ITMI20072150A1 - DEVICE FOR THE GENERATION OF A THERMAL BARRIER WITH A TEMPERATURE CONTROLLED BY THE MAGNETIC INDUCTION POLES OF A WELDING HEAD - Google Patents

Description

Description of the INDUSTRIAL INVENTION entitled:

Device for (generating a temperature thermal barrier connected to the magnetic induction poles of a welding head

Field of application of the invention

The present invention refers to the field of inductive heating machinery, and more precisely to a device for the generation of a temperature-controlled thermal barrier to the magnetic induction poles of a welding head. In the following, the invention will be described with special attention to its application to the local welding of the component layers of a printed circuit multilayer, however, this is not intended as a limitation of the scope of application. Review of the known art

The inductive spot welding of the various component layers of a multilayer printed circuit is known from! Spanish patent No. 2190757, which would seem so far to constitute the closest known art to the invention that will be described. Referring to Figure 1 (corresponding to Figures 1, 2, and 3 of the aforementioned citation), we note a perspective view of a multilayer printed circuit 1 composed, for example, of four layers with circuit layout (also called layers), respectively indicated with references 2, 4, 6 and 8, and by three insulating layers, respectively indicated with 3, 5 and 7.

The latter consist of a thermo-adhesive glue composed of resin-impregnated glass fabric, known as prepreg. In the arrangement indicated for the multilayer 1, the insulating layers are alternated with the layers with circuit layout, and all the layers are rectangular of equal size. The first and last layers of the battery have circuit tracing. The circuit layers 2, 4, 6 and 8 have a copper-free perimeter band 9 bordering a rectangular copper crown 10 which delimits the circuit layout. Regularly spaced along the longer sides of the crown 10, metal-free rectangular shaped buffer areas 11 are visible, except for two short-circuited turns 12 and 12 'adjacent to each other. The areas of respect 11 with the coils 12 and 12 'inside them are placed in the same positions in all the layers, so as to form groups of areas of respect, and coils, superimposed through the multilayer. Each group of overlapping coils constitutes a heating group. A specially generated variable magnetic flux induces strong short-circuit currents in the turns 12 and 12 'which heat the conductors due to the Joule effect, melting the surrounding prepreg. When the currents cease, the prepreg resolidifies by welding the layers together.

The same patent mentioned above also claims a machine for the welding process which consists of the following elements:

- a structure in which at least one inductor device 13 provided with a magnetic circuit, generally U-shaped, is arranged, comprising a central body 14 equipped with an inductor winding and two arms 15, placing at the outer end of each arm 15 a corresponding induction electrode 16, 17, and the two electrodes 16 and 17 being in a perpendicular position with respect to the multilayer printed circuit 1 placed on a circuit board (not shown), and in a coaxial position with respect to each other with the possibility of moving in both directions;

- a device (not indicated) for fixing the set of layers 2, 3, 4, 5, 6, 7, 8 of the multilayer printed circuit 1 to the circuit board;

- a control device (not shown) for the movement of the circuit board, suitable for placing the groups of buffer areas 11 between the pair of induction electrodes 17, 18 of the inductor device 13; And

- a control device (not shown) for each pair of induction electrodes 16, 17, suitable for placing the electrodes in contact with the group of the buffer areas 11, by exerting pressure on it.

In operation, the inductor winding 13 generates a variable magnetic field (inductor flux) within the magnetic circuit 14, 15 and the induction electrodes 16 and 17 and continues from these into the gap between the two where the multilayer 1 to be welded is placed. The flow is linked to the coils 12, 12 'of the heating group concerned. Within the coils, variable short-circuit currents are formed, therefore very intense, which heat the surrounding substrate by the Joule effect, causing a partial melting of the prepreg substrates in correspondence with the buffer areas. When the magnetic induction ceases, the heated areas cool down and remain welded together.

Technical problem highlighted

The purpose of the inductive welding illustrated above is to be able to handle the multilayer during subsequent processing without thereby generating misalignments between the circuit paths that must correspond in the different layers. For this it is necessary that the welding operation uniformly heats the different buffer areas stacked within its own group. It is also important that the temperature at the interface between the different layers is kept at! below a limit value such as to avoid burns or the formation of gaseous bubbles in the pre preg. Unfortunately, these requirements are not guaranteed with the machinery described in figure 1. The cause is due to the different thermal regime a! which the outermost layers are subjected with circuit layout 2 and 8 with respect to the remaining ones. In fact they are placed in direct contact with the magnetic induction electrodes 17 and 16, generally made up of ferrites of great mass and with a fair thermal conductivity (about one fifth compared to copper), and therefore able to rapidly cool the contact surfaces. To compensate for the differential cooling of the outer layers, it is necessary to increase the intensity of the power supply current of the inductor winding, thereby making it more probable for harmful uncontested increases in temperature in the welding areas. In the machinery of the known art, a system for regulating the heating temperature of the layers within the sealing zones is completely lacking; this can be explained by the objective difficulty that arises in measuring a temperature in areas not accessible to any measuring instrument. Wanting to avoid raising the current value, it could extend the power supply time of the inductor winding 15, but in this way it would decrease the efficiency of the process on an industrial scale in terms of both costs and time. The drawbacks highlighted for printed circuit multilayers have a broader value that could also involve other application examples of magnetic induction welding, because it is generally true that the surface of an object to be heated placed in contact with a massive graphite electrode heats up quite a lot. less rapidly than the rest of the body. And it is also true that measuring the temperature would present difficulties similar to those encountered for the multilayer. Aims of the invention

The primary purpose of the present invention is to indicate a welding head capable of obtaining a uniform inductive heating regime within the thickness of a body to be heated, in particular a multilayer to be welded, with particular regard to the surface placed in close contact with the induction electrode. Another purpose of the invention is to place an automatic limitation on the maximum value of the heating temperature directly measurable at the interface with the induction electrode.

Summary of the invention

The invention achieves its primary purpose by indicating a head for induction welding, in particular of multilayer printed circuit boards, comprising:

- a housing structure for a U-shaped inductor device provided with pole pieces and an inductor electric winding;

- two induction electrodes aligned along the same axis with the possibility of translating while remaining in contact with the terminal surface of a respective pole piece;

- means for translating the controlled induction electrodes to stop them in contact with the surface of the object to be heated by exerting pressure on it;

where the opposite ends of the induction electrodes comprise thermo-barrier means applied to the respective polar faces, each of the thermo-barrier means being constituted by:

- a sheet-like layer of electrically and thermally insulating material, having a face adhering to the polar face of the induction electrode;

- a metal laminate adhering to the other face of the sheet-like insulating layer, extended for a predetermined fraction of the surface of said polar face of the induction electrode, as described in claim 1.

The numerical value of said predetermined fraction is calculated by means of a criterion of compression between the value of the thermal power generated by the thermobarrier and the reduction of flux in the ferrite of the induction electrode, caused by the antagonistic effect of the flux generated by the currents induced in the laminate of copper. A maximum value of 0.5 was a good choice for a rectangular copper laminate, with the longest side equal to that of the polar face of the ferrite (and of the sheet-like layer). In this case the original inductive flux is not significantly diminished. The rectangular shape of the copper laminate is the simplest to obtain but not binding, other shapes could be used for the purpose, such as: square, circular, polygonal, etc.

Further characteristics of the present invention considered innovative are described in the dependent claims.

According to a further aspect of the invention, the induction welding head also comprises a thermocouple placed in contact with the surface of the metal laminate of at least one of the thermal barrier means, for the generation of an electrical signal for measuring the local temperature towards a device control of the current circulating in the inductor circuit.

According to a further aspect of the invention, the use of a lid for closing each thermo-barrier device is envisaged.

Advantageously, in thermo-barrier means the support plate of the metal laminate is in Teflon <®>, a material with low thermal conductivity and resistant to high temperatures without melting.

Advantageously, said Teflon plate <®> is glued to the polar face of its magnetic induction electrode by means of a double-sided adhesive support (tape or sheet) which is electrically insulating and has poor thermal conductivity, commonly available on the market.

Advantageously, the closing covers of the thermo-barrier devices are made of an electrically and thermally insulating material, also preferably in Teflon <®>

Advantageously, the thermocouple is of the ribbon or sheet type commonly available on the market.

Advantageously, the induction welding head is used in the manufacture of printed circuit multilayers, where the layers with circuit layout include buffer zones in fixed perimeter locations with inside them short-circuited metal coils constituting a heating element. In this elective case of use of the invention, the alignment axis of the two magnetic induction electrodes is perpendicular to the multilayer printed circuit. From the above, it is clear that having called the device made according to the invention a thermal barrier is appropriate, in fact, with particular reference to the use in the manufacture of multilayer printed circuits:

• in the copper laminate of the thermal barrier, intense short-circuit currents circulate induced by the variable magnetic field generated by the inductor, of the same intensity as the currents circulating in the coils within the areas of respect of the multilayer; these currents heat the thermo-barrier laminate which transfers heat towards the outermost layer of the multilayer, preserving it from cooling tendency;

• the Teflon <®> plate that supports the copper laminate of the thermo-barrier prevents the heat generated by the latter from dispersing within the ferrite, making heating more efficient;

The Teflon cover <®> also increases the efficiency of the thermo-barrier, as after a short thermal transient, an almost isothermal regime is created on the two faces of the cover, and no net heat transfer is therefore possible between the thermo-barrier and the multilayer (which is thus heated from the inside without dispersion of heat towards the outside).

Advantages of the invention

In applications where it is necessary to carry out localized welds in pre-established points of a printed circuit multilayer, the thermo-barrier built according to the present invention makes the transmission of the heat necessary for the melting of the resin uniform on the thickness of the layers to be coupled, consequently increasing the number of layers matchable.

Having avoided the greater cooling of the two outermost layers, allows to limit the maximum value of the current, or alternatively to reduce welding times. In both cases the yield of the production cycle is increased. It is important to note that the aforementioned advantages are obtained by means of a passive device that is easy to manufacture and of minimal cost.

The particular arrangement of the thermal barrier also makes it possible to carry out temperature control directly at the interface with the induction electrode. This is an undoubted advance that makes inductive heating more reliable and economical.

Brief description of the figures

Further objects and advantages of the present invention will become clear from the following detailed description of an example of its embodiment and from the annexed drawings given purely for explanatory and non-limiting purposes, in which:

- Figure 1 shows an axonometric view of an induction welding head belonging to a machine made according to the known art for welding the layers of a multilayer printed circuit, of which an exploded view is provided;

Figure 2 shows a perspective view of an inductive welding head made according to the present invention;

Figure 3 shows a section of the inductive welding head of Figure 2 taken along a plane of longitudinal symmetry; an enlargement of a detail is also provided which represents the cross section of a device for the generation of a temperature controlled thermal barrier, which constitutes the heart of the present invention;

Figure 4 shows a bottom view of the thermo-barrier device of Figure 3; - figure 5 shows a partial exploded axonometric view of the end part of an induction electrode of figure 3 complete with heat shield.

Detailed description of some preferred embodiments of the invention With reference to figures 2 and 3, a head for induction welding 21 can be seen, in particular for welding a multilayer printed circuit such as the one illustrated in figure 1. comprises a housing shell 22 for an inductor device consisting of a U-shaped magnetic ferrite circuit, including a central body 23 of pole pieces 23a and 23b and an inductor winding 24, equipped with a few coils of copper wire of adequate section. The terminal faces of the two arms of the U (polar expansions) are placed in contact with the lateral surface of two parallelepiped-shaped ferrite inductor electrodes 25 and 26, placed within their housings 27 and 28. The electrodes 25 and 26 are aligned along a same axis, with the possibility of translating in both directions. The sliding axis is parallel to the central arm 23 of the U-shaped inductor and perpendicular to the surface of a multilayer (not visible). The terminal faces of the pole pieces 23a and 23b are in contact with the lateral surface of the electrodes 25 and 26; the contact is sliding and remains even during their translation, thereby guaranteeing the continuity of the path of the magnetic field flux lines. The shells 22, 27, and 28 are made of Ertalon ™, a polyamide material similar to Nylon ™ that has an optimal combination of properties, such as: mechanical strength, stiffness, hardness, stress damping, sliding properties, mechanical machinability, strength wear, thermal inertia, and electrical insulation. The top ends of the induction electrodes 25 and 26 are constrained to respective sliding rods 29 and 30 of two pneumatic actuators 31 and 32, held in position by two systems of brackets 33 and 34 rigidly connected to the shells of Ertalon ™ 22, 27, and 28. The pneumatic actuators 31 and 32 allow the clamp closure of the induction electrodes 25 and 26 on the two sides of the multilayer with the exertion of a certain pressure on the external layers during welding.

The opposite faces of the induction electrodes 25 and 26 constitute polar ends which support their own Teflon ™ cover 35 and 36, connected by screws. Between each polar face of the ferrites 25 and 26 and the relative cover there is a device 37A, 37B which we have called “thermal barrier” or also “thermal barrier” due to the function it performs. Only the thermal barrier 37A, enlarged in the ellipse in the figure, comprises a thermocouple 41 from which a flexible strip 42 with two conductors directed towards a terminal board 43 screwed to the shell 27 of the upper electrode 25 starts. The thermocouple 41 is a ribbon, for example of the TT260J type, commonly available on the market. The length of the flexible strap 42 is such as to support the translation of the aforementioned electrode until the arm 29 is fully extended outside the actuator 31.

In the enlargement it can be seen that the thermo-barrier is constituted by a laminate formed by a rectangular sheet of Teflon ™ 38 and by a layer of copper 40 adhering to one face of the same. The thickness of the copper layer 40 is approximately 70 microns and covers approximately the outermost half of the plate 38 (the drawing does not reflect the actual dimensions). The latter has the same shape and size as the polar face of the induction electrode 25. A double-sided tape / sheet 39, electrically insulating and with poor thermal conductivity, is glued, on one side, to the Teflon plate 38 and, on the other, side, to the polar face of the induction electrode 25. The characteristics described up to now of the thermo-barrier 37A are common to those of the barrier 37B. The thermal barrier 37A alone mounts the thermocouple 41, as shown in figure 4 where the thermocouple 41 is seen in contact with the copper layer 40. From the thermocouple 41 two conductors 42a and 42b (of the strip 42) of different materials for the generation of an electric potential at the point of contact (Seebeck effect). The ferrite 25 on which the thermobarrier 37A is glued is tightened by the shell 27, which has four threaded holes for screwing the cover 35. Figure 5 shows an exploded view of the reciprocal relationships between the various parts around the thermobarrier 37A. As can be seen, the thermocouple 41 is clamped against the copper laminate 40 when the cover 35 is tightened, favoring its thermal contact.

the general principle on which the operation of the welding head is based does not differ from that of the machinery illustrated in figure 1, in fact in both cases it is necessary to generate a variable magnetic field of strong intensity within an air gap. In both cases, the main use is aimed at welding the layers of a printed circuit multilayer, exploiting the heating generated by the Joule effect within special short-circuit coils that link the inductive flux and are present on some layers. The fundamental difference is in the presence of thermal barriers on the polar faces of the induction electrodes and of a thermocouple which limits their heating. The benefits of having the two thermal barriers integrated into the welding head have already been abundantly illustrated in the introductory part, to which reference should be made. With the proposed solution, ferrites 25 and 26 are no longer in direct contact with the multilayer, cooling it, but rather through the thermobarriers 37A and 37B and the respective closing covers 35 and 36 which together prevent it from cooling while contributing to the uniformity of heating. Compared to the machine in figure 1, the inductor flux is slightly decreased due to the increased thickness of the air gap due to the presence of the two thermal barriers, and to the partially shielding effect due to the induced heating currents circulating in the two additional laminates, but considered at great intensity of the inductor flux and the reduced thickness of the thermal barriers, it can be safely said that the reduction is negligible, while the benefits are obvious.

For more details on the operation, the winding 24 (with 27 turns) works as an inductance fed at 500 V voltage with a square wave trend at the industrial frequency of 24 kHz. The current, being the integral of the voltage, is a triangular wave that generates a variable magnetic field in the

time whose flow lines, thanks to the magnetic permeability of the ferrite enormously higher than that of air, have a preferential course along the ferrites 23, 23a, 23b, 25, and 26, The variable magnetic field affects the copper laminates 40 of the thermobarriers 37A and 37B, where it induces a variable current equal to the conductance of the copper multiplied by a voltage proportional to the time derivative of the inductor magnetic field. The very high conductance of copper makes the thermal barriers similar to a short circuit, where the current generated is very strong, heating the copper in a short time.The function of the Teflon support 38 consists instead in creating a thermal insulation that avoids the dispersion of the heat towards the part facing the ferrites 25 and 26. The function of the thermocouple 41 is to generate information on the temperature reached within the thermal barrier towards a control system of the power supply of the induction coil 24 {both not visible in the figure),

The control system is configured for:

a) powering the inductor winding 24 and sampling the temperature values measured by the thermocouple 41, revealing that a preset temperature has been reached; b) maintain nutrition for a predetermined time;

c) interrupt the power supply at the expiration of the pre-established time;

d) restore the power supply when the measured temperature drops below the preset temperature;

e) repeat the previous cycle for a new welding operation.

In this way, burning phenomena within the thermo-barrier in the single multilayer welding operation are avoided.

Based on the description provided for a preferred example of embodiment of the invention, it is obvious that some changes can be introduced by the person skilled in the art without thereby departing from the scope of the invention as shown in the following claims

Claims (10)

CLAIMS 1. Induction welding head (21), in particular for soldering printed circuit multilayers (1), comprising: - a housing structure (22, 27, 28) for a U-shaped inductor device (23, 23a, 23b), provided with pole pieces (23a, 23b) and with an electric inductor winding (24); - two induction electrodes (25, 26) aligned along the same axis with the possibility of translating while remaining in contact with the terminal surface of a respective pole piece (23a, 23b); - translation means (31, 29; 32, 30) of the controlled induction electrodes to stop them in contact with the surface of the object to be heated (1) by exerting pressure on it, characterized in that the opposite ends of the induction electrodes (25, 26) comprise thermo-barrier means (37A, 37B) applied to the respective polar faces, each of the thermo-barrier means being constituted by; - a sheet-like layer (38) of electrically and thermally insulating material, having a face adhering to the pruned face of the induction electrode (25, 26); - a metal laminate (40) adhering to the other face of the sheet-like insulating layer (38), extended for a predetermined fraction of the surface of said polar face of the induction electrode (25, 26). 2. The welding head according to claim 1, characterized in that the numerical value of said predetermined fraction is calculated with a criterion of compression between the value of the thermal power generated by said thermo-barrier means (37A, 37B) and the reduction of flux of the magnetic field within the induction electrode (25, 26) caused by the antagonistic effect of the flux generated by the currents induced in the said metal laminate (40) 3. The welding head according to claim 2, characterized in that said numerical compromise value is limited above 0.5 for a copper laminate of rectangular shape with the longest side equal to that of said polar face. 4. The welding head according to any one of claims 1 to 3, characterized by! the fact that a double-sided layer (39) in the form of an electrically insulating tape or sheet with poor thermal conductivity is interposed between the said sheet-like insulating layer (38) and the said polar face of the induction electrode (25, 26). The welding head according to any one of claims 1 to 4, characterized in that each of the thermo-barrier means (37A, 37B) includes a containment lid (35, 36) made of thermally and electrically insulating material, anchored to the housing structure (27, 28) of a respective induction electrode (25, 26). 6. The welding head according to claim 5, characterized in that at least one of the heat shield means (37A) includes a thermocouple (41) placed in contact with said metal laminate (40) generating an electrical signal for measuring the local temperature towards a device for controlling the current flowing in the inductor electric drive (24). 7. The welding head according to any one of claims 1 to 6, characterized in that said sheet-like insulating layer (38) is made of Teflon ™. 8. The welding head according to any one of claims 5 to 7, characterized in that said containment covers (35, 36) are made of Teflon ™. The welding head according to any one of claims 1 to 8, characterized in that it includes power supply means for said inductor winding (24) capable of generating a square wave voltage at industrial frequency. 10. The welding head according to claim S, characterized in that it includes means for controlling said feeding means, configured for: a) powering the inductor winding (24) and sampling the temperature values measured by the thermocouple (41) revealing that a preset temperature has been reached, b) maintaining the power supply for a predetermined time; c) interrupt the power supply at the expiration of the pre-established time; d) restore the power supply when the measured temperature drops below the pre-set temperature; e) repeat the previous cycle ac a new welding operation