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Surface Treatment For PCB ---- Immersion Gold
Aug 19, 2017

In PCB process there are many kinds of surface treatment methods, such as HASL(LF), OSP, Immersion gold, etc., because the ENIG color on the PCB surface is stable, brightness is good, plating level off,forming good weldability nickel and gold plating, so even if the price is expensive, but still be choosen by most electronic products manufacturer.

1. Technology Introduction

The sinking process can be divided into four stages: pretreatment (oil removal, microcorrosion, activation, post-dip), immersion nickel, immersion gold, post-treatment (waste gold washing, DI washing, drying).

2. Pretreatment

There are usually several steps for ENIG pretreatment: oil removal (30% AD-482), microeclipse (60g/InaPS, 2% H2SO4), activation (10% act-354-2), post-immersion (1% H2S04).The purpose is to remove oxide on copper surface, and immersion palladium in the copper surface, to be as the center for the activation of nickel. If some processes are poorly handled, which will affect following immersion nickel and immersion gold, and lead to batch scraped. In the process of production, all kinds of potions must be analyzed and added periodically and controlled within the range of requirements. Such as: micro corrosion rate should be controlled in "25u - 40u", active potion must open new cylinder when copper content is more than 800 PPM, medicine tank cleaning and maintenance is very important to ENIG PCB quality, except for the oil cylinder, micro erosion cylinders, leaching cylinder should be changed weekly, and all the washing cylinder should also be cleaned weekly.

3. Immersion Nickel

The main components in immersion nickel potion are Ni2+(5.1-5.8 g/1) and reducing agent sodium hypophosphite (25-30g/1) as well as stabilizing agent, because chemical nickel is strict for liquid composition range, per shift in the process of production must be analyzed twice, and adding Ni2 reductant per the production board area of bare copper or experience, when feeding, should follow the principle of filling material within a small amount, spread for many times,in order to prevent the local solution response, lead to the accelerated ageing of the plating solution, PH value and solution temperature influence largely to nickel thickness, nickel liquid temperature should be controled between 85 ℃ and 90 ℃. When PH is at 5.3-5.7, no nickel cylinder production, nickel cylinder temperature should be reduced to about 70 ℃, to slow the aging. Chemical nickel plating solution is sensitive to impurities, a lot of chemical composition is harmful to chemical nickel, can be divided into the following categories:

Inhibitors including Pb,Sn,Hg...Ti,Bi (low melting heavy metal), organic impurities including S2, nitric acid and anionic wetting agent. All of these substances will reduce the activity, resulting in the reduction of chemical plating speed and miss plating,seriously will lead to the complete cessation of the electroless nickel plating process.

Organic impurities: except for the organic stabilizers mentioned above, there are also plastics and impurities from the equipment and solder. Although some impurities can be removed by continuous plating, it cannot be completely eliminated.

Unstable agent: including Pd and a small amount of copper, both of which are formed in chemical nickel instability, which makes the coating rough and is plated too much on the tank wall and heater.

Solid impurities: calcium sulphide or calcium phosphate and other insoluble substances are sunk or brought into solution. The filter can remove solid particles.

In conclusion, effective measures should be taken to reduce such impurities in the ENIG PCB production process.

4. Immersion Gold

The ENIG process is a kind of dipping gold process and the main components of the gold cylinder:Au (1.5-3.5g/l), binder (Ec0.06-0.16 mol/l), can replace  pure gold on nickel phosphorus alloy layer, making the coating smooth and crystallization meticulous, solution PH value is generally between 4-5, control the temperature in 85℃-90℃.

5. Post-processing

ENIG post-processing is also an important link, for printed circuit board, generally including: waste gold washing, DI washing, drying and other steps, conditionally can use horizontal washing machine to do further washing and drying for PCBs. Washing machine can set below process in order: liquid washing (10% sulfuric acid, hydrogen peroxide, 30 g/L), high voltage DI washing (30~50 psi), DI washing and blow drying, drying, in order to completely remove the potions and stain in printed circuit board holes or on the surface, getting uniform coating and brightness ENIG PCB boards.

6. Control in The Production Process

The problems that often appear in the process of immersion nickel, are imbalance in the plating solution composition, and additives in poor quality and plating solution magazine content exceeds bid. Prevent and improve this problem is vital for process control. Factors that should be paid attention during the producing process are the following kinds:

1) in the process of chemical nickel and gold process, as there are small holes, washing is necessary between each step and should be paid special attention.

2), between micro corrosion inhibitors and palladium as activator Micro erosion, copper fade easily, when seriously make palladium plating not uniform, resulting the malfunction in the nickel layer, if PCB washing is not good, the antioxidants from the micro erosion can prevent palladium deposition, the results is affecting the effect of ENIG, thus affect the PCB quality.

3) palladium is the most dangerous impurity in the chemical nickel process, and very micro amount of palladium will naturally decompose the tank. Although the concentration of palladium is very low, but it should be washed well before entering the chemical plating bath.

4) between palladium and chemical nickel steps, the transfer time between the chemical nickel and the leaching gold can easily blunt the nickel layer, resulting in uneven gold leaching and poor bonding force. This will easily cause the dumping of gold to present tin.

5) after leaching, to maintain solderability and malleability, fully wash after gold plating (the last wash is better with distilled water), and completely dry, especially dry hole completely.

6) the PH of the nickel cylinder shall be rised and adjusted with less than 50% ammonia water, and the PH shall be reduced by 10% V/V sulfate. All additions should be injected slowly, stirring continuously. PH measurements should be carried out in full agitation to ensure a balanced concentration of liquid. The higher the temperature, the faster the plating speed.