Abstract:
A process for mechanical assembly and electrical interconnection of the functional components of an active implantable medical device. A first step involves preparing an interconnection flex circuit ( 20 ) that is able to be electrically and mechanically linked to an electronic circuit module ( 14 ), a supply battery ( 12 ) and a series of feedthrough terminals ( 16 ) of the device, prior to being placed in a common case. The flex circuit has a series of pads ( 34 ) for linking to homologous metallizations ( 46 ) of the substrate. The mechanical assembling and electrical linking of these pads ( 34 ) to the metallizations ( 46 ) is performed without either the use of any activation flux or introduction of fusible brazing, and rather by applying an intermediate anisotropic conductive material ( 38 ) placed between the pads ( 34 ) and metallizations ( 46 ), followed by polymerizing this material. The applying and polymerizing are performed under controlled conditions of pressure, temperature and duration.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the technology of manufacturing active implantable medical devices, and more particularly to the assembly of the various functional components constituting such devices that are intended to be incorporated within a common case after being interconnected. 
     The invention will be mainly described in the context of implantable cardiac pacemakers, but it should be understood that pacemakers represent one embodiment of the invention, and the invention is more generally applicable to a great variety of “Active Implantable Medical Devices” (AIMD) as such devices are described by the Jun. 20, 1990 Directive 90/385/CEE of the Council of the European Communities. This definition includes, in addition to cardiac pacemakers, defibrillators and/or cardioverters, devices such as neurologic devices, pumps for diffusion of medical substances, cochlear implants, implanted biological sensors, etc. 
     BACKGROUND OF THE INVENTION 
     These active implantable medical devices (hereinafter referred to as “devices” or “implantable devices”) are generally in the form of a case containing a power supply (e.g., a battery) and a hybrid circuit board (hereinafter referred to as “electronic circuit”) supporting and interconnecting the various active and passive components, allowing for collecting and analyzing signals, generating electrical pulses, storing data such as medical follow-up information in memory, controlling the device&#39;s various functions, etc. 
     Manufacturing the device usually comprises a series of steps including, first, interconnecting the various functional elements, which are: the battery, the electronic circuit(s) as well as a series of feedthrough terminals intended to be subsequently connected to a corresponding terminal of a connector head. The following step concerns placing the resulting interconnected set of elements inside the case and then adding a connector head. The connector head will be used to ensure the mechanical and electrical connection of the case to external elements, preferably leads for collecting signals and/or delivering pulses. 
     The interconnection of the foregoing and similar functional elements is usually realized by a flex circuit comprising conductive tracks that are electrically and mechanically linked to the functional elements prior to integrating the functional elements within the case. 
     U.S. Pat. No. 6,245,092 proposes a cardiac pacemaker made using the flex circuit technology. The conductive tracks are present in the form of printed conductors that have a pad at their extremity which is intended to be linked through a metallization with a corresponding functional element. Brazing, the metallization technique used for the linkage, is a well-known technique providing excellent results in terms of electrical and mechanical performance and long-term reliability. 
     The use of brazing, typically Tin-lead (Sn—Pb) brazing, however, requires applying a brazing activation flux, followed by a careful cleaning of the substrate so as to remove any remainder of the flux. Until now, chlorofluorocarbon (CFC) based products could very efficiently clean flux from substrates. However, the prohibition against using CFC based products renders this cleaning step more difficult and creates an increased risk of forming metal dendrites resulting from the migration of Sn—Pb alloy to some areas of the substrates that have not have been prepared perfectly. 
     Moreover, future prohibitions against using brazing allows containing lead exacerbates these difficulties, specifically in the case of AIMD. Indeed, due to the limited room available within the case, the advanced miniaturization of these devices requires creating link pads with a very low pitch (typically 1.47 mm) and very narrow spacing between these pads (the interval between adjacent pads is typically about 250 μm). These problems, resulting from the extreme compactness of circuit area, is further amplified by the reduction of the size of the cases containing these circuits, whose volume has been recently reduced down to 8.5 cc. 
     U.S. Pat. No. 6,245,092, cited above, incidentally suggests avoiding brazing with lead, and replacing the brazing step with a gluing step. Gluing—which is a well-known and mastered technique in itself—for establishing electrical connections to the components, however, presents some major drawbacks. This technique essentially replaces brazing with a drop of conductive glue (silver glue), deposited on each of the contact pads of the flex printed circuit. This technique therefore requires that an individual deposit one drop onto each contact pad, and is therefore not appropriate for collective processing i.e., automated mass production, as contrasted with brazing (wave brazing for instance), which makes the whole process more complex and expensive. 
     Further, to avoid the risks of short-circuits between two adjacent pads, particularly if the pads are close to each other, it is highly recommended to deposit an insulating drop into the space separating the two conductive drops deposited on the adjacent pads. This gluing process becomes even more difficult as the circuit dimensions are reduced (pitch and interval between pads), which renders it inappropriate for such applications where advanced miniaturization is an essential requirement. Also, if the glue is in discrete drops, there is always a remaining risk that the silver particles that are in suspension within the conductive drops will migrate to the intermediate non-conductive drop. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to propose a process for the mechanical assembly and electrical linkage of the pads of the interconnection flex circuit to the corresponding metallizations of a functional element of an AIMD, in which the process avoids the use of brazing or gluing techniques, which involve constraints that are difficult to overcome. 
     It is another object of the present invention to propose a process that is collective, i.e., useful for automated mass production, and does not require any activation flux, cleansing of the surfaces to be linked together, or use of any material containing lead, while responding to the particular requirements of AIMD, such as:
         possible implementation with low-volume circuits, packed in high density with an extremely small interval between adjacent link pads;   possible industrialization of the embodiment, through a process that is collective and not operator-dependent;   technique that allows prompt reworking in case a defect is detected during the manufacturing process;   possible utilization with circuits on substrates that diffuse heat, for example, ceramic substrates;   very high reliability (the lifetime of an implant is usually about 10 years and any defect requires the explanation of the device through surgical intervention);   severe mechanical and electrical constraints such as pull out strength, conductivity, isolation, etc.       

     Advantageously, the present invention is directed to using an anisotropic conductive material that is heat-polymerizable instead of brazing (particularly, Sn—Pb brazing). These anisotropic materials are already well-known for similar purposes, notably for the purpose of interconnecting a flex circuit and a glass substrate, typically for liquid crystal display screens and printed circuit boards. The implementing conditions relating to such known applications are, however, very different from those encountered with AIMD. For example, the components of an AIMD are confined in a far smaller space, and the prior art implementation is done on a support (glass) that does not diffuse heat. Furthermore, until now, this type of connection has been considered, from a long-term point of view, less reliable than Sn—Pb brazing, which is known for excellent aging properties, and therefore provides a level of confidence sufficiently high to be implemented in implantable devices. 
     For this reason, this technique has so far been restricted to multimedia applications (interconnection of display screens), radiography applications (control screens) and automotive applications (instruments and control panel displays). 
     More preferably, the present invention proposes, for an active implantable medical device, a process for mechanical assembly and electrical interconnection of functional components of an active implantable medical device intended to be incorporated within a common case after being interconnected. These functional components comprise at least an electronic circuit module, a supply battery and a series of feedthrough terminals able to be linked to a connector head joined to the case. 
     The process comprises the steps of (i) preparing the interconnection flex circuit electrically and mechanically linked to the respective functional components, wherein the flex circuit comprises a series of pads for linkage to homologous metallizations of one of the functional components, and (ii) mechanical assembly and electrical linkage of the pads of the flex circuit to the homologous metallizations of the functional component. 
     In a manner characteristic of the invention, the step of mechanical assembly and electrical linkage of the pads of the flex circuit to the homologous metallizations of the functional component is a step (i) essentially not requiring any activation flux or any introduction of fusible brazing in its implementation, and (ii) comprising (a) the placement of an intermediate anisotropic conductive material between the pads and the respective metallizations, and (b) the polymerization of the intermediate anisotropic conductive material, the operations of placement and polymerization executed under controlled pressure, temperature and duration. 
     The electronic circuit module is preferably a hybrid module with a substrate, such as ceramic material or epoxy, supporting the electronic components. The functional component supporting the metallizations then comprises the module, and the metallizations are formed onto the substrate of this module. 
     In a preferred embodiment, the step of mechanical assembly and electrical linkage of the pads of the flex circuit to the homologous metallizations of the functional component is performed subsequently to a step of mechanical assembly and electrical linkage of the interconnection flex circuit to the supply battery and feedthrough terminals. 
     The functional component supporting the metallization can comprise another interconnection flex circuit, and/or a discrete electronic component on which the metallizations are formed. 
     In a preferred embodiment, the anisotropic conductive material is a film of ACF type. In such a case, the step of mechanical assembly and electrical linkage of the pads of the flex circuit to the homologous metallizations of the functional component comprises (i) a sub-step of pre-tacking by gluing an ACF-type film to the pads, or to the metallizations, and (ii) a sub-step of tacking by positioning the pads vis-à-vis the metallizations, and then polymerizing the pads vis-a-vis the metallizations under pressure, this tacking sub-step being performed under conditions of temperature, duration and pressure higher than those of the pre-tacking step. 
     In an alternate embodiment, the anisotropic conductive material may be a conductive paste. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features, advantages and characteristics of the present invention will become apparent to a person of ordinary skill in the art in view of the following detailed description of preferred embodiments of the invention, made with reference to the drawings annexed in which like reference characters refer to like elements, and in which: 
         FIG. 1  is an elevated perspective view of the various functional components interconnected by a flex circuit, prior to placing them in the generator&#39;s case; 
         FIG. 2  is front elevation view of  FIG. 1 ; 
         FIG. 3  is a plane view of the functional components after their placing in the generator&#39;s case; 
         FIG. 4  illustrates a process of pre-tacking and tacking of a typical assembly process in accordance with the present invention; and 
         FIG. 5  is a magnified sectional view of Section V of  FIG. 4 , the conductive interface between flex circuit and substrate of the electronic circuit module. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 1 , the reference  10  designates the whole of the components intended to be placed within the case of a pacemaker or implantable cardioverter defibrillator. These various components, hereinafter generically referred to as “functional components”, comprise one or more of a supply battery  12 , one or more an electronic circuit module  14  and a series of feedthrough terminals  16 , in this case represented as equipped with a cap  18  for their protection until they are placed within the case. 
     The invention can notably be applied—in a non-limiting manner—to the manufacturing of implantable medical devices marketed by ELA Medical, Montrouge—France, such as the Symphony and ELA Rhapsody brand pacemakers. 
     The electronic circuit module  14  is, in the illustrated example, a hybrid module comprising a unique substrate supporting the whole of active and passive components of the device. The substrate can notably be made of a multilayer ceramic material receiving, inside appropriate cavities, one or more integrated circuit chips, the cavity being subsequently closed by an insulating resin, so as to provide a surface for receiving the various associated discrete components. The substrate ceramic can be coated with an epoxy resin; in an alternate embodiment, the substrate can be made of an epoxide resin. 
     The various functional components are interconnected by a flex circuit  20 , for example, a polyimide flex circuit, bearing on its surface various metal conductive tracks ensuring the electrical links between the terminals  22  of battery  12 , the extremities  24  of terminals  16  and the various link terminals of module  14 , that are presented in the form of a series of surface metallizations. These metallizations are intended to be electrically and mechanically linked to the flex circuit  20 , level with the interface  26  of circuit  20  with the substrate of module  14 . 
     Once the linkage of the various functional components to the interconnection flex circuit  20  is realized, the various components are positioned in a common plane, through bending the flex circuit  20 , following the arrows illustrated in  FIG. 2 . The finished configuration is that illustrated in  FIG. 3 , with the battery  12  approximately lying in the same plane as that of flex circuit  20 , and the module  14  turned back against this same circuit  20 , which is curved at 180° in the vicinity of interface  26 . As for the terminals  16 , they are straightened by a quarter-turn relative to their initial position, and spread over the common plane of the other components positioned as explained above. 
     The various functional components, thus configured, can then be placed in the case  30  of the generator, in as confined a configuration as possible, with a minimum lost volume. In a subsequent step, a connector head  32  will be fitted onto the case  30  and the various conductive organs of the connector will be electrically connected to terminals  16 . 
     Another aspect of the present invention concerns the realization of the electrical and mechanical linkage of flex circuit  20  to the substrate of the electronics module  14  level with the interface  26  of these two elements. It should be understood that insofar as the linkage of battery  12  and terminals  16  to the flex circuit does not raise the same difficulties identified earlier, this linkage, level with the terminals  22  and  24 , can be realized in a traditional manner, by brazing. The invention is also applicable to the realization of the links described below and the special advantages should be taken into account. 
     The extremities of the conductive tracks of the flex circuit  20  shall be electrically and mechanically linked to the homologous metallizations formed on the substrate of module  14 , while respecting the various electrical, mechanical, industrial and environmental requirements cited in the introduction of the present disclosure. 
     The invention proposes, in a characteristic manner, to realize these links through a process of high temperature gluing, under pressure constraints, of the flex circuit to the substrate, with interpositioning of an anisotropic conductive film. Such anisotropic conductive films are well-known in the applications cited above (multimedia, automotive industry, etc.), but, until now, have not been considered in the domain of medical implantable devices, taking into account the much more severe restrictions in terms of aging properties, miniaturization, etc. Anisotropic conductive films are known under the denomination of “ACF films” or “Z-axis conductive films”. For example, it is possible to use the ACF-type films marketed by Hitachi Chemical Co., Ltd or any similar film. 
       FIG. 4  illustrates the various successive steps of the process according to this invention, which essentially comprises two phases, so called “pre-tacking” (steps ad) then “tacking” (steps e-g) phases. 
     The “pre-tacking” phase consists of making an ACF film adhere to one of the two parts to be assembled, such as flex circuit  20  (although substrate  14  could be used instead), as illustrated in  FIG. 4 . 
     At step a, the flex circuit  20 , which is already linked by brazing to the battery  12  and terminals  16 , is positioned by an appropriate tool. 
     Using the same tool, an ACF film  36  is positioned above flex circuit  20 . This film comprises, in a manner known per se, an anisotropic conductive layer (ACF layer)  38  deposited on a protective backing  40  which is, in one example, made of PET. The ACF layer  38  is turned toward the conductive pads  34  of flex circuit  20 , and a heating element or thermode  42  is put in the vicinity of the back of the ACF film  36 , advantageously with interpositioning of a silicon ribbon  46  so as to absorb the asperities and for a better spreading of heat. 
     The following step b consists of laminating ACF film  36  on flex circuit  20  by means of heat-adhesion under pressure. The conditions for this step are, for example as follows: heating to a temperature of about 80° C., applying the thermode for a duration of 3 to 5 seconds, and applying pressure of 0.5 to 1.5 MPa. Once this heat-adhesion step has been performed, the following step c involves, after removal of thermode  42  and intermediate ribbon  44 , extracting the protective backing  40 , so that only the thickness of the ACF film per se  38  remains on the flex circuit  20 . The configuration in the end of this pre-tacking phase is as illustrated in  FIG. 4 . 
     The following “tacking” phase, corresponds to steps e to g illustrated in  FIG. 4 . At step e, after the flex circuit  20  equipped with ACF film  38  has been turned back to back, it is placed facing the substrate  14 , so that the pads  34  (covered with ACF film  38 ) of flex circuit  20  are positioned relative to metallizations  46  that are present on the surface of substrate  14 . As with step a, the thermode  42  is positioned above the assembly, with interposition of a silicone protective ribbon  48  so as to absorb asperities and provide a better spreading of heat. Step f consists of laminating the assembly, under pressure and high temperature, so as to polymerize the resin of the ACF film. The conditions for this step are, for example: heating to a temperature of about 170° C., applying the thermode for a duration of 10 to 15 seconds, and applying pressure of 2 to 4 MPa. 
     The final configuration is as illustrated in  FIG. 4   g.    
       FIG. 5  shows, in a magnified format, the detail referred to as portion V of  FIG. 4   g . At the interface between the pads  34  of flex circuit  20  and metallizations  46  of substrate  14 , the ACF film  38  contains conductive micro-balls  50  contained within a paste  52 . The dimension and concentration of these conductive micro-balls  50  are chosen so as to establish an electrical contact of excellent quality between pad  34  and metallization  46  at the end of the process, at the interface of these two elements. 
     However, out of this interface, the concentration of micro-balls is not sufficient for them to come into mutual contact, thus preventing any electrical link between flex circuit  20  and substrate  14  (out of the interface between pads and metallizations). One can thus typically obtain a pad/metallization contact impedance lower than 10Ω and a flex circuit to substrate isolation resistance higher than 1 GΩ. These specifications are preserved, even after aging and submission to heat, humidity, etc. constraints, which provide excellent aging properties. 
     If the temperature/time/pressure profile is defined in a precise and reproducible way for the steps of pre-tacking and tacking, then the process can be industrialized in a perfectly repetitive and reproducible manner. Furthermore, in case of a defect, the product is “reworkable”. In a manner similar to a brazed link, a flex circuit that has a defective connection may be replaced by another circuit, after cleaning the hybrid circuit, the operation being carried out through pealing on a heating plate at a temperature of about 125° C. 
     In an alternate embodiment, it is also possible to replace the use of an ACF ribbon by an anisotropic conductive paste, packaged in a syringe. This paste can then preferably be deposited, at ambient temperature, on sections covering the metallizations of the substrate or conductive pads of the flex circuit, the heat sealing being then performed as described above. 
     Moreover, the present invention is not limited to the realization of a link between a flex circuit and substrate of a hybrid circuit. It can also be used for linking two flex circuits together, or to an attached discrete component, as such or on a chip carrier. 
     One skilled in the art will appreciate that the present invention can be practiced by embodiments other than those disclosed, which are presented for purposes of illustration and not limiting.