PCB surface coating and process characteristics
All electrical and electronic equipment exported to the EU must meet the requirements of RoHS, which will cause the PCB electronics manufacturing industry to change from leaded to lead-free. Joint lead is derived from device lead plating, PCB pad plating, and solder. In order for the lead in the solder joint to meet RoHS requirements (lead content less than 0.1%), PCB surface treatment must also be lead-free. Due to the high melting point characteristics of lead-free solders, lead-free tin-spraying processes are rarely applied to PCBs with more than 6 layers. A wide variety of lead-free surface treatment processes have been proposed in the industry, and ENIG, Im-Ag, Im-Sn and OSP are widely used at present.
ENIG, the abbreviation of Electroless Nickel /Immersion Gold, commonly known as chemical nickel gold in Chinese. Due to electroless plating, the Ni layer is not a pure Ni layer, and there is a certain P. At the time of immersion gold, a P-rich Ni layer is formed near the Au layer due to the displacement reaction. When the ENIG coating is soldered, Au dissolves rapidly into the solder to form an Au-Sn alloy, and the solder forms an IMC only with the Ni layer.
ENIG coating is suitable for PCBA with a large number of fine pitch devices (<063mm) and high coplanarity requirements. It can also be used as a selective coating for OSP surfaces.
(1) The surface is flat.
(2) Good compatibility with lead-free.
(3) Long storage period, up to 12 months.
(4) Good solderability (wettability)
(1) High price
(2) There is a risk of embrittlement in the solder joint/weld.
(3) There is a risk of “black disk” failure. The black disk is a defect with very low probability of occurrence. It is difficult to find by common detection methods, but the failure caused is catastrophic. Therefore, it is generally not recommended for fine pitch BGA pad surface treatment.
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