Field of application of the invention

The present invention refers to the field of apparatuses for obtaining multilayer sheets, from whose processing printed circuits can be obtained.

The multilayer sheets of the aforesaid type comprise one or more layers made of dielectric material superimposed on each other and covered on one or both faces by a layer of laminated copper. In order to obtain a printed circuit from a multilayer sheet, it is necessary to chemically remove the "excess" copper so as to transform the copper laminas into respective circuit connection networks. Specifically, a printed circuit is defined "single-layer" if it is formed by a multilayer sheet comprising only one dielectric layer covered on one or both faces by a layer of laminated copper. A printed circuit is instead defined "multilayer" if it is formed by a multilayer sheet comprising a multiplicity of dielectric layers cov- ered on one or both faces by a layer of laminated copper, and superimposed on each other with the interposition of adhesive layers. In the latter case, the stability of the alignment between the layers that make up the sheet can be obtained by mutually welding the different layers at circumscribed areas thereof belonging to a perimeter band without circuit network where several circular metalliza- tion elements are present; such metallization elements are shaped as spirals aligned between the various layers and are sites of strong induced short circuit currents capable of causing local welding. Hereinbelow in the present description, with the expression "multilayer sheet" it is intended to identify a sheet of the aforesaid type, from whose processing any one printed circuit is obtainable.

More precisely, the present invention regards an apparatus for obtaining, by means of hot pressing, multilayer sheets for printed circuits provided with suitable means for cooling, according to a pre-established gradualness, the multilayer sheets at the end of the pressing process. The present invention also regards a method for obtaining multilayer sheets for printed circuits actuatable by employing the apparatus, object of the invention.

Review of the prior art

An apparatus and a method for obtaining multilayer sheets for printed circuits are described in the patent application PCT/IT1992/000101 , owned by the same Applicant, which is given as completely known herein. The method claimed in the aforesaid patent application substantially consists in the formation of a stack of "packs", each comprising a group of support sheets impregnated with plastic material and metal laminas on one or both faces, obtained by a metal band wound as a spiral around said packs. The metal band has two ends at which it is connected to a generator of electric current with suitable power such that by generating an opportune pressure on the stack of packs and with the electric circuit of the generator closed, the various sections of the metal band opposite the faces of the packs behave as heating elements, obtaining via Joule effect the close connection between the various elements and the formation of the multilayer sheets. Specifically, the metal band is made of copper and each pack comprises sheets of glass fiber impregnated with epoxy resin. Also present are separation sheets interposed between the various packs, preferably made of stainless steel.

The use of a copper band wound as a coil, in which heating current is made to circulate, allows obtaining a more uniform distribution of heat inside the stack of packs with respect to that allowed by the preceding multi-level presses, in which only the packs at the two ends of the stack were in contact with the heating levels. While having undeniable advantages, also the process that uses the copper band for heating the packs does not lack drawbacks, in particular the packs at the ends of the stack are heated less than the packs at the center of the stack, and consequently they must often be discarded. The lesser heating of the peripheral packs is in part due to the transfer of heat from the copper band to the metal plates containing the stack, and in part to the fact that such end packs benefit to a lesser extent from the thermal mass constituted around the packs closest to the center of the stack.

In order to overcome the aforesaid drawbacks, the Applicant has conceived a new apparatus for obtaining multilayer sheets for printed circuits, which became the object of the patent application PCT/IT2012/00066, also given herein as to- tally known. In particular, a method (claimed in the aforesaid patent application) for obtaining multilayer sheets for printed circuits, actuatable by means of said new apparatus is characterized in that it provides, at the two ends of the stack, an additional heat supply by using two auxiliary heating elements constrained to the two levels of the press and thermally insulated therefrom, controlling the temperature of the auxiliary heaters in a manner so as to make the temperature increase according to a pre-established gradualness, so as to obtain, in the packs placed at the top and bottom of the stack, the same temperature measured in the stack.

The two additional heaters, respectively upper and lower, due to the thermal produced and to their thermal insulation towards the cold levels of the press, create a barrier against the dispersion of the heat generated in the stack. The packs closest to the two ends of the stack benefit most from the additional heat, and this compensates the lesser heating that they would have due to their position. It follows that there is a greater constancy of the temperature along the en- tire stack, such that the possibility of having to discard packs is definitively prevented.

During the pressing operation, the stack of packs reach a temperature preferably comprised between 180 °C and 200 °C. Nevertheless, before being able to carry out the subsequent disassembly of the obtained multilayer sheets, it is necessary to wait until the temperature of the stack of multilayer sheets has cooled, reaching a temperature suitable for allowing said disassembly, with clear slowing of the process for producing the multilayer sheets for printed cir- cuits.

Summary of the invention

Object of the present invention is to overcome the aforesaid drawbacks and indicate an apparatus by means of which it is possible to obtain multilayer sheets for printed circuits starting from a stack of packs, each of which comprising at least one dielectric layer covered on one or both faces by a layer of laminated metal, superimposed on each other with the interposition of adhesive layers, the two outermost layers of each pack being constituted by respective portions of at least one metal band repeatedly folded in opposite directions in order to form loops of a coil which systematically traverses the stack,

said apparatus comprising:

• suitable means for exerting a pressure on the stack of said packs;

• suitable means for generating an electric current, electrically connected to said metal band;

· means for controlling said generator means, programmable for varying the intensity of said current so as to regulate the temperature of the stack for the purpose of obtaining a mutual connection between the layers of each pack, wherein, according to the invention, the apparatus also comprises means for cooling said stack.

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

According to one aspect of the invention, the apparatus comprises a work space within which the stack is housable, the cooling means comprising first pneumophorous means suitable for inserting air in the work space that comes from an environment outside the space.

Preferably, the work space can be hermetically closed. In such case, the apparatus, object of the invention, also comprises suitable means for creating the vacuum in the work space, and the pressure means are suitable for generating said pressure on the stack when the latter is housed in the work space. In such a manner, the "conversion" of the multilayer packs into multilayer sheets for printed circuits can advantageously occur in vacuum conditions.

According to another aspect of the invention, the first pneumophorous means can be directed in a manner such that a first air flow can be at least partially blown on the stack (i.e. it at least partially hits the stack) when the latter is housed in the work space.

By "directing a pneumophorous means" it is intended to arrange the pneumo- phorous means in a manner so as to direct the air flow generated by the same. Advantageously, the cooling of the stack of multilayer sheets at least partially occurs via forced convection.

According to another aspect of the invention, the cooling means comprise second pneumophorous means suitable for drawing (i.e., sucking) air from the work space in order to insert it in said environment outside the space.

The air inserted in the work space by the first pneumophorous means and heated by the stack via forced convection is advantageously drawn from the space by the second pneumophorous means. This further accelerates the process of cooling the stack.

According to another aspect of the invention, the second pneumophorous means can be directed in a manner such that a second air flow can be at least partially sucked in proximity of the stack when the latter is housed in the work space.

According to another aspect of the invention, the cooling means comprise third pneumophorous means suitable for drawing air from the work space and moving said air back into the space. Preferably, the third pneumophorous means are housed within the work space.

The third pneumophorous means determine a recirculation of air inside the work space. This increases the phenomenon of the forced convection, further accel- erating the process of cooling the stack.

According to another aspect of the invention, the third pneumophorous means can be directed in a manner such that a third air flow can be at least partially sucked in proximity of the stack and/or can be at least partially blown on the stack when the latter is housed in the work space.

According to another aspect of the invention, the control means comprise means for controlling the cooling means, the control means being programmable for varying the flow rate of said air flows so as to regulate the decrease of the temperature of the stack.

Another object of the invention is a method for obtaining multilayer sheets for printed circuits by using the apparatus according to the invention. Said method includes the steps of:

a) obtaining a stack of packs, each of which comprising at least one dielectric layer covered on one or both faces by a layer of laminated metal, superimposed on each other with the interposition of adhesive layers, the two outermost layers of each pack being constituted by respective portions of at least one metal band repeatedly folded in opposite directions in order to form loops of a coil which systematically traverses the stack;

b) applying a pressure to the stack and simultaneously generating heat in the stack, in a controlled manner, so as to regulate the temperature of the stack for the purpose of obtaining a mutual connection between the layers of each pack;

c) returning to initial pressure and temperature conditions;

d) cutting the metal band flush with the opposite sides of the stack from where it exits and disassembling the multilayer sheets,

wherein, according to the invention, during step c), means are driven for forcibly cooling the stack.

According to one aspect of the method invention, during step c), the stack is cooled by means of forced ventilation.

According to another aspect of the method invention, during step c), the cooling means are driven in a controlled manner so as to make the temperature of the stack decrease according to a pre-established gradualness.

Brief description of the figures

Further objects and advantages of the present invention will be clearer from the following detailed description of an embodiment thereof and from the enclosed drawings, given as a mere non-limiting example, wherein:

- figure 1 shows a schematization of an apparatus for obtaining multilayer sheets for printed circuits, according to the present invention;

- figure 2 shows, in perspective view, the apparatus schematized in figure 1 ;

- figure 3 shows, in schematic right section, the apparatus of figure 2. Detailed description of several preferred embodiments of the invention

In the following description, a figure may also be illustrated with reference to elements not expressly indicated in that figure but in other figures. The scale and proportions of the various depicted elements do not necessarily correspond with actual scale and proportions.

Figure 1 schematically shows an apparatus 1 by means of which it is possible to obtain multilayer sheets for printed circuits starting from a plurality of multilayer packs 2 superimposed on each other to form a stack 3 placed between the lower level 4, preferably fixed, and the upper level 5, preferably movable, of a movement and compression press 6 (visible in figure 2) that is preferably of pneumatic type.

Each pack 2, proceeding from the bottom towards the top in the figure, comprises by way of example:

• a lower layer made of laminated metal, belonging to a metal band 7, prefera- bly made of copper;

• a lower adhesive layer 8, preferably made of prepreg;

• a stratification 9 comprising dielectric layers, preferably made of FR-4, variously metalized on at least one face, alternated with adhesive layers, preferably made of prepreg;

· an upper adhesive layer 10, preferably made of prepreg;

• an upper layer made of laminated metal, also belonging to the copper band 7.

The prepreg layers (8, 10 and within the stratification 9) are preferably constituted by a fabric made of glass fiber, pre-impregnated with epoxy resin that is only slightly polymerized. Said layers become adhesive following polymerization and subsequent cross-linking of the resin due to a suitable heating thereof, as is extensively described in the abovementioned patent applications PCT/IT1992/000101 and PCT/IT2012/00066.

By "dielectric layers variously metalized on at least one face" it is intended, by way of example, one or more layers made of dielectric material superimposed on each other and covered on one or both faces by a layer in laminated metal, preferably copper. Between one pack 2 and the next, a separation sheet 11 is inserted, preferably made of suitably anodized aluminum for obtaining a surface layer of thermal- radiating and electrically-insulating black oxide.

The band 7 originates from a continuous sheet wound in a roll, systematically and progressively folded as a coil, for the entire height of the stack 3, around the layers 8 and 10, to complete the packs 2, and around the separation sheets 11 adjacent thereto. As will be better illustrated hereinbelow, when an electric current is made to circulate in the band 7, the current heats the band 7 due to the Joule effect. In such a manner, it is possible to heat the stack 3 from the in- side. By way of example, in the formation of the coil 7, the loops open on the right in figure 1 belong to the packs 2 while the loops adjacent thereto open on the left in figure 1 include the sheets 11. In the stack 3, the upper and the lower loop include an identical structure comprising the following in order: a sheet 11 in contact with the band 7, a buffer layer 13 preferably constituted by a fabric made of aramid fibers and silicone binder, and two sheets of kraft paper 12. The function of the sheets 12 and the buffer 13 is that of balancing the pressure exerted by the levels 4 and 5 of the press 6 on the surfaces at the two ends of the stack 3. The band section 7 placed at the base of the stack 3 lies on a stack- carrier tray 14, preferably constituted by a non-anodized aluminum plate that can be moved forward and backward on opposite sides of the press 6. During the pressing cycle, the tray 14 rests on a heating plate 15, preferably made of non-anodized aluminum, superimposed on a dielectric sheet 16 and rigidly connected thereto. The latter lies on the lower level 4 of the press 6, where it is stably fixed. The set of the heating plate 15 and the dielectric sheet 16 forms a de- vice 17 also functioning as a "thermal barrier" against the downward escape of endothermic heat generated in the stack 3, thus preventing a lowering of temperature of the packs 2 placed in the lowest part of the stack 3 with respect to the central packs 2. By way of example, the sheet 16 is made of epoxy glass G11 and has a thickness preferably of 60 mm. Such term refers to a stratified composite material with glass fiber base arranged orthogonally within a cross- linked epoxy resin.

The heating plate 15 can be obtained in different ways. A preferred mode con- sists of making in the plate 15, e.g. 25 mm thick, a certain number of longitudinal holes that are preferably equidistant from each other over the entire width, and introducing therein respective armored resistors 18, preferably of "candle" type, connected in parallel between the terminals of a 230 V AC power supply. Such resistors 18 are constituted by a cylindrical casing, preferably made of steel, within which a resistor is encapsulated that is preferably made of ferrous- nickel or nickel-chrome. The filler material, preferably silicone or cement, is electrically insulating and is a thermal conductor. By way of example, eight preferably 2 kW resistors are equipped with cylindrical casing of the diameter, preferably 15 mm. The number of resistors and the individual absorption depends on the size and number of packs 2 in the stack 3.

A second mode consists of obtaining, in the plate 15, a rectangular slot that is as wide as the plate 15 and inserting therein a thick sheet of silicone that incorporates a resistive coil.

A third mode consists of making, in the plate 15, a cavity that is preferably rectangular, nearly as wide as the plate 15, and inserting therein the armored resistor described in the patent application No. PCT/IT2006/000121 , owned by the same Applicant. Such resistor is constituted by a resistive coil preferably made of steel bar, insulated with preferably mica sheet from the preferably anodized aluminum casing and, by way of example, with parallelepiped shape.

In the upper part of the stack 2, a sheet 19, preferably made of aluminum without anodization, rests on the band 7. The fact that the tray 14 and the plate 19 are preferably made of aluminum does not preclude the possibility of using other metals or alloys. The upper level 5 of the press 6 has, at its interior, a cavity 20 open on the lower part in order to allow the inclusion of a base 21 that is free to vertically translate without leaving the cavity 20. The base 21 delimits, in the cavity 20, a chamber with rigid walls and a variable volume at whose interior an "airbag" 22 is housed connected with a compressor 23 by means of a flexible tube 24. The airbag 22 is an air chamber which, when deflated, has a nearly rectangular flattened shape and which, being expanded, is capable of completely filling the cavity 20 to its maximum downward extension. The lower face of the movable base 21 is fixed to a dielectric sheet 25, in turn fixed to a heating plate 26 at whose interior armored resistors 27 are present, preferably of candle type. The sheet 25 and the plate 26 are both belonging to a device 28 constrained to the upper level 5 of the press 6, entirely identical to the lower device 17 and as such also acting as "thermal barrier" against the escape, this time upward, of the endothermic heat generated in the stack 3, preventing a temperature lowering therewith of the packs 2 placed in the highest part of the stack 3 with respect to the central packs 2.

The electrical power for heating the band 7 and the resistors 18 and 27 is preferably supplied by the 400 Volt three-phase electric power grid. The three- phase conductors of the grid 29 and the neutral N (grounded) are suitably selected by a disconnecting switch 30 with four contacts. Most of the power is absorbed by the band 7 through a main direct current electric generator 31 acting as a system for the modulation of the alternating power and for the transformation into direct current. By way of example, the generator 31 corresponds with the main electric generator described in detail in the abovementioned patent application PCT/IT2012/000066. The generator 31 has a positive terminal and a negative terminal, respectively connected to the heating plates 15 and 26 by means of two strong conductors 32 and 33. The conductor 32 terminates with a flexible section in order to support the translation of the upper level 5 of the press 6. The current that exits from the positive terminal flows into the upper hot plate 26, crosses through the upper sheet 19, flows from one end of the band 7 to the other, heating the stack 3 from the interior, crosses through the tray 14, flows into the lower heating plate 15, and finally reaches the negative terminal. The heating of the group of resistors in parallel 18 which heat the lower plate 15 is provided by a secondary alternating current electric generator 34 connected to one phase of the electric grid 29. Similarly, the heating of the group of resistors in parallel 27 which heat the upper plate 26 is provided by a secondary alternating current electric generator 35, also connected to one phase of the electric grid 29.

The modulations of the electric power supplied to the band 7 and to the groups of resistors 18 and 27 are driven by a control in temperature which makes use of at least three thermal probes, including a first probe 36 preferably situated at the center of the stack 3, a second probe 37 preferably situated in a niche obtained in the heating sheet 26, and a third probe 38 preferably situated in a niche obtained in the heating plate 15. The thermal probes 36, 37 and 38 are connected to a programmable logic controller, or PLC, 39 in turn connected, by means of a bus 40, to a Personal Computer, or PC, 41 which executes the actuation program of the hot pressing cycle of the stack 3.

The apparatus 1 also comprises a vacuum pump 42 and a ventilation apparatus

43, both connected to the PLC 39. The pump 42 serves for generating the desired level of vacuum in a hermetic chamber 44 (visible in figure 2 and also indi- cated with the expressions "space" and "intermediate compartment" in the present description) which contains the stack 3 during the hot pressing cycle. The ventilation apparatus 43 starts operating upon conclusion of the pressing cycle in order to more quickly cool the stack 3 by means of the conveyance and circulation of ambient air in the chamber 4, preferably close to the stack 3. As will be better described hereinbelow, the ventilation apparatus 43 preferably comprises a first group of fans 45 (shown in figures 2 and 3) for the insertion of ambient air in the chamber 44, a second group of fans 46 (shown in figures 2 and 3) for sucking air from the chamber 44 and emitting the same into the environment outside apparatus 1 , and a third group of fans 47 (shown in figure 3) for facilitat- ing the recirculation of air inside the chamber 44.

In operation, the stack 3, at the end of an initial preparatory step, rests on the tray 14 positioned below the upper level 5 of the press 6, inside the chamber

44. The level 5 is lowered, by means of preferably pneumatic actuators, not visible in the figures, until the plate 26, with airbag 22 deflated, comes into contact with the aluminum sheet 19 placed at the top of the stack 3. At this point, the PC 41 generates, by means of the PLC 39, a signal D which activates the compressor 23 for a time sufficient to inflate the airbag 22 to a pre-established pressure P on the stack 3. The PLC 39, before starting a work cycle, generates a signal E that activates the vacuum pump 42 for a time necessary to reach a level of vacuum sufficient for preventing the formation of bubbles inside the multilayer during the hot pressing. After which, the PC 41 , by means of the PLC 39, generates three signals A, B, C, respectively directed to the generators 31 , 34 and 35 in order to initiate the heating cycle for the band 7 and the resistors 18 and 27. After a certain delay due to the thermal inertia of the heating elements, the thermal probes 36, 37 and 38 generate respective analog signals Ta, Tb, Tc proportional to the measured temperatures; these signals are directed to the PLC 39 which digitizes them with suitable frequency and transfers the number values on the bus 40. The PC 41 executes a continuous monitoring of the bus 40 in order to acquire the temperature values Ta, Tb, Tc necessary for generating new values of the signals A, B, C which control via feedback the respective generators 31 , 34 and 35 according to the logic of the stored program. In sum- mary, such program is set to ensure that all the packs 2 of the stack 3 share a same growth profile of the temperature starting from ambient temperature up to complete cross-linking of the resin. Such object can be reached by knowing beforehand the time progression of an ideal growth curve of the temperature in the presence of pressure that is best suited for the type of stack 3 of packs 2 to be pressed.

A similar ideal curve can be obtained on the basis of the known chemical- physical phenomena which affect the epoxy resin present in the prepreg layers. Such phenomena comprise a reaction of polymerization (gelling) of the resin components in the presence of a catalyst and the cross-linking of the resinous gel until a vitreous state is reached. Said ideal curve rises linearly over time with a gradient that depends on the maximum electric power transferable to the band 7 in the unit of time, and is limited by the effects of the temperature on the chemical-physical reactions that occur in the epoxy resin. Preferably, once the value of about 180 °C has been reached, it is convenient to make the tempera- ture increase very slowly, by a few degrees, in order to complete the cross- linking.

At the end of the pressing cycle, the PC 41 , by means of the PLC 39, turns off the generators 31 , 34 and 35 in order to terminate the heating of the band 7 and the resistors 18 and 27. Subsequently, the PC 41 , still by means of the PLC 39, commands the lifting of the level 5 of the plate 6, deactivates the vacuum pump 42 and sends an activation signal F to the ventilation apparatus 43 in order to accelerate the cooling of the stack 3. Analogous to that which occurs during the heating of the stack 3, the thermal probes 36, 37 and 38 generate respective analog signals Ta, Tb, Tc proportional to the measured temperatures; these signals are directed to the PLC 39 which digitizes them with suitable frequency and transfers the number values on the bus 40. The PC 41 executes a continu- ous monitoring of the bus 40 in order to acquire the temperature values Ta, Tb, Tc necessary for generating new values of the signal F which controls via feedback the ventilation apparatus 43 according to the logic of the stored program. In summary, such program is set to ensure that the temperature of the stack 3 forcibly decreases, starting from the temperature possessed at the end of the pressing cycle, according to a pre-established gradualness.

The width of the band 7 is selected as a function of the requirements of the printed circuit designers and is preferably comprised between 300 mm and 1400 mm. The thicknesses of the band 7 are by now standardized and the most common are the following: ¼ ounce (9 micron); ½ ounce (12 micron); ½ ounce (18 micron); 1 ounce (35 micron); 2 ounces (70 micron); 3 ounces (105 micron). The length of the band 7 depends on the number of packs to be pressed and is preferably comprised between 10 meters and 200 meters. Upon passage of the current in the band 7, the supplied thermal energy is equal on each level of the stack 3.

With reference to figure 2, it is possible to observe that the press 6 comprises a base 50 for anchoring a strong support frame for the level 4 supporting the material to be pressed. The frame divides the press into three compartments 51 , 44 and 52 superimposed on each other. The lower compartment 51 is delimited on the upper part by the support level 4, on which four columns rest in nearly angular positions, such columns sustaining the floor of the upper compartment 52. The intermediate compartment 44 corresponds with the hermetic chamber 44 and is delimited on the side by walls 53 and 54 with pneumatic seal, of which two opposite walls 53 are fixed and include the windows 55 in order to allow an operator of the apparatus 1 to see inside the chamber 44 during processing, while the other two walls 54 are removable like doors in order to allow the insertion and extraction of the tray 14 with the stack 3 of packs 2.

Housed in the lower compartment 51 are the tension stabilizers and the appara- tuses for supporting the pneumatic functioning of the actuators, along with the vacuum pump 42 which creates the vacuum in the intermediate compartment 44. Such support apparatuses and the vacuum pump 42 could be situated outside the press 6. The aforesaid support apparatuses include the compressor 23 and relative distribution valves for carrying out the following functions: the driving of the pneumatic cylinders for the translation in the two senses of the upper level 5 of the press 6; the inflation of the airbag 22 for exerting a pressure on the stack 3; the actuation of the actuators for lifting the tray 14 at the time of the introduction of the stack 3 between the levels 4 and 5 of the press 6. Preferably, also housed in the lower compartment 51 is the electronic control system constituted by the PLC 32 and by the motherboard of the PC 41. The latter comprises a non-volatile memory in which the program is written that governs the operation of the press 6. A suitable interface connector connects the motherboard with an external station of the PC 41 , which is available for the operator to insert, in the system, the main operation parameters of the work cycle and update the contents of the program memory.

Housed in the intermediate compartment 44 are the tray 14 with the stack 3, the auxiliary heaters 17 and 28 with the relative thermal probes 38 and 37, the thermal probe 36 preferably at the center of the stack 3, the upper level 5 of the press 6 with the relative double-acting pneumatic cylinders for the movement thereof, and the airbag 22 with relative movable containment wall 21. As will be better illustrated with reference to figure 3, at the walls 54 of the intermediate compartment 44, it is possible to observe several fans 45 and 46 (schematically shown), respectively belonging to the first and to the second group of fans of the ventilation apparatus 43.

Housed in the upper compartment 52 are the magnetic switches for overload protection and the generators 31 , 34 and 35 with the relative control electronics. Two strong metal cylindrical bars, preferably made of copper, run side-by-side from the lower compartment 51 to the upper compartment 52 in order to power supply the generators 31 , 34 and 35. An insulating casing 56 encloses these bars for protection purposes.

Figure 3 schematically shows the ventilation apparatus 43 of the apparatus 1 , object of the invention. By way of example, the apparatus 43 comprises:

• four fans 45 belonging to the aforesaid first group (for the introduction of ambient air into the compartment 44);

• two fans 46 belonging to the aforesaid second group (for the suction of air from the compartment 44 and the emission thereof into the environment outside the apparatus 1);

• four fans 47 belonging to the aforesaid third group (for the recirculation of air to the interior of the compartment 44).

The fans 45 are preferably installed at the movable walls 54 of the compartment 44. Still more preferably, two fans 45 are installed at each of the walls 54, respectively in proximity of the lateral edges thereof (vertically arranged in figure 2). The fans 45 are suitable for introducing, in the compartment 44, an air flow sucked from an environment outside the apparatus 1 and preferably directable in a manner so as to at least partially hit the stack 3 of packs 2.

The fans 46 are preferably installed at the movable walls 54 of the compartment 44. Still more preferably, each of the fans 46 is installed at one of the walls 54, in an intermediate position between the fans 45. The fans 46 are suitable for introducing, into the environment outside the apparatus 1 , an air flow sucked from the compartment 44 preferably in proximity of the stack 3 of packs 2.

The fans 47 are preferably installed inside the compartment 44 and, preferably, are integrally connected to the lower level 4 of the press 6. In particular, the fans 47 preferably lie in the space comprised between the stack 3 and the fixed walls 53 of the compartment 44. Still more preferably, two fans 47 are opposite each of the walls 53. In such a manner, the fans 47 are not obstacles during the operations of insertion and extraction of the stack 3 in the compartment 44 (said operations occurring through the movable walls 54). The fans 47 are suitable for sucking an air flow from the compartment 44 and moving said air flow back into the compartment 44. Preferably the fans 47 are suitable for moving the air flow back into the compartment 44 in a manner so as to at least partially hit the stack 3 of packs 2. In an alternative embodiment not shown in the figures, the fans 47 are suitable for sucking an air flow preferably in proximity of the stack 3 and for moving said air flow back into the compartment 44. In another alternative em- bodiment not shown in the figures, the fans 47 are suitable for sucking an air flow preferably in proximity of the stack 3 and for moving said air flow back into the compartment 44, preferably in a manner so as to at least partially hit the stack 3.

The fans 47, when in operation, accelerate the cooling of the stack 3 since they force the circulation of the air in the compartment 44 at the walls of the stack 3 not opposite the movable walls 45 and 46 (i.e. not opposite the fans 45 and 46). By way of example: overall, the fans 45 are capable of introducing into the compartment 44 up to 600 m ^A 3 of air per hour; overall the fans 46 are capable of drawing from the compartment 44 up to 800 m ^A 3 of air per hour; the fans 47 have overall a maximum flow rate of 800 m ^A 3 of air per hour.

Each fan 45, 46 and 47 is programmable in an autonomous manner with respect to the other fans, as a function of the desired cooling profile (i.e. temperature decrease) of the stack 3. Preferably, the fans belonging to the same group have, at the same instant, the same flow rate.

On the basis of the description provided for a preferred embodiment, it is clear that some changes can be introduced by the man skilled in the art without departing from the scope of the invention as defined by the following claims.