Background Technology
Light-emitting diode (LED) is directly packaged on PCB board by resin, which can make the LED more miniaturized. This technology has been widely used in LCD backlight, commodity display lighting and automobile lighting, etc. It can meet the requirements of high brightness and long life of LED. In this technology, the effective luminescence of the LED elements is very important to improve the brightness, but the brightness of the LED is often reduced due to the deterioration and discoloration of the reflective surface of the mounted substrate. Therefore, in order to improve the brightness and longevity of the LED, it is necessary to choose a substrate material with high reflection coefficient and not easy to discolor.
In practical application, both white LED and high-energy blue LED or the ultraviolet light emitted by them will promote the degradation of the substrate surface and easily cause discoloration. In addition, the thermal shock of curing engineering or reflow welding engineering during installation of packaging resin can easily cause discoloration of the substrate. If the substrate is discolored, it not only reduces the brightness of the LED, but also changes the tone due to the reflected light of the substrate, which affects the lifetime of the LED. Therefore, it is required that the substrate for LED must have a high reflection coefficient, and will not deteriorate due to blue or ultraviolet light, nor will it change color due to heating. The current market of the substrate can not meet this requirement.
Weld resistance coating is an insulating and protective coating material for PCB surface layer. The solder resistance film is responsible for preventing unnecessary solder attachment on the surface of the circuit and short circuit of the circuit when the components are installed. At the same time, as a permanent protection circuit graphics from dust or heat, humidity and other external environment, to maintain the insulation between the circuits. Therefore, the welding resistance film must have various qualities, such as insulation, heat resistance, resistance to resistance, moisture resistance, reliability and adequate hardness to scratch.
A method for manufacturing an LED circuit board includes the following steps:
A. To adapt to the size of circuit boards, copper foil is fed, and solder resistance ink is printed on the surface of copper foil.
B. Two copper foil inks with good solder resistance printing ink face inward, and PP sheets are placed in the middle for pressing.
C. Laser drilling;
D. Deposition of copper and electroplating;
E. External circuit fabrication.
Supplementary Notes
- Step a also includes the steps of baking the solder resist ink.
- Step a uses white or black solder resist ink, and the ratio of the solder resist ink to boiling oil water is 1:0.01~0.03.
- Step a adopts 77T white screen printing plate.
- Step a chooses one of the surfaces to be coated with copper foil with aluminum foil, and solder resistance ink is printed on the surface of copper foil without aluminum foil.
- In step a, the thickness of solder resist ink is over 30um.
- Step C also includes the steps of opening copper windows by acid etching of inner layer before laser irradiation.
- Step E also includes the steps of dry film pretreatment by ultra-coarsening and AOI detection by single-machine scanning after circuit fabrication.
- After the outer circuit is fabricated, the gold-converting step is also needed.
- In step d, copper was deposited twice in succession.
The LED circuit board manufactured by the method can cover insulating substrates with 100% ink. The LED circuit board manufactured by the method can be used in commercial lighting, high-end automobiles, aerospace, military and other lighting places with high-reliability requirements, effectively avoiding deterioration and discoloration of substrates.
Specific implementation methods
The specific process of the manufacturing method of the LED circuit board is as follows:
Cutting of CA copper foil on the first side and on the second side→Printing anti-welding ink on copper foil→Bake→Drilling rivet hole→Two layers of copper foil with PP sheet pressed in the middle→Drill board edge tool hole→Remove aluminium foil from CA copper foil→Laser opens the copper window→Laser Drilling→Copper Deposition and Hole Filling Electroplating→Graphic Circuit Making→etching→AOI detection →Immersion Gold→Forming.
Firstly, the copper foil as the circuit layer is fed. In this example, the CA copper foil is used to fabricate the LED pad and circuit by etching process. CA copper foil refers to the structure of a layer of aluminium foil on a layer of copper foil. Aluminum foil can increase the overall hardness of copper foil, realize the printing of anti-welding ink for ultra-thin copper foil. At the same time, aluminium foil can also protect copper foil from PP powder and other impurities pollution during pressing. The adhesive between copper foil and aluminium foil is made of material with high temperature peeling property. Only one circle is coated around the edge of the plate. After laminating at high temperature, the adhesive will crack with the copper foil, which is convenient to peel off the aluminium foil. The adhesive can be peeled off together with the aluminium foil to avoid colloid residue on the copper foil. Of course, common copper foil can also be used in this step.
Copper foil is printed with anti-welding ink on the surface of copper foil after uncovering, which is used to protect insulating substrates, and black or white anti-welding ink is preferred. Black has the best light-resisting property, but its reflectivity is low and white reflectance is high, but its light-resisting property is not as good as black ink, so it can be selected according to the needs of actual terminal products. After selecting the ink, it should be mixed with boiling oil and water in a certain proportion. The ratio of general ink to boiling oil and water is 1:0.01-0.03. After mixing, it should be stirred for 15-30 minutes, and then it can be printed on the computer for 15 minutes. If black ink is used, the ratio of black ink to boiling oil water is 1:0.02; if white ink is used, the ratio of black ink to boiling oil water is 1:0.03. When printing, 77T white screen is preferred for the screen plate of anti-welding wire printing, which can effectively avoid the problem of ink sticking to the screen. The thickness of anti-welding ink for printing should be controlled above 30um.
After silk printing with anti-welding ink, the ink is kept for 20-30 minutes, then baked at 150 C for 1 hour, so that the ink can be fully cured.
In the process of pressing, the intermediate PP sheet is preferred to be BT resin with good heat resistance and wear resistance, low dielectric constant and low dielectric loss. A PP sheet is arranged in the middle of the two layers of copper foil, and one side of the copper foil printed with solder resistance ink is pressed towards the PP sheet. The number of PP sheets can be determined according to the thickness of finished products, but at least one must be placed. After pressing, drill the tool hole at the edge of the plate, and then remove the aluminum foil layer. The aluminum foil can be recycled and reused.
In the process of laser opening copper window, only one copper foil is needed to laser the copper window to facilitate laser drilling and adjust the laser parameters so as to avoid laser drilling breaking through the other copper foil. Copper window data diameter design and radium perforation aperture are large. After etching, it is necessary to ensure that the tolerance of window opening diameter is within +/-0.5mil. Inner acid etching is preferred for copper window etching. No browning process is needed before laser drilling. After laser drilling, special AOI is needed to check whether the residual glue and another layer of copper foil are broken.
In the electroplating process of copper sinking and hole filling, the horizontal copper sinking wire is adopted, the first copper foil is facing down, so as to avoid Tibetan medicine, and the non-special thin plate wire needs to be added with strips to prevent the clamping. Continuous sinking twice to ensure that copper is deposited on the insulating hole wall and ink. Special plating auxiliary frame should be used for filling hole electroplating to avoid jamming and breakage. After filling holes, slices should be made to confirm the blind hole depression, and the quality of the subsequent LED chip packaging should be guaranteed within 0.5 ml. In addition, to improve the quality of copper deposit, a plasma and a degumming treatment are needed.
The next step is to etch the copper foil. Because the surface circuit of LED carrier plate products is usually only pad and copper foil, there is no precise circuit, no need to re-screen the anti-welding layer, directly cover the dry film etching circuit, the surface of the circuit is preferred to nickel-gold treatment. Ultra-coarsening is preferred in the pretreatment of dry film, and no grinding is allowed.
LDI exposure machine is preferred for exposure, PE value is controlled by (+50um) to ensure line accuracy, acid etching is preferred to improve the uniformity of etching line and shorten the production process. After line etching, AOI detection is carried out, and single machine scanning is used to prevent the board from creasing and breaking when line scanning is completed.
In the process of gold alloying, copper foil must be retained on both sides of the auxiliary side in the design, so as to increase the hardness of the plate edge and prevent vibration damage during gold alloying. The edge of the board should be added with a gold bead hole every 100-200 mm, and the two short edges of the board should be added with priority. The diameter of the hole is 1.5-3.2 mm, which ensures that the board can be fixed when gold is melted. All bead holes are preferred to use Teflon wire string isolation beads, so that the board and the board are separated and fixed well. The PTH holes are made without cutting copper around tool holes and positioning holes at the edge of typesetting board, so as to prevent deformation and damage of the hole edge.
The method adopts special CA copper foil to realize silk printing of anti-welding ink to block LED light on the copper foil, and press the anti-welding ink under the copper foil of the circuit layer to realize 100% covering of the insulating base material, protecting the insulating base material, effectively avoiding the deterioration and discoloration caused by the LED light, so as to improve the brightness and service life of the LED. In practice, copper sinking and electroplating can be realized by using strips and special plates and frames. The gold-plating process can effectively guarantee the quality of the gold-plating process by adding bead holes separately and retaining copper foil at the edge of the plate.
