FR2723501A1 - Reflow soldering method for electronic surface mounted component cards - Google Patents

Abstract

The circuit board (20) has via holes (22) connecting an upper and lower electrical surface. To solder surface mounted components in position, a first solder pad layer (21) is formed in the component location. A second layer of lower temperature solder is deposited over the first layer, and heated to 150 degrees C with the component in place. As the heat is applied, solder runs down the via hole, forming a solid connection (26) and leaving a meniscus connection (24) around the outside of the component. A thin solder connection (5) is made to the solder pad.

Description

Electronic card for surface mounting components fixed by reflow, and corresponding manufacturing process.

 The invention relates to the manufacture of electronic canes comprising surface mount components fixed by reflow.

 As the name suggests, a surface mount component, or SMD component, attaches directly to the surface of the cane holder, as opposed to a component with pins that go right through the holder to be welded to the opposite side of this support.

 A SMD component is therefore equipped with metal contacts which are in general either metal regions formed directly on the surface of the body of the component, or folded tabs extending outside the body of the component. These metal contacts are fixed by welding to metal areas formed on the surface of the support.

 The reflow soldering technique used to secure the CMS components to the surface of the support is well known to those skilled in the art. It essentially consists of depositing, using a mask, a soldering means, such as solder paste (typically a mixture of tin and lead), on the predefined metal areas of the support which are intended to receive the metal contacts of SMD components, then depositing said metal contacts on said pads. After the installation of a component, part of the corresponding metal area is covered by the contact of the component, while the rest of the area is only covered by the solder paste.

 The support thus equipped with its components is then placed in a reflow oven and the temperature is increased according to a predetermined profile comprising in particular, over a very short period, typically less than 20 seconds, a temperature rise peak above the melting point of the welding means and then a rapid drop in temperature below this melting point.

 n results then, from a section, a weld between the lower surface of the metallic contact of the component and the corresponding part of the metallic surface of the support, and, on the other hand, the realization of a weld meniscus between the surface lateral side of the metallic contact of the component and the rest of the range. It is this meniscus which essentially ensures the solidity of the attachment of the component to the support.

 The support of such a card also includes metallized interconnection holes, commonly called "via" by those skilled in the art. These interconnection holes provide electrical interconnection between the various conductive layers of the support. Such holes may for example be through to connect two conductive layers deposited on the two opposite surfaces of the support, or else for example blind to connect a surface conductive layer and a conductive layer buried within the support.

 However, the need to provide these interconnection holes currently poses problems both in terms of circuit design and in terms of their quality.

 Indeed, these metallized holes are currently made, before depositing the solder paste, outside the metal areas of the support and are connected at least for some of them to these areas by conductive paths. However, while the technology for manufacturing SMD components makes it possible to obtain components having an area as small as 0.5 mm 2, the current diameter of a via hole surrounded by its conductive ring is of the order of 0 , 7 mm, which makes an interconnecting hole equivalent to a component from the surface point of view, and which therefore greatly increases the size of electronic cards.

 Furthermore, the internal metallization of these holes must be correctly carried out to avoid any electrical problems.

 The invention aims to provide a more satisfactory solution to these problems.

 An object of the invention is both to solve the problem of surface area of the cards and to ensure better electrical quality of the interconnection holes.

 The invention therefore first of all proposes an electronic card, comprising surface mount components fixed by reflow on predetermined metal areas of a support also equipped with metallized interconnection holes between the various conductive layers of the support.

 According to a general characteristic of the invention, at least some of the interconnection holes open inside at least some of the contact pads in predetermined areas of these pads entirely located under the corresponding components.

 The invention also relates to a method of manufacturing an electronic card comprising surface-mount components fixed by reflow on predetermined metal areas of a support further equipped with metallized interconnection holes between the various conductive layers of the support.

According to a general characteristic of the invention:
- at least some of the interconnection holes are made to open in predetermined areas of at least some of said contact pads,
- the contact pads are covered with a welding means,
the components are positioned on the pads so that they cover a first part of said pads fully incorporating said predetermined zones, leaving the rest of said pads not covered by the components,
- We proceed to the reflow of the welding means so as to simultaneously achieve a weld securing the components to the pads and to leave inside the interconnection holes a residue of solidified weld means.

 The invention also relates to a support for the manufacture of an electronic card with components for surface mounting fixed by reflow, comprising metal pads on which said components are intended to be positioned, as well as metallized interconnection holes. between the different conductive layers of the support. According to a general characteristic of the invention, at least some of the interconnection holes open inside at least some of the metal areas in predetermined areas of these areas intended to be entirely located under the corresponding components.

 According to one embodiment of the invention, when at least one of the metal pads comprising an interconnection hole is associated with a homologous metallic pad, the two pads being separated by a predetermined distance M and intended to receive two contacts of a component having a minimum length L overall of contact with predetermined contact, this component being positioned on said areas with a tolerance of laying T predetermined, the opening of the interconnection hole opening into the place considered is then located entirely in an area limited by the internal edge of said area located opposite the internal edge of the homologous area, and extending from said internal edge, over a distance Z equal to (LM) / 2 - T.

Other advantages and characteristics of the invention will appear on examining the detailed description of a nonlimiting embodiment illustrated in the accompanying drawings, in which
Figures 1 and 2 show two types of CMS components;
Figures 3 to 6 illustrate the essential steps of fixing a component on the support by reflow; and
FIG. 7 illustrates an example of positioning of an interconnection hole inside a metal pad.

 In FIG. 1, the component 1 for surface mounting, for example a resistor, comprises a body 3 surrounded at its two ends by a metallic coating forming the metallic contacts of this component.

More precisely, this metallic coating comprises a vertical end part 4 extended under the component by a lower part 5. Each lower part 5 is delimited by the two points P1 and P2 and extends over a distance D. M denotes the distance between the ends of the lower metal contacts and L denotes the minimum overall length of contact to contact of the component.

Of course, the actual overall length from contact to contact of the component may be slightly greater than the minimum length L, taking account of manufacturing tolerances.

 FIG. 2 shows another type of component 10 for surface mounting. I1 can be for example an integrated circuit box comprising n metal contacts. In Figure 2, is shown for simplicity only one pair of opposite contacts.

 Each metal contact 12 here consists of a folded tab extending from the body 13 of the component. This tab has a vertical part 14 extended square by a lower part 15 horizontal. By analogy with FIG. 1, the distance M represents the distance between contacts of the component, that is to say the distance separating the two vertical parts 14 from the two legs, while the distance L which represents the minimum overall distance of contact to contact, is counted here between the two end points P1. The distance D which is equal as in the case of FIG. 1 to (L-M) / 2, is in fact the length of the lower part 15 of each leg.

 As will be seen below, these are the lower parts 5 and 15 which are intended to come into contact with metal pads of the support of a cane.

 While it is clear in the case of a component 1, that the region situated between the points P1 and P2 is a region situated under the component, it will be accepted in the sense of the present invention that a point situated between the vertical parts 14 of the legs of a component 10 is also located under the component whether this point is actually located under the body 13 itself of the component or between an end of this body and the vertical part close to the corresponding leg.

 Such a card is illustrated diagrammatically in FIG. 3. In this figure, the reference 20 designates the insulating support of the card provided, in the present case, on its two faces, with metal pads 21 on which the surface components will be positioned. These metal areas are produced in a conventional manner, for example by screen printing.

 There are also provided interconnection holes 22, commonly known to those skilled in the art of "via", in this case connecting the two surface conductive layers of the support.

However, some of these holes could be blind, that is to say connect one of the surface conductive layers to a buried layer.

 According to the invention, as illustrated in FIG. 3, these interconnection holes 22 open inside certain metal areas in predetermined areas of these areas, which will, as will be seen in more detail below, to be located under the component.

 Once the support thus produced, a layer 23 of a solder means is deposited using a mask on the metal pads 21, typically a predetermined mixture of tin and lead (FIG. 4).

Also, the opening of the interconnection holes is consequently closed by said layer 23.

 The components 1 (for example) are then positioned (FIG. 5) on the corresponding pads 21. When this positioning is carried out, the part 21b of the surface of the pad 21 is located, taking into account the fitting tolerance, under the lower metallic contact 5 of the component and is spaced therefrom by the welding means 23. The rest 21a of the area is not located under the component and is simply covered by the welding means 23.

 When all the components have been placed on the support, it is placed in a reflow oven and the actual reflow of the welding means is carried out. This is done using a predetermined temperature profile. This profile firstly includes a rise in temperature to a first temperature value, generally around 1500C. A temperature plateau is then carried out at this value for approximately 1 minute so as to correctly stabilize the position of the components on the ranges. Then a rapid rise in temperature is carried out beyond the melting point of the solder paste followed by a very rapid reduction in temperature below this melting point. Thus, for a mixture of tin-lead solder paste in a ratio 2 / 3-1 / 3 approximately, the temperature peak is around 2350C and the gradients of rise and fall in temperature are from 1 to 4 "C per second, while the duration of the temperature peak is 10 seconds maximum.

 At the end of the reflow, the component 1 (FIG. 6) is joined to the metal pads 21 by a lateral weld meniscus 24 extending from the part of the metallic pad not covered by the component to the vertical part. 4 of the metallic contact of the component 1. It is this meniscus 24 which provides most of the attachment of the component to the support. In addition, there is also a welding film between the part S of the metal contact of the component and the corresponding surface of the metal pad completing the fixing of the component.

 Furthermore, during remelting, part of the liquefied welding means is introduced into the interconnection hole 22 and, during cooling, solidifies to leave a residue of solidified solder 26 in the hole. interconnection.

 In fact, depending on the temperature characteristics, the material used and the diameter of the interconnection hole, it may be that the latter is completely plugged with solidified solder. In other cases, only a weld film may remain attached to the metallized internal edges of the via hole. Anyway, in any case, the quantity of solder introduced into the interconnection hole allows the improvement of the quality of the interconnection hole with regard to its electrical conductivity.

 Furthermore, the method according to the invention has simultaneously made it possible to secure the component to the electrical pads, and to improve the quality of the via by introducing solder inside it.

 However, the opening of the interconnection hole 22 must be in a predetermined area of the metal area which is intended to be entirely located under the component.

 Further details on the positioning of this opening of the hole will now be provided with reference to FIG. 7, which illustrates a particular example of two metal pads 21 intended to receive a pair of opposite metal contacts of a component 1 or 10 .

 The area 21, shown on the left part of FIG. 7, is rectangular in shape and is bounded on the right by an internal edge 21i situated opposite the internal edge of the homologous area 21 (illustrated on the right part of the figure), and on the left, by an external edge 21st. A region 21 c extending from the internal edge 21i over the distance D corresponding to the length of the lower part 5 or 15 of the metal contact of the component intended to be positioned on this range is defined on this surface of the area 21.

 If the component is perfectly positioned on track 21, the lower part 5 or 15 of the metallic contact of the component therefore extends exactly over the length D of zone 21c. However, these components are intended to be positioned using a specific machine having a predetermined pose tolerance T. The component can therefore be shifted as far as possible to the right by a length T or to the left by a length T.

 Another zone 21z is therefore defined, extending from the internal edge 21i over a length Z equal to (L-M) / 2 - T. The rest of the zone 21c, of length T, is referenced 21t. Furthermore, a homologous zone, of length T, is located next to the zone 21t and to the left of the latter and is referenced 21u. The rest of the range, referenced 21a, will therefore be the area which will in any case not be covered by the component.

 According to the invention, the opening of the interconnection hole must be located in the zone 21 z and its diameter must therefore be less than or equal to Z.

 If we now refer to a fictitious abscissa axis represented in the lower part of FIG. 7, and if we consider that the internal edge 2 li is located at the abscissa zero, the different cases of positioning of the component will now be discussed.

 If the component is perfectly positioned, that is to say if the points P1 and P2 of the lower part of the metallic contact 5 or 15 of the component are located respectively on the abscissa (LM) / 2 and zero, the opening of the hole interconnection 22 is therefore located entirely under the metal contact 5 or 15.

 If the component is shifted to the right by the length T, that is to say if the point P1 is located on the abscissa (LM) / 2 - T, the opening 22 of the interconnection hole always remains positioned entirely under the metallic contact 5 or 15 of the component.

 If the component is shifted to the left by a length T, it is even if the points P1 and P2 are respectively located on the abscissa (LM) / 2 + T and T, part of the zone 21z is located under the contact metal S or 15 of the component, and the right part of this zone 21z is then located under the body 3 of the component 1, or else under the body 13 of the component 10 or possibly only under the horizontal upper part of the tab 12 of the component 10 , but in any case under the component in the general sense of the present invention.

 Thus, in all positioning cases, the zone 21z is located at least partially under the lower metallic contact of the component and, in certain cases, partially under the body of the component or at least between the two vertical parts of the metallic contacts of the component. .

 Thus, within the meaning of the present invention, the zone 21z will, in any event, always be located under the component.

 However, this positioning under the component is fundamental, because it avoids a significant migration of the welding means from the zone 21a towards the interconnection hole, which could lead to a reduction in the meniscus of welding and therefore to a poor attachment of the component. . This would happen in particular if the interconnection hole 22 overflowed into the zone 21u and the component was positioned with an offset to the right.

 On the other hand, the third case of positioning of the component (shift to the left) possibly providing for a slight clearance of the opening 22 of the interconnection hole in the right part of the zone 21z, is not detrimental to the good behavior of the meniscus of welding, because, on the one hand, the orifice thus released is only partially so and is relatively distant from zone 21a and therefore from the meniscus, and, on the other hand, the presence of the metallic contact of the component ( having a width substantially equal to that of the pad and forming a barrier) as well as the viscosity of the liquefied weld does not allow a significant migration of the weld located at the meniscus.

Claims (4)

 1. Electronic card comprising surface-mount components (1, 10) fixed by reflow on predetermined metal areas (21) of a support (20) further equipped with metallized interconnection holes (22) between the different layers conductive of the support, characterized in that at least some of the interconnection holes (22) open inside at least some of the areas (21) in predetermined zones (21 z) of these areas fully located under the components correspondents.  2. Method of manufacturing an electronic card comprising surface-mounted components fixed by reflow on predetermined metal areas of a support also equipped with metallized interconnection holes between the various conductive layers of the support, characterized by the fact than  - at least some of the interconnection holes (22) are opened in predetermined areas (21z) of at least some of the metal areas (21),  - the metal areas are covered with a welding means (23),  - The components (1) are positioned on the pads so that they cover a first part (21b) of the corresponding pads incorporating said predetermined zones (2lz) in full, leaving the rest (21a) of the pads not covered by the components,  - We proceed to the reflow of the welding means so as to simultaneously form a weld (24) securing the component to the pad and to let remain inside the interconnection hole (22) a residue (26) of means solidified weld.  3. Support for the manufacture of an electronic card with surface-mount components fixed by reflow, comprising metal pads (21) on which said components are intended to be positioned with a predetermined pose tolerance T, as well as holes d metallized interconnection (22) between the various conductive layers of the support, characterized in that at least some of the interconnection holes (22) open inside at least some of the metal pads in predetermined zones (21z) of these areas intended to be completely located under the corresponding components.  4. Support according to claim 3, characterized in that at least one of the metal pads (21) having an interconnection hole (22) is associated with a homologous metallic pad, the two pads being separated by a distance M predetermined and being intended to receive two metal contacts (5, 15) of a component having a minimum length L overall of contact with predetermined contact, said component being intended to be positioned on said pads (21) with the tolerance of laying T predetermined, and by the fact that the opening of the interconnection hole (22) opening into the range considered is located entirely in an area (21z) limited on the one hand by the inner edge (21i) of the range located in face of the inner edge of the homologous area, and extending on the other hand, from the inner edge (21i) over a distance Z equal to (LM) / 2 - T.