1. Overview of LED packaging technology and materials

LEDs are semiconductor light-emitting diodes that are now widely used in many fields such as lighting, displays, information, and sensors. According to the power and application requirements, LED devices use corresponding packaging materials, mainly including epoxy resin, silicone resin and inorganic packaging materials.

Encapsulation materials, encapsulation processes, and encapsulation equipment need to match each other, and they are basically one-to-one correspondence. The main methods of LED packaging are as follows:

1) Dispensing and potting based on liquid glue;

2) Transfer Molding based on solid-state EMc;

3) Vacuum pressing type based on semi-curing adhesive film;

4) Other special packaging methods, such as mold injection molding based on liquid resin, brushing or printing, spraying and other packaging processes based on thixotropic glue.

1.1 Dispensing potting technology

The core equipment of the dispensing process includes a dispensing machine (with air pressure, plunger pump, gear pump and other feeding methods), an integrated metal bracket with a dam or a reflective cup, and the packaging material is two-component or one-component glue. Both liquid epoxy resins and liquid silicone gels are basically packaged in two-component packaging, because two-component is conducive to the long-term storage of the material, but they need to be fully mixed to achieve uniformity before dispensing and potting. In order to mix the glue with inorganic materials such as phosphors, it is necessary to use a high-speed double planetary disperser to ensure that the inorganic material is uniformly dispersed within the organic resin. The mixed material needs to be encapsulated in accordance with the supplier's recommended operation method and used up within the specified time, otherwise, the inorganic material cannot be stably dispersed in the liquid glue for a long time, and agglomeration and sedimentation will occur. In addition, after the mixing of components A and B, even when stored at room temperature, chemical cross-linking or hygroscopicity can occur, which can affect the viscosity stability of the material. Epoxy resin mainly uses anhydride as a curing agent and is configured as an addition reactive encapsulation material, and this epoxy resin is a two-component formula of A and B. In addition, epoxy resins can be formulated as one-component glues based on the cationic reaction mechanism. This cationic reaction formulation material has more heat resistance and high temperature yellowing resistance, but due to the high cost of catalytic system, it cannot be widely used, and is only limited to the packaging field with high thixotropy requirements. The silicone solution used in LED packaging applications mainly uses metal platinum to catalyze the addition reaction system of organosiloxanes containing double bonds and organosiloxanes containing silicon hydrogen. The reaction system is usually formulated as a two-component encapsulation material with A and B, which are stable and easy to store. Most of the LED encapsulation adhesives are heat-curing materials, and some encapsulation materials use UV light curing systems for special applications. For heat-curing materials, after dispensing, the glue needs to be baked at a high temperature of 150 degrees for about 2-5 hours to achieve a fully cured package. When the resin is cured, the resin will undergo a certain volume shrinkage, resulting in shrinkage stress, which will have a certain impact on the bonding between the resin and the chip, the chip and the silver glue, the gold wire solder joint, and the bonding interface between the resin and the bracket. Therefore, the packaging material and the packaging process are directly related to the system stability of the LED device, and the packaging engineer needs to systematically and carefully study and analyze to determine the optimal packaging process and packaging material.

1.2 Transfer Molding technology based on thermosetting resin encapsulation materials

Transfer molding is a transfer molding technology, which consists of three elements: a molding machine, a chip and its support material, and an EMC (Epoxy Molding Compound) packaging resin. The classification and generation economy of the main molding machine equipment are summarized in Table 1.

There are two types of chip supports: metal bracket (leadframe) and substrate (PCB substrate). Full-mount chips are bonded to a bracket or substrate by conductive or non-conductive die bonding adhesive, and then connected to the chip and the support through gold wire (aluminum wire or copper wire for some products). Flip chips are attached to the supports by solder paste or eutectic soldering, eliminating the need for wire bonding.

Epoxy resin molding material EMC for LED encapsulation is a semi-cured, solid resin material at room temperature composed of epoxy resin, curing agent, and special additives, and is a cylindrical "cake". The industry usually refers to the diameter of 35mm, 46mm, and 48mm as "flatbread", which is suitable for traditional molding machines; The diameter of 13mm, 14mm, 16mm is "small cake", suitable for MGP mold or automatic mold machine. The EMC begins to melt at the packaging temperature, usually 150°C, and is pushed by the transfer rod of the laminator through the flow channel and injected into the cavity containing the chip. EMC will undergo curing reaction at high temperature, and lose fluidity, after the molding machine completes the transfer injection, after a few minutes of pressure holding, you can ensure that the EMC curing is complete, and the LED packaging process is completed.

Figure 1 is a schematic diagram of EMC injection molding.

1.3 Hot-pressed encapsulation technology based on semi-curing silicone phosphor film

Adhesive film lamination technology is an emerging CSP (chip scale package) packaging method for medium and high-power LEDs in the past five years. The LED CSP structure has the advantages of uniform light color, excellent heat dissipation structure, and small placement size, and is used in TV backlight, mobile phone backlight, car lights, flash, and commercial lightingIn the field of smart lighting, compared with the traditional formalized LED packaging form, the adhesive film lamination technology has irreplaceable technical advantages, which will promote the rapid development of the LED field. The concept of CSP in the LED industry is based on the concept of the IC industry, that is, the size of the packaged device is not more than 1.14 times that of the unpackaged bare chip. The CSP concept in the LED industry is slightly different from the IC in that it is closely integrated with flip chip technology, that is, it eliminates the need for wire bonding and can be directly used by the lamp factory surface mount SMT.

Figure 2: Hot-pressed encapsulation technology based on a semi-cured silicone phosphor film

The core technology of encapsulating LED CSP is to form a controllable and uniform fluorescent adhesive layer on the five gloss surfaces of the chip. Before the maturity of the adhesive film technology, the fluorescent layer was formed on the surface of the chip by spraying fluorescent glue. The spraying process is designed according to the color temperature of the LED, and it takes 7-15 times to spray repeatedly to meet the design requirements, so the production efficiency is not good. With the help of precision press-fitting equipment and press-fitting fixtures, as well as the stability and uniformity of the semi-cured fluorescent film, the fluorescent film lamination method can produce CSP with high precision and efficiency, and greatly improve production efficiency.

Figure 3 is the schematic diagram of the adhesive film lamination method

The core steps of the fluorescent film lamination process are: flip chip array crystal placement on the heat-resistant adhesive film, combination with the prefabricated fluorescent adhesive film in the vacuum pressing fixture, 5-10 minutes vacuum pressure holding and film curing, hardening of the adhesive film and chip array cutting. Vacuum pressing equipment needs to have basic process capabilities such as high-precision temperature control of upper and lower templates, rapid vacuuming, and soft mold clamping stroke control. Taking a 35mil*35mil flip chip as an example, a standard 100mm*100mm press-fit fixture can hold 6,000 chip arrays; The operating table of the mature and mass-produced pressing machine can place 4 pieces of standard pressing fixtures at a time, so it can be concluded that the CSP production capacity of a single machine (calculated in terms of pressing cycle of 10 minutes) is 144K/hr. Therefore, fluorescent adhesive film lamination is a high-efficiency, low-cost, and easy-to-control CSP manufacturing method.

2. Packaging materials in the emerging subdivision of LED packaging

According to the classification of material chemical composition, LED packaging materials mainly include epoxy resin and silicone; According to the classification of packaging application and packaging process, there are more subdivisions of packaging materials. Table 2 shows the form of packaging materials, packaging processes, packaging product applications and the competition situation of material suppliers.

2.1 Market segment 1: Epoxy EMC package for small power indication ChipLED

Low-power LEDs are used in ChipLEDs that are packaged on a substrate or metal bracket, and due to the high volume and fierce competition for yield and efficiency, manufacturers basically use the transfer molding method to package them with solid epoxy resin. Mainstream products include ChipLED 0603, 0805, 1206 for red, green, blue and yellow light, which can be monochrome, bi-color or RGB full color.