► Printed Circuit Boards (PCBs) and Their Importance
As a fundamental electronic component, Printed Circuit Boards (PCBs) are widely used in various electronic and related products. PCBs serve as both a physical support structure and a carrier for the electrical connections between components. From an appearance perspective, PCBs typically exhibit three colors on their external layers: gold, silver, and reddish copper. Based on cost, gold is the most expensive, followed by silver, with copper being the least expensive. Internally, however, the main conductive pathways of a PCB are primarily made of pure copper, also referred to as bare copper.
Interestingly, PCBs often contain valuable metals. For instance, an average smartphone contains approximately 0.05 grams of gold, 0.26 grams of silver, and 12.6 grams of copper. A laptop contains even more gold—ten times as much as a smartphone. But why are such precious metals necessary in PCB manufacturing?
► Why Precious Metals Are Used in PCBs
PCBs are designed to support electronic components and facilitate their connections. For components to be soldered onto the board, certain areas of the copper layer must remain exposed, forming what are known as "pads." These pads, usually rectangular or circular, are the only parts exposed to the air after the application of solder mask paint.
Copper is highly susceptible to oxidation. When the exposed copper pads oxidize, soldering becomes difficult, and the electrical resistance increases significantly, compromising the performance of the final product. To prevent oxidation and protect the pads, manufacturers coat them with inert metals like gold or silver or apply a protective chemical film to the copper surface. This process ensures that the pads maintain their integrity during subsequent soldering and assembly processes.
► Key Materials in PCB Manufacturing
1. Copper-clad laminates (CCLs)
Copper-clad laminates are the foundational material for PCBs. These are created by bonding copper foil to one or both sides of a substrate, such as a fiberglass cloth, using resin through hot pressing.
For instance, fiberglass cloth-based CCLs are composed primarily of copper foil, fiberglass cloth, and epoxy resin, accounting for approximately 32%, 29%, and 26% of the product cost, respectively. As the base material for PCBs, CCLs play a crucial role in circuit interconnection. Recent advancements in technology have enabled certain types of specialized CCLs to be used directly for manufacturing printed electronic components. The copper foil used in PCBs is typically refined and processed into thin, foil-like sheets.
2. Immersion Gold PCBs
When gold comes into direct contact with copper, an electrochemical reaction known as migration or diffusion occurs. To prevent this, a layer of nickel is electroplated onto the copper as a barrier, and gold is then plated onto the nickel. This process, commonly referred to as "gold plating," is technically "nickel-gold plating."
The distinction between hard gold and soft gold lies in the composition of the gold layer. During gold plating, manufacturers can opt for pure gold or a gold alloy. Pure gold, being softer, is termed "soft gold," while gold alloys such as gold-nickel or gold-cobalt, which are harder, are known as "hard gold."
Gold-plated layers are widely used in areas such as component pads, edge connectors (gold fingers), and connector contacts. Many smartphone motherboards use gold-plated or immersion gold PCBs due to their corrosion resistance and ease of soldering. Despite being expensive—gold plating can account for up to 10% of the PCB cost—it remains a preferred choice for reliability in harsh environments.
3. Immersion Silver PCBs
Immersion silver is a cost-effective alternative to immersion gold. If a PCB requires functional interconnections but also demands cost efficiency, immersion silver is an excellent choice. It offers good flatness and contact properties, making it suitable for high-frequency signal applications and products like communication devices, automotive electronics, and computer peripherals.
While immersion silver provides outstanding electrical performance, issues like tarnishing and solder voids have limited its widespread adoption. Nevertheless, its ease of assembly and inspection makes it a recommended choice for EMS (Electronics Manufacturing Services) applications.
