What is surface mount LED (SMD)

- Feb 17, 2021-

Chip LED is a new type of surface-mounted semiconductor light-emitting device. It has the advantages of small size, large scattering angle, good luminous uniformity, and high reliability. The luminous color includes various colors including white light, so it is widely used in various Kind of electronic products. PCB board is one of the main materials for manufacturing chip LEDs. The development of each new chip LED product starts from the design of the PCB board drawing. The front and back graphics of the PCB and the chip LED assembly drawing and finished product drawing should be given during the design, and then the designed PCB board drawing should be given to the professional LED PCB board manufacturers produce boards, and the quality of their design directly affects the quality of the product and the implementation of the manufacturing process. Therefore, designing a flawless chip LED PCB board is not an easy task, and many factors affecting the design must be considered. For this reason, this article will discuss the design of the chip LED PCB board from the following aspects.

    1. Chip LED PCB board structure selection

    The types of chip LED PCB boards are classified according to the structure: through-hole type structure, slotted hole type structure, etc.; according to the number of chips used in a single chip LED: single crystal type, double crystal type and triple crystal type. The difference between the through-hole structure PCB board and the slotted-hole structure PCB board is that the former needs to be cut in two directions when cutting, and the single finished electrode is half-arc; the latter only needs to be cut in one direction when cutting. The choice of what structure PCB board and chip LED uses several chips is based on the requirements of market users. When the user has no special requirements, the PCB board is usually designed with a slotted hole structure. The PCB substrate is a BT board.

    Second, the choice of slotting direction

    If you choose to design the PCB board with a slotted hole structure, you must consider which direction to choose for the slotted hole. Under normal circumstances, the slotted hole is designed along the width of the PCB board, because this can minimize the deformation of the PCB board after compression molding. As shown in Figure 1.

    Three, PCB board outline size selection

    The factors that must be considered in the selection of the size of each new chip LED PCB board: ①The number of products designed on each PCB board is required. ②Whether the deformation degree of the PCB board is within the acceptable range after compression molding.

    Without affecting the process, the number of products on each PCB should be designed as many points as possible, which is beneficial to reduce the cost of a single product. In addition, since the colloid will shrink after compression molding, the PCB board is prone to deformation, so the PCB board should be designed to consider that the number of chip LEDs in each group should not be too much, but the number of groups can be designed more. In this way, the requirement of the number of chip LEDs on a single PCB can be met, and the deformation of the PCB caused by the shrinkage of the colloid after the compression molding will not be too large. The large deformation of the PCB board will cause the PCB board to be unable to be cut and the glue and the PCB board will easily peel off after cutting.

    The thickness of the PCB board is selected according to the overall thickness of the chip LED used by the user. The thickness of the PCB board should not be too thick, too thick will cause the wire bonding after die bonding; the thickness of the PCB board should not be too thin, too thin will cause the PCB board to deform too much due to the shrinkage of the colloid after the compression molding.

    Take the ordinary chip LED product with the thickness of 0603 specification of 0.6mm as an example. If a PCB board with a thickness of 0.3mm is selected, the thickness of the colloid part can only be 0.3mm, and then a wafer with a thickness of 0.28mm is selected for die bonding. The overall thickness is already 0.58mm, and the wire bonding operation cannot be performed. If the thickness of the PCB board is 0.1mm, the thickness of the colloid part is 0.5mm, and the colloid shrinks significantly after the compression molding is formed due to the thick colloid, while the PCB board is thin, which will cause the PCB board to deform too much. Therefore, when designing the thickness of the PCB board, an appropriate thickness must be selected, which can make the same PCB board suitable for chip-type LEDs of different thicknesses, without causing excessive deformation of the PCB board after compression molding.

    Figure (2) shows the finished product of 0603 specification. Figure (3) is a PCB board with an area of 60mm×130mm, each group consists of 44 chip LEDs connected as a whole, and Figure (4) chip LED unit Figure a) is suitable for the production of ordinary single electrode chips LED, Figure b) is suitable for the production of sapphire substrate double-electrode chip chip LED, at the same time, it can also be applied to the production of single-electrode chip ordinary chip LED.

    Four, PCB board circuit design requirements

    1. Die-bonding zone: The size design of the die-bonding zone is determined by the size of the wafer. Under the condition that the chip can be securely fixed, the die bonding area should be designed as small as possible. In this way, the adhesion between the glue and the PCB board will be better after compression molding, and it is not easy to peel off the glue from the PCB board. At the same time, it is also necessary to consider the design of the die bonding area as much as possible in the middle of the single chip LED circuit board.

    2. Wire bonding area: The wire bonding area is basically larger than the size of the bottom of the magnetic nozzle.

    3. The distance between the die bonding area and the wire bonding area: the distance between the die bonding area and the wire bonding area should be determined by the wire arc. The distance will cause insufficient wire arc tension, and the small distance will cause the gold wire to contact the chip during wire bonding.

    4. Electrode width: The electrode width is generally 0.2mm.

    5. Circuit wire diameter: The size of the circuit wire diameter connecting the electrode and the die bonding area should also be considered. Using a small wire diameter can increase the adhesion between the substrate and the colloid.

    6. Via hole diameter: If the PCB board is designed with via holes, the minimum via hole diameter is generally Φ0.2mm.

    7. Slotted hole aperture: If a through hole is used to design the PCB board, the minimum width of the slotted hole is generally 1.0mm.

    8. Cutting line width: due to the existence of a certain thickness of the cutting blade during cutting, a part of the PCB board will be worn after cutting. Therefore, the thickness of the cutting blade should be considered when designing the cutting line width, and the PCB board design should be compensated. Otherwise, the width of the finished product will be narrow after cutting.

    In addition, the aperture size of the positioning hole must also be considered.

    Generally, the number of products within the designable circuit range of a PCB board is designed as an even number.

    Five, the quality requirements for PCB substrates

    When designing the PCB board, the following technical instructions should be made for the production of the PCB board:

    1. Sufficient accuracy is required: the unevenness of the board thickness is required to be <±0.03mm, and the deviation of the positioning hole to the circuit board circuit is <±0.05mm.

    2. The thickness and quality of the gold-plated layer must ensure that the tensile test of the gold wire after bonding>8g.

    3. After the PCB board is made into a finished product, it is required that the surface is free of dirt, and the adhesion between the mold and the glue is good.

Surface paste technology is commonly used in modern circuit board assembly. This kind of circuit board assembly uses many surface paste types.

     LED (ie SMD LED).

Commonly used SMDs are dome-shaped, flat-topped and ultra-small. It has the following types of pins

     (1) "Haio Wing" pin

     (2) "Bullhorn" pin

     (3) "Z-shaped" pin

     These three kinds of LED components can be pasted on the PCB with an automatic device, and then connected by convection IR reflow, steam reflow, and sometimes with sealant.

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