Discussion on Electroless Nickel Gold Technology (1)

Electroless nickel-gold plating integrates the functions of soldering, contact conduction, wire bonding, and heat dissipation. There is currently no other process that can compete with it. However, this process has been difficult to do for a long time, and the problem often occurs and it is not easy to rework. The solution of the problem must start from the source. In this regard, I will discuss the quality problems encountered in the work process with the seniors in the industry.

First, we start with the reaction mechanism of electroless nickel-gold.

1. Nickel

Nickel Salt: mainly nickel sulfate, nickel chloride, nickel acetate

Reducing Sgent: sodium dihydrogen hypophosphite, NaH2PO2

Reaction mechanism:

Description:
①The hypophosphite ion of sodium dihydrogen hypophosphite is hydrolyzed and oxidized into phosphate, and two active hydrogen atoms are released at the same time to be adsorbed on the palladium surface of the copper bottom.

② Nickel ions rapidly reduce and plated nickel metal on the activated palladium surface.

③ A small part of hypophosphite is stimulated by catalytic hydrogen to generate phosphorus atoms and deposit in the nickel layer.

④ Part of the hypophosphite will oxidize in the catalytic environment and generate hydrogen gas to emerge from the nickel surface.

2. Chemical Gold

When the nickel layer on the PCB surface is plated and put into the gold tank, the nickel surface is attacked by the tank liquid to dissolve the nickel ions, and the two thrown electrons are obtained by the gold cyanide ions to deposit the gold layer on the nickel surface.

Reaction mechanism:

Ni→Ni2+2e

Au(CN)2-+e→Au+2CN-

It can be seen that the deposition of two gold atoms can be obtained by dissolving one nickel atom, and because there are many sparse tiles on the gold layer, although the surface is covered with the gold layer, the sparse nickel surface can still be dissolved and the plating can continue. The gold layer is only getting slower and slower.

Secondly, the control of each process of chemical nickel gold.

1. Pre-processing

1.1 Brushing: Use a high-mesh nylon brush (1000-1200 mesh) to lightly brush the board surface (brushing current 2.0±0.2A) to remove dirt and oxides. Clean gloves must be worn when connecting the board after brushing Avoid touching the area to be plated in the forming line, so as to avoid spots on the board surface after the subsequent production of the board.

1.2 Degreasing: To remove grease and organic matter, only non-ionic remote interface active agent can be used, which only has a wetting effect, and too strong board-type active agent cannot be used to prevent static electricity on the surface of the solder mask or the substrate. The nickel or the solder resist developed by the water-based lye is damaged and the carbon content is dissolved out to contaminate the bath.

1.3 Micro-etching: Usually only bite the copper 30-40µm, if there is a phenomenon of star-exposed copper, it is likely to be caused by the unclean development of the wet film process or the poor washing after development. At this time, the micro-etching time can be appropriately extended to remove it.

1.4 Pickling: Removes copper salts generated by micro-etching.

1.5 Pre-Activation: When palladium chloride is used as the activator, its pre-activation can be equipped with hydrochloric acid to remove the front tank, remove the pollution of the front tank liquid, and protect the activation tank.

1.6 Activation: The activator used is ionic palladium chloride (there are also platinum group noble metals such as palladium sulfate and ruthenium sulfate). The concentration is best controlled at 30ppm-60ppm.

1.7 Washing: There must be pure water washing with strong stirring and circulation between each medicine water tank to avoid the contamination of the board surface caused by water washing.