An inkjet printhead with a robust wire bond encapsulation

Abstract

An electronic component includes a substrate having a die and a PCB mounted thereon. Bond wires interconnect bond pads of the die and contact pads of the PCB, each bond wire having a first end portion bonded to a corresponding bond pad, an opposite second end portion bonded to a corresponding contact pad, and an intermediate section extending between the first end portion and the second end portion. A dam encapsulant encapsulates each of the first end portion and the second end portion, a first fill encapsulant contacts the substrate and the dam encapsulant; and a second fill encapsulant is superimposed on the first fill encapsulant. The first fill encapsulant has a lower modulus of elasticity than the second fill encapsulant and the dam encapsulant.

Description

Field of the Invention

[0001] The present invention relates to wire bonding encapsulation, particularly in an inkjet printhead having one or more printhead dies connected to a PCB. Wire bonding encapsulation is mainly used to provide robust chemical and mechanical protection for the wire bonds while minimizing the thermo-mechanical stresses that can cause problems such as die cracking. Background of the Invention

[0003] The present applicant has previously described methods for wire bonding encapsulation in inkjet printheads. For example, as described in US8,063,318 (the content of which is incorporated herein by reference), wire bonds connecting contact pads on a printhead chip (or "printhead die") and a PCB can be encapsulated using a combination of "dam" encapsulant beads and "fill" encapsulant, where the "dam" encapsulant beads cover the contact pads at opposite ends of the wire bond and the "fill" encapsulant encapsulates the wire bond extending between these ends. As described in US 8,063,318, the dam encapsulant has a relatively high modulus of elasticity compared to the fill encapsulant, which provides sufficient control of mechanical stresses to avoid wire bond damage during thermal expansion / contraction of the printhead.

[0004] US 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 alloy) gasket. Such a printhead has been designed to be used with pigment-based inks and further enables the construction of relatively long printheads (such as A3 page-width printheads).

[0005] As mentioned above, the main function of the wire bonding encapsulant in an inkjet printhead is to protect the wire bonds from the ink. If the encapsulant breaks and the ink contacts the wire bonds, the printhead will malfunction due to an electrical short. However, it is equally important that the encapsulant itself does not introduce thermo-mechanical stresses in the printhead that can cause wire bond fracture or die cracking during manufacturing or via thermal cycling during normal use.

[0006] Relatively hard encapsulation materials (i.e., encapsulation materials having a relatively high modulus of elasticity) are generally preferred in terms of mechanical and chemical robustness, particularly against aggressive co-solvents and surfactants in certain inkjet inks. For pigment-based inks used with the printheads described in US 10,442,200, relatively hard encapsulation materials have been found to be necessary to avoid chemical erosion. On the other hand, those relatively hard materials are more likely to introduce unwanted thermo-mechanical stresses in the printhead. In particular, when using a relatively hard "filled" encapsulant, the printheads described in US 10,442,200 are very prone to die cracking during manufacturing. This is understood to be the result of mechanical linkage provided by the encapsulation material between the die and the PCB.

[0007] Accordingly, it is desirable to provide a wire bond encapsulant that resists chemical erosion while minimizing thermo-mechanical stresses in the printhead that can lead to die cracking. Summary of the Invention

[0009] An electronic component, comprising:

[0010] A substrate;

[0011] One or more dies mounted on the substrate, each die having a plurality of bond pads;

[0012] A PCB mounted on the substrate, the PCB having a plurality of contact pads;

[0013] A plurality of wire bonds interconnecting the bond pads and the contact pads, each wire bond having a first end portion bonded to a corresponding bond pad, an opposite second end portion bonded to a corresponding contact pad, and an intermediate section extending between the first end portion and the second end portion;

[0014] A dam encapsulant encapsulating each of the first end portion and the second end portion, the bond pads, and the contact pads;

[0015] A first filled encapsulant disposed on the substrate so as to contact at least the substrate and the dam encapsulant; and

[0016] A second filled encapsulant disposed on the first encapsulant so as to contact at least the first filled encapsulant and the dam encapsulant;

[0017] Wherein:

[0018] The second filled encapsulant does not contact the substrate;

[0019] At least one of the first filled encapsulant and the second filled encapsulant encapsulates the intermediate section of the wire bond; and

[0020] The first filling encapsulant has a lower elastic modulus than the second filling encapsulant and the dam encapsulant.

[0021] The electronic assembly according to the first aspect advantageously provides robust protection for the bond wires while minimizing thermomechanical stresses that may cause die cracking or wire breakage. In particular, the relatively hard second fill encapsulant provides a mechanically robust and chemically resistant outer layer, while the relatively soft first fill encapsulant weakens the mechanical linkage between the die and the PCB, so that die cracking during manufacturing or during normal use is minimized. Although the die and the PCB are not completely mechanically decoupled, the relatively soft first fill encapsulant is sufficient to minimize thermomechanical stresses to the extent that die cracking becomes unproblematic.

[0022] Preferably, the electronic component is a print head and the die is a print head chip, such as a MEMS print head chip.

[0023] Preferably, the substrate is an ink manifold for delivering ink to the print head chips. The ink manifold may comprise, for example, a polymer such as a liquid crystal polymer or a metal such as Invar.

[0024] Preferably, the print head chip is mounted on the substrate via an intervening gasket. The gasket may, for example, take the form of a double-sided tape having opposing adhesive layers disposed on a polymer support, as described in US 7,347,534 (the contents of which are incorporated herein by reference). Alternatively, the gasket may take the form of a film (e.g., a metal alloy film) bonded to the substrate, as described in US 10,442,200. Typically, the gasket has an ink through hole defined therein for delivering ink from an ink manifold to the print head chip.

[0025] Typically, the PCB is mounted directly on the substrate, for example, via bonding. The substrate may have a stepped mounting surface for accommodating the PCB.

[0026] Preferably, the dam encapsulant is configured as a peripheral wall having a pair of opposing longer walls covering the contact pad and the bonding pad, respectively, and a pair of shorter walls interconnected with the longer walls at each end thereof.

[0027] Preferably, the first filling encapsulant and the second filling encapsulant are disposed within the peripheral wall such that the dam encapsulant dams the flow of the first filling encapsulant and the second filling encapsulant during liquid deposition thereof.

[0028] Preferably, the second fill encapsulant is relatively more resistant to chemical attack than the first fill encapsulant.

[0029] Preferably, the elastic modulus of the first filling encapsulant is in the range of 20 MPa to 200 MPa, or preferably in the range of 50 MPa to 150 MPa.

[0030] Preferably, the elastic modulus of the second filling encapsulant is in the range of 500 MPa to 3000 MPa, or preferably in the range of 700 MPa to 2000 MPa.

[0031] Preferably, the elastic modulus of the dam encapsulant is in the range of 500 MPa to 3000 MPa, or preferably in the range of 700 MPa to 2000 MPa. The second filling encapsulant and the dam encapsulant may comprise the same or different materials.

[0032] Typically, the dam encapsulant, the first filling encapsulant, and the second filling encapsulant each comprise an epoxy resin. Epoxy resin encapsulants with different elastic 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.

[0033] In a related aspect, a method of encapsulating bond wires in an electronic component is provided, the electronic component having:

[0034] a substrate;

[0035] one or more die, the one or more die being mounted on the substrate, each die having a plurality of bond pads;

[0036] a PCB, the PCB being mounted on the substrate, the PCB having a plurality of contact pads; and

[0037] a plurality of bond wires interconnecting the bond pads and the contact pads, each bond wire having a first end portion bonded to a corresponding bond pad, an opposite second end portion bonded to a corresponding contact pad, and an intermediate section extending between the first end portion and the second end portion,

[0038] wherein the method comprises the steps of:

[0039] depositing a dam encapsulant on each of the first end portion and the second end portion, the bond pad, and the contact pad;

[0040] curing the dam encapsulant

[0041] depositing a first filling encapsulant so as to contact at least the substrate and the dam encapsulant;

[0042] curing the first filling encapsulant;

[0043] Deposit a second fill encapsulant on the first fill encapsulant so as to contact the first fill encapsulant and the dam encapsulant; and

[0044] Cure the second fill encapsulant,

[0045] wherein:

[0046] The second fill encapsulant does not contact the substrate;

[0047] At least one of the first fill encapsulant and the second fill encapsulant encapsulates an intermediate section of the bond wire; and

[0048] The first fill encapsulant has a lower modulus of elasticity than the second fill encapsulant and the dam encapsulant.

[0049] As used herein, the term "modulus of elasticity" refers to the modulus of elasticity of the encapsulating material at 25 degrees Celsius after being fully cured.

[0050] As used herein, the term "PCB" refers to a type of printed circuit board having a non-conductive substrate and one or more conductive traces that carry electrical signals. The non-conductive substrate can be flexible or rigid. The PCB can include additional electronic components (such as capacitors, resistors, etc.), or alternatively, the PCB can have no additional electronic components and is only used to carry electrical signals via its conductive traces.

[0051] As used herein, the term "ink" is considered to mean any printing fluid that can be printed from an inkjet printhead. The ink can contain or can be free of a colorant. Accordingly, the term "ink" can include conventional dye-based or pigment-based inks, infrared inks, fixatives (such as pre-coats and finishes), 3D printing fluids (such as binder fluids), biological fluids, functional fluids (such as sensor inks, solar inks, etc.), and so on. In the case of referring to a fluid or a printing fluid, this is not intended to limit the meaning of "ink" herein.

[0052] As used herein, the term "mount" includes both direct mounting and indirect mounting via an intervening portion. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

[0054] Figure 1 is a front perspective view of an inkjet printhead;

[0055] Figure 2 is a bottom perspective view of the printhead;

[0056] Figure 3 is an exploded perspective view of the printhead;

[0057] Figure 4 is an enlarged cross-sectional perspective view of a portion of the printhead;

[0058] Figure 5 is a bottom perspective view of a portion of the printhead;

[0059] Figure 6 is an enlarged bottom perspective view of the printhead, in which the shielding plate and encapsulant of one row of printhead chips are removed; and

[0060] Figure 7 is a schematic side cross-sectional view of the connection area between the printhead chip and the PCB. DETAILED DESCRIPTION

[0061] The present invention relates to an electronic component in its most general form. In a preferred embodiment, the electronic component takes the form of an inkjet printhead, which will be described in detail hereinafter.

[0062] Reference Figures 1 to 3 shows an inkjet printhead 1 as described in US 10,442,200, the content of which is incorporated herein by reference. The printhead 1 includes an elongate molded plastic housing 3 having ink connectors at each of its ends. The inlet connectors 7A of the multi-channel inlet coupler 8A project upwardly through an opening at one end of the housing 3; and the outlet connectors 7B of the multi-channel outlet coupler 8B project upwardly through an opening at the opposite end of the housing ( Figure 1 only two inlet connectors and two outlet connectors are shown in). The inlet connector 7A and the outlet connector 7B are configured to be coupled to complementary fluid connectors (not shown) that supply ink to and from the printhead.

[0063] The housing 3 has a first portion 3A and a second portion 3B positioned on either side of a central locator 4, and the first housing portion 3A and the second housing portion 3B are biased towards each other and the central locator 4 by a spring clamp 6 engaged therebetween. The two-piece housing 3 in combination with the spring clamp 6 enables the housing to expand longitudinally at least to some extent to accommodate a certain degree of longitudinal expansion of the body 17 of the printhead 1.

[0064] The printhead 1 receives electrical power and data signals via opposite multi-row electrical contacts 13 that extend along the respective sidewalls of the printhead. These electrical contacts 13 are configured to receive electrical power and data signals from complementary contacts of a printer (not shown) or a print module, and convey the electrical power and data to the printhead chip 70 via the respective PCB 18, as explained in more detail below.

[0065] As Figure 2As shown, the printhead 1 includes a first row 14 and a second row 16 of printhead chips 70 for printing onto a print medium (not shown) passing beneath the printhead. Each row of printhead chips is configured to print two colors of ink, such that the printhead 1 is a full-color page-width printhead capable of redundantly printing four ink colors (CMYK). The printhead 1 is generally symmetric about a longitudinal plane bisecting the first row 14 of printhead chips and the second row 16 of printhead chips, but the ink colors in the printhead are different during use.

[0066] In Figure 3 the exploded perspective view shown, it can be seen that the body 17 forms a rigid core of the printhead 1 for mounting the various other components. Specifically, the housing 3 snap-fits to the upper portion of the body 17; the inlet coupler 8A and the outlet coupler 8B (enclosed by the housing 3) are connected to opposite ends of the body; a pair of PCBs 18 are attached to the lower portion of the body (and are thereby covered by the shield 20); and a plurality of leads 22 (defining electrical contacts 13) are mounted to opposite sidewalls of the body.

[0067] The body 17 is a two-piece machined structure including an elongate ink manifold 25 and a complementary cover plate 27. The ink manifold 25 serves as a carrier substrate having an integral lower surface for mounting the first row 14 and the second row 16 of printhead chips 70 and the corresponding PCBs 18. The manifold 25 and the cover plate 27 are formed of a metal alloy material (e.g., invar alloy) having a relatively high stiffness and a relatively low coefficient of thermal expansion. When combined, the manifold 25 and the cover plate 27 provide a robust rigid structure at the core of the printhead 1 that has minimal expansion along its longitudinal axis. As described above, the housing 3 is configured not to constrain any longitudinal expansion of the body 17, thereby minimizing the arching of the printhead during use. Accordingly, the printhead 1 can be provided as an A4-length printhead or an A3-length printhead.

[0068] Referring Figure 4 , an invar alloy gasket 66 is bonded to the lower surface 52 of the manifold 25, and a plurality of printhead chips 70 of the first row 14 and the second row 16 are arranged to be bonded to the gasket 66 (only the printhead chips of the first row 14 are visible in Figure 4 ). Each row of printhead chips 70 receives ink from a longitudinal ink supply channel 40 defined in the manifold 25 via through-holes in the gasket 66.

[0069] A pair of longitudinal PCBs 18 sandwich the printhead chip 70 on opposite sides of the first row 14 and the second row 16, with each PCB bonded to the lower surface 52 of the manifold 25. Each PCB 18 includes a rigid substrate (e.g., an FR-4 substrate) for mounting various electronic components, and has an edge abutting against a step 74 defined in the lower surface 52 of the manifold 25. Each PCB 18 extends laterally outward beyond the sidewall 41 of the manifold 25. The shield 20 is bonded to the lower surface of each PCB 18 and surrounds the first row 14 and the second row 16 of the printhead chip 70 as well as the central longitudinal region between the first row and the second row. The protruding portions of each PCB 18 and the shield 20 define opposite wings 75 of the printhead 1, and the uniformly flat lower surface of the shield 20 is configured to engage with a peripheral capping member (not shown) surrounding the two rows of printhead chips.

[0070] Still referring to Figure 4 , a row of connection pads 80 extends longitudinally along the distal edge portion of the upper surface of each PCB 18. One end of each lead 22 is connected to the connection pad 80 and extends upward toward the corresponding sidewall of the body 17. The lead 22 has an upper portion and a lower portion. The upper portion is mounted to the corresponding flange 29 of the cover plate 27 via a lead retainer 24 attached thereto, and the lower portion flares laterally outward toward the connection pad 80. Each lead 22 also has a portion defining an electrical contact 13 for connection to an external power connector and a data connector of the printer.

[0071] Now referring to Figure 5 and Figure 6 , each edge of each PCB 18 near the corresponding row of printhead chips 70 has a corresponding row of pin leads in the form of contact pads 77. Each contact pad 77 is connected to a corresponding bonding pad 73 on one of the printhead chips via a wire bond (invisible in Figure 5 and Figure 6 ). In this way, each row of printhead chips 70 receives power and data from the electrical contact 13 via the leads 22 and the corresponding PCB 18 adjacent to this row of printhead chips.

[0072] The wire bonds are protected by an encapsulant 79 that extends between the proximal edge of each PCB 18 containing the contact pads 77 and the proximal edge of the printhead chip 70 containing the bonding pads 73. As described above, it is necessary that the encapsulant 79 provides robust protection for the wire bonds, especially against chemical erosion from high pH ink, which typically contains erosive co-solvents and surfactants.

[0073] Figure 7A side cross-sectional view schematically shows a connection area of a PCB 18 and a printhead chip 70 according to the present invention. The printhead chip 70 having a row of longitudinal bonding pads 73 is mounted on the lower surface 52 of the ink manifold 25 via an intervening gasket 66. The PCB 18 having contact pads 77 is mounted directly adjacent to the printhead chip 70 on the lower surface 52 of the ink manifold 25 and is received in a stepped portion 74 of the lower surface. Bonding wires 90 interconnect the bonding pads 73 and the contact pads 77. The bonding wire 90 has a first end portion 91 bonded to the bonding pad 73, an opposite second end portion 92 bonded to the contact pad 77, and an intermediate section 93 extending between the first end portion and the second end portion.

[0074] An encapsulant encapsulation 79 protects the bonding wires 90, as well as the bonding pads 73 and the contact pads 77, and includes three components: (1) beads of dam encapsulant 95 that extend longitudinally along the row of bonding pads 73 and the row of contact pads 77 and form an annular peripheral dam via transverse interconnect portions at each of its longitudinal ends; (2) a first fill encapsulant 96 having a relatively low modulus of elasticity, disposed on the lower surface of the ink manifold 25 and the exposed portion of the gasket 66, within the perimeter of the dam encapsulant 95; and (3) a second fill encapsulant 97 having a relatively high modulus of elasticity, disposed on the first fill encapsulant 96 within the perimeter of the dam encapsulant. The dam encapsulant 95 encapsulates the first end portion 91 and the second end portion 92 of the bonding wire 90, as well as the bonding pads 73 and the contact pads 77, while the first fill encapsulant 96 and the second fill encapsulant 97 together encapsulate the intermediate section 93 of the bonding wire.

[0075] Importantly, the second fill encapsulant 97 does not contact the ink manifold 25, which serves as a common support substrate for the printhead chip 70 and the PCB 18. This has the effect of reducing mechanical linkage between the printhead chip 70 and the PCB 18 via the encapsulant encapsulation 79. Since only the first fill encapsulant 96 having a relatively low modulus of elasticity contacts the ink manifold 25, any mechanical stress caused by thermal expansion of the ink manifold is minimized. On the other hand, the relatively hard second fill encapsulant 97 provides a robust outer surface that resists chemical erosion and provides a mechanically robust protective layer. Thus, the encapsulant encapsulation 79 significantly improves die encapsulation in harsh environments subject to thermomechanical stress and chemical erosion, such as those found in inkjet printheads.

[0076] In practice, it has been found that the intermediate section 93 of the bonding wire 90 can be encapsulated within the first fill encapsulant 96, the second fill encapsulant 97, or as Figure 7The shown is encapsulated within both the first fill encapsulant and the second fill encapsulant. By the relatively soft first fill encapsulant 96, the thermo-mechanical stress in the connection region is minimized and the breakage of the bond wire is minimized, regardless of whether the bond wire 90 is encapsulated by the first fill encapsulant or the second fill encapsulant.

[0077] An exemplary method for manufacturing an electronic component according to the present invention includes the following steps: (1) forming a bond wire connection between a bond pad of a die (such as a print head chip 70) and a contact pad of a PCB (such as a PCB 18); (2) dispensing beads of a dam encapsulant onto the bond pad and the contact pad and forming a peripheral wall; (3) curing the dam encapsulant using UV and / or heat; (4) dispensing a first fill encapsulant and flowing it within the peripheral wall of the dam encapsulant; (5) curing the first fill encapsulant using UV and / or heat; (6) dispensing a second fill encapsulant and flowing it within the peripheral wall of the dam encapsulant; and (7) curing the second fill encapsulant using UV and / or heat.

[0078] Of course, it should be understood that the present invention has been described by way of example only, and modifications in details may be made within the scope of the present invention as defined in the appended claims.

Claims

1. An electronic component comprising: substrate; one or more dies mounted on the substrate, each die having a plurality of bonding pads; A PCB, the PCB being mounted on the substrate, the PCB having a plurality of contact pads; a plurality of bond wires interconnecting the bond pads and the contact pads, each bond wire having a first end portion bonded to a corresponding bond pad, an opposite second end portion bonded to a corresponding contact pad, and an intermediate section extending between the first end portion and the second end portion; a dam encapsulant encapsulating each of the first end portion and the second end portion, the bonding pad, and the contact pad; a first filling encapsulant disposed on the substrate so as to at least contact the substrate and the dam encapsulant; as well as a second filling encapsulant disposed on the first filling encapsulant so as to at least contact the first filling encapsulant and the dam encapsulant; in: The second filling encapsulant does not contact the substrate; At least one of the first filling encapsulant and the second filling encapsulant encapsulates a middle section of the bonding wire; The first filling encapsulant has a lower elastic modulus than the second filling encapsulant and the dam encapsulant; and The dam encapsulant, the first filling encapsulant, and the second filling encapsulant include different materials.

2. The electronic component according to claim 1, wherein, The die is a printhead chip and the electronic component is a printhead.

3. The electronic component according to claim 2, wherein, The substrate is an ink manifold for delivering ink to the print head chip.

4. The electronic component according to claim 3, wherein, The print head chip is mounted on the substrate via an intervening pad.

5. The electronic component according to claim 4, wherein, The gasket includes a metal alloy film.

6. The electronic component according to claim 4, wherein, The PCB is directly mounted on the substrate.

7. The electronic component according to claim 1, wherein The dam encapsulant is configured as an annular peripheral wall having a pair of opposing longer walls covering the contact pad and the bonding pad, respectively, and a pair of shorter walls interconnected with the longer walls at each end thereof.

8. The electronic component according to claim 7, wherein, The first filling encapsulant and the second filling encapsulant are disposed within the perimeter wall such that the dam encapsulant dams the flow of the first filling encapsulant and the second filling encapsulant during liquid deposition thereof.

9. The electronic component according to claim 8, wherein, The second fill encapsulant is relatively more resistant to chemical attack than the first fill encapsulant.

10. The electronic component according to claim 1, wherein, The elastic modulus of the first filling encapsulant is in the range of 20 MPa to 200 MPa.

11. The electronic component according to claim 1, wherein, The elastic modulus of the second filling encapsulant is in the range of 500 MPa to 3000 MPa.

12. The electronic component according to claim 1, wherein, The elastic modulus of the dam encapsulant is in the range of 500 MPa to 3000 MPa.

13. The electronic component according to claim 1, wherein, The dam encapsulant, the first fill encapsulant, and the second fill encapsulant each include a material selected from the group consisting of epoxy resins.

14. A method of encapsulating a wire bond in an electronic component, the electronic component having: substrate; one or more dies mounted on the substrate, each die having a plurality of bonding pads; A PCB, the PCB being mounted on the substrate, the PCB having a plurality of contact pads; as well as a plurality of bond wires interconnecting the bond pads and the contact pads, each bond wire 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 end portion and the second end portion, The method comprises the following steps: depositing a dam encapsulant on each of the first end portion and the second end portion, the bond pad, and the contact pad; allowing the dam encapsulant to cure, depositing a first fill encapsulant so as to contact at least the substrate and the dam encapsulant; curing the first filling encapsulant; depositing a second filling encapsulant on the first filling encapsulant so as to contact the first filling encapsulant and the dam encapsulant; and allowing the second filling encapsulant to cure, in: The second filling encapsulant does not contact the substrate; At least one of the first filling encapsulant and the second filling encapsulant encapsulates a middle section of the bonding wire; The first filling encapsulant has a lower elastic modulus than the second filling encapsulant and the dam encapsulant; and The dam encapsulant, the first filling encapsulant, and the second filling encapsulant include different materials.

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

16. The method according to claim 14, wherein, The dam encapsulant is deposited as an annular peripheral wall having a pair of opposing longer walls covering the contact pad and the bonding pad, respectively, and a pair of shorter walls interconnected with the longer walls at each end thereof.

17. The method according to claim 16, wherein, The first fill encapsulant and the second fill encapsulant are disposed within the perimeter wall, and wherein the dam encapsulant dams the flow of the first fill encapsulant and the second fill encapsulant during respective deposition thereof.

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