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Gold Plating and Immersion Gold |
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Immersion gold and gold plating are the technologies often used in PCB surface finish, then what effect of these two kinds of surface finish will have on the circuit board? |
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What is gold plating?
There are many names of what we said the “whole board with gold plated” in the industry, such as "electroplating gold" "electroplating nickel gold " "electrolysis gold" "electric gold" "electro-nickel gold", there is a distinction of soft gold and hard gold. The principle is to dissolve the nickel and gold (commonly known as gold salt) in chemical potions, the circuit board is immersed in the electroplating tank and connected to the current to generate a layer of nickel and gold on the surface of copper foil of the circuit board. In particular, the "copper foil" mentioned here includes the track of circuit and the pads. Considering the cost, some users choose gold plated on the pads only. Electro-nickel gold plating is widely used in electronic products due to its advantages of high hardness, wear resistance, oxidation difficulty. The gold layer of electric gold can be done very thick, as long as the time is extended, which is suitable for bonding board. The discarding chance of electric gold potion is smaller than that of immersion gold, but the electric gold is required for full board conduction, and which is not suitable for special fine circuits. |
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So what is immersion gold?
Immersion gold is to generate a layer of plating through the method of chemical redox reaction, and use self-redox reaction of sodium hypophosphite to form a nickel layer, use displacement reaction to form a gold layer, it is a chemical method, so it is also called chemical gold. Immersion gold is generally very thin (less than 0.2um), the purity of gold is low. The working fluid can only be discarded after it is used to a certain extent.
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The differences between immersion gold board and the gold plating board: |
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1) The gold layer of electroplating is thicker, with high hardness, so it is often used for the plug sliding part, such as the gold fingers of switch card. The crystal structures formed by the two are different. |
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2) As the crystal structures formed by immersion gold and gold plating are different, compared with gold plating, the immersion gold is easier to be welded, which will not cause poor welding and customer complaints. At the same time, it is also because that the immersion gold is softer than gold plating, so the gold plating is generally selected for gold finger board, as hard gold is more wear resistant. |
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3) For immersion gold, the nickel and gold are only on the pads, and in the skin effect, the signal transmission is in the copper layer and will not be affected. As the frequency of the signal becomes higher and higher, the effect of signal transmission quality in multiple plating is more obvious due to the skin effect: |
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Skin effect refers to: high frequency AC current will tend to flow on the surface of the conductor concentratedly. According to the calculation, the skin depth is related to the frequency:
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4) Compared with gold plating, the crystal structure of immersion gold is denser, which is difficult to produce oxidation. |
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5) With the wiring become denser and denser, the line width and spacing have reached 3-4mil. It is easy for gold plating to produce short circuit of gold wire. For immersion gold, only nickel and gold are on the pads, so it will not produce short circuit of gold wire. |
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6) For immersion gold board, only nickel and gold are on the pads, so the combination of the solder mask on circuit and the copper layer is firmer. It will not affect the spacing when engineers do compensation. |
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7) Immersion gold is generally used for the board with higher demanding (flatness shall be good), generally, the immersion gold will not generate the phenomenon of black pad after assembly. The flatness and shelf life of immersion gold circuit boards are as good as gold plating board. |
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Now the gold price is expensive on the market, in order to save costs, many PCB users are reluctant to produce gold plating boards, but only the immersion gold boards with nickel and gold on the pads, the price is really much cheaper. |
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Why do not use Hot Air Leveling? |
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As the integration of IC gets higher and higher, the IC pins are denser and denser. However, it is difficult for the vertical hot air leveling process to blow the slender pad smooth, which brings difficulties to the mounting of SMT; what’s more, the shelf life of hot air leveling board is very short. While the gold plating board just solve these problems: |
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1)For the surface mount technology, especially for the ultra-small surface mount of 0603 and 0402, as the flatness of the pad is directly related to the quality of tin paste printing process, which has a decisive influence on the quality of the following reflow welding, so the whole board with gold plating is often seen in the high density and ultra-small surface mounting technology. |
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2)At the trial production stage, affected by factors of component procurement and so on, the board is usually not welded immediately after it’s finished, however, it has to be waited for a few weeks or even months. The shelf life of gold plating board is many times longer than lead-tin alloy, so everyone is willing to use gold-plated board. In addition, the cost of gold plating PCB in trial phase is more or less the same with lead-tin alloy board. |
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Addendum: the differences between immersion gold board and gold plating board |
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