ENIG (Electroless Nickel Immersion Gold) is a process used for the surface treatment of circuit boards (Finished). The PCBA board inside the mobile phone, and some BGA carrier boards also use ENIG.

Compared to electroplated nickel-gold, this type of nickel-immersed gold does not need to be energized on the circuit board during the process of the circuit board manufacturer, nor does it need to pull a wire on each pad to be plated to be plated. Nickel gold, therefore, its production process is relatively simple, the output is also more than multiples, so the production cost is relatively cheap.

So how do we distinguish which PCBA board needs immersion gold and which circuit board does not need immersion gold? You can use the following types of analysis and decomposition.

1. The board has gold fingers that need to be gold-plated, but the layout other than gold fingers can be selected according to circumstances such as tin spray or immersion gold, which is the usual "sink gold + gold-plated finger" process and "tin-plated + gold-plated finger" process, and occasionally a few in order to save costs or time, the designer urgently chooses the entire method of immersion gold to achieve the purpose, but the immersion gold cannot reach the thickness of the gold plating, and if the gold fingers are often inserted and peeled, a bad connection may occur.
2. The board's line width / pad spacing is insufficient. In this case, the spraying process is often difficult to produce, and short-circuits such as tin bridges are common. Therefore, for the performance of the board, processes such as immersion gold are usually used.
3. Because of the layer of gold on the pad surface, solderability is good and the board performance is stable.

Other boards do not need to choose the immersion gold process in order to save costs, if you don't have different requirements for the board's solderability and electrical properties.

1. Advantages of ENIG (nickel immersion gold) surface treatment

-The surface treatment can be used as the base metal of COB wire.
-Reflow can be repeated many times. Generally, it is required to withstand at least 3 times of high temperature welding, and the welding quality can still be consistent.
-Has excellent electrical conductivity. It can be used as a cheat wire for key conduction, and has high reliability.
-Gold metal has low activity and is not easy to react with the components in the atmosphere, so it can play a certain ability to prevent oxidation and rust. Therefore, the shelf life of ENIG can generally easily exceed six months or more. Sometimes, even if it is stored in a warehouse for more than one year, as long as it is in good storage and there is no rust problem, the circuit board is baked, dehumidified and soldered. Confirm that it is okay, you can still use it for welding production.
-Gold is not easily oxidized when exposed to the air, so you can design a large-area exposed pad for heat dissipation.
-Can be used as the contact surface of the blade. The gold plating for this application must be a little thicker. Hard gold plating is generally recommended.
-The surface of ENIG is flat, the flatness of printed solder paste is good, and it is easy to solder. Very suitable for fine leg parts and small parts, such as BGA, Flip-Chip and other parts.

2. Disadvantages of ENIG (nickel immersion gold) surface treatment

-In general, the solder joint strength of Ni3Sn4 is not as good as that of Cu6Sn5. Some parts that require welding strength in particular may not be able to withstand excessive external impact forces and there is a risk of falling.
-Because the price of gold is rising, the cost is relatively higher than the surface treatment of OSP.
-There is a risk of black pads or black nickel. Once the black pads are generated, the strength of the solder joints may decrease rapidly.
The black pad is composed of a complex NixOy chemical formula. The root cause is that the nickel surface is subjected to excessive oxidation during the immersion gold substitution reaction (the metal nickel is freely converted to nickel ions, which can be broadly referred to as oxidation). Very large gold atoms (a gold atom radius of 144pm) are irregularly deposited to form a rough and loose porous grain arrangement, which means that the gold layer does not completely cover the nickel layer underneath, allowing the nickel layer to continue to contact the air. Finally, nickel rust was gradually formed under the gold layer, which eventually caused welding obstacles. Currently, there is a process of nickel-impregnated palladium gold (ENEPIG) that can effectively solve the problem of black pads, but because its cost is still relatively expensive, it is currently only used by high-end boards, CSP or BGA manufacturers.

3. Production process of chemical nickel-gold

Pre-treatment → Degreasing → Water washing → Pickling → Water washing → Micro-etching → Water washing → Pre-immersion (H2SO4) → Activation (Pd catalyst) → Water washing → Chemical Nickel (Ni / P) → Water washing → Dip plating → Gold recovery → Water washing → drying

-Pretreatment: The purpose is to brush or sandblast to remove the oxide on the copper surface and roughen the copper surface to increase the adhesion of the subsequent nickel and gold.
-Micro-etching: sodium persulfate / sulfuric acid to remove the copper oxide layer and reduce the depth of groove marks caused by brushing during pre-treatment. Excessive brush marks often become accomplices in immersing gold in attacking the nickel layer.
-Activation: Since the copper surface cannot directly initiate the chemical nickel deposition reaction, a layer of palladium (Pd) must be applied on the copper surface as a catalyst for the chemical nickel deposition reaction. Based on the principle that Cu is more active than Pd, palladium ions are reduced to palladium metal and adhere to the copper surface.
-Chemical nickel: Ni / P, whose main function is to prevent migration and diffusion between copper and gold, and to act as an element that chemically reacts with tin to form IMC during subsequent soldering.

-Dip gold plating: The main purpose of gold is to protect and prevent the oxidation of the nickel layer. Gold does not participate in the chemical reaction during the soldering process. Excessive gold will hinder the strength of the solder, so the gold only needs to cover the nickel layer to make it. It is not easy to oxidize. If you want to make COB (Chip On Board) wire, it is another matter, because the gold layer must have sufficient thickness.