In the early PCB manufacture, HASL is the most widely used surface treatment, which has excellent welding performance and low cost.With the high precision PCBs developing, it is difficult to below the surface to smooth of HASL, which brings difficulties to SMT mounting. In addition, the service life of HASL board is very short, and the ENIG PCBs and gold plating PCB just solved the problems.
ENIG PCBs and gold plating PCBs have many advantages over HASL PCB.
- The surface is smooth.
- Good solderability, good heat dissipation, gold wire bonding can be made
- Long storage time. (Vacuum packaging can be kept for more than one year)
- Resistant reflow welding and can be reworked many times.
Speaking of gold plating, PCB factories will ask you whether you are making ENIG or gold plating, soft gold or hard gold. For PCB designers, they are confused. Today we will talk about different kinds of gold plating PCBs.
According to the different processing technology, it can be divided into electroplating gold(gold plating) and Immersion Gold (ENIG).
Electroplated gold is divided into soft gold and hard gold according to the purity of gold.
Soft gold: Pure gold is soft because of its high purity.
Hard gold: In order to increase the surface hardness, and increase other metals called hard gold.
Immersion Gold (ENIG):
Immersion Gold is a process of thin gold deposition by controlled redox reaction under the catalysis of metals.Chemical replacement method is used to produce the effect of surface gold layer, so the maximum thickness of the gold layer can not achieve the same thickness as electroplated gold in principle, and the lower the gold content, the thickness is generally 1-3 micro inches.
Immersion Gold(ENIG) process:
Whole hole – -> Degreasing – -> Washing – – >Erosion – -> Washing – -> Activation – -> Washing – – >Nickel Immerse – -> Washing – – > Gold Immerse – – >Washing – -> Drying.
Whole hole: Hydrochloric acid or thiourea is used to deactivate the remaining palladium in the non-conducting pore to prevent it from sinking nickel and gold.
Degreasing: Use acid degreasing agent to remove light grease and oxides from copper surface, so as to make copper surface clean and increase wettability.
Water washing: Back-flow water washing after oil cylinder removal.
Microetching: Sodium persulfate etching solution is used to coarsen the copper surface and increase the precision of copper and chemical nickel layers.
Activation: In the electrochemical sequence, copper is located behind nickel, so it is necessary to activate the copper surface before electroless nickel plating. PCB industry mostly uses the method of producing a layer of replacement palladium layer on the copper surface to activate it. Activators are generally sulfuric acid type and hydrochloric acid type. Now more palladium sulfate type is used to activate night, and the effect is ideal.
Nickel precipitation: Nickel ions are deposited on bare copper surface under the reduction condition of sodium hypophosphite under the activation of palladium. When nickel deposits are coated with Pd-catalyzed crystals, the autocatalytic reaction will continue until the required nickel layer thickness.
Gold Deposition: After nickel plating on PCB plate is put into the gold trough, the nickel surface is attacked by the trough solution and nickel ions are dissolved. The two electrons thrown out are obtained by gold cyanide ions and gold deposits on the nickel surface.
Immersion Gold PCB is superior to gold-plated PCB in the following aspects:
- Immersion Gold is easier to weld than gold plating, and will not cause bad welding.
- There is only nickel and gold on the pad, which has little effect on the signal transmission in the skin effect.
- Immersion Gold deposits are more dense than gold plated crystals and are not easy to oxidize.
- Immersion gold is only needed for the location of window opening, and the cost can also be well controlled.
Gold-plated PCB boards are often used in gold finger boards and bonding boards because of their excellent electrical conductivity, wear resistance and oxidation resistance.
The gold layer on PCB boards acts as a metal corrosion resistance layer, which can withstand all general etching solutions. It has a high conductivity. Because of its negative oxidation potential, it is a good metal for corrosion resistance.