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Why Is Immersion Gold and Gold Plating Needed on PCBs?
Gold and Gold Plating PCB
PCB Surface Treatment
- Antioxidant treatment, tin spraying, lead-free tin spraying, gold plating, tin plating, silver plating, hard gold plating, full-board gold plating, gold fingers, nickel-palladium-gold.
- OSP (Organic Solderability Preservatives): Low cost, good solderability, strict storage conditions, short storage time, environmentally friendly process, good soldering, and flatness.
- Tin Spraying: Tin-sprayed boards are generally multi-layer (4-60 layers) high-precision PCB samples, which have been used by many large domestic communication, computer, medical equipment, aerospace enterprises, and research units.
- Gold Fingers: Connecting components between the memory module and the memory slot on the memory module, through which all signals are transmitted. Made up of many golden conductive contact pieces arranged like fingers due to being gold-plated on the surface.
- Gold Fingers Manufacturing: Involves applying a layer of gold to the copper-clad board through a special process. Gold is chosen for its strong oxidation resistance and conductivity. However, due to the high cost of gold, many memories currently use tin plating instead, which began in the 1990s. The “gold fingers” of motherboards, memories, graphics cards, and other devices are now mostly made of tin, with only some high-performance server/workstation accessories continuing to use gold plating, which is naturally expensive.
Why Use Gold-Plated Boards
With the increasing integration of ICs, the number of IC pins is increasing and denser. Vertical tin spraying technology finds it difficult to blow the fine solder joints flat, which brings difficulties to the SMT assembly process. In addition, the shelf life of tin-sprayed boards is very short. Gold-plated boards solve these problems.
For surface mount technology, especially for 0603 and 0402 ultra-small surface mount components, the flatness of the solder joints directly affects the quality of the solder paste printing process, which has a decisive impact on the quality of the subsequent reflow soldering process. Therefore, full-board gold plating is often seen in high-density and ultra-small surface mount technology.
During the trial production stage, due to factors such as component procurement, the board is often not soldered immediately after it arrives, but often has to wait for a few weeks or even a couple of months before use. The shelf life of gold-plated boards is many times longer than that of lead-tin alloy, so everyone is willing to use them.
Furthermore, the cost of gold-plated PCBs at the sample stage is almost the same as that of lead-tin alloy boards. However, with the wiring becoming more and more dense, the line width and spacing have reached 2-4 mils. This has brought about the problem of gold wire short circuits: As the signal frequency increases, the effect of the skin effect on signal transmission between multiple plating layers becomes more apparent. The skin effect refers to: for high-frequency AC, the current tends to flow on the surface of the conductor. According to calculations, the skin depth is related to the frequency.
Why Use Gold Immersion Boards
To solve the above problems of gold plating boards, PCBs using gold immersion have the following characteristics:
1. Because the crystal structure formed by gold immersion is different from that of gold plating, gold immersion is more yellow than gold plating, which customers are more satisfied with.
2. Because the crystal structure formed by gold immersion is different from that of gold plating, gold immersion is easier to solder than gold plating and will not cause soldering defects, leading to customer complaints.
3. Because only the pads of the gold immersion board have nickel-gold, the signal transmission in the skin effect is in the copper layer and will not affect the signal.
4. Because the crystal structure of gold immersion is denser than that of gold plating, it is less likely to oxidize.
5. Because only the pads of the gold immersion board have nickel-gold, there will be no gold wire shorts, which are micro-shorts.
6. 5Because only the pads of the gold immersion board have nickel-gold, the combination of solder resist on the traces and the copper layer is more secure.
7. When CAM engineering performs compensation, the spacing will not be affected.
8. Because the crystal structure of gold immersion is different from that of gold plating, its stress is easier to control, which is more conducive to the bonding processing of bonded products. At the same time, because gold immersion is softer than gold plating, gold immersion boards are not wear-resistant when used as gold fingers.
9. The flatness and shelf life of gold immersion boards are as good as those of gold plating boards.
Gold Immersion Boards vs. Gold Plating Boards
In fact, the gold plating process can be divided into two types: electroplated gold and immersion gold. For the gold plating process, its soldering effect is greatly reduced, while the soldering effect of immersion gold is relatively better. Unless the manufacturer requires bonding, most manufacturers will choose the immersion gold process! In general, the common surface treatments of PCBs are as follows: gold plating (electroplated gold, immersion gold), silver plating, OSP, tin spraying (with and without lead). These treatments are mainly for FR-4 or CEM-3 materials, and there are surface treatments for paper-based materials and pine rosin coatings. If the poor tinning (tin eating) is excluded, the reasons include the production of solder paste by the patch manufacturer and the material process. For PCB issues, there are several reasons:
- Whether there is an oil film on the PAN during PCB printing, which can block the tinning effect; this can be verified by tinning test.
- Whether the lubrication on the PAN meets the design requirements, that is, whether the solder pad design can sufficiently ensure the support effect of the components.
- Whether the solder pad is contaminated, which can be determined by ion contamination testing.
The above three points are basically the key aspects considered by PCB manufacturers. Regarding the advantages and disadvantages of several surface treatment methods, each has its own strengths and weaknesses! In terms of gold plating, it can extend the storage time of the PCB, and the changes in the external environment temperature and humidity are relatively small (compared to other surface treatments), and it can generally be stored for about a year; tin spraying surface treatment is secondary, OSP is next, and these two surface treatments require much attention to environmental temperature and humidity storage time.
In general, silver plating surface treatment is somewhat different, with a higher price, more stringent storage conditions, requiring packaging with sulfur-free paper! And the storage time is about three months! In terms of tinning effect, immersion gold, OSP, and tin spraying are actually similar, and manufacturers mainly consider cost-effectiveness!
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