Printed Circuit Board Surface Finishing
Surface Finishing is a pivotal interface between components and the bare PCB board. Firstly, it can prevent exposed copper traces from oxidation. Secondly, it creates a surface for soldering components to the PCB. Both of which ensure good solderability and electrical properties of the PCB. There are various types of surface finishing, therefore it is necessary to select the right one to meet the customer’s requirements. When choosing the type of surface finishing, the following factors need to be considered: cost, lead-free, productivity, component type, and environmentally friendly material.
Figure 1: The PCB surface finish
Electroless Nickel Immersion (ENIG)
ENIG, as a double-layer metallic coating of nickel and gold, is currently the most popular type of surface finishing in the industry. The nickel layer acts as a barrier between the copper surface and the component soldering surface. The gold layer is used to protect the nickel layer during storage as well as to provide strong solderability. Compared to other surface finishing types, ENIG is suitable for bonding and plated through holes (PTH), has a long shelf life, and provides the flattest solderable surface. It is lead-free but expensive because of the complicated process and not re-workable property.
Figure 2: Electroless Nickel Immersion (ENIG)
Immersion Tin (I-Tin)
The I-Tin surface finishing is to immerse the circuit board into a couple of chemical baths to produce the best tin adhesion and ideal flatness. This technology is very promising since solder is based on tin, and the tin layer can be matched with any type of solder. I-Tin is lead-free, cost-effective, re-workable, and suitable for fine pitch products. However, it is not good for multiple assembly processes and plated through holes (PTH), requires sensitive handling, and has the possibility of damaging the solder mask. Tin whiskers may appear, leading to short circuits.
Figure 3: Immersion Tin (I-Tin)
Organic Solderability Preservative (OSP)
Unlike other surface finishing types, OSP acts as a barrier between the copper circuit and air. In other words, OSP is to chemically form an organic film on the clean bare copper surface before soldering. It is lead-free, creates a coplanar surface, and has a simple process with low equipment maintenance. There are also some disadvantages such as short shelf life and the possibility of exposure of the copper during final assembly.
Lead- Free Hot Air Soldering Leveling (HASL)
In this surface finishing type, the PCB board receives a flux coating and goes through a bath of molten tin solder, which is removed by hot-air knives. There are two main types of HASL, one is lead HASL and another is lead-free HASL. HASL was once favored by the industry, but it is non-environmentally friendly. The advantages of HASL are low-cost and widely available with excellent shelf life. However, its disadvantages cannot be ignored. Although the lead-free one is environmentally friendly, it creates an uneven surface, which is not acceptable for fine pitch components. In the process of lead and lead-free HASL, the PCB board is exposed to high temperature, which can create high thermal stress to the board. The HASL is also not suitable for plated through holes (PTH).
Figure 4: The HASL Manufacturing Process
Hard Gold Plating
The hard gold plating, known as electroplated, is one of the expensive surface finishing types. Due to its high cost, the complexity of the process and comparatively poor solderability, it is usually applied to high-wear areas such as edge connectors instead of solderable areas. Unlike ENIG, the thickness of the gold layer can vary according to the customer’s requirement. Other merits of hard gold surface finishing include forming a flat surface, excellent durability, and long shelf life.
Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)
ENEPIG, as a triple-layer metallic coating of nickel, palladium, and gold, is a comparatively new surface finishing type. Unlike ENIG, it has an additional palladium layer as a resistance layer to prevent nickel from oxidation and diffusion to the copper layer. ENIG is conducive to a simple process mechanism and high-temperature resistance, but ENEPIG features outstanding multiple reflow cycles and reliable wire bonding capabilities. Both ENIG and ENEPIG are costly because they provide the highest solderability for the PCB. ENEPIG is bondable and very suitable for high-frequency applications with limited spacing. Moreover, it doesn’t have corrosion and black pad risks, and has an excellent shelf life and solder joints. The figure below shows the different manufacturing processes of ENIG and ENEPIG.
Figure 5: Manufacturing Processes of ENIG and ENEPIG
Immersion Silver (I-Ag)
I-Ag is a comparatively stable surface finishing type with a moderate shelf life. For fine pitch and flat package coatings, it can be a good substitute for ENIG. It is lead-free, cost-effective, and perfect for high-speed PCB applications. However, it is sensitive to surrounding contaminants, which may lead to tarnishing issues. It also has the silver whisker and high friction coefficient problems.