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SMD LED PCB Design

23 Sep

SMD LED is a new type of surface-mount semiconductor light emitting device with the advantages of small size, large beam angle, excellent light uniformity, high reliability and so on, light color can be various colors including white, so it is widely used in variety of electronic products. PCB is one of the main materials to manufacture SMD LEDs. The development of each new SMD LED product starts from designing drawings of PCB, when designing the PCB drawings, the front and back drawings of the PCB, the assembly drawing and the finished product drawing of SMD LED shall all be given, then give the completed PCB drawing to professional LED PCB manufacturers to produce, the design quality will have a direct impact on product quality and the implementation of manufacturing process. Therefore, designing a perfect SMD LED PCB is not an easy task, many factors which affect the design shall be taken into account. Therefore, this article will discuss SMD LED PCB design from the following aspects.

1. Choose the right SMD LED PCB structure

SMD LED PCB can be sorted into two types according to their structure: via hole type structure and chamfer hole type structure; according to the wafer number that single SMD LED chip uses, SMD LED PCB can be sorted into: single wafer type, double wafer type and three wafer type. The difference of via hole type structure PCB and chamfer hole type structure PCB is: the former need to be cut in both directions when cutting, single finished electrode is semi arc-shaped; the latter only need to be cut in one direction when cutting. Choosing the design structure of PCB and the number of wafers for SMD LEDs depend on the market and users’ requirements. If the user does not have special requirements, we generally choose chamfer hole type structure to design PCB. The substrate of PCB is BT board.

2. Choosing direction for chamfer hole

If we choose to use the chamfer hole type structure to design PCB, we must consider which direction to choose for the chamfer hole. Under normal circumstances the chamfer hole is designed along the direction of PCB board width, which can minimize the PCB deformation after molding.

3. PCB dimesion selection

Selection of each new SMD LED PCB dimension must take following factors into consideration: ① the specified designed product number on each PCB. ② Whether the extent of PCB deformation after molding is in the acceptable range.

When it does not affect the workmanship, the number of products on each PCB shall be designed as much as possible, which will help reduce the cost of single product. Meanwhile because the gel will shrink after molding, PCB is easy to deform, the number of each group SMD LED can not be too much, but the the number of groups can be designed more when designing PCB. This will not only meet number requirement of SMD LED on single PCB, but also decrease the PCB deformation after molding and gel shrinking. Serious PCB deformation will cause that PCB can not be cut and the gel is easy to be stripped from PCB after cutting.

The decision of PCB thickness is based on the entire thickness requirement of SMD LED that the client uses. PCB thickness can not be too thick, too thick may cause failure to weld wire after die bonding; PCB thickness can also not be too thin, too thin may cause gel shrink and serious PCB deformation after molding.

We can take 0603 type ordinary SMD LED products with thickness of 0.6mm as an example. If we choose the PCB with thickness of 0.3mm, the thickness of gel can only be 0.3mm, and then choose the wafer with thickness of 0.28mm to die bond, the entire thickness has reached 0.58mm, then the wire welding can not be operated. If we choose the PCB with thickness of 0.1mm, the thickness of gel will be 0.5mm, since the gel is thick, the gel will obviously shrink after molding, while the PCB is thin, which will cause serious deformation of PCB. Therefore, when designing the thickness of PCB, we must choose a suitable thickness, which on the one hand can make the PCB suitable for SMD LEDs with different thickness, on the other hand will not cause serious deformation of PCB after molding.

4. PCB circuit design requirements

1). Die bonding area: the size design of die bonding area is determined by the size of the wafer. Under the condition that the wafer can be fixed safely, the die bonding area shall be designed as small as possible. The adhisive property between gel and PCB will be better and the gel is not easy to be stripped from PCB, meanwhile we shall consider to design the die bonding area in the middle of single SMD LED PCB.

2). Wire welding area: the size of wire welding area shall basically be greater than the bottom size of the magnetic mouth.

3). Distance from die bonding area to wire welding area: Distance from die bonding area to wire welding area shall be determined by the wire arc, if the distance is far, the pull of the wire arc is not enough, if the distance is near, the metal wire may contact the wafer when welding wire.

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

5). The diameter of the circuit wire: We also shall consider the diameter of circuit wire which connects the electrode and the die bonding area. Use of circuit wire with small diameter can increase the adhesive force between the substrate and the gel.

6). Via hole diameter: If we use PCB with via hole design, the minimum via hole diameter is generally Φ0.2mm.

7). Chamfer hole diameter: If we use PCB with chamfer hole design, the minimum width of chamfer hole is generally 1.0mm.

8). Width of cutting line: since the cutting blade has a certain thickness when cutting, PCB will be worn after cutting, when designing cutting line width, we should consider the thickness of the cutting blade, and compensate on the PCB, Otherwise, after cutting the width of the finished product will be narrow.

In addtion, we should also consider the diameter of positioning holes and other issues. Generally the product number design of a PCB within the circuit range shall be even number.

5. The quality requirements for the PCB substrate

When designing PCB, we shall make the following technical description on PCB:

1). Sufficient accuracy is required: the degree of uneveness of PCB thickness shall <± 0.03mm, the tolerance of positioning holes to circuit board shall <± 0.05mm.

2). the thickness and quality of gold plating layer must ensure the pull test of the gold wire > 8g after die bonding.

3). The surface of finished products of PCB shall contains no dirt, the adhisive property between it and the gel shall be strong.

 
 
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