1）OSP, or Organic Solderability Preservative, differs from other surface treatment processes in that it serves as an insulating layer between copper and air. Essentially, OSP is cultivated on a clean bare copper surface through a chemical reaction, forming an organic film. Due to its organic nature, it is more cost-effective than tin spraying, making it a preferred choice for many computer motherboards, especially those with large board sizes, where OSP technology offers significant cost savings.
2）However, OSP presents challenges. Firstly, its transparency makes it difficult to visually inspect for proper processing. Secondly, OSP is inherently insulative and non-conductive, which impacts electrical testing. Consequently, to establish electrical contact for testing, test points must be opened using a stencil and coated with solder paste to remove the OSP layer, allowing direct contact with the pin points. Moreover, OSP cannot be utilized on surfaces requiring electrical contact, such as keyboard keys.
3）Moreover, OSP is susceptible to the effects of acid and temperature. During secondary reflow soldering, it must be processed within a specific timeframe; otherwise, the soldering results may be compromised, with subsequent reflow soldering displaying reduced efficacy. Additionally, if left exposed to air over extended periods, OSP requires resurfacing. Furthermore, once opened, OSP-treated boards must be utilized within 24 hours to maintain effectiveness.
Gold Plating: Gold plating, despite being applied in a thin layer, significantly contributes to the overall cost of a circuit board, accounting for nearly 10%. Gold plating serves two primary purposes: facilitating soldering and preventing corrosion. Notably, components like memory stick gold fingers maintain their luster even after years of use, contrasting sharply with the rust-prone fate of copper, aluminum, and iron counterparts. The widespread application of gold-plated layers extends to component pads, gold fingers, and connector springs on circuit boards. Mobile phone motherboards, immersion gold boards, and various digital circuit boards frequently feature gold-plated layers, distinguishing them from non-gold-plated counterparts like computer motherboards, audio circuit boards, and small digital circuit boards.
Electroless nickel-palladium immersion gold (ENEPIG): Compared with nickel-gold, ENEPIG incorporates a palladium layer between nickel and gold. This palladium layer serves as a protective barrier during the replacement gold deposition reaction, preventing the nickel layer from being substituted by gold. By mitigating excessive corrosion, palladium adequately prepares the surface for immersion gold while thwarting corrosion resulting from substitution reactions. Nickel deposition typically ranges from 120 to 240μin (approximately 3 to 6μm) in thickness, while palladium thickness ranges from 4 to 20μin (approximately 0.1 to 0.5μm). Gold deposition typically ranges from 1 to 4μin (0.02 to 0.1μm) in thickness.
HASL (Hot Air Solder Leveling): Also known as hot air solder leveling, HASL involves coating the PCB surface with molten tin-lead solder and then using heated compressed air to level (flatten) it, creating a protective layer resistant to copper oxidation and offering excellent solderability. During the hot air leveling process, a copper-tin metal compound forms at the junction between solder and copper, typically with a thickness of about 1 to 2 mils. Applying a tin layer to the outer copper circuitry can aid in soldering, although it does not provide the long-term contact reliability of gold. While it does not affect already soldered components, its reliability diminishes for pads exposed to air over an extended period, such as grounding pads and pin sockets. Over time, oxidation and corrosion may occur, resulting in poor contact. Primarily utilized in small digital product circuit boards, spray tin boards are chosen for their cost-effectiveness.
1. Immersion silver: The immersion silver process sits between OSP and electroless nickel/immersion gold, offering simplicity and speed. It involves a displacement reaction, resulting in a nearly submicron pure silver coating (5~15μin, about 0.1~0.4μm). Occasionally, organic matter is included in the immersion silver process, primarily to prevent silver corrosion and mitigate silver migration issues. Measuring this thin organic layer is generally challenging. Analysis indicates that the organic material typically accounts for less than 1% of the total weight. Despite exposure to heat, humidity, and pollutants, immersion silver maintains good electrical properties and solderability, albeit with a loss of gloss. Notably, due to the absence of nickel beneath the silver layer, immersion silver lacks the robust physical strength of electroless nickel/immersion gold.
2. Immersion Tin: As all current solders are tin-based, the tin layer can be compatible with any solder type. However, immersion tin processes have historically been associated with tin whisker formation in PCBs, leading to reliability issues. Additionally, tin whiskers and tin migration during soldering exacerbate these problems, constraining the utility of immersion tin processes. Subsequently, organic additives have been introduced to tin immersion solutions to impart a granular structure to the tin layer, addressing previous challenges while enhancing thermal stability and solderability.
3. Each surface treatment process possesses unique characteristics and varying applications. Nevertheless, ENEPIG stands out as a versatile processing method capable of meeting the demands of diverse assembly scenarios.
