Printed Circuit Boards (PCBs) provide necessary interconnections and support for electronic components to function as a cohesive entity in modern electronic devices. As size, complexity, and performance requirements of electronic devices continue to evolve, the choice of PCB surface treatment becomes increasingly crucial. The surface quality affects not only solderability of components but also the device's reliability, durability, and overall performance.
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This article provides a thorough analysis of factors that influence the selection of the optimal PCB surface finish. It discusses available surface treatment options, their advantages, and disadvantages, and provides engineers and designers with guidelines to help them make informed decisions.
The Purpose of PCB Surface Finish
Surface finish coating applied to printed circuit boards performs several important functions:
- Ensure solderability. Solderability is a primary function of a PCB surface finish. During assembly, it provides a solderable surface for attaching electronic components. Without an appropriate surface finish, solder joints may not form accurately, leading to defects such as solder bridges or poor connections.
- Prevent oxidation. Copper, the most prevalent conductor material on PCBs, is susceptible to oxidation when exposed to oxygen. A suitable surface treatment prevents oxidation by forming a protective layer over the copper traces. Oxidation can lead to inadequate electrical connections and signal degradation.
- Improve reliability. Surface texture of PCBs plays a significant role in their dependability. It affects the performance of solder joints, which need to withstand mechanical tension, temperature fluctuations, and environmental conditions. A durable surface finish ensures the PCB's long-term reliability.
- Simplify wire bonding. In certain circumstances, semiconductor devices are connected to the PCB via wire bonding. The quality and reliability of these wire bonds are influenced by the surface finish selected.
An apparatus set up for the application of surface finish on the PCB can be seen in Figure 1.
Figure 1: Equipment for the application of surface finish on the PCB.
Types of PCB Surface Finishes
The PCB surface finish is a crucial interface between electronic components and the PCB substrate. It assures reliable soldering, prevents oxidation of copper traces, and influences electrical performance. Surface finish selection affects assembly yield, solder joint quality, and long-term dependability.
The most commonly used PCB surface finishes include:
- Organic Solderability Preservatives (OSP): A thin organic layer that protects copper and allows direct soldering.
- Electroless Nickel Immersion Gold (ENIG): A two-layer finish with a nickel undercoat and gold topcoat, providing excellent solderability and corrosion resistance.
- Immersion Tin (ImSn): A cost-effective lead-free finish that offers good solderability but requires careful handling due to tin whisker formation.
- Electroplated Nickel Gold (Ni/Au): A robust finish with excellent corrosion resistance and wire bonding capabilities.
- Hot Air Solder Leveling (HASL): A traditional finish involving a layer of solder applied over the exposed copper, suitable for simple boards.
- Electroplated Tin: A lead-free alternative to HASL, offering better planarity and surface flatness.
- Organic Surface Protectant (OSP): An environmentally friendly finish that provides solderability and protects against oxidation.
- Immersion Silver (ImAg): A finish with good conductivity and solderability, sensitive to storage conditions and forming silver sulfide.
- Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG): A complex finish with high reliability, suitable for advanced applications.
Figure 2: PCB treated with ENIG finish
Figure 3: PCB treated with HASL finish
Figure 4: PCB treated with ImAg finish
Key Guidelines for Surface Finish Selection
Choosing the appropriate surface finish for a printed circuit board (PCB) is a critical step influenced by several factors such as the intended application, soldering method (lead-free or lead-based), environmental conditions, signal integrity requirements, cost considerations, and regulatory compliance.
The main factors influencing surface finish selection include:
- The application and intended use of the printed circuit board. Consider whether the PCB will be integrated into a consumer product, automobile system, high-reliability aerospace application, or medical device. Different applications have distinct specifications regarding durability, thermal performance, and environmental resistance.
- Type of soldering: lead-free or lead-based. Regulatory standards such as the Restriction of Hazardous Substances (RoHS) directive prompt the adoption of lead-free soldering. Verify the compatibility of the selected surface finish with the soldering process employed—lead-free or lead-based. Ensure compliance with regulatory standards like RoHS and REACH regarding the use of restricted substances.
- Shelf life and storage requirements. Consider the PCB's shelf life and storage conditions. Certain surface finishes are more susceptible to the effects of storage conditions and may deteriorate over time, affecting their solderability.
- Environmental concerns. Several environmental factors, including humidity, chemicals, and extreme temperatures, can influence the selection of surface finish. Choose a finish that can endure the environmental conditions your equipment is likely to face.
- Reliability of solder joints. Assess the required level of solder joint reliability for the application. Certain surface finishes exhibit enhanced resilience to heat cycling and mechanical stress, making them suitable for applications exposed to severe environmental conditions.
- Signal integrity and impedance control. Surface finish can affect signal integrity and impedance management in high-frequency or high-speed applications. Select a coating that can fulfill the electrical performance specifications of the circuit.
- Compatibility with the assembly process. Different assembly procedures including reflow soldering, wave soldering, and wire bonding interact differently with various surface finishes. Make sure the chosen finish is compatible with your manufacturing processes.
- Cost and budget. Surface finish expenses include material, application, and associated assembly costs. Balance cost, performance, and reliability.
Comparative Analysis of Surface Finishes
Each surface finish has its own set of benefits and drawbacks, as well as best-suited use cases. Engineers and designers must select the appropriate surface treatment carefully, considering the specific requirements of their PCB designs.
Understanding the different surface finishes available and their pros and cons is essential. Below is a comparative analysis of commonly used surface treatments:
Type of Surface Finish
Advantages
Disadvantages
Organic Solderability Preservatives (OSP)
Simple process, economical, lead-free, and appropriate for fine-pitch (FPT) components
Limited shelf life and sensitivity to handling may disqualify products from harsh environments
Electroless Nickel Immersion Gold (ENIG)
Superior solderability, corrosion resistance, and flatness; suited for fine-pitch components
More complicated and expensive compared to some alternatives
Immersion Tin (ImSn)
Lead-free with a uniform surface and excellent solderability
Sensitive to handling, formation of tin whiskers, and unsuitable for multiple reflows
Electroplated Nickel Gold (Ni/Au)
Robust with excellent corrosion resistance, suitable for wire bonding
Costly and not optimal for lead-free soldering
Hot Air Solder Leveling (HASL)
Simple and cost-effective process, suitable for through-hole components
Not lead-free, may lead to uneven surface, not suitable for FPT components
Electroplated Tin
Lead-free, cost-effective, suitable for simple boards
Costly and not optimal for lead-free soldering
Organic Surface Protectant (OSP)
Lead-free, eco-friendly, and appropriate for fine-pitch components
Limited shelf life, handling sensitivity, unsuitable for harsh environments
Immersion Silver (ImAg)
Excellent conductivity, devoid of lead, and suitable for high-frequency applications
Sensitive to storage conditions and formation of silver sulfide
For more information, please visit lead-free hasl.
Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)
Superior dependability, advanced application suitability, and lead-free
Complex and costly procedure
Conclusion
Choosing the optimal PCB surface treatment is a crucial step in the PCB design process. By considering application, soldering process compatibility, reliability, environmental resilience, signal integrity, cost, and regulatory conformance, you can ensure the performance and dependability of your electronic devices. Keep in mind that every application is unique, so a thorough evaluation of your specific needs is required to make the best choice.
In today's rapidly evolving electronics industry, it is essential to remain abreast of emerging surface finish technologies and materials. Regularly evaluate your surface finish options to ensure they align with the most recent innovations and industry standards. A well-informed decision regarding PCB surface treatment will ultimately contribute to the success and durability of your electronic products.
07.11.2023
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Selecting the Right Surface Finish for Your PCB
Surface Finishes on Printed Circuit Boards
Choosing the right surface finish for your PCB is as important as choosing a manufacturer. Use the wrong surface finish, and you may face issues during assembly or longer-term reliability problems.
Surface finishes protect against oxidation and contamination. Without a surface finish, exposed copper oxidizes, making soldering difficult and leading to poor electrical connections.
Since all finishes have advantages and disadvantages, it is important to select the right one based on PCB design, storage, functionality, and assembly handling.
The finishes we offer are Lead-free Hot Air Surface Levelling (HAL), Electroless Nickel/Immersion Gold (ENIG), and Immersion Silver (ImAg). All are lead-free and can be used for RoHS designs as well as SnPb assembly.
It is recommended that edge connectors have an Electroplated Hard Gold (Ni/Au) surface finish.
HAL Lead-free (Hot Air Surface Levelling)
The Lead-free HAL (or HASL) finish provides the PCB with the highest level of solderability. It is robust for multi-step assembly and storage at a reasonable price. However, the HAL process requires submerging the PCB in liquid solder, causing additional thermal load.
Therefore, it isn’t the best choice for small via holes or boards with high aspect ratios. The surface is less flat than other finishes, making it less suitable for components with small and fine pitch pads.
Chemical Ni/Au (Electroless Nickel/Immersion Gold or ENIG)
Many customers choose this surface finish for its flat surface, good solderability, and acceptable shelf life. It has been a well-established surface finish even before Lead-free requirements. However, the ENIG process is complex with a higher risk of defects (skip plating, black pad, or interface embrittlement).
When using ENIG, the solder joint forms between the solder and the Ni layer. The Au is completely dissolved into the solder joint. This interface is more brittle than the HAL (SnCu) interface, not recommended for applications subject to shocks, bending, or strong vibrations. It is the most expensive surface finish but ideal for keypads or wire bonding.
Chemical Ni/Au Large Area (Electroless Nickel/Immersion Gold or ENIG)
Any copper area not covered by solder mask and ≥40% of the total copper area of an outer layer is classified as a large area, increasing the cost of the PCB.
Chemical Ni/Au before Soldermask (Electroless Nickel/Immersion Gold or ENIG)
If selected, the ENIG surface finish is applied before the solder mask, covering all copper with ENIG.
Chemical Ag (Immersion Silver)
This surface finish has mixed reviews. It offers a flat surface with very good solderability. The solder joint is created with the underlying Cu since the Ag dissolves during the soldering process. However, Immersion Silver is susceptible to sulfur dioxide (SO₂), which tarnishes the surface and creates an AgS₂ layer, affecting solderability.
To avoid tarnish, we pack the PCBs in silver-saving paper and hermetically sealed packing. In multi-step assembly, store partially assembled PCBs in a sulfur-free atmosphere. Depending on storage conditions, shelf life may be an issue.
Other Available Surface Finishes
Hard Gold for Edge Connectors
Edge connectors require abrasion-resistant pads, thus electroplated NiAu (Hard Gold) surface finish is recommended. This finish is processed in a specialized bath for connectors.
Carbon
Carbon combines high mechanical strength with good electrical conductivity, often substituting gold on contacts. It is printed directly on Cu and used for switch contacts, foil keyboards, and creating cross-over conductors. It withstands HAL and soldering processes without significant resistance change.
Summary Table
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