Induction soldering process for connecting FFC / PFC cables to various terminals

Abstract

A method for attaching a flat flexible cable (FFC) (10) to a plurality of terminals (180) of an electrical connector (140) includes a step of arranging a plurality of terminals (180) within a connector housing (140, 141), each terminal defining a weld area (182) electrically connectable to a conductor (12) of the FFC (10). The FFC (10) is then positioned proximate the connector housing (140, 141) such that the weld areas (182) of each terminal (180) are disposed directly adjacent to a respective one of a plurality of exposed conductors (12) of the FFC (10). At least the welding areas (182) of the plurality of terminals (180) are heated with an inductive heat source (270) to electrically connect the plurality of conductors (12) of the FFC (10) to the plurality of terminals (180).

Description

The subject matter herein relates to electrical connectors and, more particularly, to systems and methods for electrically connecting flat flexible cables to conductive terminals of electrical connectors.As understood by those skilled in the art, flat flexible cables (FFCs) or printed flexible cables (PFCs) are electrical components that consist of at least one conductor (e.g., a metallic foil conductor) embedded in a thin flexible strip of insulation. Flat flexible cables gain popularity throughout many industries due to advantages provided over their traditional "round cable" counterparts. In addition to having a lower profile and weight, FFCs in particular also allow for the implementation of large circuit paths with substantially greater ease compared to round cable based architectures. As a result, FFCs are implemented in many complex and / or large volume applications, including wire harnesses such as those used in automobile manufacturing.A critical obstacle that prevents the implementation of FFCs in these applications is the need to develop fast, robust, and low resistance termination techniques that allow the FFCs to fit with various components. One particular challenge involves the reliable and efficient connection of the sensitive conductors of the FFCs to a conductive terminal of a connector. Existing methods involve the use of laser or resistance heating techniques. However, laser heating is often less reliable and awkward to implement. Resistance heating has also proved to be relatively difficult to implement reliably, and is time consuming and requires periodic maintenance (e.g. the exchange of heating tips). The current methods are also not suitable for mass connection and / or automation desired by the industry.Accordingly, improved methods for interfacing FFC packages are desired.In one embodiment of the present disclosure, a method of attaching a flat flexible cable (FFC) to a plurality of terminals of an electrical connector includes a step of disposing a plurality of terminals within a connector housing, each terminal defining a weld area adapted to be electrically connected to a conductor of the FFC. The FFC is positioned proximate the connector housing such that the weld regions of each terminal are disposed directly adjacent a corresponding one of a plurality of exposed conductors of the FFC. At least the weld areas of the plurality of terminals are heated with an inductive heat source to electrically connect the plurality of conductors of the FFC to the plurality of terminals.The invention will now be described by way of example with reference to the accompanying drawings, in which FIG. 1 is a side perspective view of an FFC connector assembly useful for describing embodiments of the present disclosure in a fitted state; FIG. 2 is a cross-sectional view of the connector assembly of FIG. 1 in an initial alignment state; FIG. 3 is a cross-sectional view of the connector assembly of FIG. 1 in the fitted state; FIG. 4 is a bottom perspective view of a cable subassembly including an FFC and a cable stiffener installed thereon; FIG. 5 is a front view of the cable subassembly of FIG. 4 ; FIG. 6 is a bottom perspective view of a terminal used in the connector assembly of the foregoing figures; FIG. 7 is a bottom view of the connector of FIG. 6 ; FIG. 8 is a side view of the connector of FIG. 7 ; FIG. 9 is a partial side view of another terminal that may be used in embodiments of the present disclosure; FIG. 10 is a plan view of a plug of the connector assembly having a plurality of terminals inserted therein; FIG. 11 is a bottom view of the plug of FIG. 10 ; FIG. 12 is a bottom perspective view of the cable subassembly of FIGS. 4 and 5 further including solder applied over exposed conductors of the FFC; FIG. 13 is a side perspective view of a cable assembly including the cable subassembly of FIGS. 4 and 4 in an initial mating position with the plug of FIGS. 10 and 11 ; FIG. 14 is a bottom view of the cable assembly of FIG. 13, illustrating a soldering operation or method for electrically connecting the FFC to the terminals of the connector; FIG. 15 is a side view illustrating a soldering operation performed by a system according to embodiments of the present disclosure; FIG. 16 is a diagram of an example system useful for performing the soldering operations described herein; and FIG. 17 is a cross-sectional view of a connector using graphite fingers to improve induction soldering of its terminals according to another embodiment of the present disclosure.Exemplary embodiments of the present disclosure will be described in detail hereinafter with reference to the accompanying drawings, wherein the same reference numerals refer to the same elements. However, the present disclosure may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that the present disclosure will be thorough and complete, and will fully convey the concept of the disclosure to those skilled in the art.In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are shown schematically to simplify the drawing.Referring now to FIGS. 1-3, there is shown an exemplary FFC connector assembly 100 useful for describing connection methods according to embodiments of the present disclosure. The connector assembly 100 includes a flat flexible cable (FFC) 10, a stiffener 120, a plug 140, and a header 160. The assembly 100 is adapted to electrically connect the FFC 10 to a substrate 11, e.g., a printed circuit board (PCB). FIGS. 1 and 3 illustrate the connector assembly 100 in the fully fitted state, wherein a cable assembly 101 including the FFC 10, the stiffener 120, and the plug 140 is fitted with the header 160. In FIG. 2, the assembly 100 is in an initial orientation or partially fitted position in which the stiffener 120 and the plug 140 have not yet been connected or electrically engaged to the header 160. As shown, the cable assembly 101 is inserted into a front opening of the head portion 160. In this manner, the assembly 100 includes a so-called "front load" connector system.As further shown in FIG. 2, a plurality of conductive terminals 180 are retained within the plug 140 and are electrically connected to a corresponding plurality of conductors 12 of the FFC 10, for example, via a soldering process in accordance with embodiments of the present disclosure. Each terminal 180 is further connectable to a corresponding one of a plurality of header conductive tabs or contacts 190 disposed on or in the header 160 when the plug 140 is inserted therein, as shown in FIG. 3. The ends of each header tab 190 are exposed through a bottom of the header 160 so that they can be electrically connected to the substrate or PCB 11 (e.g., via soldering or welding with conductive traces or pads formed thereon).Referring now to FIGS. 4 and 5, the cable stiffener or element 120 is adapted to structurally support and securely attach the FFC 10 to the plug 140. The stiffener 120 defines a slotted opening 122 sized to receive the FFC 10 therethrough, a pair of guide protrusions 124, and a pair of locking arms 126. In one embodiment, the FFC 10 is attached (e.g., with an adhesive applied therebetween) to the stiffener in an initial step of a termination or assembly process. The guide protrusions 124 and the locking arms 126 are disposed on both sides of the stiffener 120. The guide protrusions 124 are adapted to guide the cable assembly when connected to the head shield 160. The locking arms 126 are adapted to secure or attach the cable stiffening element 120 to the plug 140, as shown in FIGS. 13 and 14. Referring to FIG. 5, the stiffener 120 may also include welding windows 121 (an example window is shown) formed therethrough to facilitate welding or soldering the FFC 10 and the terminals 180 from a top of the cable assembly 101.Still referring to FIGS. 4 and 5, the conductors 12 of the example FFC 10 are embedded in an insulating material 14. The conductors 12 may include, by way of example only, metallic sheets or foils, such as copper foil, structured in any desired configuration. The insulating material 14, e.g., a polymer insulating material, may be applied to each side of the conductors 12 via an adhesive, resulting in an embedded conductor arrangement. The insulating material 14 may be selectively removed or initially not deposited in the desired areas to expose the conductors 12, e.g., in a window 165 defined on the bottom of the exemplary FFC 10. The exposed portion of each of the conductors 12 is then connected (e.g., soldered) to a corresponding terminal 180 that is retained within the plug 140, as set forth in more detail herein.FIGS. 6-8 illustrate the example connector 180 in more detail. The terminal 180 defines a central slot or slotted opening adapted to slidably receive the header contact 190 therein in an insertion direction I. The opening includes a front end or opening 188 and a slotted contact portion 181 communicating therewith. The terminal 180 also defines a weld region 182 adapted to be electrically connected to the exposed conductor 12 of the FFC 10 by welding or soldering. More specifically, the terminal 180 may have a generally inverted U-shaped cross section comprising a top wall 183 and two generally parallel side walls 184 (or spring members) extending perpendicularly from the top wall. Weld portion 182 includes a generally planar or flattened surface defined on top wall 183. As shown in FIG. 8, an area directly below the weld area 182 defines a void 185.In the exemplary embodiment, one of the side walls 184 defines an integral bracket or support 187 that extends across the central opening and engages the other of the side walls 184. More specifically, the bracket 187 may be bent over the slotted opening or contact area 181 defined between the side walls 184 on a bottom side of the terminal 180 opposite the top wall 183. The bracket 187 may engage or be received by a corresponding recess 189 formed in the other of the side walls 184 such that its free end faces the side wall 184 in a direction perpendicular to a longitudinal axis of the contact portion 181. In this manner, the clip 187 is adapted to prevent excessive spreading or opening of the slotted contact portion 181 when the header contact 190 is inserted therein. This ensures sufficient and uniform electrical contact force between the terminal 180 and the header contact 190.As can be seen from the figures, the terminal 180 may be formed by a combination of sheet metal forming operations such as stamping and bending. Punching the area that ultimately defines the side walls 184 adjacent the weld area 182 is used to effectively broaden the weld area. Also, punching the portion corresponding to the top wall 183 is used to form the slotted contact portion 181. Each of the side walls 184 may be bent or curved inwardly toward a central axis center of the terminal in the contact portion 181 to apply an adequate elastic stress or normal force to an inserted header tab 190. In some embodiments, the side walls 184 define inwardly facing, opposing raised contact protrusions 186 that are adapted to apply a further engagement force to the header tab 190. In each embodiment, the terminal 180 and contact area 181 are adapted to produce sufficient normal force to be effectively used with tin or silver plating on a mating terminal. In still other embodiments, soldering or welding may also be used to connect the terminals 180 to the respective FFC conductors 12 without departing from the scope of the present disclosure. It should be understood that the separation between the sidewalls 184 ensures that thermal stress relaxation does not occur during welding or soldering operations performed on the terminal 180.FIG. 9 illustrates another terminal 190 that may be used in plug or connector assemblies according to embodiments of the present disclosure. As shown, the terminal 190 includes a flattened weld or solder region 192 formed proximate a first end thereof and a contact region 191 at a second end thereof (e.g., a female contact region adapted to receive a pin terminal). It should be understood that embodiments of the present disclosure are not limited to the two types of terminals illustrated in FIGS. 6-9.FIGS. 10 and 11 illustrate the plug 140 with the example terminals 180 inserted therein. As shown in FIG. 10, the connector 140 includes a connector body 141 defining a plurality of terminal openings 142. The terminals 180 are inserted into the openings 142 inserted in a rear side of the plug body in an insertion direction I'. After the terminals 180 are inserted, a slidable cover 144 can be slid from an open position (not shown) to the illustrated closed position, closing the apertures 142 and securing the terminals in the body 141. The plug body 141 is adapted to position each of the welding portions 182 of the terminals 180 in the same plane extending in the lateral direction.The plug body 141 further includes elongated alignment projections or guides 149 formed on each side surface thereof. The guides 149 are adapted to align the plug 140 with respect to the head portion 160 and to guide its insertion therein in the insertion direction (see FIGS. 1-3 ). The guides 149 also serve to align the plug body 141 and the head part 160 in the fitted state. The plug body 141 also defines locking recesses 148 formed on each of its side surfaces. The recesses 148 are adapted to receive and securely engage the locking arms 126 of the stiffener 120 to secure the stiffener 120 (and the FFC 10) to the plug 140. In the secured or locked position, the stiffener 120 is adapted to maintain or position the conductors 12 of the FFC 10 in contact with the terminals 180 (e.g., the weld areas 182 thereof).An upper wall of the plug body 141 shown in FIG. 10 defines welding tab openings 152 through which the welding portions 182 of the terminals 180 on the cable side of the plug 140 are exposed. As shown in FIG. 11, a bottom surface of the plug 140 defines a plurality of windows 154 through which the bottom surfaces of the weld portions 182 of the terminals 180 are exposed. The weld areas 182 may be heated through the windows 154, for example, to weld or solder the terminals 180 to the conductors 12 of the FFC 10. The plug body 141 further includes a plurality of slits 156 formed through a bottom wall thereof and extending from a front end of the plug 140 in a direction opposite to the insertion direction I'. The slots 156 are adapted (e.g., sized, shaped, and disposed) to slidably receive the contacts of the header contact 190 upon mating of the plug 140 and the header 160.FIG. 12 provides a bottom perspective view of the cable subassembly in an intermediate step of a termination process according to an embodiment of the present disclosure. In particular, after the stiffener 120 is attached to the FFC 10 (e.g., bonded with an adhesive as shown), solder 50 is applied to the exposed conductors 12 of the FFC 10. The solder 50 may be applied in the form of a solder foil or solder paste (each with or without flux, e.g., a flux pretreated foil or paste or a foil or paste with separate dosed flux). In other embodiments, the FFC 10 may be provided with pre-coated solder, e.g., in the form of solder balls or solder pads.In a particularly advantageous embodiment, the solder 50 is applied continuously and uniformly from one lateral side of the FFC 10 to the other or from an outer exposed conductor 12 of the FFC 10 to an outer exposed conductor on the opposite side of the FFC. This arrangement simplifies the solder deposition process (e.g., either a single solder foil strip or a single deposition of solder paste) compared to embodiments that deposit the solder only on the exposed conductors. As will be discussed in more detail herein, the soldering process according to embodiments of the present disclosure is adapted to draw out the solder from between the conductors 12 of the FFC 10 and from between the corresponding terminals (e.g., the terminals 180 to be welded thereto).Referring now to FIG. 13, a process for manufacturing the cable assembly 101 of the connector assembly 100 is provided. As shown in FIG. 13, the FFC and the stiffener 120 are fitted to the plug 140 with the cable stiffener 120 fixed to the FFC 10 (e.g., by adhesive). Specifically, biasing the stiffener 120 from a separated position in a downward direction V snap-fits the locking arms 126 into the recesses 148 of the plug body 141. This positions the exposed conductors 12 of the FFC 10 opposite or immediately adjacent (e.g., abutting) the weld regions 182 of the terminals 180. As shown in FIG. 14, the conductors 12 and the terminals 180 are heated from the bottom of the connector 140 through the windows 154 by an inductive heating element. In an embodiment, this target heating region A has a length of at least 3 mm in an axial direction of each terminal 180 and / or conductor 12.Referring to FIG. 15, the terminal 180 and the FFC 10 are shown during a soldering operation. For clarity, the plug 140 has been removed and the terminal and cable reversed from their orientations as described above. Melting of the solder 50 disposed between the conductors 12 of the FFC and the weld portion 182 of each terminal 180 is accomplished by at least one inductive heating coil or induction coil 270. The example coil 270 is disposed on a side of the terminal 180 opposite the FFC 10 (i.e., on a bottom side of the plug or terminal as shown in FIG. 14). The molten solder 50 transfers heat to the conductors 12 of the FFC 10 to create a stable, fully wetted solder joint. For thicker conductors and / or unique terminal geometries, a second inductor 270 may be placed on the other side of the terminal 180 to heat the conductors as needed and avoid overheating of the terminal. Surface tension and the suction of the solder 50 pulls the solder from between the terminals 180 and the conductors 12 and into each connection space. During soldering, at least one of the plug 140 or the FFC 10 may be vibrated to promote the suction of the solder from between the terminals 180 and into the connection space. After completion of a brazing cycle cool air could be injected to further ensure that the terminal is not heated to the point where stress relaxation occurs. In other embodiments, cool air could be continuously injected, even during brazing operations, to ensure that a front portion of the plug remains relatively cool.Referring to FIG. 17, in some embodiments, depending on the terminal geometry, graphite fingers may be used to conduct or concentrate heat during the inductive soldering operations of the FFC 10 to where it is needed. In the exemplary simplified embodiment, a connector 300 is shown having a cover or housing 310 formed with an access opening 320. The access opening 320 may expose weld areas 382 of a plurality of terminals 380 as described above in the previous embodiments. As would be understood by one of ordinary skill in the art, induction heating is caused by circulating currents within a workpiece. Graphite requires less current by orders of magnitude than, for example, copper in order to achieve the same heating effect. For this reason, the use of graphite fingers 330 positioned over the welding regions 382 of the terminals 380 may increase the soldering efficiency and improve the target accuracy of the regions to be heated. It should be understood that the connector 300 of FIG. 17 is a simplified illustration to illustrate the use of the graphite fingers 330. It should be understood that many other variations may be made to the illustrated embodiment, e.g., the number of openings 320 exposing the terminals 380 and / or FFC conductors may be varied (e.g., there may be such an opening for each weld area 382 or terminal 380. Likewise, a single graphite element may be inserted into the opening 320 while a plurality of graphite fingers 330 are shown. The graphite fingers 330 may also be present on both sides of the connector 300 or used in any of the connectors described above without departing from the scope of the present disclosure.While the above embodiments of the present disclosure describe the use of inductive soldering techniques for connecting FFCs to terminals, it should be understood that other forms of soldering are used with the terminals described herein. For example, resistance brazing techniques or brazing may be used without departing from the present disclosure. As would be understood by those skilled in the art, this technique would require access to either side of the joint as well as the use of a fusible link alloy between the elements to be joined (i.e., the terminals and the FFC conductors). Also, other techniques such as laser soldering (e.g., so-called "BLUE" laser soldering) may offer distinct advantages in speed and accuracy over other soldering techniques and typically result in very small spot size and low solder consumption.Soldering methods according to embodiments of the present disclosure may be performed in whole or in part by one or more automated control systems that implement and / or control a soldering system or machine, as well as additional hardware and software functions. For example, referring generally to FIG. 16, an example control system 200 of a soldering system or machine 202 useful for performing the operations of the embodiments of the present disclosure is shown. The control system 200 may be under fully automated control, or may be fully or partially controlled via one or more user input devices 205 (e.g., touch screen / buttons / keyboards, etc.). The control system 200 includes at least one processor 210, e.g., a digital microprocessor, responsive to instructions stored in a storage device 220 to perform the methods or operations described herein. The processor 210 is operatively coupled to the inductor 270 and / or to the power supply thereof to selectively operate the inductor under voltage and / or current control. The system 200 may further include a current and / or frequency monitor or sensor 230 that may cooperate with the processor 210 to monitor and / or control the frequency and current in or through the induction coil 270.The system 200, and in particular the processor 210, may control the operation of feed wheels, vibration generators, and / or blowers 260 of the machine 202 to selectively pass the cable through the machine, vibrate the cable and connector assembly during soldering, and cool the connection. Similarly, the control system 200 may include one or more actuators 240 (e.g., a linear actuator) operatively attached to the induction coil 270 to selectively move the coil relative to the connector assembly to be soldered. In one embodiment, the one or more actuators 240 may be multi-directional, having the ability to vary not only the longitudinal position of the induction coil 270 along a length of a connector assembly, but also the radial or lateral distance between the connector assembly and the induction coil, which further promotes the ability to accurately control heat generation in predetermined areas of the assembly.The control system 200 further includes a temperature sensing device and / or an imaging device, such as a thermal imaging device and, more particularly, an infrared (IR) temperature sensor and / or a camera 250, for example only. In other embodiments, the control system 200 may include separate temperature sensing devices and imaging devices. Further, the imaging device 250 may be optical, such as a digital camera or a video capturing device, without departing from the scope of the present disclosure. Thermal imaging device 250 may be mounted to induction coil 270 or to another portion of machine 202 suitable for achieving appropriate operation. As shown, each of the components of the control system 200 and / or the machine 202 may communicate via a shared power / data bus 215.The control system 200, including the processor 210 operating with associated instructions prestored in the storage device 220, enables multiple additional modes of operation to those described above with reference to the previous figures. For example, the processor 210 operates using the current and / or frequency monitoring device 230 as well as predetermined values stored in the memory device 220 to determine or estimate a characteristic, such as a size of the FFC conductors and / or the terminals, and automatically adjust various operating parameters according to this determination. The system 200 may vary heating times, periodic cycle parameters, frequency, voltage, and / or current associated with operation of the induction coil 270 according to a detected characteristic to achieve optimal operation. These parameters may be prestored in the storage device 220, such that after a determination by the processor 210 regarding the relevant characteristics of the connector and / or the FFC arrangement, the function of the coil 270 may be automatically controlled without requiring further user inputs.According to embodiments, the inductor 270 is energized for a certain amount of time based on the application. The frequency of the induction coil 270 may be varied to control the depth of heating, e.g., the higher frequency enables the depth of induction heating to be controlled such that the penetration of the induction heating is flat. In contrast, a lower frequency allows for deeper penetration of induction heating into the elements to be soldered.

Claims

A method of attaching a flat flexible cable (FFC) (10) to a plurality of terminals (180), comprising: disposing a plurality of terminals (180) in a connector housing (140), each terminal (180) defining a weld region (182) adapted to be electrically connected to a conductor (12) of the FFC (10); positioning the FFC (10) proximate the connector housing (140) such that the weld region (182) of each terminal (180) is disposed directly adjacent a respective one of a plurality of exposed conductors (12) of the FFC (10); heating at least the weld area (182) of each of the plurality of terminals (180) with an inductive heat source (270) to electrically connect the plurality of conductors (12) of the FFC (10) to the plurality of terminals (180).The method of claim 1, further comprising the step of, prior to positioning the FFC (10) proximate the connector housing (140), fitting a stiffener (120) on a side of the FFC (10) opposite the plurality of exposed conductors (12).The method of claim 2, further comprising the step of, prior to the heating step, securing the stiffener (120) to the connector housing (140) such that the exposed conductors (12) of the FFC (10) are aligned with the weld areas (182) of the terminals (180).The method of claim 3, wherein the connector housing (140) includes: a plurality of first openings (152) defined on a first side thereof and exposing the weld area (182) of each of the terminals (180); and a plurality of second openings (154) defined on a second side thereof and exposing a bottom of each weld area (182).The method of claim 4, wherein the inductive heat source (270) is disposed proximate the second side of the connector housing (140).The method of claim 5, wherein the heating step is additionally performed by a second inductive heat source (270) disposed proximate the first side of the connector housing (140).The method of claim 6, wherein the stiffener (120) includes a plurality of stiffener openings (121) formed therethrough and adapted to be aligned with each of the plurality of conductors (12) of the FFC (10), wherein the second inductive heat source (270) heats the weld areas (182) of the plurality of terminals (180) through the plurality of stiffener openings (121).The method of claim 4, further comprising a plurality of third openings (156) defined in the second side of the housing (140) and exposing a contact region (181) of each of the terminals (180) to electrically engage a respective one of a plurality of contacts (190) of a complementary mating connector (160).The method of claim 1, wherein at least one of solder balls or solder pads (50) is formed on the exposed conductors (12) of the FFC (10).The method of claim 1, further comprising the step of applying the solder (50) to the exposed conductors of the FFC (10) prior to the step of positioning the FFC (10).The method of claim 10, wherein the step of applying solder (50) includes applying a single continuous layer of solder over the plurality of exposed conductors (12).The method of claim 11, wherein the step of heating is controlled to draw out the solder (50) from between adjacent ones of the plurality of exposed conductors (12) and the plurality of terminals (180).The method of claim 11, wherein the step of applying the single continuous solder layer (50) includes applying a solder foil over the plurality of exposed conductors (12).The method of claim 11, wherein the step of applying the single continuous solder layer (50) includes applying a solder paste over the plurality of exposed conductors (12).The method of claim 1, further including the step of blowing air (260) onto at least one of the plurality of terminals (180) or the connector housing (140).The method as recited in claim 1, further comprising the steps of: detecting a temperature (250) of the weld area (182) of at least one of the plurality of terminals (180); and controlling (210) at least one of a duration, current, or frequency of operation of an inductive coil of the inductive heat source (270).

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