The PCB industry knows that flexible PCBs have a copper coating. Copper will oxidize without protection. The circuit board will become unusable. Surface finish is the interface between printed circuit boards and components. Surface finish has two functions: It protects the exposed circuitry and it provides a solderable surface when attaching components to the board. Flex PCB surface finishes can be divided into two categories: natural and metallic coatings.
Understand the different surface finishes of PCBs
1. Electroless nickel immersion gold (ENIG OR ENi/IAu )
Electroless Nickel Immersion gold is a metal plating used to improve solderability and avoid oxidation of copper contacts.
ENIG is a two-layer metal covering, with 2-8 μin Au on top of 120-240 μin Ni. It's typically applied using rapid immersion into a solution that contains a gold salt. The gold protects the nickel from oxidation, while the nickel protects and secures the base metal surface and allows for the circuit to be soldered securely to it. ENIG has become the most popular surface finish in the PCB Industry due to the implementation and development of RoHS regulations.
Advantages: It is not easily oxidized, can be stored for a long period, its surface is flat and smooth, and is easy to solder onto PCB. Due to its thinness, it is better suited for FPC. It is suitable for soldering precision pins, components, and small solder joints. The reflow soldering process can be repeated many times without affecting the solderability. It is RoHS-compliant and lead-free. The plated holes can maintain tighter tolerances.
Disadvantages: Black pad problems can occur because of the chemical nickel plating. Over time, the nickel layer oxidizes and reliability over a long period is compromised.
Electroless Nickel Electroless Palladium Infusion Gold (ENEPIG).
ENEPIG, which was first introduced in the late 1990s, is a relatively recent addition to the surface finish market for circuit boards. ENEPIG and ENIG are similar. Palladium is used to protect the nickel layer and prevent it from oxidizing. ENEPIG is a good choice for applications that require high frequency and limited pitch. It also meets packaging requirements and RoHS regulations. ENEPIG, however, is not used as a surface finish on printed circuit boards due to its high cost and low demand.
Immersion Silver(ImAg)
Immersion silver is an electrolytic treatment that involves immersing copper PCBs in a bath of silver ions. The surface of the copper PCBs has a thickness of between 5 and 18 microns. It is compliant with current environmental standards, such as WEEE and RoHS, is superior in terms of the environment to HASL, and ENIG, and costs less. Immersion silver is an alternative treatment to immersion gold on electroless nickel. It has a range of thicknesses from 0.12 to 0.40mm and is suitable for components with fine pitch or flat surfaces. The silver is chemically applied to the base metal on the PCB. It does not need a diffusion barrier and is especially suitable for applications that require the avoidance of magnetic materials. Immersion silver is a silver coating that is free of lead and protects copper from corrosion. However, it is sensitive to airborne contaminants and certain surfaces. It has a shelf-life of between 6 and 12 months and requires a good circuit board factory to control the packaging and ensure quality. Immersion silver is used for many applications, including EMI shielding and dome contacts. It can also be used to bond wires, bond aluminum wires, and create very fine traces and membrane switches.
Advantages: RoHS-compliant flat soldering surface. Suitable for lead-free and SMT. Low loss, tighter tolerances for plated holes. Low cost and simple process. Suitable for very critical circuits.
Disadvantages: Surface oxidizes and tarnishes. Storage requirements are high, and it is easy to contaminate. Solder strength can be affected by microcavities. Solder masks are prone to electromigration, electrochemical corrosion, and other problems.
4.Immersion Tin(ISn)
Immersion Tin is a metal coating deposited by a chemical replacement reaction and applied directly to the copper of the circuit board. ISn protects copper during its expected shelf-life. Immersion tin typically has a thickness between 20-50 μin. The interaction between tin, copper, and other metals will cause them to diffuse into one another. This will affect the shelf-life of the coating and its performance. Immersion Tin works well for components with small geometries and fine pitches. Immersion Tin costs less than ENIG or immersion silver and is RoHS-compliant.
Advantages: Lead-free, RoHS-compliant, and flat soldering surface. Plated holes maintain tighter tolerances. Suitable for horizontal production. Precision routing processing.
Disadvantages: PCBs can be damaged and have soldering issues when handled. Tin whiskers are also a problem. Shelf life is shorter than ENIG. Storage conditions are higher. The contact switch design is not suitable. Solder masks must be processed in a way that is very strict, or else they will come off. For multiple soldering, it is best to use the N2 gas. Electrical measurement can also be a problem.
Organic Solderability Preservative(OSP)
OSP involves chemically growing a thin organic film over the clean, bare copper surface. This film is anti-oxidation and thermal shock resistant, as well as moisture resistant, to protect the copper from rusting. This protective film is not necessary in normal environments, but it is required in high-temperature welding.
Advantages: Flat surface for soldering, non-toxic and environmentally friendly, RoHS compliant. Cheaper than other surface treatments. Easy process suitable for SMT and lead-free soldering.
Disadvantages: Limited reflow time, poor high-temperature resistance. Not suitable for wire bonding, or crimping. Short soldering window, slightly poor reliability over the long term. Not suitable for multiple-soldering processes.
Electrolytic Nickel/Gold (Hard Au/Soft Au)
The electrolytic nickel/gold is a nickel base with a gold layer overlaid. This coating is classified as either hard gold (99.6% purity) or soft (99.9% purity). The type of application will determine whether you choose hard gold or soft.
Hard Gold finish
This type of PCB surface finish is an alloy of gold containing cobalt or nickel. Hard gold is a gold coating over a nickel layer. Between the gold-plated layer and copper, low-stress nickel is used. Hard electrolytic is made up of two layers: a gold-plated layer and a nickel barrier layer. Hard gold is very resistant and is ideal for components with a high wear risk, such as connectors on the edge, interconnect carriers, and contacts. Due to its cost and poor solderability, it is not recommended for solderable areas.
Hard gold finishes will vary in thickness depending on their application. If hard gold is being used for military applications, then the thickness must be between 50 and 100 μin. For non-military uses, 25 to 50 μin thickness is required.
Advantages: Lead-free, RoHS compliant, and a long shelf life. Storage period >12 months. Ideal for electrical testing, contact switch design, and gold wire bonding.
Disadvantages: Electroplating fingers with gold is extremely expensive and requires a special circuit to conduct electricity. Soldering may be difficult due to the inconsistent thickness of gold. Electroplating problems with uneven surfaces. Nickel-plated gold does not wrap around wire edges.
Soft Gold Metallic Finish
This printed circuit board surface finish is a gold finish that has a higher purity of gold on the outer layer. Soft gold is 99.9% pur. Circuit boards that need wire bonding and high solderability are often finished in soft gold. Soft gold creates stronger solder joints compared to hard gold.
In general, the electrolytic nickel/gold finish is the most costly PCB finish.
Hot air solder leveling (HASL)/Lead-Free HASL
HASL, or a traditional method of surface treatment, is a way to treat the surface. The board is dipped into a molten-solder bath to coat all surfaces exposed with solder. The board is placed between hot-air knives to remove any excess solder. The solder used in this case is a mix of tin, lead, and zinc. The HASL method has an unexpected benefit: it exposes the PCB up to 265°C temperatures, allowing any delamination problems to be discovered before expensive components are attached. HASL makes the device more resistant to drops. The HASL method is identical to the lead-free HASL. The only difference is that the solder is made of lead-free alloys such as tin-copper-nickel-germanium, tin-silver-copper (SAC), and tin-copper-cobalt. HASL used to be the industry standard but has seen a decline in popularity due to possible RoHS compliance concerns.
Advantages: Low price, long shelf-life, and HASL can be reworked.
Disadvantages: Uneven soldering surfaces, contains lead (not RoHS compatible), can't maintain tight tolerances for plated holes.
ENIG, electrolytic gold, and electrolytic tin are the most common flex PCB surface finishes. Flex Plus also offers other surface treatments. Please contact us if you have questions or want to know more.
Flex Plus (Xiamen) Co., Ltd. is a manufacturer specializing in the production of flexible circuit boards. The products cover single-sided, 2 layers flex PCB, multilayers flex circuit board, rigid-flex PCB, split and dual access (back bare) flex PCB, sculpturer flexible printed circuit, and other types of flexible circuit boards, which can meet the needs of customers for all kinds of products.