Immersed gold plate

The immersion gold board is a PCB board made by the immersion gold process. Most of these immersion gold boards are multi-layer PCB boards. The immersion gold process deposits on the surface of printed circuits a stable color, good brightness, smooth coating, and good solderability. With the nickel-gold plating, the PCB can be used for a long time without oxidation problems.

1. Process introduction

The purpose of the immersion gold process is to deposit a nickel-gold coating with stable color, good brightness, smooth coating and good solderability on the surface of printed circuits.

It can basically be divided into four stages: pre-treatment (oil removal, micro-etching, activation, post-immersion), nickel immersion, gold immersion, and post-treatment (waste gold washing, DI washing, drying).

2. Pre-processing

Immersion gold pre-treatment generally has the following steps: degreasing (30% AD-482), micro-etching (60gInaPS, 2% H2SO4), activation (10% Act-354-2), and post-immersion (1% H2S04). To remove the oxide on the copper surface and deposit palladium on the copper surface to serve as a nickel precipitation activation center. If one of the links is not properly handled, it will affect the subsequent nickel and gold sinking, and lead to batch scrapping. During the production process, various potions must be analyzed and replenished regularly and controlled within the required range. More important examples include: the micro-etching rate should be controlled at “25U-40U”. When the copper content of the activation potion is greater than 800PPM, a new tank must be opened. The cleaning and maintenance of the potion tank also has a greater impact on the quality of the PCB. In addition to the oil tank and the micro-etching tank, The post-immersion tank should be replaced every week, and each water washing tank should also be cleaned every week.

3. Nickel sinking

The main components of the nickel solution are Ni2+ (5.1-5.8g1), the reducing agent sodium hypophosphite (25-30g1) and the stabilizer. Since chemical nickel has strict requirements on the composition range of the solution, two samples must be analyzed and tested per shift during the production process. times, and add Ni2 reducing agent according to the bare copper area of the production board or experience. When adding material, the principle of small amount and dispersed multiple times should be followed to prevent local plating solution from reacting violently, resulting in accelerated aging of the plating solution and pH value. , The temperature of the plating solution has a greater impact on the nickel thickness, and the temperature of the nickel solution is controlled at 85°C-90°C. When the PH is 5.3-5.7, when the nickel cylinder is not in production, the temperature of the nickel cylinder should be reduced to about 70°C to slow down the aging of the plating solution. The electroless nickel plating solution is sensitive to impurities. Many chemical components are harmful to electroless nickel, which can be divided into the following Several categories:

Inhibitors: including Pb.Sn..Hg.Ti.Bi (low melting point heavy metals), organic impurities including S2, nitric acid and anionic wetting agents. All these substances will reduce the activity, resulting in reduced electroless plating speed and missed plating. In severe cases, the electroless nickel plating process will be completely stopped.

Organic impurities: including: in addition to the organic stabilizers mentioned above, there are also plastic agents and impurities from equipment and solder. Although some impurities can be removed by continuous plating, they cannot be completely removed.

Instable agent: including Pd and a small amount of copper. These two components make chemical nickel unstable, making the coating rough, and excessive plating on the tank wall and heater.

Solid impurities: including calcium sulfate or calcium phosphate and other insoluble substances sinking or being brought into the solution. Filtration removes solid particles.

In short: effective measures should be taken during the production process to reduce such impurities from being mixed into the plating solution.

4. Immersed Gold Source

The immersion gold process is an immersion gold process. The main component of the immersion gold cylinder is Au (1.5-3.5gl), and the binding agent is (Ec0.06-0.16molL), which can replace pure gold plating on the nickel-phosphorus alloy layer. , making the coating smooth and crystallized, the pH value of the plating solution is generally between 4-5, and the control temperature is 85℃-90℃.

5. Post-processing

Immersion gold post-processing is also an important step. For printed circuit boards, it generally includes: scrap gold washing, DI washing, drying and other steps. If conditions permit, the immersion gold board can be further washed with a horizontal plate washing machine. drying. The horizontal surface plate washer can be set up according to the sequence of chemical washing (10% sulfuric acid, 30gL hydrogen peroxide), high-pressure DI water washing (30~50PSI), DI water washing, blow drying, and drying to completely remove the chemical liquid in the holes and surface of the printed circuit board. and water stains, to obtain an immersion gold plate with uniform coating and good brightness

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6. Control during production process

Problems that often occur in the production process of immersed nickel gold are often unbalanced components of the plating solution, poor additive quality and excessive content of the plating solution magazine. Preventing and improving this problem plays a great role in process control. The production process is now The factors that should be paid attention to include the following:

1) In the electroless nickel gold process, because there are small holes, water washing between each step is necessary and special attention should be paid to it.

2) After micro-etching between the micro-etching agent and the palladium activator, the copper will easily fade. In severe cases, the palladium plating will be uneven, leading to failure of the nickel layer. If the circuit board is not washed well, the oxidant from the micro-etching will prevent The deposition of palladium will affect the effect of immersion gold, thus affecting the quality of the board.

3) Between palladium activator and chemical nickel Palladium is the most dangerous impurity in the chemical nickel process. A very small amount of palladium will cause the bath liquid to naturally decompose. Although the concentration of palladium is very low, it should be washed well before entering the electroless plating tank. It is recommended to use two water washes with air agitation.

4) Between electroless nickel and immersion gold. Between these two steps, the transfer time will easily passivate the nickel layer, resulting in uneven immersion gold and poor bonding force. This can easily lead to the loss of gold and gold.

5) In order to maintain solderability and ductility after immersion in gold, wash it thoroughly with water after gold plating (it is best to use distilled water for the last wash), and dry it completely, especially the inside of the hole.

6) PH and temperature of the nickel immersion cylinder. The nickel immersion cylinder should increase the PH and adjust it with less than 50% ammonia water. Lower the PH and adjust it with 10% VV sulfuric acid. Make all additions slowly, stirring continuously. pH measurements should be performed with sufficient stirring to ensure a balanced bath concentration. The higher the temperature, the faster the plating speed. When plating thick layers, low temperatures are used to slow down needle emergence.

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PCB using immersed gold board mainly has the following characteristics:

1. Because the crystal structures formed by immersion gold and gold plating are different, immersion gold will be golden yellower than gold plating, and customers will be more satisfied.

2. Because the crystal structures formed by immersion gold and gold plating are different, immersion gold is easier to weld than gold plating and will not cause poor welding or customer complaints.

3. Because the immersion gold board only has nickel and gold on the pad, the signal transmission in the skin effect is in the copper layer and will not affect the signal.

4. Because the crystal structure of immersion gold is denser than gold plating, it is less likely to cause oxidation.

5. Since the immersion gold plate only has nickel gold on the pad, it will not produce gold wires and cause short spots.

6. Because the immersion gold plate only has nickel gold on the pad, the solder resist on the circuit is more firmly bonded with the copper layer.

7. The project will not affect the spacing during compensation.

8. Because the crystal structures formed by immersion gold and gold plating are different, the stress of the immersion gold plate is easier to control. For products with bonding, it is more conducive to bonding processing. At the same time, it is precisely because immersion gold is softer than gold plating, so gold fingers made of immersion gold plates are not wear-resistant.

9. The flatness and service life of the immersed gold plate are as good as those of the gold-plated plate.

Welding strength comparison

After three times of high temperature, the solder joints of the immersed gold plate are full and bright.

After three times of high temperature treatment on the OSP board, the solder joints are dark in color, similar to the color of oxidation.

After three high-temperature weldings, it can be seen that the solder joints of the immersion gold board are full and bright, and the soldering is good and will not affect the activity of the solder paste and flux. However, the solder joints of the OSP process board are dark and dull, which affects the solder paste and flux. activity, easily causing empty welding and increased repairs

Heat dissipation comparison

The thermal conductivity of gold is good, and the pad made of it has the best heat dissipation due to its good thermal conductivity. The better the heat dissipation, the lower the temperature of the PCB board, and the more stable the chip operation will be. The immersion gold plate has good heat dissipation, and comprehensive heat dissipation holes can be used in the CPU bearing area and BGA component soldering base on the Notebook board, while the OSP and silver plate have average heat dissipation.

Comparison of electrical testability

The immersion gold plate can be directly measured before and after production and shipment. The operation technology is simple and is not affected by other conditions; the OSP plate has an organic solderable film on the surface, and the organic solderable film is a non-conductive film, so it cannot be measured directly at all. Measurement must be done before OSP, but excessive micro-etching may easily occur after OSP, resulting in poor welding. The surface of the silver plate has a film with average stability and strict requirements on the external environment.

Process difficulty and cost comparison

The immersion gold process board has a complex process, high equipment requirements, strict environmental protection requirements, and the cost is the highest among lead-free process boards due to the extensive use of gold elements; the silver process board has a slightly lower process difficulty and has considerable water quality and environmental requirements. Strict, the cost is slightly lower than the immersed gold board; the OSP board has the simplest process, so the cost is also the lowest

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