Printed board surface plating (coating) process

First, the printing plate surface plating (coating) method: a, plating method. b, electroless plating.

Second, the printed circuit board surface coating common elements:

a, nickel, element symbol Ni, atomic weight 58.7, density 8.88g/cm3, Ni2+ electrochemical equivalent 1.095g/AH.

b. Gold, the symbol Au, the atomic weight 197, the density 19.32 g/cm3, and the electrochemical equivalent of Au+ 0.1226 g/A·m.

c, tin, element symbol Sn, atomic weight 118.69.

d, silver, element symbol Ag, atomic weight 107.88.

e. Palladium, symbol Pd, atomic weight 106.4.

Third, the plating overview:

a. Nickel: The nickel coating used for printed boards is classified into semi-bright nickel (also known as low-stress nickel or dumb nickel) and bright nickel. It is mainly used as the bottom layer of plated gold or plug gold plating, and can also be used as the surface layer as required. The thickness of the plating layer is not less than 2~2.5 μm according to the IPC-6012 (1996) standard. Nickel plating should be uniform, fine, low porosity, good ductility, etc., and low-stress nickel should have the appropriate function of brazing or pressure welding.

b. Gold: There are two types of gold plating used for printed board production; gold plating on the board and gold plating on the plug.

1) Gold plating on the board: The gold plating on the board is 24K pure gold. It has a columnar structure and it has excellent electrical conductivity and solderability. Coating thickness 0.01 ~ 0.05μm.

The gold-plated layer of the board is based on low-stress nickel or bright nickel as the bottom layer, and the nickel plating thickness is 3-51xm. The nickel plating layer acts as an intermediate layer to serve as a barrier layer between gold and copper, which can prevent the mutual diffusion between gold and copper. Prevent copper from penetrating into the gold surface. The presence of a nickel layer corresponds to an increase in the hardness of the gold coating.

The gold-plated layer on the board is not only the protective layer for alkaline etching, but also the final surface plating for IC aluminum wire bonding and button-type printed boards.

2) Gold plating of the plug: The gold plating of the plug is also called hard gold plating, commonly known as "gold finger". It is an alloy plating containing alloy elements such as Co, Ni, Fe, and Sb. Its hardness and wear resistance are higher than those of pure gold plating. Hard gold plating has a layered structure. Plugs used for printed boards are typically 0.5 to 1.5 μm or thicker. Alloy element content ≤ 0.2%. The gold plating of the plug is used for the connection of highly stable and highly reliable electrical contacts; there are requirements for plating thickness, wear resistance and porosity.

Hard gold plating uses low-stress nickel as a barrier layer to prevent the mutual diffusion between gold and copper. In order to improve the bonding strength of the hard gold plating layer and reduce the porosity, and in order to protect the plating solution to reduce pollution, a pure gold layer of 0.02 to 0.05 p,m is required to be plated between the nickel layer and the hard gold layer.

c. Tin: Electroless tin plating on bare copper PCBs is also a solderable coating that has received widespread attention in recent years.

The electroless tin plating on the copper substrate is essentially a chemical immersion tin, which is a displacement reaction between the complex tin ions in the copper and the plating solution, and a tin plating layer is formed. When the copper surface is completely covered with tin, the reaction stops.

d. Silver: The electroless silver plating layer can be both soldered and "bonded" (pressure welding), and thus it has received universal attention. The electroless silver plating layer is also impregnated with silver. The standard electrode potential of copper is 0Cu+/Cu=0.51 V, and the standard electrode potential of silver is 0 Ag+/Ag=0.799 V. Therefore, copper can replace the silver ions in the solution and copper. The surface of the deposited silver layer: Ag + Cu → Cu + Ag to control the reaction rate, Ag + solution will exist in the form of complex ions, when the copper surface is completely covered or the solution reaches a certain concentration of Cu, the reaction is completed.

e. Palladium: Electroless Pd plating is an ideal copper and nickel protective layer on PCBs. It can be both welded and "bonded." It can be directly plated on copper, and because Pd is self-catalytic, the coating can grow thick. Its thickness can reach 0.08~0.2μm. It can also be plated on electroless nickel plating. The Pd layer has high heat resistance, stability, and can withstand multiple thermal shocks.

In the assembly welding, the Ni/Au coating is formed. When the gold plating layer is in contact with the molten solder, gold is melted into the solder to form AuSn4. When the weight ratio of the solder is up to 3%, the solder will brittle and affect the reliability of the solder joint. The molten solder does not form a compound with Pd, and Pd floats on the solder surface and is stable.

Because Pd's price is too high, it has limited its application to a certain extent. As IC integration increases and assembly technology advances, electroless plating Pd will play a more effective role in chip-pole assembly (CSP).

Source: PCB Network City