Printed Circuit Board design and manufacturing is itself a technical and precision-based approach. Each component and process has an equal and key contribution towards the overall quality of the PCB. One of the critical aspects of PCB manufacturing is the outer surface finish of the PCB. The surface finish is done in line with the required product quality and performance. Before manufacturing PCBs, a PCB surface finish comparison is needed. Choosing the wrong PCB surface finish will have a direct effect on the subsequent PCB performance.
PCB Surface Finish & its Importance
PCB surface finish is a thin layer (ranging in microns), carried out to the outer parts of the PCB’s components, copper traces, pads & substrate. PCB and its components are very sensitive to external and environmental factors. PCB surface finish is a dedicated chemical solution developed for PCBs, which does not react with the PCB substrate, components & copper in any way. It prepares a protective shield for the PCB to keep it secure from hazardous factors consisting of dust, moisture, oxidising agents, and other harmful ingredients. Apart from protecting the PCBs, it also helps in the solderability of the PCB components. A good surface finish removes oxidised content from the PCB surface, making it suitable for good soldering connections. So that’s why PCB surface finish comparison is important in actual PCB manufacturing.
PCB Surface Finish Comparison
Primarily based on the material availability and end-project requirements, there are numerous options available for the PCB surface finishes for PCB assembly solutions providers. The cost of the surface finish varies according to the project volume and quality requirements. In critical applications, a high-end surface finish is a must without much considering the costing part. Here is the PCB surface finish comparison.
1. Immersion Tin
Immersion Tin is a lead-free surface finish solution for PCBs. In this method, a thin layer of the Tin (Sn) is.- applied on the PCB surface by immersion process. Immersion is one of the affordable solutions due to being less expensive. This is also suitable for the reflow soldering process by proving a stale base for SMT components. But the Immersion Sn has a lower shelf life and tarnishing issues over time. So, it is best for low & medium applications with high volumes typically for consumer electronics products. However, this method can not guarantee the flatness of PCB compared to ENIG HASL.
2. Immersion Silver
Another process with immersion to provide PCB surface finish is the Immersion Silver process. This surface finish involves the coating of the silver on the PCB surface by an immersion process. Silver does not easily react with the PCB components, copper pads & traces and acts as a strong protective layer. It is also a lead-free and environmentally friendly solution due to absence of the hazardous substances. However due to the presence of the silver material, this process is quite costly to deal with. Silver also tarnishes over time, which degrades the solderability of the components on the PCB. Thus, Immersion Silver surface finish is used in high-density, high-performance & precision applications like telecommunication and computing. Compared to HASL ENIG and any other surface treatment method, this method has a drawback, in that silver is easily oxidized.
3. ENIG (Electroless Nickel Immersion Gold)
ENIG is a popular surface finish option in PCB assembly. ENIG stands for Electroless Nickel Immersion Gold, which includes two processes for PCB surface finish – Nickel electroless and gold immersion. In the first part, the Nickel is deposited over the PCB surface with an electroless process and further gold is applied over the Nickel surface by an immersion process. This hybrid solution provides an excellent protection for PCBs. ENIG is a lead-free & corrosion-resistant surface finish and provides a smooth and flat surface after the complete surface finish process resulting in the most suitable option for SMT component soldering. In some cases, Nickel migration may occur but can be avoided if proper processing is carried out. ENIG surface is mostly applicable for complex and high-density compact PCBs. Also in some applications where cost is not a major concern product reliability is most required. Between the ENIG HASL and OSP, the ENIG is the expensive surface finish method but it can enhance PCB electrical performance.
4. HASL (Hot Air Solder Leveling)
HASL (Hot Air Solder Leveling) is considered the most initial type of PCB surface finish but it is also being used in some applications at present. In this process, the PCB is dipped into the molten solder solution and then the solder is levelled with the help of hot air flow. The airflow is done to create a uniform layer over the PCB surface. HASL is a cost-effective solution and can be easily implemented into any PCB manufacturing process due to the availability of the solder. The solderability of the PCB also increases due to HASL surface finish. A few concerns in this method are non-uniformity & inconsistency of the solder layer over the PCB. Solder is mainly categorized into lead and lead-free versions. The usage of Lead solder causes environmental concerns. As the initial versions of the PCBs were not very complicated and had basic configurations only without any complexity and high-density components, HASL became the initial version of the PCB surface finish. At present, the PCBs with easy designs and simple configurations still use the HASL surface finish method. HASL ENIG and the other PCB surface finish can be used according to the applications.
5. Organic Solderability Preservative
Due to the increase in environmental concerns in PCB manufacturing practices, innovators are focusing on eco-friendly and cost-effective solutions. Since most of the PCB surface finish processes have hazardous chemicals and wastages, a new method has been developed for PCB surface finish, known as organic solderability preservative. An organic solution is applied over the PCB surface for complete protection. Being environmentally friendly, it is cost-effective and has good solderability. The only disadvantages are lower shelf life and more vulnerability towards scratches. Compared to HASL ENIG ENEPIG and HASL, organic solderability preservatives are being used in many applications as a better option.
Summary
With improvements within the PCB era, many surface finish options have been developed for PCBs. Selecting the best choice for the required project is dependent on many elements including price, volume, material properties & environmental aspects. There are many surface finish methods, like HASL ENIG ENEPIG and OSP. By means of carefully analyzing the strengths and weaknesses of each surface finish kind, an informed decision can be made for the PCB application. The right PCB surface finish for PCB will always be in line with the end application requirement, improving PCB performance effectively.
