LED flip chip packaging structure and packaging method based on low-temperature welding slurry

Abstract

The invention discloses an LED flip chip packaging structure and packaging method based on low-temperature welding slurry, the packaging structure comprises an LED chip, the LED chip comprises an electrode surface, and the electrode surface is provided with two electrodes; the fluorescent adhesive film is coated outside the LED chip, and the fluorescent adhesive film on the electrode surface is flush with the end surface of the electrode to form a coplanar surface; and the low-temperature welding slurry layers are respectively printed on the two electrodes and cured to form two bonding pads, and each bonding pad completely covers the corresponding electrode and at least partially covers the fluorescent adhesive film. Expensive semiconductor sputtering / photoetching equipment is replaced by a mature printing process, so that the investment is small, the efficiency is high, and no heavy environment-friendly burden exists; through the printing pattern design, the shape and the size of the expansion electrode can be flexibly and accurately controlled, and the mounting requirement of a downstream PCB is perfectly met.

Description

Technical Field

[0001] This application relates to the fields of semiconductor devices and semiconductor packaging technology, specifically to the packaging structure and packaging method of LED flip chips based on low-temperature soldering paste. Background Technology

[0002] Achieving reliable and low-cost interconnection between micron-level chip electrodes and external circuits in LED chip-scale packaging has always been a key technological challenge for the industry. The current mainstream technological approaches and their main limitations are as follows: Magnetron sputtering / electroplating combined with photolithography: Although this solution has the advantage of high precision, it relies on expensive equipment, has a complex process flow, a long production cycle, and involves environmental issues such as electroplating wastewater treatment. Therefore, it is not suitable for cost-sensitive large-scale manufacturing scenarios.

[0003] Conductive silver paste: This type of material generally has a low solids content, resulting in limited conductivity and high resistivity after curing. More importantly, the cured silver paste surface usually lacks good solderability, making it difficult to form a reliable metallurgical bond with the solder paste in subsequent SMT processes, thus affecting the long-term reliability of the interconnect.

[0004] Solder paste printing and reflow soldering: The adhesion between solder paste and encapsulant is poor. During reflow soldering, the solder paste printed in the electrode gap area tends to migrate and accumulate towards the electrode area instead of forming a stable electrical connection or effectively expanding the electrode structure, leading to connection failure or short circuit risk. Summary of the Invention

[0005] The present invention aims to at least solve one of the technical problems existing in the prior art. To this end, the present invention proposes: A packaging structure for LED flip chips based on low-temperature soldering paste, comprising: An LED chip includes an electrode surface having two electrodes. A fluorescent adhesive film is used to cover the LED chip, and the fluorescent adhesive film on the electrode surface is flush with the end face of the electrode to form a coplanar structure. A low-temperature solder paste layer is printed on two electrodes and cured to form two solder pads. Each solder pad completely covers the electrode and at least partially covers the fluorescent adhesive film.

[0006] One embodiment of the present invention addresses its technical problem by employing the following technical solution: the low-temperature welding slurry layer is formed by curing a low-temperature welding slurry, wherein the low-temperature welding slurry comprises, by weight percentage: 92%~93% copper core and silver shell metal particles, the remainder is organic carrier.

[0007] One embodiment of the present invention employs a technical solution to solve its technical problem: the D50 of the copper core and silver shell metal particles is less than 10 micrometers.

[0008] This application also proposes a packaging method for the above-mentioned LED flip chip, comprising: Step 1: Place the LED chip on the fixture with its electrode side facing down, cover the LED chip with fluorescent adhesive, press and cure it to make the fluorescent adhesive cover the LED chip and form a composite. Step 2: Remove the composite from the fixture and process it so that the end faces of the two electrodes of the LED chip are exposed, and the phosphor adhesive and the electrode end faces form a flat coplanar surface. Step 3: Using a printing process, low-temperature solder paste is printed onto the two electrode faces of the LED chip to form solder pads, and each solder pad completely covers the electrode and at least partially covers the phosphor film. Step 4: Bake the composite after printing the low-temperature welding paste at a low temperature. The low-temperature welding paste will solidify to form solder pads.

[0009] One embodiment of the present invention adopts the following technical solution to solve its technical problem: In step 4, the low-temperature baking temperature is 150℃~200℃ and the baking time is 10~30 minutes.

[0010] The beneficial effects of this invention are: It replaces expensive semiconductor sputtering / lithography equipment with mature printing technology, which requires less investment, is more efficient, and has no heavy environmental burden; through printing pattern design, the shape and size of the extended electrodes can be flexibly and precisely controlled, perfectly adapting to the mounting requirements of downstream PCBs.

[0011] The low-temperature conductive paste uses copper core and silver shell metal particles with a particle size of less than 10 micrometers as the base, which is the main body of the paste's conductivity and welding function. It utilizes the low cost and high conductivity of copper, and the silver layer prevents the oxidation of copper, thus ensuring the stability and solderability of the paste.

[0012] The organic carrier includes epoxy resin, latent amine curing agent, accelerator, diluent, thixotropic agent, coupling agent, dispersant, and antioxidant, which gives the low-temperature welding paste suitable printing rheological properties and enables it to cure during low-temperature baking.

[0013] Because the metal content in the low-temperature soldering paste is greater than 92%, it ensures that the volume resistivity of the cured solder pads is extremely low, as low as ≤12×10⁻⁶. -6 Ω·cm, after curing, a strong ohmic contact is formed between the pad and the chip electrode, which has excellent conductivity and mechanical bonding strength.

[0014] The thermal expansion coefficient of the pad material matches that of the fluorescent adhesive film, which can significantly reduce interfacial stress during temperature changes, thereby reducing the overall internal stress level of the package and improving the long-term reliability and temperature resistance of the device.

[0015] The presence of a silver layer on the surface of the copper-core, silver-shell metal particles and the dense metal structure after the paste has solidified enable it to be perfectly soldered with conventional SMT solder paste, solving the industry pain point that traditional silver paste cannot be soldered. Attached Figure Description

[0016] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which: Figure 1 This is a schematic diagram of the LED flip chip packaging structure described in this embodiment; Figure 2 This is a schematic diagram of step 1 in this embodiment; Figure 3 This is a schematic diagram after completing step 1 in this embodiment; Figure 4 This is a schematic diagram after step 2 is completed in this embodiment; Figure 5 This is a cross-sectional view after step 2 is completed in this embodiment; Figure 6 This is a schematic diagram after step 3 is completed in this embodiment; Figure 7 This is a cross-sectional view after step 3 is completed in this embodiment. Detailed Implementation

[0017] This section will describe in detail specific embodiments of the present invention. Preferred embodiments of the present invention are shown in the accompanying drawings. The purpose of the drawings is to supplement the textual description with graphics, so that people can intuitively and vividly understand each technical feature and overall technical solution of the present invention, but they should not be construed as limiting the scope of protection of the present invention.

[0018] This application proposes a packaging structure for LED flip chips based on low-temperature soldering paste, including: An LED chip includes an electrode surface having two electrodes. A fluorescent adhesive film is used to cover the LED chip, and the fluorescent adhesive film on the electrode surface is flush with the end face of the electrode to form a coplanar structure. A low-temperature solder paste layer is printed on two electrodes and cured to form two solder pads. Each solder pad completely covers the electrode and at least partially covers the fluorescent adhesive film.

[0019] In this embodiment, the low-temperature welding slurry layer is formed by curing a low-temperature welding slurry, and the low-temperature welding slurry comprises, by weight percentage: Copper-core, silver-shell metallic particles: 92.0%~93.0% Epoxy resin 2.8%~3.5%; Latent amine curing agents: 0.6%~0.8%; Accelerator 0.15~0.2%; Diluent 1.0~1.5%; Thixotropic agent 1.0%~1.3%; Coupling agent 0.3%~0.5%; Dispersant 0.8%~1.2%; Antioxidant content: 0.05%~0.1%.

[0020] The preparation method of the low-temperature welding slurry includes: Step 1: Dissolve the dispersant in the diluent; Step 2: Under low-speed stirring, slowly add all the copper core and silver shell metal particles into the above solution to initially wet each powder and form a uniform paste. Step 3: After mixing the remaining organic carrier components evenly, add the above paste and mix. Then, grind the mixture 3 to 5 times using a three-roll mill. Step 4: Use a planetary degassing machine to degas for 10-15 minutes at low speed revolution / high speed rotation.

[0021] Based on the above, embodiments of this application are proposed. The composition and formulation of the low-temperature welding slurry in this embodiment are shown in Table 1.

[0022] Table 1

[0023] The low-temperature soldering paste provided in Table 1 is used for LED chip packaging. Specific methods include: Step 1: Arrange LED chips with a size of 125 micrometers × 450 micrometers with their electrode surfaces facing down on a fixture. The surface of the fixture is covered with high-temperature double-sided adhesive. Cover the LED chips with fluorescent adhesive and press them together to form a fluorescent silicone film with a thickness of 250 micrometers. Cure at 150°C for 3 hours to form a composite in which the LED chips are encapsulated in the adhesive film. Step 2: Cast the entire composite material, so that the LED chip electrodes face upward and are exposed, and the phosphor adhesive and the electrode end face form a flat coplanar surface; Step 3: Using a 20-micron thick steel mesh, print the above-mentioned low-temperature soldering paste on the coplanar surface. Print the low-temperature soldering paste on the two electrode faces of the LED chip to form pads. Each pad completely covers the electrode and at least partially covers the phosphor film, so that the paste pattern extends the electrode of each LED chip to 350 microns × 450 microns. Step 4: Bake the composite after printing the low-temperature soldering paste in a 180°C oven for 15 minutes. After the paste solidifies, it forms solder pads.

[0024] After testing, the volume resistivity of the pads is ≤12×10⁻⁶. -6 Ω·cm, thermal expansion coefficient is 150~200 ppm / ℃, and the soldering strength between the pad and SAC305 solder paste is greater than 15MPa.

[0025] Of course, the present invention is not limited to the above-described embodiments. Those skilled in the art can make equivalent modifications or substitutions without departing from the spirit of the present invention. All such equivalent modifications and substitutions are included within the scope defined by the claims of this application.

Claims

1. A packaging structure for LED flip chips based on low-temperature soldering paste, characterized in that, include: The LED chip (10) includes an electrode surface having two electrodes (11). A fluorescent adhesive film (20) is wrapped around the LED chip (10), and the fluorescent adhesive film (20) on the electrode surface is flush with the end face of the electrode (11) to form a coplanar structure. The low-temperature welding paste layer (30) is printed on the two electrodes (11) and cured to form two pads, each pad completely covering the electrode (11) and at least partially covering the fluorescent adhesive film (20).

2. The LED flip chip packaging structure based on low-temperature soldering paste according to claim 1, characterized in that, The low-temperature welding slurry layer (30) is formed by curing low-temperature welding slurry, which comprises, by weight percentage: 92%~93% copper core and silver shell metal particles, the remainder is organic carrier.

3. The LED flip chip packaging structure based on low-temperature soldering paste according to claim 2, characterized in that, The D50 of the copper-core silver-shell metal particles is less than 10 micrometers.

4. A method for packaging LED flip chips as described in any one of claims 1-3, characterized in that, include: Step 1: Place the LED chip (10) on the fixture with its electrode face down, cover the LED chip (10) with fluorescent adhesive, press and cure it so that the fluorescent adhesive covers the LED chip and forms a composite. Step 2: Remove the composite from the fixture and process it so that the end faces of the two electrodes (11) of the LED chip (10) are exposed, and the fluorescent adhesive and the end faces of the electrodes (11) form a flat coplanar surface; Step 3: Using a printing process, low-temperature solder paste is printed onto the two electrode (11) end faces of the LED chip (10) to form solder pads, and each solder pad completely covers the electrode (11) and at least partially covers the fluorescent film (20); Step 4: Bake the composite after printing the low-temperature welding paste at a low temperature. The low-temperature welding paste will solidify to form solder pads.

5. The LED flip chip packaging method according to claim 4, characterized in that, In step 4, the low-temperature baking temperature is 150℃~200℃, and the baking time is 10~30 minutes.

Images