Patent Publication Number: US-2021170708-A1

Title: Pressing group for a sintering press for sintering electronic components on a substrate

Description:
The present invention relates to sintering presses of electronic components on a substrate, and in particular it relates to a pressing group for such presses. 
     As is known, in some electronic applications, integrated electronic components, such as diodes, IGBTs, thermistors, MOSFETs, are fixed to a substrate by the interposition of a layer of sintering glue. 
     In order for each component to be sintered correctly, it must be pressed with a force proportional to its projection surface and subjected to a predetermined temperature for a predetermined period of time. Since the electronic components to be fixed on a substrate can have dimensions, that is, the projection surfaces of the relative casings and thicknesses, even considerably different from each other, the application of a pressure on a single pressing member acting on all the components of a substrate does not allow the desired strength to be imparted on all components. 
     Furthermore, it is necessary to consider the further complications due to the fact that the glue layer may have a thickness that is not perfectly identical and homogeneous for all the components. 
     In a patent application no. PCT/IB2017/058520, on behalf of the same Applicant and currently still secret, a sintering press has been proposed for sintering electronic components on a support, able to overcome the drawbacks and limitations of the devices according to the prior art. Such a press is provided, in particular, with a pressing group comprising a multi-stem cylinder having a front head and a rear head which jointly delimit a compression chamber. 
     In the front head, pressing stems parallel and independent of each other are slidingly supported. Each pressing stem is coaxial to a respective electronic component to be sintered and has a thrust section proportional to the force to be applied to the respective electronic component, the area of each electronic component to be sintered and for a predetermined sintering pressure being known. 
     In the compression chamber, a sealing membrane extends which hermetically divides the compression chamber into a rear chamber and a front chamber. The rear chamber is in fluidic communication with an inlet passage of a fluid under pressure to achieve the sintering pressure. The rear ends of the pressing stems protrude in the front chamber. The sealing membrane is placed in contact with the rear ends of the pressing stems so that when the rear chamber is pressurized to the sintering pressure, the sealing membrane is deformed by abutting against the rear ends for a transfer of the sintering pressure on each pressing stem. 
     Such a press therefore allows exerting a thrust force on each electronic component to be sintered proportional to the surface thereof, due to the use of a pressing stem for each component, the section of said pressing stem being chosen according to the surface to be pressed. 
     The sealing membrane, which is in practice formed by a flat gasket, performs both a sealing function for all the pressing stems and a piston function which acts on all the pressing stems, deforming itself so as to adapt to their dimensions. 
     The sealing membrane is held peripherally between the front and rear heads of the multi-stem cylinder. Therefore, the flat gasket deforms when subjected to the action of the fluid under pressure, but does not translate axially with respect to the compression chamber. 
     However, it has been found that, over time, due to the high pressure to which it is subjected in the non-retained central zone between the two heads of the multi-stem cylinder, which can even reach hundreds of bars, the flat gasket tends to lose its original elasticity or even to become ruined and tear. After a certain number of pressing cycles, the flat gasket must therefore be replaced. 
     The object of the present invention is to propose a pressing group of the type described above, which is able, however, to overcome the drawbacks described above in relation to the flat gasket. 
     Said object is achieved by a pressing group according to claim  1  and by a sintering method according to claim  12 . 
    
    
     
       The features and the advantages of the pressing group according to the invention shall be made readily apparent from the following description of preferred embodiments thereof, provided purely by way of a non-limiting example, with reference to the accompanying figures, in which: 
         FIG. 1  is a schematic plan view of a sintering island which uses a sintering press; 
         FIG. 2  is an elevation view of the press; 
         FIG. 3  is an axial section view of the pressing group of the press closed on a substrate support unit; 
         FIG. 4  is an axial section view of only the substrate support unit of the press; 
         FIG. 5  is a top perspective view of the front head of the multi-stem cylinder of the pressing group, without the flat actuating gasket and the relative frame for anchoring it to the front head; 
         FIG. 5 a    is a view similar to the preceding one, with the actuating flat gasket positioned on the front head; 
         FIG. 5 b    is a perspective view similar to the previous one, with the anchoring frame which blocks the actuating flat gasket to the head; 
         FIG. 5 c    is a perspective view of the assembled multi-stem cylinder, with the rear head shown in transparency; 
         FIG. 6  is an axial section of only the pressing group of the press; 
         FIG. 7  is an enlarged schematic view of part of the pressing group and of the substrate support unit, showing in particular the non-pressurized compression chamber and some pressing stems and heating pressing members in contact with respective electronic components of a substrate; 
         FIG. 7 a    is a view similar to the preceding one, but with the compression chamber pressurized; and 
         FIG. 8  is schematic a perspective view of some pressing stems and heating pressing members in contact with electronic components of a substrate. 
     
    
    
     With reference to the accompanying drawings, a sintering island will be described hereinafter using a press  1  having a pressing group according to the invention to perform the sintering of electronic components  10  on a substrate  12 . 
     The substrate  12  arrives at the sintering press  1  contained in a pallet  2 , for example adapted to contain six substrates  12 . 
     The substrates  12  are positioned in respective seats formed in the pallet  2  and adapted to support the substrates  12  on suitable projections. 
     The substrates  12  carry the electronic components  10  to be sintered (for example, IGBTs, diodes, thermistors, MOSFETs) positioned on a layer of sintering glue. The components  10  must be processed with a predefined surface pressure, for example of 30 MPa, at a predefined temperature, for example of 260° C., for 180÷300 seconds. 
     The pallets  2  with the non-sintered substrates  12  must be handled smoothly without shocks at moderate speeds, in order not to modify the positions of the electronic components  10 . 
     The electronic components  10  must be pressed with a force directly proportional to their projection surface, taking into account that the components have a thickness diversified by family. 
     Furthermore, any non-parallelism of the substrate-glue-electronic component assembly, for example of 1 μm over 10 mm, must be compensated. 
     As illustrated in  FIG. 1 , in one embodiment, the sintering island comprises at least one, preferably two preheating stations  3  of the pallets  2  to 150° C., and at least one, preferably three cooling stations  4  of the pallets  2  to 50° C. 
     In one embodiment, the sintering island comprises an anthropomorphic robot  5 , for example of the 6-axis type, of 10 kg, provided with a six-jaw self-centering gripper. 
     The robot  5  manipulates the pallets  2  between:
         an inlet conveyor  6  and reader of the two pre-heating stations  3 ;   the two pre-heating stations  3  and the sintering press  1 ;   the sintering press  1  and the three cooling stations  4 ;   the three cooling stations  4  and an outlet conveyor  7 .       

     The two pre-heating stations  3  provide to raise the temperature of the respective substrates  12  to a temperature of 150° C. 
     For example, the two pre-heating stations  3  are heated by armored electric cartridges controlled by PID and PWM thermoresistances PT100. 
     The sintering press  1 , illustrated globally in  FIG. 2 , comprises a vertically extending frame  8  which supports a pressing group  14  at the top and a support unit  60  at the bottom for at least one substrate  12 , preferably six in the example shown. 
     The frame  8  is provided with longitudinal guides  9  along which the support unit  60  slides, operated by a hydraulic or electric actuator  80 . 
     In one embodiment, the support unit  60  performs a stroke of 200 mm, with uniformly accelerated and decelerated motion. 
     In particular, the upper position of the support unit  60  is absolutely irreversible and is sized to counteract forces up to 250 KN without moving. 
     In one embodiment, the stroke of 200 mm is implemented in 3 seconds, with a 1 Kw brushless geared motor  80 , and can be implemented in jog at reduced speed for mold changing and maintenance functions. 
     In one embodiment, the frame  8  also supports a motorized winder  90  and an unwinder  92 , for example with brushless gear motors, for the replacement of a protective film  52 , for example PTFE, to be interposed between the substrates  12  and the pressing group  14  during sintering. 
     In one embodiment, the protective film  52  is supported by a peripheral frame  50 . This peripheral frame  50 , in one embodiment, also supports suction/blowing means directed towards the protective film  52 . 
     Furthermore, the press  1  is provided with a vacuum pump—not shown—for sucking the air between the PTFE film and the pressing group  14 . 
     In one embodiment, the press  1  is also provided with a blow ionizing device—not shown—to remove any electrostatic charges in the sintering area. 
     In one embodiment, the frame  8  also supports horizontal sliding guides that can be inserted and removed with a right-hand and a left-hand handgrip  94 , to facilitate the replacement of the support unit  60 . 
     In an embodiment, shown in particular in  FIGS. 3 and 4 , the support unit  60  comprises a fixed reaction block  62 , wherein six deformation cylinders  68  are accommodated, provided with respective deformation sensors  70  able to operate at 350° C. 
     The support unit  60  further comprises a heating block  64  in which armored heating resistances  641  are accommodated. 
     The support unit further comprises six movable reaction blocks  66 , each provided with a respective temperature sensor  662 . 
     The pressing group  14  comprises a multi-stem cylinder  20  having a front head  22  and a rear head  24  which jointly delimit a compression chamber  26 . 
     Between the front head  22  and the rear head  24  there is interposed an annular sealing element  25  positioned around the compression chamber  26 . 
     For example, the annular sealing element  25  is placed in a respective seat  25 ′ formed between a flat portion of the rear end surface  22 ′ of the front head  22  and a flat portion of the front end surface  24 ′ of the rear head  24 , such flat portions being facing and parallel to each other. 
     The annular sealing element  25  therefore performs the function of preventing the escape, between the two side-by-side heads  22 ,  24  of a fluid under pressure present in the compression chamber  26 . 
     For example, the annular sealing element  25  consists of an O-ring. 
     In the rear head  24  an inlet passage  32  is made for introducing a pressurized fluid into the compression chamber  26 . 
     In the front head  22 , pressing stems  28  parallel and independent of each other are slidingly supported. The rear ends  28 ′ of such pressing stems  28  protrude in the compression chamber  26 . 
     In the compression chamber  26 , an actuating flat gasket  30  extends over the rear ends  28 ′ of the pressing stems  28  in such a way that, when the compression chamber  26  is pressurized, the actuating flat gasket  30  acts on such rear ends  28 ′ to transfer pressure in the compression chamber on each individual pressing stem  28 . 
     For example, each rear end  28 ′ of the pressing stems  28  ends with a flat surface on which the flat gasket  30  is pressed. 
     According to an aspect of the invention, the actuating flat gasket  30  is fixed to the front head  22  by means of an anchoring frame  34  which engages a peripheral portion  30 ′ of the actuating flat gasket  30 . 
     In other words, a peripheral portion  30 ′ of the flat gasket  30  is at least partially inserted between the front head  22  and a base or front surface  34   a  of the anchoring frame  34 . 
     The anchoring frame  34  is housed completely in the compression chamber  26  so that the pressurized fluid that is introduced into the compression chamber  26  acts, as well as on the flat gasket  30 , also on the anchoring frame  34 . 
     As mentioned, the flat gasket  30 , held in position by the anchoring frame  34 , performs the double function of sealing gasket and actuating membrane on the pressing stems  28 . 
     In particular, when the compression chamber  26  is pressurized, the pressure of the fluid weighs not only on the central portion of the flat gasket, but also, through the anchoring frame  34 , on the peripheral portion  30 ′. Consequently, the flat gasket  30  is stressed uniformly and at the same time in compression over the entire surface thereof. This avoids undesired deformations of the gasket, considerably increasing the life thereof. 
     More in detail, the rear head  24  forms a bottom wall  27  for the compression chamber  26 ; the anchoring frame  34  has a rear surface  34   b  facing towards and spaced from such a bottom wall  27 . 
     In one embodiment, the compression chamber  26  is radially delimited by a side wall  29 ; the anchoring frame  34  has a side surface  34   c  facing towards and separated from said side wall  29 . 
     In this way, the pressurized fluid acts on all the exposed sides of the anchoring frame  34 , so as to prevent it from moving or deforming. 
     In one embodiment, the anchoring frame  34  is screwed to the front head  22  by means of anchoring screws  35 . 
     It should be emphasized, however, that the fixing, for example by means of the anchoring screws  35 , of the anchoring frame  34  to the front head  22  does not have the function of retaining the flat gasket  30  when the compression chamber  26  is pressurized, but only to hold it in the correct position when the compression chamber is not pressurized. In fact, in the presence of the pressurized fluid in the compression chamber  26 , it is the pressure exerted by the fluid which allows the flat gasket  30  to be locked in position. 
     In one embodiment, the flat gasket  30  divides the compression chamber  26  into a front portion  26 ′, in which the rear ends  28 ′ of the pressing stems  28  protrude, and a rear portion  26 ″ in fluidic communication with the inlet duct  32 . 
     In one embodiment, the front portion  26 ′ of the compression chamber  26  is formed in a lowered portion  22   a  of the rear end surface  22 ′ of the front head  22 . 
     In the absence of pressure in the compression chamber  26 , the flat gasket  30  is substantially in a flat resting configuration and is positioned so as to slightly touch the rear ends  28 ′ of the pressing stems  28  ( FIG. 7 ). 
     When the rear portion  26 ″ is pressurized at the sintering pressure, the flat gasket  30  is deformed, thereby abutting against the rear ends  28 ′ of the pressing stems  28  for a transfer of the sintering pressure on each pressing stem  28  ( FIG. 7 a   ). 
     More precisely, the projection of the rear ends  28 ′ of the pressing stems  28  with respect to the lowered portion  22   a  of the front head  22 , and thus the distance between the flat gasket  30  and said lowered portion  22   a , are selected such that when the rear portion  26 ″ is pressurized at the sintering pressure and the flat gasket  30  is deformed, the gasket abuts not only against the rear ends  28 ′ of the pressing stems  28 , but also against the lowered portion  22   a  of the front head  22 , as shown in particular in  FIG. 6   a.    
     Due to this conformation of the compression chamber  26 , the flat gasket  30  behaves as if the control fluid acts directly on the individual rear ends  28 ′ of the pressing stems  28  at the desired sintering pressure. In other words, the flat gasket  30  simulates the behavior of a plurality of independent cylinder-piston systems. 
     Returning now to the pressing stems  28 , each of them is coaxial and barycentric to a respective electronic component  10  to be sintered and has a thrust section proportional to the force to be exerted on the respective electronic component  10 , the area of each electronic component to be sintered being known and for a predetermined sintering pressure. 
     The term “barycentric” means that each pressing stem  28  has a stem axis which coincides with the center of gravity of the respective electronic component  10 . 
     In one embodiment, the pressing group  14  further comprises a heating block  40  integral with the multi-stem cylinder  20  and which slidingly supports heating pressing members  42 . Each heating pressing member  42  can be actuated by a respective pressing stem  28  to act on a respective electronic component  10  to be sintered. 
     The perspective view in  FIG. 8  shows four pressing stems  28 , of different sections, which act on respective heating pressing members  42 , also of different section. 
     The heating pressing members  42  are shown in abutment on the respective electronic components  10 , resting on a substrate  12  supported by the support unit  60 . 
     In one embodiment, the pressing stems  28  have rounded front ends  28 ″ which are in contact with flat surfaces  42 ′ of the heating pressing members  42 . For example, the front ends  28 ″ of the pressing stems  28  are spherically rounded to concentrate the compressive force in the barycentric point of the corresponding electronic component to be sintered and at the same time to make a thermal break between the pressing stems  28  and the pressing members  42 . In this way, the heating pressing members  42  can easily adapt to the surface of the electronic components to be pressed and heated. 
     In one embodiment, the heating pressing members  42  are in the form of bars having a cross section substantially corresponding to or greater than the area of the casing of the respective electronic component  10  to be sintered. 
     In one embodiment, the multi-stem cylinder  20  is capable of imparting to the single pressing stems  28  a force proportional to the cross-section thereof, to develop a pressure corresponding to 30 MPa on all the electronic components  10 . 
     In one embodiment, for each substrate  12  there are provided four pressing stems and members for IGBTs, pressing stems and members for diodes, a pressing stem and members for a thermistor, a pressing stem and a member for MOSFET, for a total of 48 pressing members and relative pressing stems, all independent and of differentiated section proportional to the thrust to be developed. 
     In one embodiment, the compression chamber  26  can be pressurized up to 35 MPa (350 bar). 
     As said, the heating pressing members  42  have the task of transmitting the compression force and the heating to the electronic components  10  to be sintered on the substrate  12 . 
     In one embodiment, the heating block  40  is provided with armored resistances  44  and temperature sensors  45 . 
     The pressing members  42  are pushed barycentrically with respect to the corresponding electronic component  10  to be sintered on the substrate, and slide into the heating block  40  with a suitable clearance to allow them an adjustment to a parallelism of 1 μm over 10 mm. 
     It should be noted that if the thicknesses of the electronic components  10  change, they do not cause changes in the sintering pressure of the other components. 
     It should be noted that, as mentioned above, while the axis of each pressing stem  28  must coincide with the center of gravity of the respective electronic component  10 , the shape, the section and the position of the pressing members  42  with respect to the axis of the respective pressing stems  28  can be selected according to the shape of the electronic components  10  and/or the position thereof on the substrate. 
     For example, the pressing members  28  can be selected in such a way as to sinter in a reliable and precise manner electronic components  10  which are also very close to each other or of a non-rectangular or very different size. 
     It should also be noted that the implementation of each pressing member of the pressing group  14  in two separate components, the pressing stems  28  and the pressing members  42 , placed in contact through a substantially spherical surface  28 ″, allows obtaining a thermal break between the multi-stem cylinder  20  and the heating block  40 . For example, while the heating block  40  operates at a temperature that can reach 300° C., the multi-stem cylinder  20 , connected to a cooling circuit, can be kept below 100° C. 
     The multi-stem cylinder  20  thus cooled undergoes less wear than the heating block  40  and can be replaced much less frequently. 
     In a general embodiment, the method for sintering electronic components on the substrate using the press described above comprising the steps of:
         establishing a constant sintering pressure to be applied to all of the electronic components to be sintered;   equipping the pressing group with pressing stems with respective thrust sections according to the pressing surface of the respective electronic components;   positioning in the press a substrate with sintering glue and electronic components to be sintered so that the electronic components come into contact with the end feet of the pressing members;   heating the pressing members to a predefined sintering temperature;   pressurizing the compression chamber to the predefined sintering pressure;   maintaining the sintering pressure and sintering temperature for a predetermined sintering time;   removing the substrate from the sintering press.       

     In one embodiment, a protective film is adhered to the ends of the pressing members before the substrate is positioned. 
     In one embodiment, during the step of pressurizing the compression chamber to the predetermined sintering pressure, the sealing membrane is deformed in such a way as to come into contact both on the rear ends of the pressing stems and on a substantially flat bottom wall of the front portion of the compression chamber. 
     A sintering cycle will now be described in more detail. 
     While a sintering step is in progress, the robot  5  manages the pallets  2  which arrive in the inlet conveyor  6 , picks them up and deposits them in one of the two free pre-heating stations  3 . 
     During the sintering step, the robot  5  also manages the pallets  2  which are stationed in the three cooling stations  4 . At the end of the programmed cooling cycle, the robot picks the pallet from the corresponding cooling station and deposits it in the outlet conveyor  7 . 
     At the end of the sintering step, the following steps are carried out in succession. 
     The multi-stem cylinder  20  is depressurized from about 35 MPa to about 0.5 MPa. In this way, the heating pressing members  42  act as blank holders having a predefined force. 
     The depressurization of the multi-stem cylinder is controlled by the pressure sensors contained in the reaction block of the support unit  60 . 
     The support unit, from the advanced pressing position, descends with a uniformly accelerated and decelerated movement completing a stroke of 200 mm. 
     The peripheral frame  50  drops by 10 mm, thus releasing the protective film  52 . 
     The vacuum pump of the pressing group  14  is then gently pulse-pressurized to detach the film from the pressing members. 
     The unwinding reel and winding reel of the protective film are actuated, in order to position a portion of integral film under the pressing members of the pressing group. This operation must preferably be carried out with the pallet containing the newly sintered substrates still present. This prevents glue or film residues from falling on the support unit  60  or worse on the substrates yet to be sintered. 
     The robot picks the pallet from the press and deposits it in a free station of the three cooling stations. 
     The robot picks the pallet with the substrates to be sintered, which has completed the preheating cycle at 150° C., and deposits it gently in the support unit  60 . 
     The vacuum is then activated between the pressing group  14  and the protective film, making the film adhere to the pressing members of the pressing group  14 . 
     As soon as the robot leaves the collision area, the support unit  60  rises with a uniformly accelerated and decelerated motion. In particular, in one embodiment, in the last 15 millimeters of travel of the support unit  60  with the pallet and the relative substrates, the support unit  60  impacts with the peripheral frame  50  of the pressing group  14 , thus lifting it. In the last 2 millimeters of travel of the support unit  60 , the substrates centered in the pallet housings and resting directly on the respective movable reaction blocks of the support unit  60  impact with the pressing members covered by the protective film. 
     The pressing members  42 , which in this step are slightly pressurized, retract, thus acting as blank holders. 
     The support unit  60  reaches the upper stroke end position. 
     The multi-stem cylinder is pressurized at the programmed pressure. 
     The pressurization of the multi-stem cylinder is controlled by the corresponding deformation sensors, which measure the cumulative compressive force applied by the pressing members on each substrate. 
     In one embodiment, the sintering temperature of the substrates is generated by armored cartridge resistances contained in the lower  64  and upper  40  heating blocks. 
     In one embodiment, the sintering temperature is controlled independently in the pressing group and in the support unit, for example with suitably positioned thermoresistances PT100. 
     Preferably, the substrates are heated from below through the movable reaction blocks, and from above through the pressing members and the protective film. Since the heating conditions are different, it is possible to differentiate the heating temperatures below and above in order to compensate for differences in the heat transmission to the substrates. 
     In one embodiment, the electronic components on the substrates are maintained for a programmed time (for example 300 seconds) at the temperature of 260° C., and pressed at 30 MPa, to carry out the sintering of the components on the substrates. 
     Once the sintering time has elapsed, the multi-stem cylinder is depressurized and the press re-opens as described above. 
     A man skilled in the art may make several changes, adjustments, adaptations and replacements of elements with other functionally equivalent ones to the embodiments of the pressing group and of the sintering press according to the invention in order to meet incidental needs, without departing from the scope of the following claims. Each of the features described as belonging to a possible embodiment can be obtained independently of the other embodiments described.