Inkjet printhead with robust encapsulation of wire bonds - Patent Application 20070122997

Abstract

The electronic assembly includes a substrate having a die mounted thereon and a PCB. Wire bonds interconnect bond pads of the die to contact pads of the PCB, each wire bond having a first end portion bonded to a respective bond pad, an opposite second end portion bonded to a respective contact pad, and an intermediate section extending between the first and second end portions. A dam encapsulant encapsulates each of the first and second end portions, a first fill encapsulant contacting the substrate and the dam encapsulant, and a second fill encapsulant covering the first fill encapsulant. The first fill encapsulant has a lower modulus of elasticity than the second fill encapsulant and the dam encapsulant.

Description

Technical Field

[0001] The present invention particularly relates to the encapsulation of wire bonds in an inkjet printhead having one or more printhead dies connected to a PCB. The present invention was primarily developed to provide strong chemical and mechanical protection for wire bonds while minimizing thermomechanical stresses that can lead to problems such as die cracks.

Background Art

[0002] The present applicant has previously described a method of encapsulating wire bonds in an inkjet printhead. For example, as described in U.S. Patent No. 8,063,318 (the content of which is incorporated herein by reference), wire bonds that connect conductor pads on a printhead chip (or "printhead die") to a PCB can be encapsulated using a combination of "dam" encapsulant beads that coat the conductor pads at both ends of the wire bond and a "fill" encapsulant that extends between them. As described in U.S. Patent No. 8,063,318, the dam encapsulant has a relatively higher modulus of elasticity than the fill encapsulant, which sufficiently controls mechanical stress and avoids breakage of the wire bond during thermal expansion / contraction of the printhead.

[0003] U.S. Patent No. 10,442,200 (the content of which is incorporated herein by reference) describes a printhead having a plurality of printhead dies attached to a metal alloy manifold via a metal alloy (e.g., Invar) shim. Such printheads are designed for use with pigment-based inks and further enable the construction of relatively long printheads, such as A3 page-width printheads.

[0004] As mentioned above, the primary function of the wire bond encapsulant in an inkjet printhead is to protect the wire bond from ink. If the encapsulant is damaged and ink comes into contact with the wire bond, the printhead will subsequently fail due to a short circuit. However, it is equally important that the encapsulant itself does not impose thermomechanical stress on the printhead that could lead to wire bond failure or die cracking, either during manufacturing or during normal use via thermal circulation.

[0005] Relatively harder encapsulating materials—that is, encapsulating materials with a relatively high modulus of elasticity—are generally preferred in terms of mechanical and chemical robustness, particularly against aggressive cosolvents and surfactants found in certain inkjet inks. For pigment-based inks used in printheads described in U.S. Patent No. 10,442,200, relatively harder encapsulating materials have been found necessary to avoid chemical erosion. On the other hand, these relatively harder materials tend to impose undesirable thermomechanical stresses on the printhead. In particular, the printhead described in U.S. Patent No. 10,442,200 is highly susceptible to die cracking during manufacturing when relatively harder "fill" encapsulating materials are used. This is understood to be a result of the mechanical bond provided by the encapsulating material between the die and the PCB.

[0006] Therefore, it is desirable to provide a wire bond encapsulant that is resistant to chemical corrosion while minimizing the thermomechanical stress on the print head that can cause die cracks. [Overview of the project]

[0007] Electronic assembly is circuit board and One or more dies mounted on the substrate, each die having a plurality of bond pads, A PCB mounted on the aforementioned substrate, the PCB having a plurality of conductor pads, A plurality of wire bonds for interconnecting the bond pads and the conductor pads, each wire bond having a first end portion joined to the respective bond pad, a second end portion on the opposite side joined to the respective conductor pad, and an intermediate section extending between the first end portion and the second end portion, A dam sealing material that encloses the first end portion, the second end portion, the bond pad, and the conductor pad, A first fill encapsulant is disposed on the substrate so as to be in contact with at least the substrate and the dam encapsulant, The system comprises at least the first fill-fill material and a second fill-fill material disposed on the first fill-fill material so as to be in contact with the dam-fill material, The second fill encapsulation material does not come into contact with the substrate. At least one of the first fill encapsulation material and the second fill encapsulation material encloses the intermediate section of the wire bond, The first fill-fill material has a lower modulus of elasticity than the second fill-fill material and the dam-fill material.

[0008] The electronic assembly according to the first embodiment advantageously provides robust protection of the wire bond while minimizing thermomechanical stress that could lead to die cracking or wire bond failure. In particular, the relatively harder second fill encapsulant provides a mechanically robust and chemically resistant outer layer, while the relatively softer first fill encapsulant weakens the mechanical bond between the die and the PCB to minimize die cracking during manufacturing or normal use. Although the die and PCB are not completely mechanically separated, the relatively softer first fill encapsulant is sufficient to minimize thermomechanical stress to the extent that die cracking is not a problem.

[0009] Preferably, the electronic assembly is a print head, and the die is a print head chip such as a MEMS print head chip.

[0010] Preferably, the substrate is an ink manifold for supplying ink to the print head chip. The ink manifold may be made of, for example, a polymer such as a liquid crystal polymer or a metal such as Invar.

[0011] Preferably, the printhead chip is mounted on the substrate via an interposed shim. The shim may take the form of a double-sided adhesive tape having adhesive layers on both sides disposed on a polymer support, for example, as described in U.S. Patent No. 7,347,534, the contents of which are incorporated herein by reference. Alternatively, the shim may take the form of a thin film (e.g., a metal alloy thin film) bonded to the substrate, for example, as described in U.S. Patent No. 10,442,200. Typically, the shim has ink through-holes defined therein for supplying the ink from the ink manifold to the printhead chip.

[0012] Typically, the PCB is mounted directly onto the substrate, for example, via adhesive bonding. The substrate may have a stepped mounting surface for accommodating the PCB.

[0013] Preferably, the dam sealing material is configured as a perimeter wall having a pair of opposing long walls that cover the conductor pad and the bond pad, respectively, and a pair of short walls that interconnect the long walls at both ends thereof.

[0014] Preferably, the first fill-fill material and the second fill-fill material are arranged within the peripheral wall such that the dam-fill material blocks the flow of the first fill-fill material and the second fill-fill material during the liquid phase deposition.

[0015] Preferably, the second fill encapsulation material has relatively higher resistance to chemical corrosion than the first fill encapsulation material.

[0016] Preferably, the first fill encapsulant has an elastic modulus in the range of 20 to 200 MPa, or preferably 50 to 150 MPa.

[0017] Preferably, the second fill encapsulant has an elastic modulus in the range of 500 to 3000 MPa, or more preferably 700 to 2000 MPa.

[0018] Preferably, the dam sealing material has an elastic modulus in the range of 500 to 3000 MPa, or preferably 700 to 2000 MPa. The second fill sealing material and the dam sealing material may be composed of the same material or different materials.

[0019] Typically, the dam encapsulant, the first fill encapsulant, and the second fill encapsulant are each composed of epoxy resin. Epoxy resin encapsulants with different moduli are commercially available from various suppliers (e.g., ResinLab, Chase Corporation, Engineering Materials Systems, Inc., etc.) and are well known to those skilled in the art.

[0020] In related embodiments, a method is provided for encapsulating a wire bond within an electronic assembly, wherein the electronic assembly is circuit board and One or more dies mounted on the substrate, each die having a plurality of bond pads, A PCB mounted on the aforementioned substrate, the PCB having a plurality of conductor pads, A plurality of wire bonds for interconnecting the bond pads and the conductor pads, wherein each wire bond has a first end portion joined to the respective bond pad, a second end portion on the opposite side joined to the respective conductor pad, and an intermediate section extending between the first end portion and the second end portion, comprising a plurality of wire bonds, The aforementioned method, The steps include depositing dam sealing material on the first end portion, the second end portion, the bond pad, and the conductor pad, The steps include: hardening the dam sealing material, Depositing a first fill encapsulant so as to contact at least the substrate and the dam encapsulant; Curing the first fill encapsulant; Depositing a second fill encapsulant on the first fill encapsulant so as to contact the first fill encapsulant and the dam encapsulant; Curing the second fill encapsulant, comprising: The second fill encapsulant does not contact the substrate; At least one of the first fill encapsulant and the second fill encapsulant encapsulates the intermediate section of the wire bond; The first fill encapsulant has a lower elastic modulus than the second fill encapsulant and the dam encapsulant.

[0021] As used herein, the term "elastic modulus" refers to the elastic modulus of the encapsulating material at 25 degrees Celsius after being fully cured.

[0022] As used herein, the term "PCB" is construed to mean a type of printed circuit board having a non-conductive substrate and one or more conductive tracks for transmitting electrical signals. The non-conductive substrate may be flexible or rigid. The PCB may include additional electronic components (e.g., capacitors, resistors, etc.), or alternatively, the PCB may have no additional electronic components and may simply serve to transmit electrical signals through its conductive tracks.

[0023] As used herein, the term “ink” is construed to mean any printing fluid that can be printed from an inkjet printhead. Ink may or may not contain colorants. Therefore, the term “ink” may include conventional dye-based or pigment-based inks, infrared inks, fixatives (e.g., precoats and finishers), 3D printing fluids (e.g., binders), biological fluids, functional fluids (e.g., sensor inks, sow inks, etc.). Any reference to fluids or printing fluids is not intended to limit the meaning of “ink” as used herein.

[0024] As used herein, the term “implemented” includes both direct implementation and indirect implementation via an intermediary component. [Brief explanation of the drawing]

[0025] Embodiments of the present invention will be described as merely examples with reference to the attached drawings.

[0026] [Figure 1] Figure 1 is a front perspective view of an inkjet print head. [Figure 2] Figure 2 is a bottom perspective view of the print head. [Figure 3] Figure 3 is an exploded perspective view of the print head. [Figure 4] Figure 4 is an enlarged cross-sectional perspective view of a portion of the print head. [Figure 5] Figure 5 is a bottom perspective view of a portion of the print head. [Figure 6] Figure 6 is an enlarged bottom perspective view of a printhead with the shield plate and encapsulating material removed from one row of printhead chips. [Figure 7] Figure 7 is a schematic side cross-sectional view of the connection area between the print head chip and the PCB. [Modes for carrying out the invention]

[0027] The present invention relates to the most common form of an electronic assembly. In one preferred embodiment, the electronic assembly takes the form of an inkjet printhead, which is described in detail below.

[0028] Referring to Figures 1 to 3, an inkjet printhead 1 is shown as described in U.S. Patent No. 10,442,200, the contents of which are incorporated herein by reference. The printhead 1 comprises an elongated molded plastic casing 3 having ink connectors at both ends. The inlet connector 7A of a multi-channel inlet coupling 8A protrudes upward through an opening at one end of the casing 3; the outlet connector 7B of a multi-channel outlet coupling 8B protrudes upward through an opening at the opposite end of the casing (only the two inlet connectors and the two outlet connectors are shown in Figure 1). The inlet connector 7A and the outlet connector 7B are configured to couple to a complementary fluid coupling (not shown) that supplies ink to and from the printhead.

[0029] The casing 3 has a first portion 3A and a second portion 3B positioned on either side of the central locator 4, and the first casing portion 3A and the second casing portion 3B are biased toward each other and toward the central locator 4 by a spring clip 6 that interlocks between them. The two-part casing 3 combined with the spring clip 6 allows the casing to expand longitudinally to at least a certain extent to correspond to the degree of longitudinal expansion of the body 17 of the print head 1.

[0030] The print head 1 receives power and data signals via opposing rows of electrical contacts 13 extending along each side wall of the print head. The electrical contacts 13 are configured to receive power and data signals from complementary contacts of a printer (not shown) or print module and to transmit power and data to the print head chip 70 via their respective PCBs 18, as will be described in more detail below.

[0031] As shown in Figure 2, the printhead 1 comprises a first row 14 and a second row 16 of printhead chips 70 for printing on a printing medium (not shown) passing below the printhead. Each row of printhead chips is configured to print two colors of ink, so that the printhead 1 is a full-color page-width printhead capable of printing four ink colors (CMYK) in overlapping mode. The printhead 1 is substantially symmetrical with respect to a longitudinal plane that bisects the first row 14 and the second row 16 of the printhead chips, despite the different ink colors of the printhead being used.

[0032] As can be seen in the exploded perspective view shown in Figure 3, the main body 17 forms the rigid core of the print head 1 for mounting various other components. Specifically, the casing 3 snaps onto the top of the main body 17; the inlet coupling 8A and outlet coupling 8B (surrounded by the casing 3) are connected to both ends of the main body; a pair of PCBs 18 are mounted on the bottom of the main body (these are covered by the shielding plate 20); and several lead wires 22 (defining the electrical contacts 13) are mounted on both side walls of the main body.

[0033] The main body 17 is a two-part machined structure comprising an elongated ink manifold 25 and a complementary cover plate 27. The ink manifold 25 functions as a carrier substrate with a single bottom surface for mounting the first row 14 and second row 16 of the print head chip 70 and their respective PCBs 18. The manifold 25 and cover plate 27 are formed from a metal alloy material (e.g., Invar) having relatively high rigidity and a relatively low coefficient of thermal expansion. The manifold 25 and cover plate 27, in combination, provide the core of the print head 1 with a rigid structure that minimizes expansion along its longitudinal axis. As described above, the casing 3 is configured not to restrain the longitudinal expansion of the main body 17, thereby minimizing bending of the print head during use. Therefore, the print head 1 may be provided as an A4-length or A3-length print head.

[0034] Referring to Figure 4, an inverse shim 66 is bonded to the lower surface 52 of the manifold 25, and multiple printhead chips 70 arranged in the first row 14 and the second row 16 are bonded to the shim 66 (only the first row 14 of the printhead chips shown in Figure 4). Each row of printhead chips 70 receives ink from the longitudinal ink supply channels 40 defined in the manifold 25 through through holes in the shim 66.

[0035] A pair of longitudinal PCBs 18 are adjacent to the first row 14 and second row 16 of the print head chips 70 on their respective sides, and each PCB is bonded to the lower surface 52 of the manifold 25. Each PCB 18 has a rigid substrate (e.g., an FR-4 substrate) for mounting various electronic components and has one edge that abuts against a step 74 defined on the lower surface 52 of the manifold 25. Each PCB 18 extends laterally outward beyond the side wall 41 of the manifold 25. A shield plate 20 is bonded to the lower surface of each PCB 18 and surrounds the first row 14 and second row 16 of the print head chips 70 and the central longitudinal region between the first and second rows. The protruding portions of each PCB 18 and the shield plate 20 define the opposing wings 75 of the print head 1, while the uniformly flat lower surface of the shield plate 20 is configured to engage with a peripheral capper (not shown) surrounding both rows of print head chips.

[0036] Continuing to refer to Figure 4, the row of connection pads 80 extends longitudinally along the distal edge of the upper surface of each PCB 18. Each lead wire 22 has one end connected to a connection pad 80 and extends upward toward each side wall of the main body 17. The lead wire 22 has an upper part attached to each flange 29 of the cover plate 27 via a lead wire holder 24 mounted thereon, and a lower part that extends laterally outward toward the connection pads 80. Each lead wire 22 also has a portion that defines an electrical contact 13 for connecting to the printer's external power and data connectors.

[0037] Referring now to Figures 5 and 6, the edges of each PCB 18 adjacent to each row of printhead chips 70 have each row of pinouts in the form of conductive pads 77. Each conductive pad 77 is connected to the respective bond pad 73 on one of the printhead chips via wire bond connections (not visible in Figures 5 and 6). In this way, each row of printhead chips 70 receives power and data from the electrical contacts 13 via the lead wires 22 and each PCB 18 adjacent to the row of printhead chips.

[0038] The wire bonds are protected by an encapsulation material package 79 that extends between the adjacent edges of each PCB 18, including the conductor pads 77, and the adjacent edges of the printhead chip 70, including the bond pads 73. As implied above, it is essential that the encapsulation material package 79 provides robust protection for the wire bonds, particularly against chemical erosion from high-pH inks, which typically contain aggressive cosolvents and surfactants.

[0039] Figure 7 schematically shows a side cross-sectional view of the connection area between the PCB 18 and the print head chip 70 according to the present invention. The print head chip 70, which has longitudinal rows of bond pads 73, is mounted on the lower surface 52 of the ink manifold 25 via interposed shims 66. The PCB 18, which has conductive pads 77, is mounted directly on the lower surface 52 of the ink manifold 25 adjacent to the print head chip 70 and is received within a stepped portion 74 on the lower surface. A wire bond 90 interconnects the bond pads 73 and the conductive pads 77. The wire bond 90 has a first end portion 91 bonded to the bond pads 73, a second end portion 92 on the opposite side bonded to the conductive pads 77, and an intermediate section 93 extending between the first end portion and the second end portion.

[0040] The dam dam dam package 79 protects the wire bond 90 and the bond pads 73 and conductor pads 77 and comprises the following three components: (1) Dam dam dam dam beads 95 extending longitudinally along the rows of bond pads 73 and rows of conductor pads 77, forming an endless peripheral dam at each of its longitudinal ends via transverse interconnection portions; (2) A first fill dam dam 96 having a relatively low modulus of elasticity, disposed within the periphery of the dam The dam filling material 95 encloses the first end portion 91 and the second end portion 92 of the wire bond 90, as well as the bond pad 73 and the conductor pad 77, while the first fill filling material 96 and the second fill filling material 97 both enclose the intermediate section 93 of the wire bond.

[0041] Importantly, the second fill encapsulation material 97 does not come into contact with the ink manifold 25, which serves as a common support substrate for the printhead chip 70 and PCB 18. This has the effect of weakening the mechanical bond between the printhead chip 70 and PCB 18 via the encapsulation package 79. Since only the first fill encapsulation material 96, which has a relatively low modulus of elasticity, comes into contact with the ink manifold 25, the mechanical stress caused by the thermal expansion of the ink manifold is minimized. On the other hand, the relatively harder second fill encapsulation material 97 not only provides a mechanically robust protective layer but also a robust outer surface that is resistant to chemical erosion. Thus, the encapsulation package 79 significantly improves die packaging in harsh environments subject to thermomechanical stress and chemical erosion, such as those found in inkjet printheads.

[0042] In practice, it has been found that the intermediate section 93 of the wire bond 90 may be encapsulated in the first fill encapsulant 96, in the second fill encapsulant 97, or, as shown in Figure 7, in both the first and second fill encapsulants. The relatively softer first fill encapsulant 96 minimizes thermomechanical stress in the connection region, and failure of the wire bond is minimized regardless of whether the first or second fill encapsulant encapsulates the wire bond 90.

[0043] An exemplary method for manufacturing an electronic assembly according to the present invention includes: (1) forming a wire bond connection between a bond pad of a die (e.g., a print head chip 70) and a conductor pad of a PCB (e.g., a PCB 18); (2) distributing beads of dam encapsulation material onto the bond pad and the conductor pad to form a perimeter; (3) curing the dam encapsulation material using UV curing and / or thermal curing; (4) distributing a first fill encapsulation material and flowing it into the perimeter of the dam encapsulation material; (5) curing the first fill encapsulation material using UV curing and / or thermal curing; (6) distributing a second fill encapsulation material and flowing it into the perimeter of the dam encapsulation material; and (7) curing the second fill encapsulation material using UV curing and / or thermal curing.

[0044] Needless to say, the present invention has been described merely as an example, and it is understood that further modifications may be made within the scope of the invention as defined in the appended claims.

Claims

1. A substrate; one or more dies mounted on the substrate, each die having a plurality of bond pads; a PCB mounted on the substrate, the PCB having a plurality of conductor pads; a plurality of wire bonds interconnecting the bond pads and the contact pads, each wire bond having a first end portion bonded to a respective bond pad, an opposite second end portion bonded to a respective contact pad, and an intermediate section extending between the first and second end portions; a dam encapsulant encapsulating each of the first end portion, the second end portion, the bond pads, and the contact pads; a first fill encapsulant disposed on the substrate so as to contact at least the substrate and the dam encapsulant; a second fill encapsulant disposed on the first encapsulant so as to contact at least the first fill encapsulant and the dam encapsulant; the second fill encapsulant does not contact the substrate; at least one of the first fill encapsulant and the second fill encapsulant encapsulates the middle section of the wire bond; The electronic assembly, wherein the first fill encapsulant has a lower modulus of elasticity than the second fill encapsulant and the dam encapsulant.

2. 10. The electronic assembly of claim 1, wherein the die is a printhead chip and the electronic assembly is a printhead.

3. 3. The electronic assembly of claim 2, wherein the substrate is an ink manifold for supplying ink to the printhead chips.

4. 4. The electronic assembly of claim 3, wherein the printhead chips are mounted on the substrate via intervening shims.

5. The electronic assembly of claim 4 , wherein the shim comprises a metal alloy thin film.

6. The electronic assembly of claim 4 , wherein the PCB is mounted directly onto the substrate.

7. 2. The electronic assembly of claim 1, wherein the dam encapsulation is configured as an endless peripheral wall having a pair of opposing long walls covering the contact pads and the bond pads, respectively, and a pair of short walls interconnecting the long walls at each end thereof.

8. 8. The electronic assembly of claim 7, wherein the first fill encapsulant and the second fill encapsulant are disposed within the peripheral wall such that the dam encapsulant blocks the flow of the first fill encapsulant and the second fill encapsulant during liquid deposition thereof.

9. The electronic assembly of claim 8 , wherein the second fill encapsulant has a relatively higher resistance to chemical attack than the first fill encapsulant.

10. 10. The electronic assembly of claim 1, wherein the first fill encapsulant has a modulus of elasticity in the range of 20 to 200 MPa.

11. 10. The electronic assembly of claim 1, wherein the second fill encapsulant has a modulus of elasticity in the range of 500 to 3000 MPa.

12. 10. The electronic assembly of claim 1, wherein the dam encapsulation has a modulus of elasticity in the range of 500 to 3000 MPa.

13. 10. The electronic assembly of claim 1, wherein the dam encapsulant, the first fill encapsulant, and the second fill encapsulant are each comprised of a material selected from the group consisting of multiple types of epoxy resins.

14. 1. A method of encapsulating wire bonds in an electronic assembly, the electronic assembly comprising: A substrate; one or more dies mounted on the substrate, each die having a plurality of bond pads; a PCB mounted on the substrate, the PCB having a plurality of conductor pads; a plurality of wire bonds interconnecting the bond pads and the conductor pads, each wire bond having a first end portion bonded to a respective bond pad, an opposite second end portion bonded to a respective conductor pad, and an intermediate section extending between the first end portion and the second end portion; The method comprises: depositing a dam encapsulant over each of the first end portion, the second end portion, the bond pads, and the contact pads; allowing the dam encapsulant to cure; depositing a first fill encapsulant in contact with at least the substrate and the dam encapsulant; curing the first fill encapsulant; depositing a second fill encapsulant on the first fill encapsulant so as to contact the first fill encapsulant and the dam encapsulant; and curing the second fill encapsulant; the second fill encapsulant does not contact the substrate; at least one of the first fill encapsulant and the second fill encapsulant encapsulates the middle section of the wire bond; The method, wherein the first fill encapsulant has a lower modulus of elasticity than the second fill encapsulant and the dam encapsulant.

15. 15. The method of claim 14, wherein each curing step is selected from the group consisting of thermal curing and UV curing.

16. 15. The method of claim 14, wherein the dam encapsulation is deposited as an endless peripheral wall having a pair of opposing long walls covering the contact pads and the bond pads, respectively, and a pair of short walls interconnecting the long walls at each end thereof.

17. 17. The method of claim 16, wherein the first fill encapsulant and the second fill encapsulant are disposed within the peripheral wall, and the dam encapsulant blocks the flow of the first fill encapsulant and the second fill encapsulant during their respective deposition.

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