GB2036624A

GB2036624A - Soldering together two elements arranged on opposite sides of a substrate

Info

Publication number: GB2036624A
Application number: GB7929888A
Authority: GB
Original assignee: CII HONEYWELL BULL
Current assignee: CII HONEYWELL BULL
Priority date: 1978-10-19
Filing date: 1979-08-29
Publication date: 1980-07-02
Also published as: NL7905081A; IT7926531A0; DE2942344A1; FR2439322A1; GB2036624B; JPS5556694A; SE7906963L; FR2439322B1

Abstract

In an arrangement wherein first and second elements (e.g. of copper) arranged on opposite sides of a substrate (e.g. electronically insulating) are provided with respective adjoining parts which are connected together by connecting means passing through a hole formed in the said substrate, the part (31a) of the first element (31) blocks the hole (35), (35) in the substrate (33), whilst the part (32a) of the second element (32) is fixed in solder (36) which at least partly fills the said hole and which forms the said connecting means (34). The solder may be introduced into the hole in paste form by screen printing and remelted by hot gas. An electrically heated bit may deform the second element part (32a) into the solder while melting the latter. <IMAGE>

Description

SPECIFICATION Method and arrangement for connecting together two elements and tool for carrying out the method The present invention has as object to provide an arrangment and a method for connecting together two elements situated one on either side of an intermediate member and provided with respective portions close to a hole formed in the intermediate member; as well as a tool for soldering by Joule effect for carrying out the said method.

Connections of this kind are used in particular in the production of interconnecting circuits between devices for processing electrical signals (integrated circuit devices for example) which are fitted to a member carrying printed circuits generally referred to in the art as an "interconnecting substrate". The method adopted for making such connections is termed the "metalised hole" method. Since this method reveals virtually all the problems which may be posed when it is put into practice in certain applications, it has been taken as an example in the following description, although it should be pointed out that the invention is not restricted to this particular case.

In printed circuit techniques, the interconnecting substrate intended to receive a metallised hole is a board of insulating material which can easily be perforated without damage and which is provided on both faces with conductive elements of the interconnecting circuit. To make an electrical connection between respective parts of two of the interconnecting conductors respectively situated on opposite faces of the substrate, a hole is formed in the substrate near the two parts of the conductors.

The walls of the hole are covered uniformly with a conductive metal lining (normally formed by the electrochemical deposition of copper) which is connected by its upper and lower edges to respective ones of the two parts of the conductors by linking strips commonly formed by the electrolytic deposition of copper. The lining of the hole and the linking strips form the connecting arrangement.

It can thus be seen that metallised hole connections first of all require the hole to have walls sufficiently smooth to receive a thin and continuous metal lining, and then require two layers to be deposited in succession by different methods (electro-chemical and electrolytic) to form the lining and the linking strips. Also, they can only be formed in practice if the two conductive elements to be connected are layers secured to the substrate.

However, it would sometimes be advantageous to arrange an integrated circuit device provided with output leads on one face of the substrate in cases where the conductors associated with the leads are all on the other face on the substrate, by fixing the output leads of the device directly to the corresponding conductors on the other face via holes pierced in the substrate. With metallised holes, it would be necessary to insert the appropriate ends of the output leads of the device in the holes and to solder each of the ends to the linings of the holes with a blob of solder. Attachments of this kind present so many difficulties that they are hardly ever used in practice.To overcome the said difficulties, a conductive element is applied near each metallised hole, generally by deposition, on the face which is to carry the integrated circuit device, one end of this conductor is connected to the lining of the hole by a linking strip, and the other end of the conductor is formed into a connected island which is intended to receive the end of one of the output leads of the device.

Thus, the use of metallised holes makes it necessary to increase the dimensions of the substrate to allow additional conductors provided with connecting islands to be arranged on it. whilst the additional operations required to deposit the additional conductors and their links to the metallised holes increase the time taken to equip a substrate and cause a corresponding increase in the cost of the substrate when equipped.

The invention overcomes all these disadvantages by means of a direct soldered connection between the two elements in a hole having no conductive lining.

In effect, an arrangement according to the invention is of the kind comprising first and second members elements arranged one on either side of an intermediate member and provided with respective adjoining parts which are connected together by connecting means passing through a hole formed in the said intermediate member, and is characterised in that the said part of the first element blocks the hole whilst the said part of the second member is fixed in place in solderwhich at least partly fills the hole and which forms the said connecting means.

The invention thus also provides a method for connecting first and second elements situated on either side of an intermediate member and provided with respective parts adjoining a hole formed in the intermediate member, characterised in that it consists in: blocking the said.hole with the said part of the first element; at least partly filling the hole with solder; arranging the said part of the second element in such a way as at least partly to overlap the hole; bringing the said part of the second element into contact with liquid solder contained in the hole; and allowing the solder to cool in order to fix the second element to the solder.

The invention also has as an object a tool for micro-soldering by Joule effect in order to carry out the above method, this tool being of the kind intended for connection to two terminals for supplying electrical current which are carried by a tool shank, and comprising a heating bit formed by at least one elongated bit member having a generally plane soldering face, which member connects together the corresponding ends of two electrically conductive side-pieces whose other ends are intended to be connected to respective ones of the said supply terminals, the tool being characterised in that the said soldering face of the bit member includes means forming protuberances.

The features and advantages of the invention will be more clearly apparent from the following description, which is given with reference to the accompanying drawings. In the drawings: Figure 1 is a sectional view of a prior art substrate which is provided with a metallised hole to connect together two conductors arranged respectively on the two faces of the substrate; Figure 2 is a sectional view of an embodiment of an arrangement according to the invention; Figure 3a to 3d il lustrate respectively four successive phases of an example of a method which may be adopted to produce an arrangement according to the invention, such as that shown in Figure 2; Figure 4 is a sectional view of a modified embodiment of an arrangement according to the invention;; Figure 5 is a perspective view of a tool suitable for carrying outthe method illustrated in Figures 3a to 3d; Figure 6 is a sectional view on line VI-VI of a bit member of the tool shown in Figure 5; and Figure 7is a graph illustrating a soldering cycle which may be used with the tool of Figure 5, the graph showing the changing temperatures and pressures applied by the tool with respect to time.

In order that the features and advantages of the invention may be more clearly demonstrated, the prior art arrangement discussed earlier in the text is shown in Figure 1. The arrangement 10 shown in Figure 1 comprises a first conductive element 11 and a second conductive element 12 which are arranged on either side of an electrically insulating intermediate member 13 forming what is referred to in the art as an interconnecting substrate. The conductors 11 and 12 have respective physically adjacent parts 1 1a and 12a which are connected together by connecting means 14 passing through a hole 15 formed in the substrate 13.In conformity with the prior art technique for metallised hole connections, the connecting arrangement 14 comprises a conductive lining 16 which covers the walls of the hole 15, the linking strips 17, 18 which connecting parts 118 and 12a of conductors 11 and 12 to the corresponding edges of the lining 16. Should it be desired to connect the output lead 19 of an integrated circuit device for example (not illustrated) to a conductor 11 situated on the other side from that carrying the integrated circuit device, the prior art calls for the deposition of a conductor, such as conductor 12, provided with a connecting island 20, the deposition of the lining 16 and the connecting strips 17 and 18, and the connection of the output lead 19 to the island 20 by solder 21.

Figure 2 shows an arrangement 30 which, in accordance with the invention, solves the same problem as the arrangement illustrated in Figure 1, namely the attachment of an output lead of an integrated circuit device to a conductor situated on the other side of the interconnecting substrate. Like the arrangement 10, arrangement 30 comprises a first conductive element 31 and a second conductive elements 32 which are arranged on either side of an electrically insulating intermediate member 33 which forms an interconnecting substrate as in the previous case. The conductors 31 and 32 have respective physically adjacent parts 31a and 32a which are connected together by connecting means 34 passing through a hole 35 formed in the substrate 33.In accordance with a feature of the invention, part 318 of conductor 31 partially blocks the hole 35, whilst part 32a of conductor 32 is fixed in a blob of solder 36 which at least partly fills the hole 35 and which forms the said connecting means 34. In this case the conductor 32 may be one of the output leads of a signal processing device, part 32a representing the end of this lead. As to part 31a, this may be an end of the conductor 31, or part of a conductor which continues past the hole, as shown. Also, if the general direction of the lead 32 prevents its end 32a from being fixed directly in the solder 36, the end 32a is angled to enter the hole 35 and make contact with the solder 36 as shown.

Figures 3a to 3d illustrate an example of a method for obtaining the arrangement 30 shown in Figure 2.

Referring to Figure 3a, the conductor 31 is first of all applied to the bottom face of the substrate 33 in such a way as to blockthe hole 35. In practice, this may be achieved by bonding the conductor 31 to the substrate 33, or by thermal fixing by heating the conductor 31 to cause it to adhere to the substrate 33, or by any other technique familiar to the man skilled in the art. In the actual arrangement shown, a sheet of copper has been bonded to the bottom face of the substrate and etched chemically to form, inter alia, the conductor 31, and the adhesive has been removed from part 31a. Once the conductor 31 is in place in accordance with the invention, solder 36a in paste form is introduced into the hole 35 by screen printing.The paste in question in an amalgam of beads of solder in an alloy of lead-tin in cases where the conductors 31 and 32 are made of copper. The paste fills the hole 35. In the next phase, the paste 36a is re-melted to produce the compact block of solder 36 by means of a jet 37 of a hot reducing gas such as hydrogen or nitrogen (Figure 3b). This re-melting enables the solder 36 to adhere to part 318 of conductor 31 and makes it easier for conductor 32 to be soldered subsequently. It can be seen in Figure 3b that, because of the re-melting, the solder 36 only partly fills the hole 35.

The output lead 32 of an integrated circuit device (not shown) is then arranged in such a way that its end 32a at least partly overlaps the hole 35, as shown in Figure 3c. Finally, the end 32a is brought into contact with the solder 36 at the same time as the latter is made liquid. These two operations are performed at the same time in the case illustrated in Figure 3d, by simultaneously making a bend 38 in end 32a and re-melting the solder 36 by means of a lug 39 on the heating bit of a tool 40, such as is shown in Figure 5. It is then merely necessary to allow the solder 36 to cool and then to raise the lug 39 to obtain the arrangement 30 shown in Figure 2.

A tool highly suitable for carrying out the method which has just been described is illustrated in Figure 5. It is a tool for micro-soldering by Joule effect of the kind described in our copending patent application No. 13390178.

Thus, the tool 40 shown in Figure 5 is coupled to a tool shank 41 composed of two massive conductive uprights 41a and 41b which are separated from one another by an electrically insulating strip 41c. The uprights 41a and 41b form the two terminals for supplying electrical current to the tool 40. They are therefore normally connected to a source of electric al current which is not shown in the drawings. As indicated in Figure 3d, the tool 40 has a heating bit 42 comprising two members 42a and 42b which connect together the corresponding ends of two electrically conductive side pieces 43a and 43b whose other ends are respectively coupled to the supply terminals 41 a and 41b by attachment means which in the present case are screws 44. Figures 5 and 6 show that the bit members 42 have a generally plane soldering face 45.In accordance with a feature of the invention, the soldering face 45 of each bit member 42 includes means which form proturberances, such as the four lugs 39 shown in Figures 3d, 5 and 6.

In addition, since the side pieces 43a and 43b are each of substantially greater cross-sectional area than the bit member 42a and 42b, the heat which is generated virtually uniformily in each bit member would tend to dissipate into the side pieces 43a and 43b, with the result that the temperature at the ends of the bit members 42 would be lower than in the central region of these members, assuming the cross section of the bit members and the current intensity to be otherwise equal. In accordance with a feature of the invention, the tool 40 includes means which produce a selective increase in electrical resistance in the regions connecting each bit member 42 to the side pieces 43, so as to make the temperature of the soldering faces 45 substantially constant for virtually their entire length.In the embodiment illustrated, it can be seen that these means are formed by a constriction 46 at each of the said connecting regions between the bit members and the side pieces. It can in fact be seen in Figure 5 that the region 46 is of smaller cross-sectional area than both the members 42, so that the heat loss by diffusion into the side pieces is compensated for by an increase in temperature at the regions 46.

In accordance with another feature of the invention, the two bit members are connected together in their central regions by a conductive bridge 47.

Experience has shown that the effect of adding this bridge is to harmonise to an even greater degree the soldering temperatures generated by the two bit members 42. It is for this reason that the soldering temperature is detected by a member 48 arranged on or close to the bridge 47.

Figure 7 shows an example of a soldering cycle which can be used with the tool 40 to obtain the arrangement 30 shown in Figure 2. It can be seen that a pressure of 3009 is exerted on end 32a of conductor 32 at the same time as each bit member is raised to a temperature of approximately 2200 for the above mentioned soldering operation. The current is shut off before the bit is withdrawn so that the end 32 has time to become firmly fixed to the solder 36.

To demonstrate that the invention is not restricted merely to something similar to the arrangement 30 of Figure 2, Figure 4 shows a modified embodiment of the invention. In the arrangement 50 shown in Figure 4, the first conductive element 51 is part of a conductive layer deposited on an electrically insulating interconnecting substrate 52. The second conductive element 53 rests on an insulating layer 54 which, in this modification, forms the intermediate member through which passes a hole 55 by means of which a connection is made between conductors 51 and 53 by solder 56. It will be seen that in the arrangement 50, the part 53a which is connected to part 51a of conductor 51 is not the end of an output lead of a signal processing device (an integrated circuit device for example) but is part of a member extending beyond the edges of hole 55.The conductor 53 may thus be a member which is applied to the insulating layer 24, and which is fixed to the layer on one side of the hole or which is totally unattached to it before the part 53a is brought into contact iwth the solder 56.

In the arrangements 30 and 50, parts 32a and 53a are angled to allow them to be brought into contact with the solder 36 and 56. However, it is clear from the above description that this is only one particular case. In effect, in the case of Figure 2, if the conductor 32 were originally closerto the axis of hole 35 and/or if the solder 36 filled the hole 35 to a greater extent, it is clear that the angling operation would be no longer be necessary. In the case of Figure 4 is would be possible to envisage filling the hole 55 completely or applying a projection to part 53a of the conductor 55 so that the said part would no longer have to be deformed to fix it to the solder 56. It is also clear that the angling could be performed before the conductor is arranged on the intermediate member.

In other words, the invention is in no way restricted to the embodiments described and illustrated and in fact covers all the means forming technical equivalent, of the means described, as well as combinations thereof if these are made within the scope of the following claims.

Claims

1. An arrangement of the kind comprising first and second elements arranged one on either side of an intermediate member and provided with respective adjoining parts which are connected together by connecting means passing through a hole formed in the said intermediate member, characterised in that the said part of the first element blocks the said hole, whilst the said part of the second element is fixed in solder which at least partly fills the said hole and which forms the said connecting means.

2. Arrangement according to claim 1, characterised in that the part of the second element which is fixed in the hole is angled to allow it to enter the hole.

3. Arrangement according to claim 1 or 2, characterised in that the said part of the second element is an end thereof.

4. Arrangement according to any of claims 1 to 3, characterised in that the said part of the first element is secured to the intermediate member close to the said hole, whilst the second element is an element which is not secured to the intermediate member except at the said hole.

5. Arrangement according to any of claims 1 to 3, characterised in that the said respective parts of the first and second elements are both secured to the intermediate member.

6. Arrangement according to any of claims 1 to 5, characterised in that the said first and second elements are electrically conductive elements, whilst the intermediate member is an electrically insulating member.

7. Arrangement according to claim 6, characterised in that the intermediate member is an interconnecting substrate.

8. Arrangement according to claim 6, characterised in that the intermediate member is an insulating layer which covers at least the said part of the first element, and which is itself a layer formed on top of an interconnecting substrate.

9. Method of connecting first and second elements situated one on either side of an intermediate member and provided with respective parts adjoining a hole formed in the intermediate member, characterised in that it consists in: blocking the said hole with the said part of the first element; at least partly filling the said hole with solder; arranging the said part of the second element in such a way as at least partly to overlap the said hole; bringing the said part of the second element into contact with the liquid solder contained in the hole; and allowing the solder to cool in order to fix the second element thereto.

10. Method according to claim 9, characterised in that the said filling consists in introducing solder in paste form into the hole by screen printing and in re-melting the paste.

11. Method according to claim 9 or 10, characterised in that the bringing of the second element into contact with the solder comprises soldering this element at the point where the hole is situated.

12. Method according to any of claims 9 to 11, characterised in that it consists in re-melting the solder to produce the said liquid solder.

13. Tool for micro-soldering by Joule effect so as to carry out the method according to any of claims 9 to 12, of the kind intended to be coupled to two terminals for supplying electrical current which are carried by a tool shank, and including a heating bit formed by at least one elongated bit member which has a generally plane soldering face and which connects together the corresponding ends of two electrically conductive side pieces whose other ends are intended to be coupled to respective ones of the said supply terminals, characterised in that the said soldering face of the bit member includes means forming protuberances.

14. Tool according to claim 13, characterised in that the said side pieces are each of substantially greater cross-sectional area then the bit member, and in that the said tool includes means which produce a selective increase in electrically resistance in the connecting regions between the bit element and the side pieces, in order to compensate for the loss of heat to the side pieces at the ends of the bit member and thus to render the temperature of the soldering face of the bit element substantially constant for approximately the whole of its length.

15. Tool according to claims 13 or 14, characterised in that it has at least two bit members which are connected in their central regions by a conductive bridge.

16. Tool according to claim 15, characterised in that the said bridge includes means for detecting the temperature of the bit.

17. Connecting arrangements substantially as hereinbefore described with reference to Figures 2 to 7 7 of the accompanying drawings.

18. Methods of connection substantially as hereinbefore described.

19. Atool substantially as hereinbefore described with reference to Figures 5 and 6 of the accompanying drawings.