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Three typical features related to PCB pads black plate

158 Published by FASTPCBA 7月 10,2019

Three typical features related to PCB pads black plate

1.Definition

Because the solder joint fracture surface is gray and black in SMT processing plant , so that it is called black plate.

The black plate-related cracks in the pcb pads of the SMT processing plant occur under the IMC of the Sn–Ni interface.The darker the pad color, the thinner the IMC, from black to dark gray color!

pcb

2.Three typical features related to PCB pads black plate

(1) Corrosion cracks (mud cracks) can be observed from the fracture surface. The more cracks there are, the lower the link strength of the solder joints of the SMT processing plant (the cracks are completely non-wetting) and the more difficult to form the IMC. Since the lead-free solder joints are relatively hard and are directly conducted to the interface after suffering stress, the lead-free solder joints are more likely to be seen due to the black plate failure in the pcb pads of the SMT processing plant.

(2)It can be observed from the polished cross section that “corrosion” has permeated and even permeatethe Ni layer to the Cu matrix, commonly known as the “golden thorn” feature. The corrosion “golden thorn” exhibited by the crack or cross section pattern formed on the Ni layer is usually used as one of the judgment criteria of the black plate.

(3)The P content on the near surface of Ni is relatively high (about 20%, which is twice time of the normal), but it should be pointed out that phosphorus enrichment is not the direct cause of solder joint failure. If the rich P is used as the basis for the black plate, it should be measured before welding, because the welding process will increase the enrichment of P due to the migration of Ni, and the high P is not an abnormal condition.

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