Composition for dielectric filler, chip-embedded printed circuit board, and method for embedding chips in printed circuit boards

Abstract

The chip-embedded printed circuit board 100 includes a cavity 110 in the printed circuit board, a chip 104 in the cavity of the printed circuit board, and a thixotropic dielectric filler 106 in a gap in the cavity to encapsulate the chip in the printed circuit board.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Patent Application No. 17 / 515,870, filed November 1, 2021, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to hybrid UV-curable composite materials, and more particularly to nanocomposites for UV-curable direct-write semiconductor applications. [Background technology]

[0003] In a conventional printed circuit board, electronic components or chips are mounted on the surface of the bottom wiring layer, and an insulating layer and a top wiring layer are stacked on top of the chip, with via holes formed to electrically connect the wiring layer and the chip.

[0004] In response to the demand for higher performance and smaller electronic devices, recent technological trends are moving toward higher density and smaller electronic components. This has led to an increasing demand for smaller printed circuit boards that can accommodate higher density electronic components. Therefore, development of multilayer circuit boards, which electrically connect wiring layers or wiring formed on different layers to electrical components through via holes, is underway. Multilayer circuit boards reduce the number of wiring layers connecting electronic components to each other and increase the wiring density, thereby increasing the surface area of ​​the printed circuit board and providing superior electrical properties. Summary of the Invention

[0005] According to one or more embodiments, a chip-embedded printed circuit board includes a cavity in a printed circuit board, a chip in the cavity of the printed circuit board, and a thixotropic dielectric filler in a gap in the cavity to encapsulate the chip in the printed circuit board.

[0006] According to another embodiment, a composition for making a dielectric filler includes a plurality of particles having an aspect ratio of about 2:1 to about 30:1, an acrylate oligomer, a photoinitiator that absorbs light at a wavelength of about 280 to about 320 nanometers, and a photoinitiator that absorbs light at a wavelength of about 320 to about 400 nanometers.

[0007] Furthermore, according to another embodiment, a method for embedding a chip in a printed circuit board includes forming a cavity in the printed circuit board, placing a chip in the cavity in the printed circuit board, and disposing a thixotropic dielectric filler in a gap in the cavity to seal the chip in the cavity in the printed circuit board.

[0008] Additional features and advantages are realized through the techniques of the present disclosure. Other embodiments and aspects of the present disclosure are described in detail herein and are considered a part of the claimed disclosure. For a better understanding of the present disclosure, together with its advantages and features, reference is made to the description and drawings. [Brief explanation of the drawings]

[0009] For a more complete understanding of the present disclosure, reference is now made to the following brief description taken in conjunction with the accompanying drawings and detailed description, wherein like reference characters represent like parts.

[0010] [Figure 1A] FIG. 1 is a top view of a chip-embedded printed circuit board. [Figure 1B] FIG. 1B is a side cross-sectional view of FIG. 1A. [Figure 2] 1 is a side cross-sectional view of a method for embedding a chip in a printed circuit board using a gap filler. [Figure 3] FIG. 10 shows images of gap filler traces with and without boron nitride. [Figure 4] 1 is a graph showing dynamic viscosity in centipoise (cps) as a function of shear rate (s −1 ) for materials with and without boron nitride. DETAILED DESCRIPTION OF THE INVENTION

[0011] As mentioned above, conventional printed circuit boards have stacked chips solder-mounted to their surface, with metal-filled vias connecting the wiring layers above and below them. However, creating such via holes requires complex processing steps.

[0012] Thus, described herein is a method for embedding a chip in a printed circuit board that eliminates via wiring, thereby reducing processing costs and device complexity. A cavity is formed in the printed circuit board, a chip is placed in the cavity, and a direct-write UV-curable composite dielectric ink is provided to fill the gap in the cavity between the chip and the circuit board, providing an embedded die encapsulated with a printable gasket material. An interposer is placed over the cavity to provide electrical connections to the chip. This composite dielectric material / ink is designed to provide ideal gap-filling capabilities using the thixotropic shear-thinning effect, excellent thermal conductivity to promote heat dissipation, and opacity to enable laser detection of Z-height in the printing system.

[0013] FIG. 1A is a top view of a chip-embedded printed circuit board 100. FIG. 1B is a side cross-sectional view of FIG. 1A. The chip-embedded printed circuit board 100 includes a printed circuit board substrate 102. The printed circuit board substrate 102 is a laminated structure of conductive layers (such as copper) and insulating layers. The printed circuit board can be single-sided (e.g., one copper layer), double-sided (two copper layers on either side of one substrate layer), or multi-layered (outer and inner layers of copper alternating with layers of insulation).

[0014] To embed the chip 104 in the substrate 102, a cavity 110 is formed in the substrate 102. The cavity 110 is formed by a processing method that depends on the material forming the substrate 102. The cavity 110 may be formed, for example, by etching, stamping, drilling, laser machining, or a combination thereof. The formed cavity 110 has length 112 and width 114 dimensions (FIG. 1A) that are greater than the chip 104 to be embedded. In some embodiments, the height 116 (or thickness, FIG. 1B) of the cavity 110 is substantially the same as the height (or thickness) of the chip 104, such that the surface of the embedded chip 104 is substantially flush with the surface of the substrate 102. In yet other embodiments, the height 116 (or thickness) of the cavity 110 is less than the height (or thickness) of the chip 104, and the gap filler 106 is positioned or stacked to act as an increment to the chip 104. After forming the cavity 110 in the substrate 102, the chip 104 is inserted into the cavity 110. The chip 104 is also called a die or an electronic component. The chip 104 is a resistor or a capacitor.

[0015] The chip-embedded circuit board 100 includes a gap filler 106 in a gap in a cavity 110 to form a gasket that seals the chip 104 in the substrate 102. The filler 106 is a thixotropic dielectric material. The gap filler 106 includes an inert inorganic filler, which is a plurality of inorganic particles with a high aspect ratio. In a low-viscosity liquid, the high aspect ratio of the particles causes them to align in the direction of flow when shear is applied, creating a shear-thinning effect. This shear-thinning results in smooth, high-resolution prints. The inorganic particles of the gap filler 106 are thermally conductive, opaque, and do not absorb UV light, so they do not interfere with UV curing.

[0016] According to one or more embodiments, the high aspect ratio particles are hexagonal boron nitride particles. In some embodiments, the high aspect ratio particles have an aspect ratio of about 2:1 to about 30:1. In other embodiments, the high aspect ratio particles have an aspect ratio of about 10:1 to about 20:1.

[0017] According to one or more embodiments, the high aspect ratio particles have an average diameter of about 0.5 to about 1.2 micrometers. In other embodiments, the high aspect ratio particles have an average diameter of about 0.8 to about 1.0 micrometers.

[0018] In one or more embodiments, the high aspect ratio particles are present in the gap filler 106 composition in an amount of about 5 to about 20 weight percent, and in other embodiments, the high aspect ratio particles are present in the gap filler 106 composition in an amount of about 10 to about 15 weight percent.

[0019] The gap filler 106 further includes a polymer matrix. In one or more embodiments, the polymer of the polymer matrix is ​​a cross-linked polymer. In some embodiments, the polymer of the polymer matrix is ​​an acrylate polymer, a cross-linked acrylate polymer, or a cross-linked epoxy acrylate polymer. Other non-limiting examples of the polymer of the polymer matrix include trimethylolpropane ethoxylate triacrylate, urethane acrylate, epoxy acrylate, polyester acrylate, or any combination thereof.

[0020] In one or more embodiments, the polymer matrix is ​​formed from a combination of at least two oligomers, including a first oligomer with low viscosity and rapid initiation and a second oligomer that is a crosslinker that introduces crosslinks into the matrix to increase modulus and thermal stability. In some embodiments, the polymer matrix is ​​formed from a combination of an epoxy acrylate oligomer and an epoxy triacrylate crosslinker oligomer, which creates a strong three-dimensional polymer network as a result of three unsaturated sites. The low-viscosity oligomer, in some embodiments, has a viscosity of about 50 to about 1000 centipoise (cps). In other embodiments, the low-viscosity oligomer has a viscosity of about 200 to 600 centipoise.

[0021] In one or more embodiments, the gap filler 106 is formed from a combination of a first low-viscosity oligomer and a second cross-linkable oligomer. In embodiments, the first oligomer is an epoxy acrylate and the second oligomer is an epoxy triacrylate. The first low-viscosity oligomer, in embodiments, is present in the gap filler 106 in an amount of about 50 to about 95% by weight. In yet other embodiments, the low-viscosity oligomer is present in the gap filler 106 in an amount of about 60 to about 85% by weight.

[0022] The gap filler 106 further includes one or more photoinitiators, photosensitizers, or a combination thereof. The photoinitiator absorbs ultraviolet energy to generate free radicals, which react with the monomers and / or oligomers to initiate polymerization and curing. The photoinitiator can cure the gap filler 106 using ultraviolet light from a UV source, such as a mercury lamp or a light-emitting diode lamp. When irradiated with ultraviolet light, Type I photoinitiators cleave to generate free radicals. When irradiated with ultraviolet light, Type II photoinitiators abstract hydrogen from a co-initiator (amine synergist) to generate radicals.

[0023] In one or more embodiments, the gap filler 106 includes one or more photoinitiators that absorb ultraviolet light (wavelengths of 200 to 450 nanometers), e.g., light having a wavelength range of 280 to 320 nanometers, 320 to 400 nanometers, or a combination thereof. In some embodiments, a combination of a Type 1 photoinitiator having long-wavelength ultraviolet absorbance (320 to 400 nanometers) is combined with a Type 1 photoinitiator having mid-wavelength ultraviolet absorbance (280 to 320 nanometers). Non-limiting examples of suitable photoinitiators include phenylbis(2,4,6,7-trimethylbenzoyl)phosphine oxide, 2-methyl-4'-(methylthio)-2-morpholinopropiophenone, 4-benzoyl-4'-methyldiphenyl sulfide (benzyl dimethyl ketal), 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-1-butanone, or a combination thereof.

[0024] In embodiments, the gap filler 106 further includes a photosensitizer in addition to one or more photoinitiators. The photosensitizer absorbs ultraviolet light and transfers its energy to another molecule, specifically the photoinitiator, to generate radicals and initiate polymerization of the polymer. In one or more embodiments, the photosensitizer absorbs ultraviolet light. In some embodiments, the photosensitizer absorbs ultraviolet light at wavelengths between about 320 and about 400 nanometers. Non-limiting examples of photosensitizers include isopropylthioxanthone, benzophenone, or combinations thereof.

[0025] In some embodiments, a composition for making the gap filler 106 includes a plurality of particles having an aspect ratio of about 2:1 to about 30:1, an acrylate oligomer, a photoinitiator that absorbs light at a wavelength of about 280 to about 320 nanometers, and a photoinitiator that absorbs light at a wavelength of about 320 to about 400 nanometers. In embodiments, the composition further includes an acrylate crosslinker, a photosensitizer, or a combination thereof. The gap filler composition is sonicated and / or stirred until no clumps are visible. The gap filler is disposed or printed on a substrate, and then cured by exposing the gap filler to ultraviolet light from a UV source.

[0026] The chip-embedded printed circuit board 100 further includes a plurality of interposers 108 printed on the gap filler 106 to provide electrical connections between the chip 104 and the substrate 102. The interposers 108 include, for example, silver.

[0027] 2 is a cross-sectional side view illustrating a method for embedding a chip 204 in a printed circuit board 202. The method includes forming a cavity 210 in the printed circuit board 202 and placing the chip 204 in the cavity in the printed circuit board 202. The method further includes disposing a thixotropic dielectric filler 206 in a gap in the cavity 210 to seal the chip 204 in the cavity of the printed circuit board 202. The method further includes forming an interposer 208 on the gap filler 206 to provide an electrical connection between the chip 204 and the printed circuit board 202.

[0028] The thixotropic dielectric ink is injected into the gap around the chip 204 by a syringe 220 in some embodiments. Any method can be used to place the thixotropic dielectric filler 206 into the cavity 210. The thixotropic properties of the filler 206 mean that its shear thinning is time-dependent. The filler 206 is thick or viscous under static conditions or low shear, but becomes less viscous and more flowable upon application of shear stress. In some embodiments, the filler 206 may be injected into the gap around the chip 204 by a syringe 220. -1 The viscosity of filler 206 at a shear rate of 50 s -1 According to one or more embodiments, the viscosity of the filler 206 at a shear rate of 5s -1 It has a viscosity of about 10,000 to about 50,000 cps at a shear rate of 50 s -1 According to another embodiment, the filler 206 has a viscosity of about 1,000 to about 10,000 cps at a shear rate of 5 s. -1 It has a viscosity of about 15,000 to about 30,000 cps at a shear rate of 50 s -1 It has a viscosity of about 2,500 to about 7,500 cps at a shear rate of 100 rpm.

[0029] According to one or more embodiments, the filler 206 is -1 According to another embodiment, the filler 206 has a viscosity of about 25,000 to about 50,000 cps at a shear rate of 0 s -1 It has a viscosity of about 30,000 to about 45,000 cps at a shear rate of 1000 rpm.

[0030] The dynamic viscosity of the filler 206 allows it to flow into the gap between the chip and the printed circuit board under shear stress and provide a smooth surface, making it an ideal electronic gap filler for embedding chips in printed circuit boards. The high content of high aspect ratio particles (e.g., boron nitride) gives the filler a strong thixotropic effect. The relatively high concentration of high aspect ratio particles also benefits thermal conductivity and laser detection, which are attractive properties for flexible gasket applications.

[0031] The thixotropic shear thinning effect causes the filler to decrease in viscosity when shear is applied to extrude it from a syringe, then return to high viscosity when the shear is removed, leaving behind smooth patterns with high resolution. In contrast, commercially available electronic fillers have undesirable surfaces due to their high viscosity, which results in a rough finish that cannot be printed on the surface, or their low viscosity, which results in minimal shear thinning, resulting in capillary wetting of the sidewalls of joints and even losses in gaps at the bottom of devices.

[0032] In addition to the gap-filling applications described above, the dynamic viscosity of the fillers described herein also makes them suitable for other electronic applications, including any gap-filling electronic application or other electronic applications where a dielectric material is required to provide a sloped or elevated surface. [Example]

[0033] Comparative Example 1: Commercially available adhesive Two commercially available materials were analyzed for favorable properties for filling the gap in the cavity between a chip and a printed circuit board. Norland Electronic Adhesive 121 (NEA 121) is a mercaptoester / benzophenone composition used for tacking, filling, sealing, conformal coating, and tamper-evident adhesive applications for precision products. Creative Materials 119-48 (CM 119-48) is a screen-printable, flexible, UV-curable dielectric coating. A cavity was formed in the printed circuit board, and a chip was inserted into the cavity. NEA 121 and CM 119-48 were deposited into the gap between the chip and the printed circuit board to evaluate their ability to encapsulate the chip and form an embedded printed circuit board.

[0034] NEA 121 had too low a viscosity (~300 cps) and exhibited minimal shear thinning, resulting in undesirable wetting of the capillaries on the junction side and loss of material through the gap at the bottom of the device. CM 119-48 had the opposite problem, exhibiting an undesirably high viscosity (~150,000 cps), which caused the material to retain its shape and left a rough surface finish. The rough surface finish made it difficult to print silver on the surface. Both NEA 121 and CM 119-48 exhibited undesirable thermal conductivity, reducing heat transfer from the device. Neither NEA 121 nor CM 119-48 are opaque, which reduces the ability of detection systems used to map surface topology.

[0035] Example 2: Ink Formulation Table 1 below shows examples of composite inks having the indicated preferred properties for embedding chips in printed circuit boards as described herein. [Table 1]

[0036] The oligomer, boron nitride, photoinitiator, and sensitizer were added to a glass bottle covered with aluminum foil. The bottle was probe sonicated for 2 hours with 10-second on-off cycles. The sample was checked every 30 minutes for clumping around the probe. If clumping was observed, the mixture was mixed by hand until the clumps broke up, and the probe was re-administered. The final mixture should be free of visible clumps.

[0037] Example 3: Characterization of dynamic viscosity Composite ink formulations containing and not containing boron nitride were analyzed for preferred viscosity. Ink Formulation #1, containing 15.50% boron nitride, from Table 1 above, was compared to a formulation without boron nitride (98.5% CN131, 0.5% phenylbis(2,4,6,7-trimethylbenzoyl)phosphine oxide, 0.5% 2-methyl-4'-(methylthio)-2-morpholinopropiophenone, 0.5% isopropylthioxanthone). Lines of ink were deposited using a syringe, as shown in Figure 3. Without boron nitride (left), the ink lines are shorter (492-567 micrometers) and wider (384-576 micrometers), while with 15.5% boron nitride (right), the ink lines are longer (885-1020 micrometers) and thinner (211-221 micrometers). Both inks were printed using the same printing procedure. This behavior demonstrates the effect of boron nitride on print resolution. The ink containing 15.5% boron nitride was able to print the intended shape with greater precision compared to the ink without boron nitride.

[0038] Figure 4 shows the relationship between the viscosity of the ink (cps) and the shear rate (s -1 The viscosity of the ink containing no boron nitride is 215 cps at all shear rates tested. The viscosity of the ink containing 15.5% boron nitride is 5 cps. -1 Up to 18,181 cps at a shear rate of 50 s -1 The viscosity of the solution was up to 3,996.67 cps, thus showing thixotropic behavior. -1 The ratio of the viscosities at two different shear rates was 4.55.

[0039] The corresponding structure, material, acts, and equivalents of all means or step-plus-function elements in the following claims are intended to include any structure, material, or acts for performing the function as specifically claimed in combination with other claimed elements. The description of the present disclosure has been presented for purposes of illustration and description, but is not intended to be exhaustive or to limit the disclosure to the form set forth. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the present disclosure. The embodiments have been selected and described to best explain the principles and practical applications of the present disclosure and to enable others skilled in the art to understand various embodiments, with various modifications suitable for the particular use intended.

[0040] While preferred embodiments have been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements that fall within the scope of the following claims, which should be construed to maintain appropriate protection for the disclosure as originally described.

Claims

1. A chip-embedded printed circuit board, a cavity in the printed circuit board; a chip in the cavity of the printed circuit board; a thixotropic dielectric filler in the gap in the cavity for sealing the chip in the printed circuit board; The thixotropic dielectric filler is a plurality of particles having an aspect ratio of 2:1 to 30:1; an acrylate oligomer; an epoxy triacrylate crosslinker oligomer; a photoinitiator that absorbs light at a wavelength of about 280 to about 320 nanometers; a photoinitiator that absorbs light at a wavelength of about 320 to about 400 nanometers.

2. 2. The chip-embedded printed circuit board according to claim 1, wherein the plurality of particles having an aspect ratio of 2:1 to 30:1 are boron nitride particles.

3. The chip-embedded printed circuit board of claim 1 , wherein the thixotropic dielectric filler further comprises one or more photosensitizers.

4. The chip-embedded printed circuit board of claim 1 , wherein the thixotropic dielectric filler comprises a cross-linked epoxy acrylate polymer.

5. A thixotropic dielectric filler, comprising: a plurality of particles having an aspect ratio of 2:1 to 30:1; an acrylate oligomer; an epoxy triacrylate crosslinker oligomer; a photoinitiator that absorbs light at a wavelength of about 280 to about 320 nanometers; a photoinitiator that absorbs light at a wavelength of about 320 to about 400 nanometers.

6. 6. The thixotropic dielectric filler of claim 5, wherein the plurality of particles having an aspect ratio of 2:1 to 30:1 are boron nitride particles.

7. The thixotropic dielectric filler of claim 5 further comprising a photosensitizer.

8. The thixotropic dielectric filler of claim 5 , wherein the acrylate oligomer is an epoxy acrylate oligomer.

9. 6. The thixotropic dielectric filler of claim 5, wherein the plurality of particles having an aspect ratio of 2:1 to 30:1 are present in the thixotropic dielectric filler in an amount of about 5 to about 20 weight percent.

10. 6. The thixotropic dielectric filler of claim 5, wherein the plurality of particles having an aspect ratio of 2:1 to 30:1 is present in the thixotropic dielectric filler in an amount of about 10 to about 15 weight percent.

11. 1. A method for embedding a chip in a printed circuit board, comprising: forming a cavity in the printed circuit board; placing the chip in the cavity in the printed circuit board; and disposing the thixotropic dielectric filler of claim 5 in a gap in the cavity of the printed circuit board to encapsulate the chip in the cavity.

12. 12. The method of claim 11, wherein the plurality of particles having an aspect ratio of 2:1 to 30:1 are boron nitride particles.

13. The method of claim 11 , wherein the thixotropic dielectric filler further comprises one or more photosensitizers.

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