The surface finish of a PCB (Printed Circuit Board) refers to the type of coating or treatment applied to the exposed copper traces and pads on the board’s surface. Surface finish serves several purposes, including protecting the exposed copper from oxidation, enhancing solderability, and providing a flat surface for component attachment during assembly. Different surface finishes offer varying levels of performance, cost, and compatibility with specific applications.
Gold-plating and immersion gold are commonly used processes in modern circuit board production. With the increasing integration of ICs and the growing number of pins, the vertical solder spraying process struggles to flatten small solder pads, posing challenges for SMT assembly. Additionally, the shelf life of sprayed tin plates is short. Gold-plating or immersion gold processes offer solutions to these issues.
In surface mount technology, especially for ultra-small components like 0603 and 0402, the flatness of solder pads directly impacts solder paste printing quality, which in turn significantly influences the quality of subsequent reflow soldering. Therefore, the use of full-board gold-plating or immersion gold is often observed in high-density and ultra-small surface mount processes.
During the trial production phase, due to factors like component procurement, boards are often not soldered immediately upon arrival. Instead, they may wait for weeks or even months before being used. The shelf life of gold-plated and immersion gold boards is much longer than that of tin-plated boards. Consequently, these processes are preferred. The cost of gold-plated and immersion gold PCBs during the sampling stage is comparable to that of lead-tin alloy boards.
1. Electroless Nickel Immersion Gold (ENIG): This is a common PCB surface treatment method. It involves applying a layer of electroless nickel as an intermediary layer on the solder pads, followed by a layer of immersion gold on the nickel surface. ENIG offers benefits like good solderability, flatness, corrosion resistance, and favorable soldering performance. Gold’s characteristics also aid in preventing oxidation, thus enhancing long-term storage stability.
2. Hot Air Solder Leveling (HASL): This is another common surface treatment method. In the HASL process, solder pads are dipped into a molten tin alloy and excess solder is blown away using hot air, leaving behind a uniform solder layer. HASL’s advantages include lower cost, ease of manufacturing and soldering, though its surface precision and flatness might be comparatively lower.
3. Electroplating Gold: This method involves electroplating a layer of gold onto the solder pads. Gold excels in electrical conductivity and corrosion resistance, thereby improving soldering quality. However, gold plating is generally more expensive compared to other methods. It’s especially applied in gold finger applications.
4. Organic Solderability Preservatives (OSP): OSP involves applying an organic protective layer to solder pads to shield them from oxidation. OSP offers good flatness, solderability, and is suitable for light-duty applications.
5. Immersion Tin: Similar to immersion gold, immersion tin involves coating the solder pads with a layer of tin. Immersion tin provides good soldering performance and is relatively cost-effective compared to other methods. However, it might not excel as much as immersion gold in terms of corrosion resistance and long-term stability.
6. Nickel/Gold Plating: This method is similar to immersion gold, but after electroless nickel plating, a layer of copper is coated followed by metalization treatment. This approach offers good conductivity and corrosion resistance, suitable for high-performance applications.
7. Silver Plating: Silver plating involves coating the solder pads with a layer of silver. Silver is excellent in terms of conductivity, but it might oxidize when exposed to air, usually requiring an additional protective layer.
8. Hard Gold Plating: This method is used for connectors or socket contact points that require frequent insertion and removal. A thicker layer of gold is applied to provide wear resistance and corrosion performance.
Differences between Gold-Plating and Immersion Gold:
1. The crystal structure formed by gold-plating and immersion gold is different. Gold-plating has a thinner gold layer compared to immersion gold. Gold plating tends to be more yellow than immersion gold, which customers find more satisfactory.
2. Immersion gold has better soldering characteristics compared to gold-plating, reducing soldering defects and customer complaints. Immersion gold boards have more controllable stress and are more suitable for bonding processes. However, due to its softer nature, immersion gold is less durable for gold fingers.
3. Immersion gold only coats nickel-gold on the solder pads, not affecting signal transmission in copper layers, whereas gold-plating might impact signal transmission.
4. Hard gold plating has a denser crystal structure compared to immersion gold, making it less susceptible to oxidation. Immersion gold has a thinner gold layer, which might allow nickel to diffuse out.
5. Immersion gold is less likely to cause wire short circuits in high-density designs compared to gold-plating.
6. Immersion gold has better adhesion between solder resist and copper layers, which doesn’t affect spacing during compensatory processes.
7. Immersion gold is often used for higher-demand boards due to its better flatness. Gold-plating generally avoids the post-assembly phenomenon of black pad. The flatness and shelf life of immersion gold boards are as good as those of gold-plating.
Selecting the appropriate surface treatment method requires considering factors like electrical performance, corrosion resistance, cost, and application requirements. Depending on specific circumstances, suitable surface treatment processes can be chosen to meet design criteria.
Post time: Aug-18-2023
