The core purpose of PCB surface finishes is to guarantee excellent solderability of pads or deliver stable electrical contact performance. Bare copper readily oxidizes and sulfidizes when exposed to air for extended periods. Oxide layers directly cause cold solder joints and poor contact. Therefore, all exposed copper traces require surface protection treatment.

Commonly known as hot-air leveling. The PCB is immersed in molten solder, and excess liquid solder is blown off by high-temperature air knives to form a tin protective layer on copper surfaces. A copper-tin alloy layer forms at the interface between solder and the copper substrate, providing both anti-oxidation protection and good solderability.Process principle: Circuit boards are dipped into molten solder. Air knives flatten the surface rapidly before solder solidifies, reducing solder fillet curvature and preventing short circuits caused by solder bridges between adjacent traces.✅ Advantages: Mature, well-established process, low cost, strong solder joint strength⚠️ Disadvantages: Uneven surface finish; not suitable for fine-pitch BGA devices below 0.5 mm; severe thermal shock may warp thin boards📌 Applications: General power supply boards, through-hole circuit boards, low-density consumer electronics

OSP complies with RoHS environmental standards. An ultra-thin organic protective film is formed on clean bare copper via chemical reactions.The thin film isolates air under normal conditions to prevent copper oxidation and sulfidation. During soldering, flux quickly dissolves the organic film, exposing pure copper to form reliable joints with molten solder.✅ Advantages: Extremely flat surface, low cost, no precious metals, low loss for high-frequency signals⚠️ Disadvantages: Thin coating vulnerable to scratches; limited shelf life of 3–6 months; typically supports only one reflow cycle📌 Applications: Smartphone motherboards, high-density HDI boards, RF & WiFi circuit boards, mass-produced low-cost digital products

A nickel layer is electroplated onto copper traces first, followed by an electroplated gold layer. Nickel acts as a barrier to prevent interdiffusion between gold and copper.Two variants:• Soft gold (pure gold): Matte finish, primarily used for semiconductor wire bonding;• Hard gold: Alloyed with trace elements such as cobalt, featuring a dense, wear-resistant, glossy surface, widely adopted for electrical contacts requiring repeated plugging and unplugging.📌 Applications: Gold fingers, connector contacts, semiconductor packaging substrates

Nickel and thin gold layers are deposited on copper via chemical displacement reactions to build a stable nickel-gold protective structure for long-term circuit protection. The nickel layer blocks copper diffusion, while the top gold layer continuously resists oxidation and avoids soldering defects induced by copper dissolution during lead-free assembly.✅ Advantages: Uniform flat surface, strong corrosion resistance, long shelf life, compatible with fine-pitch BGA/QFN⚠️ Disadvantages: Relatively high cost; risk of black pad defects without strict process control📌 Applications: Industrial control boards, medical electronics, communication modules, high-end products requiring prolonged storage

Solder materials are tin-based, so immersion tin coatings exhibit excellent compatibility with all types of solder paste. The process creates a flat copper-tin alloy interface with superior solderability and solves unevenness issues seen with HASL.⚠️ Note: Immersion tin PCBs cannot be stored for long periods. Production and assembly should follow the first-in, first-out principle, and whisker growth risks must be controlled.✅ Advantages: Flat surface, suitable for fine-pitch components⚠️ Disadvantages: Limited storage life; risk of tin whisker formation📌 Applications: High-speed digital boards, compact fine-pitch component modules

Immersion silver has a process complexity between OSP and ENIG. The coating maintains solderability under high-temperature and humid conditions; its only drawback is tarnishing after long-term storage.Immersion silver consists solely of copper and silver, with no nickel barrier layer, resulting in weaker mechanical wear resistance compared to ENIG. It delivers outstanding conductivity and low high-frequency signal loss.✅ Advantages: High flatness, excellent conductivity, superior high-frequency performance⚠️ Disadvantages: Silver layers easily sulfide and darken; risk of silver migration in humid environments📌 Applications: High-speed circuits, RF instruments, impedance-sensitive equipment

Compared with conventional ENIG, an extra palladium layer is added between nickel and gold. The palladium layer effectively suppresses corrosion from displacement reactions and prevents nickel oxidation. The top gold layer fully covers palladium to form a stable soldering and contact interface.✅ Advantages: Supports wire bonding, powerful anti-corrosion performance, compatible with multiple reflow cycles📌 Applications: High-frequency RF boards, automotive electronics, high-reliability military and communication products

Thick electroplated hard gold plating greatly enhances surface wear resistance and withstands tens of thousands of plug-in friction cycles. It generally adopts selective plating and is only applied locally on contact regions and gold fingers.📌 Applications: Memory card contacts, gold fingers for various interfaces, connector zones with frequent plugging and unplugging