PCB Surface Finishes
Different types of surface treatments are being used for circuit protection
PCB surface treatment is a crucial process in manufacturing and assembly. It is usually applied to the exposed pads and gold fingers to protect them from the environment while enhancing solderability and corrosion resistance. This process involves coating a layer of metal or chemical substances on the PCB’s surface to protect its electrical and mechanical properties. Moreover, PCBs without a good finish are susceptible to rapid oxidation, which can affect the copper and impair its electrical performance.
To sum up, when selecting a PCBA manufacturer, electronics manufacturers should ensure that they offer the necessary treatment service. Additionally, upon receiving the board, one must verify that the surface finish is flawless to extend the PCB’s life.
Types of PCB Finishes
While the layout design and material specifications of substrates, laminates, stacks, and components are critical concerns for PCB engineers, they should not overlook the importance of the circuit board surface treatment. In fact, the engineer’s focus should also include the selection of the appropriate surface finish as it impacts the PCB assembly and board reliability by protecting copper traces and strengthening solder connections. Unfortunately, engineers often rely on the default option provided by the design software without realizing its impact. Thus, knowing the differences between the different types of PCB surface finishes is essential in making the right selection for your electrical project. The following are explanations for different types.
The Hot Air Solder Leveling process, abbreviated as HASL, is utilized to apply a layer of molten tin-lead solder onto the surface of a printed circuit board (PCB). This molten solder is then flattened and smoothed using compressed hot air, producing a thin layer of solder on the PCB’s surface that protects it from oxidation and improves solderability. The success of this process is dependent on various factors, including the proper soldering temperature, air knife pressure, air knife temperature, lift speed, and DIP soldering time. HASL PCB finishes are available in two variations: traditional and lead-free. Although lead-free HASL is more environmentally friendly and compliant with RoHS regulations, it poses a higher challenge to the reflow soldering process, which requires higher temperatures and may result in difficulties with fine-pitch components. Here are its features:
- Cost-effective: It is a relatively mature surface finish technology that is widely recognized in the PCB manufacturing industry due to its simplicity and applicability.
- Sufficient shelf life: PCBs with a HASL finish have a longer shelf life because the solder mask protects the exposed copper from oxidation.
- Good Thermal and Electrical Conductivity: The thin layer of solder applied during the process provides good thermal and electrical conductivity, making it suitable for high-power applications.
- Good Solderability: It has excellent solderability and is suitable for most through-hole components and low-density surface mount components.
- Certain limitations: Fine-pitch components, wire bonding, capacitive touch sensor switches, and thin panels are not recommended with HASL.
OSP, also known as Organic Solderability Preservative, is a commonly used surface treatment agent in PCB manufacturing. The process involves spraying an organic protective film on the copper surface pad to form a protective layer that resists oxidation, moisture, and heat, which helps prevent damage to copper surfaces in normal operating environments. This protective layer acts as a barrier between the air and copper. The OSP surface treatment process consists of several steps, including degreasing, microetching, pickling, pure water cleaning, application of an organic coating, and a final cleaning step. OSP is gradually becoming more popular and shows a trend of replacing traditional HASL. The following are its characteristics:
- Environmentally friendly and RoHS and WEEE compliant.
- Compared to HASL, OSP has a longer shelf life because it doesn’t require a tin-lead finish that oxidizes over time.
- While OSP can provide some protection for plated through holes, it is less effective than other PCB surface treatment processes.
- OSP is suitable for fine-pitch surface-mount components, but it may not be able to withstand high temperatures, making it unsuitable for precision components like ICs.
- No waste or by-products, making it a clean and efficient process.
- While it can be reworked, it is a difficult process that requires care to avoid damaging the copper traces.
- Although OSP wettability may be slightly inferior, it can be improved by strictly controlling process parameters during the manufacturing process.
Silver is a suitable metallic substance for use in PCBs, and immersion silver refers to the deposition of a thin layer of silver on the copper surface of the PCB. This chemical process ensures that silver-coated PCBs maintain perfect electrical performance and solderability even when exposed to contamination, moisture, and high temperatures. The process involves a displacement reaction that deposits a layer of pure silver on copper. Occasionally, the process may involve combining silver with an OSP coating to prevent the silver from reacting with environmental sulfides.
- Uniformity and Smoothness: The immersion silver process results in a uniform and smooth finish, which is critical for fine-pitch surface mount components.
- Excellent Solderability: The thin layer of pure silver on the copper surface provides excellent solderability for good and reliable solder joints.
- Limited shelf life: PCBs with an immersion silver finish have a limited shelf life due to the silver’s tendency to migrate, resulting in increased surface resistance.
- Protection against oxidation: The silver layer acts as a barrier to protect the copper from oxidation even in high temperature and high humidity environments.
- Good Conductivity: It has high conductivity, making it a popular choice for high-frequency applications requiring high-speed signals and radio-frequency circuits.
- Limitations: This finish is not recommended for PCBs with plated through holes.
All solders are based on tin, so immersion tin finishes are compatible with any type of solder. The process produces a flat tin-copper intermetallic compound, which eliminates flatness and intermetallic issues. When the tin layer is combined with organic additives, a granular structure is formed, which solves the problems caused by whiskers and tin migration while also providing good solderability and thermal stability.
- Mass production: Suitable for use in horizontal production line applications.
- Ease of processing: Immersion tin is a relatively simple and easy-to-process method compared to other surface finish methods, and can be processed using conventional PCB manufacturing processes.
- Short lifespan: After prolonged use, the tin layer may undergo oxidation reactions, resulting in the formation of tin crystals.
- Good flatness: Its superior flatness makes it widely used in the SMT assembly field and improves the performance of component installation.
- High sensitivity: The immersion tin surface is susceptible to fingerprint discoloration, so the storage conditions for PCBs must be more stringent.
Immersion Gold, also known as ENIG, is the abbreviation for Electroless Nickel Immersion Gold. It is a surface finish process for PCBs that involves two layers of metal. The process involves depositing a thin layer of nickel on the copper pad of the PCB using an electroless plating process, followed by the replacement reaction method to coat a layer of gold atoms on the copper surface. The nickel inner layer is typically 3 to 6 microns thick, while the gold outer layer is deposited at 0.05 to 0.1 microns. The nickel layer acts as an isolation barrier between copper and solder, while the gold layer protects the nickel from oxidation and provides optimal surface flatness.
- Extensibility: ENIG has extensive applications in various electronic equipment such as aerospace, consumer electronics, military and medical electronic equipment, and industrial and medical PCBs.
- Manufacturing complexity: ENIG involves a complex manufacturing process, pre-clean → copper deposition → nickel deposition → gold deposition. The process may encounter black pad problems.
- Cost-effectiveness: It is relatively expensive compared to other surface finish, but it offers better corrosion resistance, solderability, and flatness.
- Longevity: It provides a durable protective layer to the copper surface of the PCB, which lasts longer depending on the quality of the plating process, layer thickness, and usage and storage conditions of the PCB.
ENEPIG, or electroless nickel electroless palladium immersion gold, is a surface finish process that differs from the two-layer structure of ENIG by having a three-layer structure. The process involves three steps: first, electroless nickel plating; second, electroless palladium plating; and third, immersion gold coating. The palladium layer acts as a barrier to prevent the nickel layer from corroding during the black pad and ENIG surface treatment. ENEPIG is commonly used in high-reliability applications, such as aerospace and military electronics, due to its excellent corrosion resistance and wire bonding properties. It is also utilized in electronic components for the automotive industry where long-term reliability is required.
- Zero Corrosion Risk: ENEPIG has a super corrosion-resistant palladium layer, ensuring high reliability even in harsh environments.
- Good Wire Bonding Performance: It provides excellent wire bonding performance compared to ENIG due to the flat and smooth surface of the palladium layer.
- Compatibility with Various Substrates: Compatible with various substrates, including copper, gold, and aluminum.
- Uniform Plating Thickness: It provides uniform plating thickness, ensuring a consistent coating across the entire surface of the PCB.
- Immature Technology: Despite its excellent performance, ENEPIG is still an emerging surface treatment method.
- Surface finish is a crucial process in manufacturing that enhances the durability of manufactured boards.
- To prevent copper oxidation and improve board solderability.
- Several methods are used in the surface finish process, including HASL, Immersion Silver, Immersion Tin, OSP, ENIG, and ENEPIG.
- HASL involves coating the PCB surface with molten tin-lead solder and flattening it.
- Immersion Silver coats the PCB copper layer with silver to improve its electrical properties.
- OSP process sprays an organic protective film on the copper pad surface to offer protection.
- Immersion Tin adds a thin layer of tin with organic additives to the copper layer surface.
- ENIG involves adding a thin layer of gold on top of the nickel immersion to protect the nickel against oxidation.