Electronic Design - PCB process COB requirements for PCB design

PCB process COB requirements for PCB design

Since COB does not have a lead frame for IC packaging, it is replaced by PCB. Therefore, the design of PCB pads is very important. Moreover, Fihish can only use electroplated gold or ENIG, otherwise gold wire or aluminum wire, or even The latest copper wire will have the problem of not being able to reach it.

1. The surface treatment of the finished PCB boardmust be electroplated gold or ENIG, and it must be a little thicker than the general PCB gold plating layer to provide the energy required for Die Bonding to form a gold-aluminum or gold-gold co-gold.

2. In the wiring position of the solder pads outside the COB Die Pad, try to make sure that the length of each solder wire has a fixed length, that is to say, the distance between the solder joints from the wafer to the PCB solder pads should be as consistent as possible. The position of each welding wire can be controlled, and the problem of short-circuiting of the welding wires can be reduced. Therefore, the diagonal pad design does not meet the requirements. It is suggested that the PCB pad spacing can be shortened to eliminate the appearance of diagonal pads. It is also possible to design elliptical pad positions to evenly disperse the relative positions between the welding wires.

3. It is recommended that a COB wafer has at least two positioning points. The positioning points should not use the traditional SMT circular positioning points, but use the cross-shaped positioning points, because the Wire Bonding machine is doing automatic Basically, the positioning will be done by grasping a straight line. I think this is because there is no circular positioning point on the traditional lead frame, but only a straight outer frame. Some Wire Bonding machines may be different. It is recommended to design with reference to the performance of the machine first.

Fourth, the Die Pad size of the PCB should be slightly larger than the actual wafer. One can limit the deviation when placing the wafer, and it can also prevent the wafer from rotating too much in the die pad. It is recommended that the wafer pads on each side are 0.25~0.3mm larger than the actual wafer.

5. It is best not to have via holes in the area where COB needs to be filled with glue. If it cannot be avoided, then the PCB factory is required to completely plug these via holes 100%, in order to avoid the penetration of the via holes into the PCB during Epoxy dispensing. On the other side, causing unnecessary problems.

Sixth, it is recommended to print the Silkscreen logo on the area that needs glue, which can facilitate the dispensing operation and the control of the dispensing shape.

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