Material
The five raw materials of LED are: chip, bracket, silver glue, gold wire, epoxy resin
Chip
The composition of the chip: It is composed of gold pad, P pole, N pole, PN junction, and back gold layer (double pad chip without back gold layer). The wafer is a PN combination composed of P-layer semiconductor elements and N-layer semiconductor elements rearranged and combined by the movement of electrons. It is this change that enables the wafer to be in a relatively stable state. When the wafer is applied with a certain voltage to the forward electrode, the holes in the forward P area will continuously swim to the N area, and the electrons in the N area will move to the P area relative to the holes. While the electrons and holes move relative to each other, the electrons and holes pair with each other to excite photons and generate light energy.
Main classification, surface emitting type: Most of the light is emitted from the surface of the wafer. Five-sided luminous type: There are more light emitted from the surface and sides according to the luminous color, red, orange, yellow, yellow-green, pure green, standard green, blue-green, and blue.
Bracket
The structure of the bracket is 1 layer of iron, 2 layers of copper plating (good electrical conductivity, fast heat dissipation), 3 layers of nickel plating (anti-oxidation), 4 layers of silver plating (good reflectivity, easy wire bonding)
Silver glue (because there are many types, we take H20E as an example)
Also called white glue, milky white, conductive adhesion (baking temperature: 100°C/1.5H) silver powder (conductive, heat dissipation, fixed chip) + epoxy resin (cured silver powder) + thinner (easy to stir). Storage conditions: Silver glue manufacturers generally store silver glue at -40 °C, and application units generally store silver glue at -5 °C. Single agent is 25 °C/1 year (in a dry and ventilated place), mixed agent is 25 °C/72 hours (but due to other factors "temperature, humidity, and ventilation conditions" during online operation, to ensure the quality of the product is average The use time of the mixture is 4 hours)
Baking conditions: 150 °C/1.5H
Stirring conditions: Stir evenly in one direction for 15 minutes
Gold wire (take φ1.0mil as an example)
The gold wires used in LEDs are φ1.0mil, φ1.2mil, and the material of the gold wire. The material of the gold wire for LED generally contains 99.9% gold. The purpose of the gold wire
Utilizing its high gold content, soft material, easy deformation, good electrical conductivity, and good heat dissipation characteristics, a closed circuit is formed between the chip and the support. (Conversion relationship: 1 mil = 0.0254mm, 1 in = 25.4mm)
Epoxy resin (take EP400 as an example)
Composition: A and B two components:
Glue A: It is the main agent, composed of epoxy resin + defoamer + heat-resistant agent + thinner
Agent B: It is a curing agent, composed of acid + release agent + accelerator
Conditions of Use:
Mixing ratio: A/B=100/100 (weight ratio)
Mixing viscosity: 500-700CPS/30 °C
Gel time: 120 °C*12 minutes or 110 °C*18 minutes
Usable conditions: room temperature 25 °C about 6 hours. Generally, according to the production needs of the production line, we set its use condition as 2 hours.
Hardening conditions: initial hardening 110 °C—140 °C 25—40 minutes
Post-curing 100 °C*6-10 hours (can be adjusted according to actual needs)
Craft
Chip inspection
Microscopic inspection: Whether there is mechanical damage on the surface of the material and whether the size of the lockhill chip and the electrode size meet the process requirements. Whether the electrode pattern is complete.
LED expansion
Since the LED chips are still arranged tightly and with a small pitch (about 0.1mm) after dicing, it is not conducive to the operation of the subsequent process. The film expanding machine is used to expand the film of the bonding chip, so that the distance between the LED chips is stretched to about 0.6mm. Manual expansion can also be used, but it is easy to cause undesirable problems such as chip drop and waste.
LED dispensing
In the LED holder on the corresponding position of the point of silver paste or an insulating adhesive. For GaAs , SiC conductive substrates, red, yellow, and yellow-green chips with back electrodes, silver glue is used. For blue and green LED chips with sapphire insulating substrates, insulating glue is used to fix the chips.
The difficulty of the process lies in the control of the amount of glue. There are detailed technological requirements for the height of the colloid and the position of the glue. Since silver glue and insulating glue have strict requirements for storage and use, remind: the waking, mixing, and use time of silver glue are matters that must be paid attention to in the process.
LED glue
Contrary to glue dispensing, glue preparation is to use a glue preparation machine to first coat the silver glue on the back electrode of the LED , and then install the LED with silver glue on the back of the LED bracket. The efficiency of glue preparation is much higher than that of glue dispensing, but not all products are suitable for glue preparation process.
LED manual puncture piece
Place the expanded LED chips (with or without glue) on the fixture of the piercing table, place the LED bracket under the fixture, and use a needle to pierce the LED chips to the corresponding positions under the microscope . Compared with automatic rack mounting, manual piercing piece has an advantage, it is convenient to change different chips at any time, and it is suitable for products that need to install multiple chips.
LED automatic mounting rack
The automatic mounting is actually a combination of the two steps of glue (dispensing) and chip mounting. First , apply silver glue ( insulating glue ) on the LED bracket , and then use a vacuum nozzle to suck up the LED chip in the moving position, and then place it on The corresponding bracket position. In the process of automatic racking, it is mainly necessary to be familiar with the equipment operation programming, and at the same time adjust the glue and installation accuracy of the equipment . In the selection of nozzles, try to use bakelite nozzles to prevent damage to the surface of the LED chip, especially bakelite must be used for blue and green chips. Because the steel nozzle will scratch the current diffusion layer on the surface of the chip.
LED sintering
The purpose of sintering is to solidify the silver paste , and sintering requires temperature monitoring to prevent poor batch performance. The sintering temperature of silver paste is generally controlled at 150°C, and the sintering time is 2 hours. According to the actual situation, it can be adjusted to 170°C for 1 hour. Insulating glue is generally 150°C for 1 hour.
The silver glue sintering oven must be opened every 2 hours (or 1 hour) to replace the sintered product according to the process requirements , and it must not be opened at will. The sintering oven must not be used for other purposes to prevent pollution.
LED pressure welding
The purpose of pressure welding is to lead the electrode to the LED chip to complete the connection of the inner and outer leads of the product.
The pressure welding process of LED has two types: gold wire ball bonding and aluminum wire bonding. The process of aluminum wire bonding is to first press the first point on the electrode of the LED chip, then pull the aluminum wire onto the corresponding bracket, press the second point and then tear the aluminum wire. The gold wire ball bonding process burns a ball before pressing the first point, and the rest of the process is similar.
Pressure welding is a key link in the LED packaging technology. The main things that need to be monitored in the process are the shape of the pressure-welded gold wire (aluminum wire), the shape of the solder joint, and the tensile force.
LED sealant
There are LED package dispensing , potting , molding three. Basically, the difficulties of process control are bubbles, lack of material, and black spots. The design is mainly the selection of materials, and the choice of epoxy and brackets that are well-combined. (General LED cannot pass the air tightness test).
LED dispensing TOP-LED and Side-LED are suitable for dispensing packaging. Manual dispensing packaging requires high levels of operation (especially white LEDs). The main difficulty is the control of the dispensing volume, because the epoxy will thicken during use. Dispensing for white light LEDs also has the problem of phosphor precipitation causing color difference in light output.
LED potting encapsulation Lamp-LED is encapsulated in the form of potting. Potting process is to injection mold cavity in the LED liquid epoxy, and good bonding insert LED holder, placed in an oven so that the epoxy cured, i.e., the LED extrusion molding from the mold cavity.
LED molding package Put the pressure-welded LED bracket into the mold, clamp the upper and lower molds with a hydraulic press and vacuum, put the solid epoxy into the entrance of the injection channel, and press the hydraulic ejector into the mold channel for heating. The epoxy enters each LED molding groove along the glue channel and cures.
LED curing and post curing
Curing refers to the curing of the encapsulated epoxy, and the general epoxy curing conditions are at 135°C for 1 hour. Molded packages are generally at 150°C for 4 minutes. The post-curing is to fully cure the epoxy while thermally aging the LED . Post-curing is very important to improve the bonding strength between epoxy and PCB. The general conditions are 120°C, 4 hours.
LED cutting and dicing
Since the LEDs are connected together (not individually) in production, the Lamp packaged LED adopts cutting ribs to cut the connecting ribs of the LED bracket. SMD-LED is on a PCB board , the need to complete the separation dicing work.
LED test
Test the photoelectric parameters of the LED , inspect the external dimensions, and sort the LED products according to customer requirements.
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