The pros and cons of different PCB coatings:
To begin, it’s important to understand the necessity of nickel plating, silver plating, tin plating, chrome plating, zinc plating, and gold plating. Typically, tin plating and gold plating are commonly used.
Shared advantages: These plating layers generally enhance corrosion resistance (improving oxidation resistance) and serve a decorative purpose.
Here are the distinctions:
1. Zinc Plating: Primarily aims to prevent corrosion. It boasts low cost, convenient processing, and effective outcomes. However, it’s unsuitable for frictional parts, which can impact PCB welding performance, thus limiting its widespread use.
2. Nickel Plating: Provides excellent chemical stability in atmospheric and alkali environments, resisting color change. It remains oxidatively stable up to 600 degrees Celsius, exhibits high hardness, and is easily polished. However, it suffers from porosity.
3. Tin Plating: Offers high chemical stability, being nearly insoluble in dilute solutions of sulfuric acid, nitric acid, and hydrochloric acid. It also ensures good solderability.
4. Chrome Plating: Comes in decorative and hard variants. Decorative chrome is primarily for aesthetics and corrosion resistance, though it lacks wear resistance. Hard chrome enhances hardness and corrosion resistance.
5. Gold and Silver Plating: Primarily used for decoration and corrosion resistance. However, they are costly.
Why opt for gold-plated PCBs?
A. With increasing IC integration, IC pins become denser. Traditional PCB tin spray processes struggle to flatten thin pads, complicating SMT placement.
B. Moreover, tin-sprayed PCBs have a short shelf life. Gold-plated boards resolve these issues: In surface PCB mounting, especially for ultra-small 0603 and 0402 SMT components, pad flatness directly impacts solder paste printing quality, crucial for subsequent reflow soldering quality. Hence, full-board gold plating is common in high-density and ultra-small SMT processes.
C. During trial production, due to component procurement factors, boards might not be immediately soldered but stored for weeks or months. Gold-plated boards offer a longer shelf life compared to lead-tin alloys, making them cost-effective for sample stage production.
3. Why use heavy gold?
The gold plating process is divided into two types (generally nickel gold, not pure gold): electroplating gold and immersion gold (chemical method). In the gold plating process, the tin effect is significantly reduced, with Immersion Gold offering superior tin compatibility. Nowadays, most manufacturers opt for Immersion Gold technology unless specific bonding requirements are stipulated by the manufacturer.
1. The crystal structure of immersion gold differs from that of electroplated gold, resulting in a richer golden color that enhances customer satisfaction.
2. Immersion gold features a crystal structure that facilitates easier soldering compared to electroplated gold, minimizing instances of poor welds that could lead to customer complaints.
3. PCBs with immersion gold only have nickel and gold on the pads, preventing skin effect interference with signals on the copper layer.
4. The denser crystal structure of immersion gold reduces susceptibility to oxidation compared to electroplated gold.
5. Immersion gold PCBs, with nickel and gold confined to the pads, avoid gold wire formation that could potentially cause minor short circuits.
6. Immersion gold PCBs ensure stronger adhesion between the solder mask on the circuit and the copper layer, thanks to the absence of other metals on the pads.
7. This process does not impact compensation distances within the project.
8. The differing crystal structures of immersion gold and electroplated gold allow for easier stress management in immersion gold PCBs, particularly beneficial for products requiring bonding. However, immersion gold’s softer nature makes it less wear-resistant on gold fingers.
9. PCB immersion gold boards exhibit comparable flatness and shelf life to gold-plated boards.
