention, the shielding effect of the inventive arrangement of the circuit board is advantageously supported also by a correspondingly selected design, respectively layout, of the circuit board.
The invention will now be described on the basis of examples of preferred forms of embodiments of the invention, with reference to an accompanying drawing, the figures of which show as follows:
a a schematic representation of a connection location of a connection wire of a component in a usual populating and soldering process;
b a schematic representation of a connection location of a connection wire of a component in a populating and soldering method of the invention;
For simplification, equal devices, components, etc. are provided with equal reference characters in the drawing.
For clarification of the components used in a conventional circuit board and the problems which arise because of their differing abilities to withstand heat,
For simplification, the components are not shown as such, but, instead, are indicated by the populating imprint, or top overlay, of the circuit board 1. Among these components not shown in greater detail are transformers 2, special plugs 4 with large housings, rotary switches 5 and resistors 6. Additionally provided on the circuit board 1 are angle plugs 7, and semiconductor components in TO-package housings 8 and in DIL-package housings 9. The illustrated components are either wired or have connection pins, with the connection wires or pins being stuck through metallized holes at the solder connections of the circuit board 1; they are, therefore, referenced as “THT-components” herein. THT is short for “Through Hole Technique”. Such THT-components are usually soldered in the wave solder bath, or, if they cannot withstand the temperatures existing there or if they deform, they are manually soldered. As already described above, a resort to manual soldering is very costly.
Some of the components illustrated in
Another example of a conventional circuit board is shown schematically in
In conventional manner, the circuit board 20 of
Among the so-called exotic components are included also those which, due to their non-uniform distribution of mass, require special securement to the circuit board, since they, for example, cannot be sufficiently secured against tipping by a simple adhesive procedure. These components must be held in position on the circuit board by means of snap-in technology or by insertion into a socket or the like, until the soldering is accomplished or even beyond that. Following subsequent fluxing 37, the circuit board 20, usually together with other circuit boards, is sent to wave soldering bath 38, where the components populated in step 36, together with the SMD-components 27 and 28, are soldered. If required, the circuit boards go through an additional cleaning, subsequent to the wave soldering 38.
Even in the case of this
To complete the just described matters,
As already explained above, the internal temperature in usual reflow ovens is a major problem, especially for components whose housings cannot withstand such temperatures over the course of the residence time in the oven. In this connection, attention must be paid to the fact that, in a usual reflow oven 40, such as is illustrated, for example, in
For a dressing 54 of the connection wires, respectively connection pins, of the THT-components, the circuit board is turned such that its first side is above and the so-called exotic components are below, thus the exotic components are located beneath the circuit board. If necessary, the connection wires, respectively connection pins, of the THT-components are shortened and/or so clinched, i.e. so spread or bent, that the THT-components in their upside down position do not fall out of the circuit board but, instead, are held in their positions. The shortening of the connection wires, respectively connection pins, of the THT-components additionally means that they then extend only slightly out of the circuit board, so that they cannot interfere with a subsequent application 55 of the solder paste, preferably by means of printing. With long, protruding connection wires, respectively connection pins, there is the danger that they protrude into the plane of the printing screen required for the application of the solder paste or that they interfere with the positioning of the printing screen.
Naturally, it is also possible to secure particularly heavy THT-components or such with an unfavorable mass distribution by adhesive on the first side of the circuit board.
Following the dressing of the connection wires, or connection pins, as the case may be, of the THT-components, an automated populating 56 of SMD-components and then of PIH-components on the second side of the circuit board is performed. Preferably, such PIH-components are used, which can be held by a sort of “wet adhesive attraction” of the solder paste and which do not require any additional measures for securing them in their proper orientation and at the desired location. Then, the circuit board, now populated on the second side, is sent into a reflow oven of the invention, for example one such as is illustrated in
A reflow oven 60 shown in
The invention allows, however, also THT-components with thermally critical housings and other items, such as THT-components which are themselves thermally sensitive, to be transported through the reflow oven 60 and soldered there. An essential idea in this is that the second sides of the circuit boards 66, thus there where soldering is to occur, are exposed to the action of the flow of heat energy required for the soldering, while their first sides, with the THT or other “exotic” and thermally critical components located thereon, face toward the conveyor belt 65. The circuit boards 66 themselves screen the thermally critical components against the heat energy. In order to achieve this, the circuit boards 66 are preferably, as shown in the case of the reflow oven 60 in
Depending on the space available in the individual chambers 62 of the reflow oven 60 and on the arrangement of the heat exchangers and blowers, the circuit boards can also be transported through the reflow oven arranged in some other way, provided that it is assured that the heat energy required for the soldering impinges in desired manner on the side of the circuit boards that is to be soldered and the circuit boards themselves cover the thermally critical components and shield them from the flow of heat energy. Thus, the heat sources, respectively feeds, can be arranged laterally in the reflow oven and caused to act from the side onto the side of the circuit boards to be soldered, with the circuit boards 66 being, in this scenario, inclined or even vertically arranged during transport through the reflow oven.
In comparison to the reflow oven 40 illustrated in
It has been found that alone by arranging the thermally critical components on the first side of the circuit board 66, to thermally separate them by the circuit board 66 itself from the heat energy required for the soldering, a temperature difference between the first and second sides of the circuit board amounting from about 28° C. to about 35° C. can be achieved. In this connection, it is of advantage when the circuit board does not have too much copper, thus conductor paths, or traces, on the surface.
In the case of many components with a housing critical relative to the temperatures existing on the upper side of circuit boards during soldering, the above-mentioned temperature difference of 28 to 35° C. between the first and second sides of the circuit board is already sufficient for enabling the soldering of the thermally critical THT-components in the reflow oven, without damaging or destroying the housing, respectively the component itself, by the temperature. Should this temperature difference not be sufficient, it is, for example, possible to apply the blowers 64 and/or heat exchangers 63 located in the reflow oven 60 of
On their first side 71, the circuit boards 70 shown here by way of example are populated by two different SMD-components 73a and 73b, which, for example, as described above, are the first components soldered in the reflow oven. The THT-resistances 75 and a THT angle plug 76 subsequently populated on the first side 71 (see
It has been found that the described soldering method of the invention can also be used for soldering thermally critical PIH-components. This is depicted by the circuit board 70 in
a and 10b illustrate a special additional advantage achieved with the soldering- and populating-method of the invention in the case of soldering THT-components.
In the case of a THT-component suited in this case for soldering in a usual reflow oven and charged into the reflow oven in the position illustrated in
The great advantage of the solder and populating method of the invention, wherein the THT-components, and especially thermally critical THT-components, can be soldered in the reflow oven upside-down, is to be seen in the result illustrated in
[In order to achieve a temperature difference between the upper side of the circuit board 90, which is facing towards a feed of heat energy (indicated by arrows 96) required for the soldering, and the opposite, under side of the circuit board facing away from the feed of heat energy, that assures that the thermally critical components on the under side will not be damaged, various means 98, 99 of covering for the upper side of the circuit board can be used according to the invention, as illustrated in
There are basically two possibilities for setting the desired temperature difference required for protecting the thermally critical components 91 on the under side of the circuit board 90. On the one hand, the temperature arising on the upper side of the circuit board 90 from the feed 96 of heat energy can be set exactly to the minimum temperature required for the soldering of the selected solder paste. This permits, with appropriate layout of the circuit board, as above described, achievement of temperature differences between upper and under sides of the circuit board 90 of about 28° C. to 35° C. alone by the shielding effect of the circuit board itself. Since the soldering temperature was already set at the lower limit, this is already sufficient in some cases to prevent a damaging of the thermally critical components 91 on the under side of the circuit board 90.
If this is not sufficient, then there is the possibility of improving the thermal separation between the upper and lower sides of the circuit board 90.
In contrast, the covering 99 presented in
Number | Date | Country | Kind |
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102 11 647.4 | Mar 2002 | DE | national |
Filing Document | Filing Date | Country | Kind |
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PCT/EP03/02627 | 3/15/2003 | WO |