WO2004070887A1

WO2004070887A1 - Smt-enabled component and circuit board

Info

Publication number: WO2004070887A1
Application number: PCT/EP2003/014829
Authority: WO
Inventors: Robert Standar; Johannes Bast
Original assignee: Siemens Aktiengesellschaft
Priority date: 2003-02-06
Filing date: 2003-12-23
Publication date: 2004-08-19
Also published as: DE10304906A1; AU2003294949A1

Abstract

The invention relates to a component (1) which can be connected to a circuit board (3). The component (1) comprises a solder foot (5, 7, 9) which can be connected to an SMT pad (6, 8, 10) of the circuit board (3) by means of a soldered connection. According to the invention, the shape of the solder foot (5, 7, 9) is such that it is adapted to the shape of the SMT pad (6, 8, 10), whereby a mutual, reciprocally oriented force of attraction is produced between the solder foot (5, 7, 9) and the SMT pad (6, 8, 10) by the adhesion forces of liquefied solder disposed therebetween. The solder foot (5, 7, 9) and SMT pad (6, 8, 10) can, for example, have congruent contours. The invention also relates to a circuit board (3) comprising an SMT pad (6, 8, 10) which is adapted in the above-manner to the solder foot (5, 7, 9) of a component (1). The invention enables automatic, precise positioning of the component (1) on the circuit board (3) during soldering without the need for any additional positioning aids.

Description

description

SMT-compatible component and circuit board

The invention relates to an SMT-compatible component and a printed circuit board which are matched to one another.

Hybrid circuits connect different types of components on a common circuit board. The components can be active or passive electrical or non-electrical components. Depending on the circuit design, they are connected to the circuit board in an electrically conductive manner and can also be connected to one another via the metallization of the circuit board. Electrically conductive connections between the component and the printed circuit board can be made using soldering processes. The solder connections are created on solderable contact surfaces, so-called pads. In addition, solder connections can also be made via holes in the circuit board into which a pin or leg of the component to be connected is inserted and soldered. However, these connections require special technologies for drilling the holes. In addition, they have the disadvantage that the basically liquid- and gas-tight circuit board loses its sealing properties due to the bore and can no longer be used as a sealing component.

In the course of the increasing integration of components and in the course of the progressive rationalization of manufacturing processes, connection technologies via holes in the printed circuit board are avoided as far as possible. Instead, hybrid circuits are largely manufactured using purely surface-based connection technologies, so-called surface mount technology (SMT). The aim of rationalization through SMT processes is to directly as many components as possible by means of solder previously applied, for example using a film printing process, directly and without the need for additional ones to connect electrical wire connections (wire bonds) to the printed circuit board. This one-step soldering process is called reflow soldering. The construction of an SMT circuit by means of a reflow soldering process enables the number of process steps to be reduced to the printing on the SMT pads with solder, the fitting with components and the production of the connections in a single reflow soldering step.

The positioning of SMT components, whose position and orientation must be matched to other components outside the circuit board, is to be viewed as critical. For the placement with such position-critical components, positioning steps must be provided. For example, arrangements of several individual plug connectors, which are used to connect a single, common opposite plug connector, a so-called sum plug, can be critical with regard to their positioning. In addition, mechanical connection elements such as screw nuts or screw threads, the position of which must be exactly matched to the position of the respective connection counterpart of another component or another printed circuit board, can also be critical, as can optical display elements, laser components, direction-dependent sensors or components whose position for further manufacturing steps of the circuit board is important.

The difficulties with the SMT assembly with position-critical components cause additional work steps and therefore additional effort.

It is an object of the invention to provide a component and a printed circuit board which are matched to one another in such a way that a soldered connection can be produced between them with a minimum of process steps, and which can be positioned exactly with respect to one another at the same time. The invention solves these problems by a component with the features of the first claim and by a circuit board with the features of the sixth claim.

A basic idea of the invention is to provide a component which can be connected to a printed circuit board, the component having a solder foot which can be connected to an SMT pad of the printed circuit board by a solder connection, the shape of the solder foot matching the shape of the SMT pad is adapted so that a mutual, mutually aligned force of attraction is generated between the soldering foot and the SMT pad by the adhesive forces of the liquefied intermediate solder. The respective contact surfaces of the component and the printed circuit board, between which a soldered connection is to be produced, must therefore be designed in such a way that their shape automatically aligns the component and the printed circuit board with one another during the soldering process. During the soldering process, solder that is located between the contact surfaces to be connected must be liquefied by melting it. The materials of the

The solder and the contact surfaces are designed in such a way that the liquefied solder wets the contact surfaces so that the adhesive forces outweigh the cohesive forces. Due to the pronounced adhesive forces, the opposite contact surfaces are drawn towards each other. The invention uses these forces to effect the automatic alignment of the contact surfaces to be connected to one another and thus to the printed circuit board and the component with respect to one another. This eliminates the need for additional alignment and positioning measures. Instead, the adhesive forces that occur anyway during the soldering process are used for positioning by the shape of the contact surfaces which are adapted to one another.

In an advantageous embodiment of the invention, the solder foot forms a contact surface, the shape of which essentially corresponds to the shape of the SMT pad. Component and printed circuit board thus have contact surfaces, the contours of which are in each other essentially the same. The contact surfaces are so designed that they can be placed on top of one another in a congruent manner. Due to the congruent, congruent contact surfaces, the adhesive forces that occur between them due to the molten solder become so effective that the contact surfaces are drawn into a congruent position with respect to one another. At the same time, the desired surfaces of the contact surfaces can be easily implemented in terms of design, and their implementation does not require any additional manufacturing effort.

In a further advantageous embodiment of the invention, the component and the printed circuit board have contact surfaces whose surfaces are essentially the same, but the contact surfaces of the component are smaller by a predetermined amount than that of the printed circuit board. This predetermined amount can be selected depending on the tolerance required for the positioning of the component. The positioning of the component is made easier by the size differences of the contact surfaces, since they can also be used as a positioning tolerance when the components are fitted. This is particularly advantageous if the contact surfaces of the printed circuit board are arranged in depressions which are formed, for example, by the surrounding lacquer. Then it must be avoided that the component is placed on the surrounding paint, stuck on it and therefore no longer automatically aligned by the adhesive forces.

In a further advantageous embodiment of the invention, the surface of the printed circuit board-side contact area is delimited by a coating surrounding the contact area. Coatings on the circuit board side that are sufficiently heat-resistant for the soldering process can be structured photolithographically without any effort. As a result, the shape of the contact area can be implemented with little effort by means of process steps which are used in any case in the production of printed circuit boards. In addition, by the possibility of selecting the area of the metallization on which the contact area is formed to be larger than the contact areas formed on it photolithographically. This enlarged metallization, like a foundation, increases the mechanical stability of the contact surfaces.

In a further advantageous embodiment of the invention, the soldering foot of the component is designed such that the component can stand thereon. As a result, the component can stand freely on the printed circuit board after being positioned, which can be done, for example, by an automatic placement machine, and no longer has to be held by the automatic machine or a positioning aid. This simplifies handling during the soldering process.

In a further advantageous embodiment of the invention, the solder foot of the component has contact surfaces that are at least partially concave, and the printed circuit board has contact surfaces that are convex, or vice versa. This results in an additional force to the adhesive forces of the liquid solder, which causes the component to be automatically aligned with the circuit board during the soldering process.

Further advantageous refinements are the subject of the dependent claims.

Exemplary embodiments of the invention are explained in more detail below with reference to figures. Show it:

FIG. 1 flat plug and printed circuit board with matched contact surfaces,

FIG. 2 variant of the flat plug with an enlarged circumference of the contact surfaces, Figure 3 flat connector with connector partner without circuit board.

1 shows a component 1 and a printed circuit board 3 in accordance with the invention. The component 1 shown in FIG. 1 is a flat plug. However, the term “component” should equally be understood to mean any other electrical or non-electrical component that is to be connected to the printed circuit board 3. For example, it also means screw connections which are to be screwed to the respective screw connection partners on another printed circuit board, frame or housing, or spacers which must come to rest on corresponding abutments of adjacent printed circuit boards or components. In addition, components such as laser diodes that have to generate a light beam of a certain orientation or sensors that have to be in a certain orientation with respect to the source of a signal to be detected can be meant.

The positioning of the flat connector 1 shown can be critical if the plug connection in the further manufacturing process is not to be closed manually but automatically or if the flat connector 1 is to be closed together with further flat connectors a plug connection with a common plug-in socket, a so-called sum plug. The knives 13, 15, 17 of the flat connector 1 must then be oriented within a predetermined tolerance in the direction of the corresponding receptacles of the sum connector. The positioning of all other components mentioned above can also be critical for their own reasons.

The flat connector 1 is shown in FIG. 1 detached from the printed circuit board 3 in order to be able to show the arrangement of the respective contact surfaces. The flat connector 1 has solder feet 5, 7, 9, via which a solder connection to contact surfaces 6, 8, 10 on the printed circuit board 3, so-called SMT pads. In this case, a solder foot 5, 7, 9 is arranged in pairs opposite an SMT pad 6, 8, 10, so that a solder connection can be produced between the respective pairs.

In addition, the solder foot 5, 7, 9 is constructed in three parts in such a way that the individual solder foot parts form a tripod. The solder foot can stand freely on the tripod formed by the solder foot 5, 7, 9. This property can be used, for example, in the soldering process in that the flat plug 1 no longer has to be held by an automatic placement machine or a positioning aid after positioning on the printed circuit board 3. This reduces the handling effort during the soldering process.

The shape of the SMT pads 6, 8, 10 which can be seen in FIG. 1 has a rectangular contour, which is the contour of the contact areas which are formed by the soldering feet 5, 7, 9 and are not visible in the viewing perspective chosen in FIG. 1 , correspond.

The contact areas of the soldering feet 5, 7, 9 are therefore also rectangular, but they are slightly smaller than the SMT pads 6, 8, 10. The size difference between the SMT pads 6, 8, 10 and the contact areas of the soldering feet 5, 7 , 9 takes into account manufacturing tolerances and positioning tolerances when assembling the printed circuit board 3 with the tab connector 1. In the case of the size difference, those tolerances are taken into account which the position of the flat connector 1 may have after the soldering process has been completed.

In addition to rectangular, other, easily realizable surfaces for the contact surfaces are also conceivable, for example round, oval or square contours. In addition, non-planar shapes, for example semicircular or semi-ellipsoidal shapes, can also be selected for the contact surfaces, the respective one then Forming opposing contact surfaces in pairs form negatives of one another, for example, concave contact surfaces correspond to the opposite convex contact surfaces.

The SMT pads 6, 8, 10 are formed by a common metallization, which is partially covered by paint 11. The paint is applied to the underlying metallization and then, e.g. structured by means of photolithographic processes for forming the SMT pads 6, 8, 10. It covers the underlying metallization and slightly exceeds it. When assembling the printed circuit board 3 with the flat plug 1, the flat plug 1 must therefore be positioned such that the contact surfaces of the soldering feet 5, 7, 9 essentially lie on the SMT pads 6, 8, 10 and not on the lacquer 11. To comply with this condition, the size of the SMT pads 6, 8, 10 must slightly exceed that of the opposite contact surfaces of the tab connector 1. The size of the metallization 11 can at the same time be chosen to be larger than that of the SMT pads 6, 8, 10 and sufficiently large to ensure stable anchoring of the flat connector 1, since smaller metallizations, e.g. in the size of the SMT pads 6, 8, 10 themselves, could be detached from the printed circuit board more easily under mechanical stress.

To increase flexibility, the flat connector 1 is designed with three different knives 13, 15, 17, which enable the plug connection to flat receptacles of different widths. For example, flat receptacles with the standardized widths of 2.8 mm, 4.8 mm or 6.3 mm could be used. As a result, a common layout of the SMT pads 6, 8, 10 and the flat plug 1 can be used to produce different plug connections. In addition, the flat connector 1 is also suitable for use as a heat sink, since the knife arrangement 13, 15, 17 forms a large cooling surface for heat exchange. The versatility compensates for manufacturing disadvantages that result from the fact that the design of the SMT pads 6, 8, 10 and the contact surfaces of the tab 1 to each other is relatively complex.

The circuit board 3 can be any substrate for hybrid circuits, which is based in a known manner on a ceramic or polymeric material. Such circuit board materials can have sealing properties that can be used for sealing against gas or liquid. In this respect, the printed circuit board 3 can, in addition to its function as a circuit carrier or component carrier, have an additional sealing functionality which makes it suitable as a sealing housing part. The invention enables the mounting of tabs, connecting screws or spacers on the circuit board by SMT processes without drilling through the circuit board 3. In this way, the sealing properties of the circuit board 3 can be obtained, which would otherwise be lost through the holes.

However, it must be taken into account here that mechanically loaded components such as plugs, spacers or, in particular, connecting screws must have a sufficiently stable connection to the printed circuit board 3. While this is conventionally accomplished by screwing through holes in the printed circuit board 3, only the solder connections between the SMT pads 6, 8, 10 and the solder feet 5, 7, 9 are available for this purpose according to the invention. The area mutually connected by the soldered connection must therefore be dimensioned sufficiently large to ensure the mechanical stability of the connection under load, for example due to shear and torsional forces. The dimensions of the metallization on which the SMT pads 6, 8, 10 are formed by partial covering with lacquer 11 also contribute in particular to this. The metallization 11 represents a large-area anchoring in the printed circuit board 3, which increases the stability like a foundation. FIG. 2 shows a variant of the flat connector 1 described in the previous FIG. 1 from a different perspective and without a printed circuit board 3. The variant has constrictions 35 on the solder feet 5, 7, 9, which enlarge the circumference of the respective contact area. Although the usable contact area itself is reduced by the constrictions 35, the enlargement of its circumference nevertheless brings about better adhesion of the soldered connection between the flat connector 1 and the printed circuit board 3. The better adhesion results in greater mechanical stability of the soldered connection, for example against shear and torsional forces. Other shapes to increase the scope are also conceivable, e.g. B. recesses or bulges.

FIG. 3 shows the flat plug 1 in the variant previously described in FIG. 1 and without a printed circuit board 3. The changed viewing perspective reveals the contact surfaces 29, 31, 33 lying on the undersides of the solder feet 5, 7, 9. In particular, it can be seen that the contact surfaces 29, 31, 33 have a rectangular contour which corresponds to the contour of the SMT pads 6, 8, 10 arranged on the printed circuit board.

The knives 13, 15, 17 of different widths, which make the flat connector 1 usable for flat receptacles 25 of different widths, can also be seen in FIG. The flat receptacle 25 has a sleeve 27, the width of which corresponds to the knife 13. The plug connection can be closed by sliding the sleeve 27 over the knife 13 as far as the stop 19. The flat connector 1 has a further knife 15 with its own stop 21, which together with the knife 13 could be encompassed by a wider sleeve. An even wider sleeve can be pushed up to the stop 23 via the further knife 17 together with the knife 15 and the knife 13. The manufacturing method described below is used to produce a soldered connection between the flat connector 1 and the printed circuit board 3. The manufacturing method is based on the fact that flat connectors 1 and printed circuit boards 3 which are matched to one another are used according to the invention. More specifically, the respective contact surfaces 29, 31, 33 of the flat connector 1 have to be matched to the respectively opposite SMT pads 6, 8, 10 of the printed circuit board 3. For example, they can have congruent contours.

In a first step, at least one of the opposing contact surfaces of printed circuit board 3 and flat connector 1 is provided with solder. Solder is preferably printed on the SMT pads 6, 8, 10 of the printed circuit board 3, in which appropriately dimensioned openings in printing foils are printed with a precise fit. The SMT pads 6, 8, 10 are printed by means of slightly oversized openings in the printing film which extend slightly beyond the edges of the SMT pads 6, 8, 10. In special cases in which more solder has to be offered, the contact surfaces 29, 31, 33 of the flat connector 1 can also be printed with solder.

In a second step, the flat connector 1 is placed on the printed circuit board 3 in such a way that the SMT pads 6, 8, 10 and the contact surfaces 29, 31, 33 lie opposite one another in pairs. The printed circuit board 3 is preferably oriented horizontally and the flat plug 1 is placed on the printed circuit board 3 by an automatic assembly machine coming from above. As a result, the flat connector 1 is pressed onto the printed circuit board 3 by gravity during the subsequent soldering step. Due to the three-legged shape, the flat connector 1 is free-standing, so that it is not necessary to hold it even further after it has been placed on the printed circuit board 1 by the automatic assembly machine or a positioning aid. In a subsequent third soldering step, the solder, which is located between the opposing contact surfaces, is liquefied, in which it is melted. This can be done by heating the entire printed circuit board 3. As a result of the heating, the solder flows between the opposing contact surfaces and wets them.

As a result of the adhesive forces leading to wetting, adhesive forces occur between the opposing contact surfaces of the printed circuit board 3 and the component 1, which cause mutual attraction. This attractive force causes the congruent contact surfaces to be drawn congruently to one another and thereby to be aligned with one another.

The tab connector 1 and the printed circuit board 3 are thereby aligned with one another in an orientation predetermined by the shape of the contact surfaces. The accuracy of the positioning does not depend on the accuracy of the positioning when mounting. When assembling, it only has to be ensured that the flat connector 1 is placed on the printed circuit board 3 in such a way that the contact surfaces lying opposite one another in pairs come to lie one above the other and the flat connector 1 remains movable when the intermediate solder melts in order to be able to follow the adhesive forces.

Claims

claims

1. Component (1) which can be connected to a printed circuit board (3), the component (1) having a solder foot (5, 7, 9) which is connected to an SMT pad (6, 8, 10) Printed circuit board (3) can be connected by a solder connection, characterized in that the shape of the solder foot (5, 7, 9) is adapted to the shape of the SMT pad (6, 8, 10) in such a way that between the solder foot (5, 7, 9) and SMT pad (6, 8, 10) a mutual, mutually aligning attraction force is generated by the adhesive forces of liquefied intervening solder.

2. Component (1) according to claim 1, since you rchgek characterized in that the solder foot (5, 7, 9) forms a contact surface (29, 31, 33) whose shape is essentially the shape of the SMT pad (6, 8, 10) corresponds.

3. The component (1) according to claim 2, characterized in that the shape of the contact surface (29, 31, 33) of the soldering foot (5, 7, 9) has dimensions which are smaller than the dimensions of the shape by a predetermined amount SMT pads (6, 8, 10).

4. Component (1) according to one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that the solder foot (5, 7, 9) is designed such that the component (1) can stand thereon.

5. The component (1) according to any one of the preceding claims, that the soldering foot (5, 7, 9) has three contact surfaces (29, 31, 33) which are arranged essentially in a triangle.

6. The component (1) according to one of the preceding claims, characterized in that the solder foot (5, 7, 9) has contact surfaces (29, 31, 33) which are at least partially concave or convex in shape.

7. Printed circuit board (3) for connecting to a component (1), the printed circuit board (3) having at least one SMT pad (6, 8, 10), which has a solder foot (5, 7, 9) of the component (1 ) can be connected by a solder connection, characterized in that the shape of the SMT pad (6, 8, 10) is adapted to the shape of the soldering foot (5, 7, 9) such that between the soldering foot (5, 7, 9 ) and SMT-Pad (6, 8, 10) a mutual, mutually aligning attraction force is generated by the adhesive forces of liquefied intermediate solder.

8. Printed circuit board (3) according to claim 7, so that the SMT pad (6, 8, 10) forms a contact surface (29, 31, 33), the shape of which corresponds to the shape of the soldering foot (5, 7, 9).

9. Printed circuit board (3) according to claim 8, characterized in that the shape of the SMT pad (6, 8, 10) has dimensions which are larger by a predetermined amount than the dimensions of the shape of the contact surface (29, 31, 33) of the solder foot (5, 7, 9).

10. Printed circuit board (3) according to one of claims 7 to 9, so that the shape of the SMT pad (6, 8, 10) is formed by a coating surrounding the SMT pad (6, 8, 10).

11. Printed circuit board (3) according to one of claims 7 to 10, d a d u r c h g e k e n n z e i c h n e t that the SMT pad (6, 8, 10) is at least partially concave or convex.