ELECTRONIC CARD COMPRISING AN INTERMEDIATE CIRCUIT WITH METALIZED HALF-HOLES

Description

[0001] The present invention relates to an electronic card according to the preamble of claim 1.

[0002] Such an electronic card results from the replacement of an initial component by an interposed printed circuit carrying a replacement component, when the initial electronic component present in a printed circuit becomes obsolete, and there is no replacement electronic component whose footprint on the circuit matches perfectly.

[0003] Designing and manufacturing a new printed circuit board to fit the updated electronic component is an expensive method, especially if it needs to be renewed regularly.

[0004] This is particularly important in railway applications, where replacing a printed circuit board requires complex disassembly operations, and where the operating time of an electronic device is long compared to the speed of evolution of the capabilities of the components that compose them.

[0005] In this case, it is known to use an interposed printed circuit board (PCB) that accommodates the replacement electronic component. This interposed printed circuit board is connected by soldering to the main electronic circuit. Such a replacement operation is much less expensive and simpler to implement than a complete retracement.

[0006] Electronic cards comprising interposed circuits are described in US 4412642 A, US 2008 / 278920 A1, US 2014 / 124256 A1 and US 2002 / 093803 A1, which describes the preamble of claim 1.

[0007] However, the operating conditions of an electrical circuit in railway equipment are often harsher than in conventional electronics. Electronic boards must therefore meet stricter standards of resistance to environmental stress and corrosion.

[0008] An aim of the invention is to provide an electronic card in which an obsolete electronic component has been replaced and which has characteristics of resistance to environmental constraints and corrosion suitable for use in a railway environment.

[0009] To this end, the invention relates to an electronic card of the aforementioned type, according to claim 1.

[0010] The electronic card according to the invention may comprise one or more of the characteristics of the dependent claims, taken in isolation or in any technically possible combination.

[0011] The invention will be better understood from reading the following description, given solely by way of example and with reference to the appended drawings, among which: there figure 1 is a perspective view of an interposed printed circuit board, carrying an electronic component; and the figure 2 is a sectional view of a part of an electronic card according to the invention.

[0012] An interposed printed circuit 10 is shown in the figure 1 This printed circuit comprises an insulating substrate 12, in the form of a substantially rectangular plate having an upper face 14 and a lower face 16, as well as four lateral edges 18. The substrate has a length and a width of between 2 mm and 40 mm depending on the applications, for a surface area of ​​between 4 mm 2< and 1600 mm 2<.

[0013] To ensure sufficient rigidity of the interposed printed circuit 10, the ratio of the length to the width of the substrate 12 is chosen to be less than 6. In addition, the substrate 12 has a thickness of 0.8 mm, which gives it sufficient rigidity for good durability. The rigidity is however not too great, to limit local mechanical stresses due to temperature variations.

[0014] The substrate 12 is made from an insulating material, such as a composite of epoxy resin and woven glass fibers. Advantageously, the insulating material is flame retardant by the addition of a brominated compound to the epoxy resin. For example, the composite material forming the substrate 12 is of class FR-4 as described in the NEMA, “National Electrical Manufacturers Association” standard LI 1-1998 (R2011).

[0015] The interposed printed circuit 10 comprises a plurality of metallized half-holes 20 located on at least two lateral edges 18. In the example shown in the figure 1 , the intermediate printed circuit 10 comprises fourteen metallized half-holes 20 distributed equally on two opposite lateral edges 18. As a variant, the intermediate printed circuit comprises for example twenty-eight metallized half-holes 20 distributed equally on the four lateral edges 18. The half-holes 20 are positioned according to a predetermined pitch of between 0.4 mm and 1.27 mm.

[0016] Each metallized half-hole 20 is a notch of semi-circular section, extending in the lateral edge 18 from the upper face 14 to the lower face 16. Each metallized half-hole has a metallic layer 22, conductive, for example copper, which extends on the inside of the notch of semi-circular section and extends on the upper face 14 of the substrate 12 to form a tab 24 there and on the lower face 16 to form a tab 25 there.

[0017] The intermediate printed circuit 10 also comprises on its upper face 14 a plurality of conductive metal pads 26, for example made of copper.

[0018] The intermediate printed circuit 10 further comprises a plurality of tracks 28, for example made of copper, on its upper face 14. Each track 28 electrically connects either at least one metal pad 26 to a tab 24, or several metal pads 26 to each other, and is capable of transmitting electrical signals.

[0019] Advantageously, the intermediate printed circuit 10 also comprises a plurality of tracks 28, for example made of copper, on its lower face 16, as well as conduits 29 pierced through the substrate 12, opening onto the upper face 14 and the lower face 16.

[0020] Each track 28 of the lower face 16 electrically connects at least one conduit 29 to one of the tabs 25, or several conduits 29 between them, and is capable of transmitting electrical signals.

[0021] The conduits 29 contain, for example, copper and electrically connect at least one of the tracks 28 of the upper face 14 to one or more of the tracks 28 of the lower face 16.

[0022] As shown in the figures 1 And 2 , the intermediate printed circuit 10 receives at least one electronic component 30 on the side of the upper face 14. The electronic component 30 is for example an integrated circuit comprising a housing 32 and metal legs 34. The housing 32 contains electronic equipment capable of performing processing and calculation functions from the electronic input signals received via some of the legs 34 of the component 30, and of emitting electrical output signals via other legs 34.

[0023] Each leg 34 of the electronic component 30 is fixed to one of the metal pads 26 by a solder 36 located between the leg 34 and the metal pad 26. In the example shown figures 1 And 2 , the welds 36 are substantially conical, and are produced by a simple soldering process, or by reflow.

[0024] As shown in the figure 2 , the intermediate printed circuit 10 is positioned on a main printed circuit 40 to form a multi-level electronic card 41. The main circuit 40 has tracks 44, capable of transmitting electrical signals.

[0025] The interposed printed circuit 10 is electrically connected to the main printed circuit 40 by solders 46 located at the metallized half-holes 20 and bonding the metal layer 22 of each metallized half-hole 20 to a track 44. The solders 46 partially fill the interior space of each metallized half-hole 20, as shown in the figure 2 , and are made by a simple soldering process or by reflow.

[0026] The welds 46 provide the electrical connection between the tracks of the main printed circuit 40 and the intermediate printed circuit 10, as well as the mechanical connection of the main 40 and secondary 10 printed circuits.

[0027] The welds 46 are made with a first filler material composed of a eutectic alloy, that is to say at constant melting temperature, that is to say which melts and solidifies at constant temperature.

[0028] The first filler material with which the welds 46 are made is for example a tin-lead alloy. The first filler material comprises a mass content of lead of between 33% and 40%, for example 37% and a mass content of tin of between 60% and 67%, for example 63%.

[0029] Alternatively, the first filler material with which the welds 46 are made is composed of a tin-silver or tin-silver-copper or tin-bismuth alloy.

[0030] The method used to produce the welds 36 uses a second filler material. The second filler material has a lead content by mass of between 33% and 40%, for example 37%, and a tin content by mass of between 60% and 67%, for example 63%.

[0031] Alternatively, the second filler material is a tin-lead-silver alloy. The mass content of tin is, for example, 62%, while the mass content of lead is 36% and the mass content of silver is 2%.

[0032] The presence of lead in the first and second filler materials gives them a melting temperature of between 160°C and 200°C, particularly close to 180°C, lower than the melting temperatures of materials normally used in the soldering of electronic components.

[0033] The presence of lead in the filler materials of the solders 36, 46 also reduces the susceptibility of the solders 36, 46 to corrosion and reduces the risk of premature breakage of the electronic card 41.

[0034] Variants of substrate dimensions 12 and pitch between half-holes 20 were tested and the results are grouped in the following Table 1. These variants correspond to replaced components of different sizes and having different numbers of legs.

[0035] The lines in Table 1 correspond to the different dimensions of the substrate 12 tested, for which the surface area is between 6 mm 2< and 1089 mm 2< , and the ratio of length to width is between 1 and 4.

[0036] Intermediate circuits 10 that have passed environmental resistance tests, particularly salt spray, correspond to a box indicating a validated result. Intermediate circuits 10 that have failed these tests correspond to a box indicating a rejected result. The boxes included vertically between two validated boxes are validated a priori. Table 1 Not 0.4 mm 0.5 mm 0.65 mm 0.8 mm 1 mm 1.27 mm Substrate 2x3 mm Valid Valid 3x4 mm Valid 5x5 mm Valid Valid 7x8 mm Valid Valid 7x10 mm Valid 10x10 mm Valid 9x12 mm Valid 10x18 mm Valid Valid Valid 13x13 mm Valid 5x20 mm Rejected 15x15 mm Rejected 17x20 mm Rejected Valid 10x29 mm Valid 20x27 mm Valid Valid 33x33 mm Valid

[0037] The pitch associated with the half-holes 20 fixes the number of welds 46 between the intermediate circuit 10 and the main circuit 40. This number of welds must be sufficient to ensure good resistance of the electronic card 41. However, it must not be too large to limit the rigidity of the electronic card 41 and local mechanical constraints, in particular during temperature variations causing expansion of the substrate 12.

[0038] Large pitch values ​​are therefore more suitable for large substrate dimensions 12, and small pitch values ​​for small dimensions. Thus, values ​​close to the first diagonal of the table represent more robust assemblies for the electronic card 41.

Claims

1. An electronic board (41) including: - a main printed circuit (40) including tracks (44); - an interposed printed circuit (10) positioned on the main printed circuit (40), including: - a substantially rectangular substrate (12), having a length and a width smaller than the length, including an upper face (14), a lower face (16) and at least one edge (18); - at least one metal track (28) located on at least one of the upper face (14) and of the lower face (16), able to transmit electric signals; - a plurality of half-holes (20) connecting the upper face (14) and the lower face (16), located at the edge (18), having a metal layer (22) covering an internal surface of each half-hole (20); and - at least one electronic component (30) located on the upper face (14), electrically connected to the tracks (28) of the interposed printed circuit (10) through at least one solder (36) ; the interposed printed circuit (10) being connected to the tracks (44) of the main printed circuit (40) through at least one solder (46) made with a first supply material, located on the metal layer (22) of each half-hole (20), the substrate (12) having a surface area comprised between 4 mm2 and 1,600 mm2, the ratio between the length and the width of the substrate (12) being less than or equal to 6, characterized and in that each solder (36) connecting the electronic component (30) to the interposed printed circuit (10) is made with a second supply material including a lead mass content of more than 33%, and in that the combination of the width of the substrate, the length of the substrate and the pitch separating each half-hole (20) from the closest half-hole (20) is selected among the following combinations : (2 mm, 3 mm, 0.5 mm), (2 mm, 3 mm, 0.65 mm), (3 mm, 4 mm, 0.5 mm), (3 mm, 4 mm, 0.65 mm), (3 mm, 4 mm, 0.8 mm), (5 mm, 5 mm, 0.5 mm), (5 mm, 5 mm, 0.65 mm), (5 mm, 5 mm, 0.8 mm), (5 mm, 5 mm, 1 mm), (5 mm, 5 mm, 1.27 mm), (7 mm, 8 mm, 0.5 mm), (7 mm, 8 mm, 0.65 mm), (7 mm, 8 mm, 0.8 mm), (7 mm, 10 mm, 0.5 mm), (7 mm, 10 mm, 0.65 mm), (7 mm, 10 mm, 0.8 mm), (7 mm, 10 mm, 1 mm), (10 mm, 10 mm, 0.65 mm), (10 mm, 10 mm, 0.8 mm), (10 mm, 10 mm, 1 mm), (9 mm, 12 mm, 0.65 mm), (9 mm, 12 mm, 0.8 mm), (9 mm, 12 mm, 1 mm), (10 mm, 18 mm, 0.65 mm), (10 mm, 18 mm, 0.8 mm), (10 mm, 18 mm, 1 mm), (13 mm, 13 mm, 0.8 mm), (13 mm, 13 mm, 1 mm), (15 mm, 15 mm, 0.8 mm), (15 mm, 15 mm, 1 mm), (17 mm, 20 mm, 0.8 mm), (17 mm, 20 mm, 1 mm), (10 mm, 29 mm, 0.8 mm), (10 mm, 29 mm, 1 mm), (20 mm, 27 mm, 0.8 mm), (20 mm, 27 mm, 1 mm), (33 mm, 33 mm, 1.27 mm).

2. The electronic board (41) according to claim 1, wherein the substrate (12) is made from a composite material comprising an epoxy resin and glass fibers.

3. The electronic board (41) according to any of the preceding claims, wherein the thickness of the substrate (12) is substantially equal to 0.8 mm.

4. The electronic board (41) according to any of the preceding claims, wherein the tracks (28) are located on the upper face (14) of the substrate (12) and include at least one metal pad (26) receiving a solder (36) connecting the metal pad (26) to a pin (34) of the electronic component (30).

5. The electronic board (41) according to claim 5, wherein the second supply material is an alloy of lead and tin comprising a lead mass content comprised between 33% and 40%.

6. The electronic board (41) according to claim 6, wherein the second supply material consists of an alloy of lead and tin comprising a tin mass content comprised between 60% and 67%.

7. The electronic board (41) according to any of the preceding claims, wherein the first supply material consists of a eutectic alloy selected from among a tin-lead alloy, a tin-silver-copper alloy, a tin-silver alloy and a tin-bismuth alloy.

8. The electronic board (41) according to any of the preceding claims, wherein the first additional material is a tin-lead alloy and comprises a lead mass content comprised between 33% and 40%, preferably equal to 37% and a tin mass content comprised between 60% and 67%, preferably equal to 63%.

9. The electronic board (41) according to any of the preceding claims, wherein the first and second supply materials have a melting temperature comprised between 160°C and 200°C, preferably of the order of 180°C.