Hybrid interconnect lead frame for thermal ink jet printhead and methods of manufacture and connection

Abstract: Disclosed herein is a method and apparatus for interconnecting a flexible (flex) circuit, printed circuit or the like to an ink jet printhead or other similar electronic device. This approach includes providing a flexible lead frame member (10) between the flex circuit and printhead, with the lead frame member (10) having plurality of conductive leads or fingers (14) extending at a predetermined angle ⊖ with respect to the final plane of interconnection. During the interconnection process, these leads or fingers (14) are spring biased through this predetermined angle to provide good compressive electrical contact between the two members interconnected by the lead frame (10) in a single plane of interconnection.

Description: Technical Field This invention relates generally to thermal ink jet printing and more particularly to a new and improved electrical interconnect lead frame for supplying power to an ink jet printhead or similar electronic device. Background Art In the art of thermal ink jet printing, it is known to use flexible (flex) circuits to provide driving current to a plurality of heater resistors which are photolithographically defined on a thin film resistor (TFR) type thermal ink jet printhead. This flex circuit connection is sometimes accomplished by providing dimples at preselected areas in the flex circuit and using these dimples to mate with corresponding electrical contact areas on the thermal ink jet printhead. Pressure pads are applied on the reverse side of the flex circuit and in the concave reverse surfaces of the dimples to provide the necessary compressive forces to make good electrical contact between the thermal ink jet printhead and the flex circuit. Thus, good firm and continuous electrical connections are maintained between the flex circuit and the thermal ink jet printhead as the printhead moves rapidly back and forth along a platen during a thermal ink jet printing operation. During this latter motion, a loop in the flex circuit rolls back and forth along the length of the platen at a relatively high speed and with frequent jerking and direction reversal motions. Accordingly, it is imperative that this good firm electrical connection between the flex circuit and the printhead be continuously maintained once the thermal ink jet printhead and associated ink reservoir housing are locked into position in the printhead carriage. This type of printhead and flex circuit connection is disclosed, for example, in the Hewlett-Packard Journal , Volume 36, Number 5, May 1985, incorporated herein by reference. Whereas the above approach to making flex circuit contacts to thermal ink jet printheads has proven generally satisfactory in many respects, the above requirement for applying pressure pads to the dimples in the flex circuit necessitates providing a corresponding space for these dimples and pads. A part of this space requirement is defined by a vertical space dimension in the Z direction with respect to the XY plane of the printhead, and such vertical dimension represents a significant percentage of the total vertical space available within the printhead carriage. Furthermore, the lateral dimension of the dimples and the corresponding space requirement therefor in the horizontal or XY plane of the printhead imposes a corresponding limitation on how much the printhead surface area can be reduced. Also in the prior art, US 3803709 describes a test probe device for making connections to a semiconductor chip, and a manufacturing method for making the text probe. The probe includes a plurality of leads embedded in a fixed plastics lead-support frame, and which are bent to extend in steps pyramidally within the material of the frame to exposed connection tips. The tips are turned downwardly and are of sufficient length to make downward connections to terminals of an integrated circuit chip without interfering with the body of the chip, when the test probe is placed above the chip. The method of making the test probe as described in US 3803709 includes forming step shaped projections extending pyramidally downwardly in a generally flat array of leads. The outer ends of the leads are joined by expansion elements to accommodate lead movement as the projections are formed. The shaped leads are then embedded in material of fixed a plastics lead-support frame, and the expansion elements are removed to leave the ends of the leads electrically isolated. Disclosure of Invention The invention is defined in the appended claims. In particular, the invention has been devised to enable a new and improved approach to interconnecting thermal ink jet printheads to flex circuits or printed circuits. Using this approach, the above space requirements in all of the X, Y and Z directions relative to the pressure pad dimple interconnect technique of the prior art have been substantially reduced. In addition, the requirement for dimpled flex circuits and corresponding pressure pads has been totally eliminated. To accomplish this purpose, we have discovered and developed a novel interconnect circuit and method of manufacture wherein we provide a lead frame with flexible leads extending at a predetermined angle ϑ with respect to a chosen plane of interconnection. One end of the lead frame is electrically connected to an external set of contacts, and an electrical device is brought into contact with ends of the flexible leads to thereby move the leads through the predetermined angle ϑ and to the plane of interconnection. In this manner, good compressive forces in electrical connection are established at the plane of interconnection between the electrical device and the lead frame. The lead frame itself includes a plurality of electrical contact areas at one location thereon, and these contact areas are electrically connected to the corresponding plurality of flexible leads at another location within the lead frame. Thus, these leads may be flexed and spring biased through the angle ϑ to the plane of interconnection when receiving an electrical device in good compressive electrical contact, and the external contacts, the lead frame contacts, amd the contacts on the electrical device all lie substantially in a single plane of interconnection. In the manufacture of this electrical interconnect lead frame member, a metal pattern is provided within a lead frame type structure and includes plurality of metal fingers or leads extending in parallel towards a central area within the lead frame. This interconnect circuit includes connection tips on the ends of these leads which define areas of connection to a corresponding plurality of mating contact pads on a thermal ink jet printhead. These fingers or leads further extend to a remote area of the lead frame and terminate at a plurality of electrical contact areas to which flex circuits or printed circuits may be connected. The fingers or leads are insulated one from another with a suitable insulating material, and the leads extend toward the central area of the lead frame at a predetermined angle with respect to the plane of electrical interconnection with the printhead. Thus, the movement of the printhead into contact with the tips of these leads will flex the leads to a spring biased position and create compressive forces between the tips of the leads and the mating contact pads on the thermal ink jet printhead. These compressive forces eliminate the need for dimples in a flex circuit and the corresponding pressure pads therefor, and thus the distance between the printhead surface on one side of the interconnect circuit and the paper on the other side of the interconnect circuit is minimized. Additionally and most advantageous is the fact that the wiping action of the sharp edges of the tips of the leads causes scratching at the contact pads on which they come to rest. This wiping action thus further improves the electrical contact between these abutting contact members by breaking through any contamination or oxidation layers which may form on the surfaces of these contact pads. The above advantages and novel features of this invention will become better understood from the following description of the accompanying drawings. Brief Description of the Drawings Figure 1A is an isometric view of the hybrid interconnect lead frame according to the invention. Figure 1B is a greatly enlarged fragmented view of an end portion of the lead frame of Figure 1A and illustrating the path of the conductive leads extending through outer insulating material. Figure 1C is an enlarged view of the end portions of the leads and contact areas in Figure 1A. Figure 1D is a cross section view of an alternative contact tip construction for the lead frame according to the invention. Figure 2 is an isometric view of the lead frame of Figure 1A and showing its mating spatial relationship to a thermal ink jet printhead prior to electrical interconnection of these two members. Figure 3 is a cross section view taken along lines 3-3 of Figure 2. Figure 4 is an isometric view similar to Figure 2 and taken after insertion of the printhead into the printhead carriage where it makes electrical interconnection to the lead frame member. Figures 5A through 5F illustrate the process sequence used to make the lead frame interconnect circuit construction thereof along the interior portions of the lead frame in the cross section of lines 5-5 of Figure 4. Figures 6A-6D illustrate a process sequence corresponding to Figures 5A-5D and taken along portions of the lead frame in the plane of lines 6-6 of Figure 4. Figure 7 is a schematic view of the lead frame, in contact with the ink jet printhead in its in-carriage print position and making electrical interconnection between the printhead and a flexible (flex) circuit. Figure 8 shows the spatial relationship between the lead frame and the flex circuit to which it connects. Figure 9 is a cross section view taken along lines 9-9 of Figure 8. Figure 10 is a cross section view taken along lines 10-10 of Figure 8. Best Mode for Carrying Out the Invention Referring now to Figure 1A, the hybrid interconnect lead frame 10 includes an outer frame member 12 of rectangular geometry and surrounding a plurality of laminated flexible lead members 14 which extend as shown at an angle ϑ into a central region of the frame. Here the tips 16 of the individual flexible leads are exposed for spring biased electrical connection to an ink jet printhead as will be further described. The tips 16 of the leads extending from opposite ends of the frame 10 are separated by a dimension indicated as 18 which corresponds to the width dimension of a thermal ink jet printhead. Electrical contact pads of this printhead will make contact with the tips 16 of the leads as these leads are moved through the angle ϑ of approximately 15° and to the plane of interconnection with the printhead. The lead frame of Figure 1A further includes index or alignment openings 24 and 26 on each side thereof and elongated slots 28 and 30 on each end. Until the final stage of the manufacturing process, the leads 14 are connected to the frame member 12 in order to facilitate gold plating of the lead frame. The slots 28 and 30 are then cut out to separate the leads 14 from the outer metal frame member 12 to thereby electrically isolate these members 12 and 14. The lead frame 10 is constructed of a beryllium copper (Be.Cu) alloy interior substrate which is selectively coated with a polyimide on both surfaces thereof after the etching process described below to expose the tip portions of the metal fingers 14. Preferably, this polyimide material is available under the tradename "Kapton" and is sold by the DuPont Company of Wilmington, Delaware. In a typical lead frame member 10 of the type described, the Be.Cu substrate will be about 0.2µm (8 mils) in thickness and the layer of Kapton on each side thereof, including an adhesive, will be in the range of 0.05-0.13µm (2-5 mils) in thickness. The individual leads 14 are insulated one from another by the polyimide and extend between the two layers of polyimide through regions 31, 32 and terminate at exposed contact pads 33, 34 which are located adjacent the rectangular slots 28 and 30. This is illustrated in Figure 1B, in which the dashed lines indicate the presence of the BeCu leads and frame under the covering layer of polyimide. The leads 14 extend to the contact pads 33, 34 but are connected to the frame with polyimide only. In Figure 1C, the Be.Cu leads 14 are shown in more detail and are laminated with Kapton 35 on their upper and lower surfaces and in the regions between adjacent leads in the geometry shown. These leads 14 are formed individually from a Be.Cu substrate by the etching process described below, and the center-to-center spacing of these leads will typically be on the order of 1.0 to 1.1 millimeters. The lead frame portion in Figure 1B is shown in its spring biased L-shaped position, with the ends 16 of the leads spring biased to the plane of interconnection where contact is made to an ink jet pen body. This pen body may, for example, be of the type shown in Figure 2. It is important that the polyimide Kapton be sufficiently rigid so that it holds the lead fingers together and maintains a constant separation therebetween, while simultaneously allowing these fingers to flex independently of one another and to accomodate for differences in contact pad height, for example, produced by variations in manufacturing tolerances. Referring to Figure 1D, the lead tip 17 of the flexible lead frame may be constructed to have a sharp edge 19 which serves to make an abrasive contact with the surface of a contact pad 21. Thus, the wiping motion indicated by the arrow 23 is capable of breaking through any contamination or oxidation layer which forms on the surface of pad 21. This wiping motion is produced by the deflection of the lead frame during pen latching and causes the edge 19 of the lead tip 17 to travel approximately 0.2 millimeters in the direction of the arrow 23. In addition, the width of the lead edge 19 is kept at a minimum, e.g. about 0.25 millimeters, in order to avoid any trapping of fibers underneath it. Referring now to Figure 2, the pen body 36 shown therein will typically be of the type described and claimed in copending application Serial Number 880,774 of Jeffrey Baker et al, filed July 1, 1986 and assigned to the present assignee. This copending application is incorporated herein by reference and describes the pen body 36 having a rectangularly shaped end housing structure 38 for receiving a thin film resistor (TFR) substrate (not shown) which is covered by an ink ejection orifice plate 40. The orifice plate 40 will typically have three (3) circularly shaped orifice clusters 42 for ejecting ink colors of cyan, magenta and yellow during an ink jet color printing operation, and may also include an additional orifice (not shown) for ejecting black ink. The thin film resistor (TFR) substrate which is shielded from view by the orifice plate 40 includes a plurality of individual contact pads 44 positioned around the periphery of the substrate. These contact pads 44 are positioned to receive the corresponding plurality of lead frame tips 16 of the Be.Cu lead frame described above. In one type of thin film resistor (TFR) substrate and process particularly suitable for practicing this invention, the substrate contacts are fabricated using a novel peripheral contact and lead-in structure. This structure and process are disclosed and claimed in copending application Serial no. 902,287 of John L. Stoffel filed August 28, 1986 and entitled "Process for Manufacturing Thermal Ink Jet Printheads and Integrated Circuit (IC) Structures Produced Thereby", assigned to the present assignee and also incorporated herein by reference. Referring now to Figure 3, there is shown a cross section view of the hybrid interconnect lead frame taken along lines 3-3 of Figure 2. The exposed lead tips 16 extend toward the central area of the lead frame in the unbiased position and at an angle ϑ with respect to the plane of interconnection as previously described. The Be.Cu alloy leads 48 are laminated on both sides, with the polyimide laminate Kapton 49 extending on the top surface of the leads to near the ends 16 thereof. This polyimide laminate material 50 on the lower surface of the Be.Cu lead 48 extends only to an area adjacent the L shaped bend thereof. This configuration is chosen because it is desirable to minimize the amount of polyimide in the bend area of the lead frame in order to improve bending tolerances. Also, a single layer of polyimide in the region of the lead frame that flexes is typically an optimum amount for holding the leads together while still allowing them to flex somewhat independently. Referring now to Figure 4, the pen body 36 is shown in its in-carriage position where the Be.Cu leads are spring biased to the plane of interconnection. In this position, the hybrid interconnect lead frame makes good compressive electrical connection with the tips 16 of the Be.Cu leads, and these metal alloy tips 16 will wipe the surfaces of the contact pads 44 of the printhead shown in Figure 2. Referring now to Figures 5A through 5F, these schematic cross section views correspond to the fabrication process for creating the final cross section taken along lines 5-5 of Figure 4. Figure 5A shows a cross section of the Be.Cu substrate 48 taken along the length dimension thereof. The substrate 48 is typically 0.15-0.2µm (6-8 mils) in thickness and is provided with a Kapton protective and insulating polyimide layer 49 in the geometry shown in Figure 5B on the topside of the substrate 48. Another protective Kapton coating 50 is provided in the geometry shown in Figure 5C on the lower surface of the substrate 48. The Be.Cu substrate is then cut or etched in the center thereof as shown in Figure 5D to form the opening 52 therein. The Kapton layers 49 and 50 are typically comprised of 25nm (1 mil) of polyimide, with an additional 0.13-0.15µm (5-6 mils) total of adhesive divided between the two layers of Kapton. In the next two processing steps shown in Figures 5E and 5F, the tips 16 are bent slightly at the ends (Figure 5E) and then are bent again as shown in Figure 5F to give the final cross section geometry of the Be.Cu leads in a cross section taken along lines 5-5 of Figure 4. The bending of the tip 16 in Figure 5E enhances the scratching action of the tip and improves the electrical contact thereof. The bend made in Figure 5F sets the bend angle ϑ as previously described. At this point the exposed BeCu is gold plated to prevent corrosion of the contact tips and thereby enhance contact reliability, and the slots indicated as 28 and 30 of Figure 1A are cut out. Referring now to Figures 6A - 6D, there is shown a process sequence in the lead frame fabrication to create the final cross section taken along lines 6-6 of Figure 4. This process sequence provides a more complete understanding of exactly how the lead frame of Figure 1A is formed and laminated with the polyimide Kapton. In Figure 6A, the Be.Cu substrate 48 is initially masked with a photoresist etch mask 64 in the geometry shown and then exposed to a ferric chloride etch. Here the metal in the exposed regions of Figure 6A is etched away to leave the metal lead frame 66 in the geometry shown in Figure 6B. Then, the lead frame 66 of Figure 6B is covered on both sides with layers 68 and 72 of Kapton, each having a thin adhesive layer (not shown) thereon. Next, the structure of Figure 6C is transferred to a heat staker or press (not shown) wherein both Kapton layers 68 and 72 are exposed to heat and pressure for a predetermined time to cause the adhesive to flow into the openings 74 between adjacent metal leads. This process step leaves the resultant laminated lead frame structure as shown in Figure 6D. In the above lamination process, the Be.Cu leads should not reach 148°C (300°F) in less than 5 minutes in order to avoid premature edge sealing and entrapped air in the laminated structure. The pressure applied to the Kapton should be about 1380 KPa (200 psi) and the lead frame should then be cured for about one hour at 177°C (350°F). In a preferred embodiment of the invention, the center-to-center spacing of adjacent leads is on the order of 1.0 to 1.1 millimeters and the laminating polyimide Kapton is about 0.18µm (7 mils) in total thickness for both sides. The Kapton includes about 25nm (1 mil) polyimide on each side and 0.13µm (5 mils) total adhesive for both sides. This hybrid interconnect lead frame was fabricated according to Hewlett Packard's specification by the Century Circuits and Electronics Company of St. Paul, Minnesota, and this Kapton-metal fabrication technology is described in some detail in their Data Bulletin No. 1280 entitled "High Technology Flexible Printed Circuits", published in 1986 and incorporated herein by reference. Referring now to Figure 7, there is shown the TFR substrate support member 38 of the pen body 36 which receives the thin film resistor substrate 80 on the upper surface thereof. The substrate 80 includes a plurality of adjacent metal contact pads (not shown in Figure 7) spaced around the periphery thereof for making electrical contact to the ends 16 of the Be.Cu leads 14. The orifice plate 40 is positioned as shown in the center of the thin film resistor substrate 80. Advantageously, these contact pads which are indicated as 44 in Figure 2 may be made in accordance with the teachings of the above-identified Stoffel patent application Serial no. 902,287. The Be.Cu leads 14 are connected as shown to the end 84 of a flex circuit 86. This connection is accomplished using state of the art reflow soldering techniques for making contact to individual contact pads 33, 34 on the end of the hybrid lead frame of Figure 1A. Referring to Figure 8, the small windows 88 expose individual conductive traces (not shown) of the flex circuit 86 and occur in the same pattern and spacing as the exposed pads 33, 34 of the lead frame of Figure 1A. When the lead frame 10 is lowered on to the flex circuit 86 in the orientation shown, each exposed lead frame pad 33, 34 (See Figure 1A) meets the mating pad exposed by windows 88 on the flex circuit. The alignment of the lead frame 10 to the flex circuit 86 is facilitated by matching the alignment openings 24 and 26 in the lead frame 10 with the corresponding openings 90 and 92 on the flex circuit 86. Figure 9 shows a cross section view of the lead frame 10 taken along lines 9-9 of Figure 8, and Figure 10 is a cross section view taken along lines 10-10 of Figure 8. In Figure 10, an exposed pad 94 is coated with a mound of solder 96 and then aligned with the exposed area 98 in the lead frame 10. When the lead frame 10 is lowered onto the flex circuit 86, the solder mound 96 touches the lead frame pad contact area 98 and these two parts are then clamped together. An infrared lamp is then used to heat the assembly until the solder melts and flows over the pad contact area 98. Once the parts have cooled the clamp is removed. Various modifications may be made in the above described embodiments without departing from the scope of this invention. For example, the invention is not specifically limited to the interconnection of flex circuits to thermal ink jet printheads. This invention may instead be used to provide electrical connection between any two members where it is desired to minimize the required contact space requirements in the x,y and Z dimensions as described above and simultaneously provide good compressively forced electrical contact between two adjacent contact members in a single plane of interconnection.

Claims: An electrical interconnect suitable for use in establishing a substantially flat or a substantially two-dimensional electrical contact relative to a contact member in a single plane of interconnection, the interconnect comprising: a lead frame (10) having a plurality of electrically isolated contacts (33, 34) at one area thereon and connected to a corresponding plurality of metal leads or fingers (14) extending from another area thereon, the leads or fingers having connection tips (16) defining the area of connection to the contacts of the contact member, and the leads or fingers being capable of flexure, the interconnect further comprising means (35) insulating the fingers or leads one from another and holding the fingers or leads together when flexed, the leads or fingers extending at a predetermined angle (ϑ) with respect to the plane of interconnection with the contact member, such that the movement of the interconnect against the contact member causes the leads or fingers to flex to a spring biased position to create compressive forces between the tips of the leads or fingers and the mating contact pads in the plane of interconnection. An interconnect according to claim 1, wherein the means (35) insulating the fingers or leads holds the fingers or leads together when flexed, and still allows the fingers or leads to flex somewhat independently. An interconnect according to claim 2, wherein the means (35) insulating the fingers or leads comprises an adhesive insulating layer coated on the leads or fingers (14). An interconnect according to claim 1, 2 or 3, wherein the insulating layer (35) comprises a polyimide, and the fingers (14) comprise a Be.Cu alloy. An interconnect according to claim 1, 2, 3 or 4, wherein the tips (16) of the leads or fingers have a sharp abrasive edge (19) or a bent tip (16) to ensure good electrical contact with the contact pads (21). An interconnect circuit for providing electrical connection to a thermal ink jet print head (40) which has contact pads (21) arranged in a single plane, the interconnect circuit comprising an interconnect as claimed in any of claims 1 to 5, the interconnect fitting against the surface of the print head to minimise the distance between the print head on one side of the interconnect and the print paper on the other side of the interconnect. A method for making electrical connections to a contact member having contact pads (21) arranged in a single plane of interconnection, the method comprising: providing an interconnect comprising a lead frame (10) including a plurality of insulated metal fingers or leads (14) capable of flexure and extended at a predetermined angle (ϑ) relative to the plane of interconnection with the contact member, the fingers (14) having conductive tips (16) for making connections with the contact pads (21); and moving one of the contact member and the interconnect relative to the other of the contact member and the interconnect so that the conductive tips (16) engage the contact pads (21), the fingers or leads flexing to a spring biased position as they move to the plane of interconnection, to create compressive forces between the conductive tips and the contact pads. A method according to claim 7, wherein the contact member comprises a thermal ink jet print head. A method for making an interconnect as defined in claim 7, the method comprising: a. providing a metal substrate (48) with good spring properties, b. forming a pattern of individual metal leads (14) from said substrate, c. forming an adhesive insulating layer (49, 50) of predefined mask geometry surrounding said leads, leaving selected regions of said leads exposed in a central area of said interconnect circuit, d. forming an opening (52) in said leads and in said central area of said interconnect circuit while leaving the tips thereof separated one from another in a common plane and spaced apart by a predetermined distance, and e. bending the leads at a predetermined angle (ϑ) with respect to said common plane, whereby the tips of said leads may be spring biased toward said common plane to thereby establish good electrical contact with an ink jet printhead or the like. A method according to claim 9, wherein said leads (14) are a Be.Cu alloy and said insulating layer includes a polyimide. A method according to claim 9 or 10 which further includes the step of forming a plurality of exposed contact pads (33, 34) at the fixed ends of said leads which are adapted for connection to a flexible circuit or the like, whereby said flexible circuit and the surface of contact member may be electrically interconnected in said common plane. A method according to claim 11, further comprising providing each exposed contact tip (16) with a sharp abrasive edge (19) or with a bent tip (16) to ensure reliable electrical connections to a said contact member (21).

Inventors: CHAN LAWRENCE W, LA DUONG TON, MEBANE JANET E, NEVAREZ RUBEN, CHAN, LAWRENCE W., LA, DUONG TON, MEBANE, JANET E., NEVAREZ, RUBEN
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1988
Main classification: B41J2
IPC codes: B41J29, H01R11, B41J2, H01R33, H01R29, H05K3, H05K1