Patent Publication Number: US-2022227133-A1

Title: Interconnection structure for a print head

Description:
BACKGROUND 
     Field 
     The disclosure relates to a print head having one or more droplet jetting devices as well as to a method for connecting a droplet jetting device to a controller. 
     Description of the Related Art 
     An ink jet printer typically has an array of narrowly spaced droplet forming devices each of which has an electro-mechanical transducer, e.g. a piezoelectric actuator, for creating acoustic pressure waves that result in the ejection of ink droplets from nozzles of the droplet jetting devices. The print head or at least an array of ejection units of the print head may be constituted by a MEMS chip (Micro-Electro-Mechanical System) which is manufactured by means of photolithographic techniques. Such a print head is known from e.g. EP 3362289 A1. In order to print an image, the transducers in the MEMS chip must be controlled individually by means of an electronic controller that is connected to the chip via a connection element in the form of a printed circuit sheet. The interconnection structure discussed here has the purpose to connect each individual transducer to a track on the printed circuit sheet which connects the chip to the electronic controller. Typically, the printed circuit sheet is a flexible sheet, e.g. a so-called FPC (Flexible Printed Circuit). 
     Since an ink jet printer operates with liquid ink and the electrical connections of the interconnection structure are sensitive to moisture and ink, respectively, it is necessary to protect the electrical connections on the connection surface of the chip against the ingress of ink. Additionally, the electrical connections are sensitive to damage due to the displacement of the flexible sheet during print head production and/or operation. Pulling forces on the sheet may cause one or more electrical connections in the interconnection structure to disconnect, which prevents the controller from actuating the respective transducers. This is known as nozzle failure and may result in visible artefacts in the printed image. Poor bonding at the interconnection structure thus reduces the reliability of the print head. 
     SUMMARY 
     Disclosed is a reliable print head, an interconnection structure, and a method. 
     According to an aspect of the present disclosure, a print head includes a body holding at least one droplet jetting device provided with an actuator for jetting a droplet of liquid from a nozzle of the at least one droplet jetting device, wherein the actuator is in electrical connection to a first contact pad structure positioned on an outer surface of the body, a flexible electrical connection element having a second contact pad structure at one end, wherein the second contact pad structure is mounted onto the first contact pad structure, and the first and second contact pad structures each have an interface surface which contacts the interface surface of the other of the first and second contact pad structures to form an electrical connection, and an adhesive covering the first and second contact pad structures and securing them together, wherein at least one of the first contact pad structure and the second contact pad structure includes a recessed portion as compared to a wider portion positioned between the recessed portion and the interface surface. 
     In an example, a first and/or second contact pad structure are a recessed portion. The wider portion is positioned between the recessed portion and the interface surface. The respective contact pad structure has been provided with one or more recesses which results in the recessed portion being narrower than the wider portion. The recessed portion and the wider portion are at different positions in a direction perpendicular to the interface surface and/or the outer surface of the body on which the first contact pad structure is provided. The adhesive inwardly extends towards a center of the respective contact pad structure beyond the contour or circumference of the wider portion, either locally or around the full circumference. The wider portion thereby forms a protrusion which extends over the portion of the adhesive in the recess. The adhesive is thereby effectively ‘anchored’ or ‘hooked’ in the recess against forces in the perpendicular direction by the portion of adhesive underneath the wider portion. This result in a secure contact between the first and second contact pad structures, which is resistant against forces exerted on the connection element. The bonding is particularly effective as it may be applied to each individual bond pad structure. This results in a higher yield for manufacturing print heads as well as a more reliable operation and lifetime of individual print heads. 
     In an embodiment, a cross-sectional area of the recessed portion parallel to the interface surface is smaller than a corresponding cross-sectional area of the wider portion. The cross-sections are preferably taken parallel to the interface surface and/or the outer surface on which the first contact pad structure is positioned. 
     In an embodiment, the wider portion extends over a recess adjacent the recessed portion, the recess being substantially filled with adhesive. The recessed portion is narrowed with respect to the wider portion due to the presence of at least one recesses positioned at the recessed portion. The recess and the recessed portion are positioned at the same or similar positions or heights in the direction perpendicular to interface surface. The interface surface is preferably parallel to the outer surface of the body. The wider portion is positioned ‘higher’ than the recessed portion and the recess, i.e. further removed from the outer surface, in a direction moving away from the interface surface. Due to its larger cross-sectional area the wider portion extends over the recess. During its application the liquid adhesive has flowed into the recess, such that a portion of the adhesive is ‘underneath’ a section of the wider portion, which extends over the recess. After hardening this portion of the adhesive form a bump or protrusion, which is trapped by the overhanging section of the wider portion, at least in the direction perpendicular to the outer surface of the body. This adhesive anchor portion preferably extends along two or more sides of the respective contact pad structure. In a preferred embodiment, the recess and thus the adhesive anchor portion extend circumferentially around the respective contact pad structure, specifically along the full circumference. This further improves the holding together of the first and second contact pad structures. 
     In an embodiment, when viewed parallel to outer surface, the adhesive extends beyond a circumference of a maximum cross-sectional area of the respective contact pad structure towards a center of the first contact pad structure. Preferably, the inner circumference of the adhesive anchor portion is smaller than that of the wider portion, specifically the circumference of the maximum cross-sectional area of the respective contact pad structure. The adhesive anchor portion locally includes an opening, the area of which is smaller than that of the maximum cross-sectional area. This prevents the adhesive anchor portion from passing over the wider portion, securing the first and second contact pad structures together. 
     In an embodiment, a portion of the respective contact pad structure tapers in a direction away from the interface surface. The portion may be the bottom side of the first contact pad structure facing the outer surface of the body. The recessed portion is formed by the tapered bottom portion, which tapers or narrows from the wider portion towards the outer surface of the body. Alternatively or additionally, the second contact pad structure may further be tapered towards the connection element. A tapered contact pad structure is relatively easy to manufacture using MEMS lithographic deposition and etching techniques. 
     In an embodiment, the actuator is connected to the first contact pad structure via a lead extending at least partially over the body, the lead being narrower than the first contact pad structure. The lead is formed of a conductive material for transmitting an energizing signal or pulse to the actuator. In consequence of the high resolution determined by the number of droplet jetting devices per unit length (e.g. dpi), the leads are relatively narrow, making it difficult to align the second contact pad structures directly thereon. The contact area is enlarged by having the leads extend into first contact pad structures. 
     In an embodiment, the first and second contact pad structures are in pressure contact with one another. The contacts pad are preferably pressed together to achieve an electrical connection. No (or at least little) adhesive is preferably present between the interface surfaces of the first and second contact pad structures. After the pressure contact has been established all contact pad structures are covered with an adhesive. Adhesive may be applied to pairs of first and second contact pad structures individually, in groups thereof, or in the form of a single adhesive coating or layer for all contact pad structures. This allows for a relatively easy manufacturing of the print head according to the present disclosure. 
     In an embodiment, the flexible electrical connection element extends away from the first contact pad structure in a direction perpendicular to the interface surface. The flexible connection element, preferably in the form of a flexible printed sheet provided with conductive tracks, extends away from the first contact pad structure upwards from the outer surface of the body towards a controller positioned remote from the droplet jetting device. The outer surface is preferably perpendicular to the droplet jetting direction of the droplet jetting, which is generally the vertical direction. 
     In an embodiment, the area of a contact surface between the first contact pad structure and the outer surface of the body is smaller than a maximum cross-sectional area of the first contact pad structure. The contact surface is the bottom surface of the first contact pad structure. The contact surface is narrowed compared to a wider portion of the first contact pad structure, positioned higher up in the perpendicular direction or more remote from the outer surface. In an embodiment, the contact surface is smaller than the interface surface of the first contact pad structure. 
     The present disclosure further relates to interconnection structure for a print head, having a first contact pad structure provided on a contact surface, a flexible electrical connection element having a second contact pad structure at one end, the second contact pad structure being mounted onto the first contact pad structure, the first and second contact pad structures each having an interface surface which contacts the interface surface of the other to form an electrical connection, an adhesive covering the first and second contact pad structures and securing them together, wherein at least one of the first contact pad structure and the second contact pad structure includes a recessed portion as compared to a wider portion, which wider portion is positioned between the recessed portion and the interface surface. The interconnection structure and its components may be embodied as in any of the above described embodiments. 
     The present disclosure further relates to a method for connecting a droplet jetting device to a controller, that includes forming a first contact pad structure on a contact surface, such that a recess accessible in a direction parallel to the contact surface is formed in at least one side of the first contact pad structure, positioning a second contact pad structure of a flexible electrical connection element on the first contact pad structure, wherein one of the first and second contact pad structures is provided on a body holding a droplet jetting device and the other is provided at the end of a flexible connection element, applying adhesive to the first and/or second contact pad structures for securing these together, wherein adhesive flows into the at least one recess. 
     The first contact pad structure herein may be positioned on the droplet jetting device or on the connection element and may be shaped or formed as described in any of the previously discussed embodiments. After applying the adhesive, the adhesive is hardened and/or cured including a portion of the adhesive, which is present in the recess. The adhesive portion in the recess is positioned between a wider portion of the first contact pad structure and the outer surface of the body. In its hardened form this adhesive portion acts as an anchor which prevents the second contact pad structure from breaking contact with the first contact pad structure. It will be appreciated that adhesive may be applied to either one or both contact pad structures before and/or after bringing the contact pad structures together. During the pressing together of the contact pad structures, the adhesive is driven out of the interface between the contact pad structure to enable the electrical connection. 
     In an embodiment, the step of applying the adhesive includes the adhesive at least partially filling the recess. The recess is at least partially filled with adhesive. In a further step, the adhesive may be hardened by a suitable process, such as drying or curing. A portion of the adhesive inside the recess is hardened cured as well. The adhesive is preferably selected such that it adheres or bonds well to the flexible connection element and outer surface, which contributes to a secure holding. Providing each first contact pad structure with a recess provides an additional holding per individual bond pad. 
     In embodiment, the first contact pad structure is formed on a removable layer, and the at least one recess is formed by at least partially removing the removable layer. The removable layer is preferably etchable. The removable layer may be applied, such that the first contact pad structure upon its formation is formed with recesses and/or such that the removable layer is partially removable below the first contact pad structure, such that its bottom surface is partially exposed. 
     Further scope of applicability of the present disclosure will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the present disclosure, are given by way of illustration only, since various changes and modifications within the spirit and scope of the present disclosure will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present disclosure, and wherein: 
         FIG. 1  is a sectional view of a droplet jetting device; 
         FIG. 2  is an enlarged view of the right hand side of the droplet jetting device in  FIG. 1  connected to the flexible connection element; 
         FIG. 3  is a schematic view of an embodiment of the interconnection structure according to the present disclosure; 
         FIG. 4  is a schematic view of another embodiment of the interconnection structure according to the present disclosure; 
         FIG. 5  is a schematic top-down, see-through view of the interconnection structure in  FIG. 3  or  FIG. 4 ; 
         FIG. 6  is a schematic representation of the steps of an embodiment of forming the interconnection structure according to the present disclosure; and 
         FIG. 7  is a schematic representation of the steps of another embodiment of forming the interconnection structure according to the present disclosure. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The present disclosure will now be described with reference to the accompanying drawings, wherein the same reference numerals have been used to identify the same or similar elements throughout the several views. 
       FIG. 1  shows a single droplet jetting device  10  which is one of a plurality of jetting devices that have an identical design and are integrated into a common MEMS chip that may be used in an ink jet print head, for example. The MEMS chip and, accordingly, the jetting devices  10  have a layered structure comprising as main layers a distribution layer  12 , a membrane layer  14  and a nozzle layer  16 . 
     The distribution layer  12  is a single silicon layer having a relatively large thickness of at least 200 micron, preferably 300 micron and more preferably more than 400 micron. In the present example, the thickness is 400 micron. The distribution layer  12  defines an ink supply line  18  which has been shown only schematically in  FIG. 1  through which liquid ink may be supplied from an ink reservoir  19  to a pressure chamber  20  that is formed on the bottom side of the membrane layer  14 . The ink reservoir  19  which has been shown only schematically in  FIG. 1  is common to a plurality of jetting devices and is formed separately from the distribution layer  12  on the top side of the distribution layer, i.e. on the side opposite to the membrane layer  14 . This has the advantage that the distribution layer  12  is not weakened by any cavity forming the reservoir. 
     The membrane layer  14  is obtained from a SOI wafer having an insulator layer  22  and silicon layers  24  and  26  formed on both sides thereof. In this embodiment, the final membrane layer  14  may have a thickness of about 75 micron. The pressure chamber  20  is formed in the bottom silicon layer  26 . The top silicon layer  24  and the insulator layer  22  form a continuous flexible membrane  30  with uniform thickness which extends over the entire area of the MEMS chip and is pierced by an opening  28  only at the position of the ink supply line  18  so as to connect the ink supply line to the pressure chamber  20 . A piezoelectric actuator  32  is formed on the top side of the part of the membrane  30  that covers the pressure chamber  20 . The actuator  32  is accommodated in an actuator chamber  34  formed at the bottom side of the distribution layer  12 . 
     An electrically insulating silicon oxide layer  36  insulates the actuator  32  and its electrodes from the silicon layer  24  and carries electric leads  38  arranged to contact the electrodes on the top and bottom sides of the actuator  32 . The leads  38  are exposed and contactable in a contact region  40  where the distribution layer  12  has been removed. 
     The nozzle layer  16  is obtained from a double-SOI wafer and has a top silicon layer  42  and a thinner silicon layer  44  interposed between two insulator layers  46  and  48 . In this embodiment, the final nozzle layer may have a thickness of about 125 micron. A nozzle  50  is formed in the two insulator layers  46  and  48  and in the silicon layer  44  intervening between them, so that the thickness of these three layers defines the length of the nozzle. The top silicon layer  42  of the nozzle layer  16  defines a feedthrough  52  which connects the pressure chamber  20  to the nozzle  50  but has a cross-section that is significantly larger than that of the nozzle  50 . 
     It will be understood that the droplet jetting devices  10  of the MEMS chip are arranged such that their nozzles  50  define a nozzle array consisting for example of one, two or even more parallel nozzle lines with uniform nozzle-to-nozzle spacings which will determine the spatial resolution of the print head. Within the contact region  40 , each of the leads  38  can be contacted, e.g. via contacts pads  54  formed as bumps, so that energizing signals in the form of electric voltage pulses may be applied individually to each actuator  32 . When a voltage is applied to the electrodes of the actuator  32 , the piezoelectric material of the actuator is caused to deform in a bending mode, thereby flexing the membrane  30  and consequently changing the volume of the pressure chamber  20 . Typically, a voltage pulse is applied to the actuator to cause a deformation that increases the volume of the pressure chamber  20 , so that ink is sucked-in from the supply line  18 . Then, when the voltage pulse drops off or changes into a pulse with opposite polarity, the volume of the pressure chamber  20  is decreased abruptly, so that an acoustic pressure wave is generated which propagates through the pressure chamber  20  and through the feedthrough  52  to the nozzle  50 , with the result that a droplet of ink is jetted-out from the nozzle  50 . 
     In the design that is proposed here, the relatively large thickness of the distribution layer  12  is utilized for arranging the restrictor  56  to extend vertically through the distribution layer  12 . That is, the longitudinal axis of the restrictor  56  is normal to the plane of the layers  12 ,  14  and  16  of the device. This permits a compact design with small dimensions of the jetting device  10  in the plane of the layers  12 - 16 . This has the advantage that a larger number of MEMS chips can be produced from a single wafer having a given diameter. Further, the compact design permits a close packing of the individual devices  10  within the chip, and therewith a high nozzle density and, consequently, a high spatial resolution of the print head. Another advantage of the vertical arrangement of the restrictor  56  is that the length and cross-sectional area of the restrictor can be controlled with high precision by using well-established lithographic techniques. 
     As has been shown in  FIG. 1 , the distribution layer  12  is connected to the membrane layer  14  by a bonding layer  62 . Similarly, the membrane layer  14  is connected to the nozzle layer  16  by a bonding layer  64 . The bonding layers  62  and  64  being layers of adhesive, their physical properties are difficult to control. However, in the design that has been proposed here, the bonding layers are arranged such that their properties do not significantly affect any of the critical parameters of the design. 
       FIG. 2  shows in more detail the right hand side of  FIG. 1 , where the first contact pad structure  54  is positioned on the body of the print head  10 . The first contact pad structure  54  in  FIGS. 1 and 2  is positioned on the top silicon layer  24 , though within the present disclosure it may be positioned on any suitable position on the print head  10 . The first contact pad structure  54  is electrically connected to the actuator  32  as well as to the flexible, electrical connection element  70 . The connection element  70  in turn is in electrical connection to a controller (not shown). This electrical connection allows the controller to transmit energizing signals or pulses to the actuator  32  for jetting the droplets. The connection element  70  preferably comprises a large number of parallel, conductive tracks or lines corresponding to the number of droplet jetting devices  10  in the respective print head unit. A suitable connection element  70  may be formed by a flexible printed sheet, which comprises a plurality of parallel conductive tracks. The connection element  70  is provided with at least one second contact pad  72  for each droplet jetting device  10  in the print head. A second or ground connection to each actuator  32  may be provided by either a common contact pad or individual pads. Additional leads and contact pad structures may be provided for additional functionality, such as sensing or detecting certain parameters of the droplet jetting device  10  during operation. This allows for sensing temperature, actuator performance, etc. The connection element  70  is flexible to allow it to be easily mounted in the print head and to reduce forces being exerted on the interconnection structure. Generally, the connection element  70  extends upwards away from the droplet jetting device  10  in the direction D. The direction D is preferably the upward vertical direction. The first and second contact pad structures  54 ,  72  are in contact with one another by pressing their interface surfaces ( 54 D in  FIG. 3 ) together. The contact pad structures  54 ,  72  are held together by an adhesive  80  covering both contact pad structures  54 ,  72 . Any suitable adhesive  80  may be applied. Preferably a moisture resistant adhesive  80  is applied is such a manner that the adhesive  80  forms a protective layer over the contact pad structures  54 ,  72 . 
       FIG. 3  illustrates a first embodiment of the interconnection structure  100  according to the present disclosure. The first contact pad structure  54  is provided on the body  36  in electrical connection with the lead  38  extending to the actuator  32 . The second contact pad structure  72  of the connection element  70  is positioned on top of the interface surface  54 D of the first contact pad structure  54 . Both contact pad structures  54 ,  72  are held together by adhesive  80 . The first contact pad structure  54  is provided with one or more recesses  55  at one or more of its sides. The recess  55  locally reduces the diameter and/or cross-sectional area  54 AA of the first contact pad structure  54 , the cross-section being parallel to the outer surface of the body  36 . The cross-sectional area  54 AA of the recessed portion  54 A is smaller than the respective area  54 CA of a wider portion  54 C of the first contact pad structure  54 . In consequence the wider portion  54 C is present between the recessed portion  54 A and the interface surface  54 D, where the second contact pad structure  72  begins. The wider portion  54 C extends outwardly beyond the circumference of the recessed portion  54 A. The wider portion  54 C overhangs the recessed portion  54 A like an outwardly extending protrusion above the recesses  55 . The applied adhesive  80  has crept into in the recesses  55  below the wider portion  54 C (when the direction D is parallel to the upward vertical direction). The adhesive  80  in the recesses  50  forms an adhesive anchor  80 A which anchors the interconnection structure  100  against upwards forces in the direction D. Such forces may be applied for example by pulling on the connection element  70 . In consequence the first and second contact pad structures  54 ,  72  are kept in good electrical contact, ensuring a reliable operation of the droplet jetting device  10 . 
     In  FIG. 3 , the recessed portion  54 A is positioned at the bottom side of the first contact pad structure  54 , which side contacts the body  36 . The first contact pad structure  54  is shaped, specifically tapered, such that its body contact area  54 AA is smaller than its maximum cross-sectional area  54 CA of the wider portion  54 C. The wider portion  54 C in  FIG. 3  is formed by the top surface of the first contact pad structure  54 . The body contact area  54 AA of the first contact pad structure  54  faces and/or contacts the body  36 . The one or more recesses  55  in  FIG. 3  are formed by respective skewed or inclined side surfaces  54 B. The side surface  55  from the wider portion  54 C to the recessed portion  54 A curves inward towards a centre (C in  FIG. 5 ) of the first contact pad structure  54 . The curvature or shape of the side surface  54 B may be determined by the manufacturing process, specifically the etching properties of the applied materials. 
       FIG. 4  illustrates another embodiment of the interconnection structure  200  according to the present disclosure. In  FIG. 4 , the first contact pad structure  154  is substantially mushroom-shaped. The first contact pad structure  154  has the recessed portion  154 A and the wider portion  154 B. The wider portion  154 B is positioned between the top surface and bottom surface  154 AA of the first contact pad structure  154  in the direction D. The top portion is narrowed with respect to the wider portion  154 B, which may be a result of the surface tension in the material used for the first contact pad structure  154  during its deposition process. Similarly the recesses  55  not need be positioned at the bottom of the first contact pad structure  154 , but may in another embodiment be positioned between the top and bottom surface of the first contact pad structure  154 , which surfaces respectively contact the body  36  and the second contact pad structure  172 . 
       FIG. 4  further illustrates that mutatis mutandis the recessed portion may further be applied in the second contact pad structure  172  on the connection element. Similar recesses  175  as discussed for the first contact pad structure  154  may be applied in the second contact pad structure  172 . 
       FIG. 5  schematically illustrates the first contact pad structure  54  in a top down view in the direction D. The area  54 CA of the wider portion  54 C extends outwardly from the centre C of the first contact pad structure  54  beyond the area  54 AA or circumference of the recessed portion, such that the recess  55  is formed. The recess  55  is positioned between the wider portion  54 C and the outer surface of the body  10 . The adhesive  80  is applied covering and/or surrounding the wider portion  54 C and recessed portion  54 A. The adhesive locally extends or protrudes into the recess  55  inwards towards the centre C beyond the circumference of the area  54 CA of the wider portion  54 C. Thereby, in  FIG. 5 , a bump or protrusion of adhesive is formed in the recess  55 , which acts as an anchor  80 A against forces in the direction D. The adhesive anchor  80 A in the recess  55  is prevented from upward movement in the direction D due to the overhanding wider portion  54 C. This effectively forms a seal which secures the first and second contact pad structures  54 ,  72  together. 
       FIG. 6  illustrates schematically the steps for forming an interconnection structure  100  according to the present disclosure. A body of a droplet jetting device  10  is provided. In  FIG. 6 , the body of a droplet jetting device  10  is formed of a silicon substrate  34 , which is provided with an isolating layer  36 , which may be formed of SiN x , SiO x , or other suitable isolators. A lead  38  has further been deposited on the substrate  34 . Suitable conductive materials may be applied for the lead  38 , such as metals, specifically gold, copper, platinum, etc. or alloys thereof. Additionally a seed layer  37  may be provided to enhance the deposition and bonding of additional layers or structures. The materials for the seed layer  37  are selected in correspondence to the materials to be deposited thereon. In step (a) a removable or etchable layer or structure  39  is provided. Preferably, the removable layer  39  is formed of a photo-resistant material as commonly applied in MEMS processing. The removable layer  39  is structured, such that it comprises an opening  39 A positioned over the lead  38 . Subsequently in step (b) conductive material is locally deposited in the area in and around the opening  39 A to form the first contact pad structure  54 . Similar materials as for the lead  38  may be applied for the first contact pad structure  54 . The structured or local depositing of the first contact pad structure  54  and/or the removable layer  39  may be controlled by using a mask provided with openings corresponding to the desired location for depositing the respective materials. The thickness of the first contact pad structure  54  is greater than that of the removable layer  39 . The first contact pad structure  54  is further deposited in the opening  39 A as well as on the adjacent top surface of the removable layer  39 . This results in a mushroom shaped first contact pad structure  54 . In step (c) the removable layer  39  is removed by etching, preferably photolithographic etching, creating the recess  55  on the bottom side of the first contact pad structure  54 . In step (d) the connection element is connected to the droplet jetting device  10  by contacting the first and second contact pad structures  54 ,  72 . While in contact, the adhesive  80  is applied covering the contact pad structures  54 ,  72 . In its liquid state, the adhesive  80  creeps into the recess  55 , such that it forms an anchor portion  80 A which protrudes inwardly beyond the circumference of the wider portion  54 C. Subsequently the adhesive  80  is hardened, thereby forming a secure bond between the first and second contact pad structures  54 ,  72 . 
       FIG. 7  is another embodiment of the method for forming the interconnection structure  100 ,  200  according to the present disclosure. A similar body of the droplet jetting device  10  as prior to the application of the removable layer  39  in  FIG. 6  is provided. In step (a) in  FIG. 7  the top contact pad portion  254 C is deposited on the seed layer  37  (without first applying the removable layer  39  as in  FIG. 6 ). In step (b) the seed layer  37  is removed by etching. The etch process conditions are selected such that so-called under-etching of the seed layer  37  below the first contact pad structure  54  occurs. The recess  255  is created by removing the seed layer  37  below the bottom edges of the top contact pad portion  254 C. In step (c) the connection element is bonded to the body  10  similar to step (d) in  FIG. 6 . In contrast to the embodiments in  FIGS. 3 to 6 , the first contact pad structure  254  in  FIG. 7  is formed of two different materials, specifically the seed layer portion  254 A and the top portion  254 C, wherein the recessed portion is formed by the etched seed layer portion  254 A. 
     It will be appreciated that each print head may comprise a large number of droplet jetting devices  10  applied in e.g. parallel or staggered configuration. The leads of the droplet jetting devices preferably all extend to an interconnection area, where the respective first bond pads structures for the leads are grouped together. Likewise, the second contact pad structures are preferably grouped together on a single connection element, such that all droplet jetting devices may be connecting with a single alignment of the connection element on the body. The adhesive is shown in  FIGS. 3 to 7  as applied individually to each contact pad pair, but may also be applied as a wider layer covering multiple pairs contact pad structures. 
     Although specific embodiments are illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations exist. It should be appreciated that the exemplary embodiment or exemplary embodiments are examples only and are not intended to limit the scope, applicability, or configuration in any way. Rather, the foregoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing at least one exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents. Generally, this application is intended to cover any adaptations or variations of the specific embodiments discussed herein. 
     It will also be appreciated that in this document the terms ‘comprise’, ‘comprising’, ‘include’, ‘including’, ‘contain’, ‘containing’, ‘have’, ‘having’, and any variations thereof, are intended to be understood in an inclusive (i.e. non-exclusive) sense, such that the process, method, device, apparatus or system described herein is not limited to those features or parts or elements or steps recited but may include other elements, features, parts or steps not expressly listed or inherent to such process, method, article, or apparatus. Furthermore, the terms ‘a’ and ‘an’ used herein are intended to be understood as meaning one or more unless explicitly stated otherwise. Moreover, the terms ‘first’, ‘second’, ‘third’, etc. are used merely as labels, and are not intended to impose numerical requirements on or to establish a certain ranking of importance of their objects. 
     The present disclosure being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present disclosure, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the claims. 
     This application claims the benefit of European Patent Application No. 21152840.1, filed Jan. 21, 2021, which is hereby incorporated by reference herein in its entirety.