ENIG (Electroless Nickel Immersion Gold) is a process used for circuit board surface finishing, generally referred to as ‘electroless nickel immersion gold’, also referred to as ‘electroless nickel immersion gold’ or ‘gold immersion gold’, is widely used in mobile phone circuit boards, and some BGA package IC carrier boards will also use ENIGE or NEPIG (electroless nickel palladium gold). ENIGE or NEPIG (Nickel-Palladium-Ink-In-Gold) is also used for some BGA packages.

The appearance of ENIG is very easy to identify, currently on the market to see the surface of the circuit board if you can see the gold solder pads is probably it, in fact, there is another kind of ‘electro-nickel plated gold’ board will also be presented with gold solder pads, but now the gold is very expensive, unless the special needs of the need to use a thicker layer of gold or hard gold, otherwise now has been less than the electro-nickel-plated gold boards.

Compared to electroless nickel plating, this kind of nickel immersion gold in the process of circuit board manufacturers do not need to do plating on the circuit board, and do not need to be plated on each of the pads (pads) to pull a wire to connect to the nickel plating, so the ENIG process is relatively simple, the output is also a multiple of electroless nickel plating, so the production cost is relatively cheaper than electroless nickel plating.
However, ENIG surface treatment also has its drawbacks and problems, such as low solder strength compared to copper-based PCBs, and easy to generate black pads, which are often criticised for their quality.

Thickness requirements of ENIG nickel and gold layers

According to the requirements of IPC-4552, it is recommended that the thickness of the chemical gold layer of ENIG circuit boards should be between 2µ‘-5µ’ (0.05µm-0.125µm), while the thickness of the chemical nickel layer should be between 3µm (118µ‘) and 6µm (236µ’).
With the price of gold is getting more and more expensive, nowadays PCB board manufacturers in order to save costs, it is very easy to cheat on the gold layer, it is recommended to focus on the control of the thickness of the gold layer, especially the large area of the solder pads. However, the thicker the gold layer, the higher the cost, according to the past experience of the working bear, we have relaxed the thickness of the gold layer to the thinnest 1.2µ’ and still can operate normally.

The production process for chemical nickel gold is broadly as follows:

Pre-treatment → oil removal → water washing → acid washing → water washing → micro-etching → water washing → pre-impregnation (H2SO4) → activation (Pd catalyst) → water washing → chemical nickel (Ni/P) → water washing → gold plating by immersion → gold recovery → water washing → drying

1.Pre-treatment: The purpose of brushing or sandblasting is to remove the oxides on the copper surface and to roughen the copper surface to increase the adhesion of the subsequent nickel alloy.

2.Micro-etching: Sodium persulphate/sulphuric acid is used to remove the oxide layer from the copper surface and to reduce the depth of scratches caused by brushing during pre-treatment. Excessively deep brush marks are often the cause of nickel attack by immersion gold.

3.Activation: Since the copper surface cannot directly activate the deposition reaction of chemical nickel, a layer of palladium (Pd) must be applied to the copper surface first as the catalyst for the deposition reaction of chemical nickel. Using the principle that Cu is more active than Pd, the palladium ions are reduced to palladium metal and adhered to the copper surface.

4.Chemical Nickel: Ni/P, its main role is to block the migration and diffusion between copper and gold, and as a subsequent welding action with tin chemical reaction to produce IMC elements.

Dip gold plating: The main purpose of gold is to protect and prevent the oxidation of the nickel layer, gold in the soldering process does not participate in the chemical reaction, too much gold rather hinder the strength of the solder, so the gold as long as enough to cover the nickel layer so that it is not easy to oxidise can be, if you want to make COB (Chip On Board) wiring is another matter, because the gold layer should be sufficient thickness.

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