HASL vs. ENIG: Choosing the Right PCB Surface Finish
PCBs or printed circuit boards are a basic necessity in electronic equipment design and manufacturing. For PCBs to function flawlessly over their entire life span, a crucial decision lies in selecting the most suitable surface finish. Rush PCB Inc. offers numerous options, with two popular choices being HASL or Hot Air Solder Leveling, and ENIG or Electroless Nickel Immersion Gold. The basic aim of both methods is to prevent the exposed copper surface on the PCB from oxidation, thereby ensuring excellent solderability. In this article, we offer deep insights into the differences between HASL and ENIG and discuss in detail the advantages, disadvantages, and applications of the two methods.
What is HASL or Hot Air Solder Leveling?
HASL Process Overview
As a widely employed PCB surface finish, HASL is the acronym for Hot Air Solder Leveling. In this process, the operator coats the exposed copper surfaces, like traces and pads, on a PCB with a layer of liquid solder. They evenly distribute this solder layer using a sharp blast of hot air knives. This results in a surface finish that protects the copper from oxidation, at the same time ensuring good solderability. Based on the type of solder used, the HASL process is of two types:
Lead-Based HASL
Utilizing a solder alloy containing tin and lead, the lead-based HASL surface finish offers excellent solderability along with an extended shelf-life. However, the presence of lead raises health and environmental concerns.
Lead-Free HASL
This process employs a lead-free solder alloy composed of tin and copper, silver, or bismuth as a substitute for lead. Although compliant with the RoHS standards, the application of lead-free HASL surface finish requires higher processing temperatures and is more susceptible to oxidation.
Advantages of HASL Surface Finish
There are several advantages of using HASL as a surface finish. For instance, the process is simple, has low-cost implications, is readily available, has good visible inspectability, and is suitable for through-hole components.
- In comparison to other surface finish methods, HASL application is a straightforward process, and a wide range of PCB manufacturers can access it.
- As the cost implications of the HASL process are on the low side, it is a cost-effective surface finish solution, making it eminently suitable for budget-conscious projects.
- The elements lead and tin is widely available and accessible, allowing many PCB manufacturers to commonly offer the HASL surface finish.
- The HASL surface finish is bright and reflective, offering good visual inspectability for quality and coverage.
- Offering a robust coating, the HASL surface finish is eminently suitable for through-hole components. It is well-suited for assembly of through-hole components on a PCB, as it offers good wetting, and consequently, strong solder joints.
Disadvantages of HASL Surface Finish
The HASL surface finish can offer several disadvantages as well. For instance, the surface finish can exhibit unevenness, limitations of dimensional tolerances, incompatibility in wire bonding, and also suffer thermal stress.
- As the HASL method exhibits an uneven surface, it poses a severe challenge for SMT or Surface Mount Technology components, especially those with fine pitch.
- The inability to control and achieve a precise thickness of the HASL finish may cause problems with dimensional tolerances, especially when applied to very thin or thick PCBs. Similarly, HASL also cannot meet the tight tolerances that plated-through holes require. This has a serious effect on the overall quality of the PCB.
- Wire bonding applications and HASL finish are not compatible, and this limits the use of HASL finishes in certain electronic designs.
- HASL surface finish does not withstand high temperatures and can suffer damage during thermal processing. This limits its use for sensitive applications.
Applications of HASL
HASL is eminently suited for many applications that include:
Prototyping
The decent durability of HASL and its easy applicability leads to quick turnaround times preferable for prototyping PCBs.
Through-hole Electronics
The affordability and wettability of HASL make it suitable for mass-producing through-hole and budget consumer electronic devices.
Longevity
The adequate lifespan offered by HASL is suitable for products with short to medium lifespans, especially as they need not incur additional costs.
Hobby and Education
The ease of application and accessibility of HASL make it a suitable choice for PCBs fabricated for hobby and educational use.
What is ENIG or Electroless Nickel Immersion Gold?
ENIG Process Overview
ENIG, an acronym for Electroless Nickel Immersion Gold type of surface finish, has two layers of metallic coating. The operator primarily applies a base layer of electroless nickel and covers it with a fine layer of immersion gold. While the nickel layer functions as a barrier to protect the copper, it also provides a suitable surface for the gold to bond to. Apart from providing a solderable surface, the gold layer also helps in preventing oxidation.
Advantages of ENIG
There are several advantages of using ENIG as a surface finish. For instance, it is lead-free, has high conductivity, is durable, offers a flat surface, and has a good shelf life.
- As ENIG has no lead, it is intrinsically compliant with RoHS requirements, besides being environmentally friendly.
- As the top surface layer is gold, the electrical conductivity of the joint is excellent, enhancing the overall board performance.
- The process of electroless deposition of nickel and immersion gold ensures a flat and smooth surface, eminently suitable for precise placement of SMT components, especially those with fine pitch.
- Boards with a surface finish of ENIG offer exceptional resistance to physical wear and environmental stresses over long periods.
- PCBs with ENIG surface finish offer high solderability for longer periods, offering extended shelf life.
Disadvantages of ENIG
Disadvantages of ENIG include its higher cost, a more complex application process, difficulty of reworking, problems with signal integrity, and a risk of black pad formation.
- The materials and processing methods for ENIG raise its cost in comparison to HASL. For cost-sensitive projects, the higher cost can be a limiting factor.
- The complex nature of the ENIG surface finish on a PCB makes it more difficult to remove and replace components.
- Skin effect at high frequencies can cause signal loss in the thin gold layer of ENIG. This can impact high-frequency applications.
- A weak gold-nickel bonding in the ENIG surface finish can lead to the potential formation of the black pad. Such defects affect PCB reliability.
Applications of ENIG
ENIG is eminently suitable for many applications including:
Advanced Assembly
The flat and smooth finish of the ENIG surface is eminently suitable for the placement of delicate and tiny SMT components, especially those with fine pitch, making ENIG ideal for tightly spaced and high-density advanced assembly designs. This makes ENIG suitable for flexible circuits, RF and microwave circuits, and automotive electronics.
High-Temperature Capability
ENIG’s ability to withstand high-temperature processing allows it to be useful for lead-free soldering processes.
Durability
The long-term durability of the ENIG surface finish allows PCBs to work consistently over extended periods and are not subject to degradations.
Harsh Environments
The ENIG surface finish offers corrosion protection from harsh environments in mechanically and thermally demanding conditions. This ensures long-term performance for the board.
Difference Between HASL and ENIG
The following table summarizes the comparison between the two surface finishes, HASL and ENIG, and helps in the decision-making process:
| PARAMETER | HASL | ENIG |
| Copper Adhesion | Good, metallurgical bond | Good, Nickel barrier layer |
| Plating Thickness | Thick solder layer | Thin Nickel and Gold layers |
| Metal Coating Type | Tin-Lead or Tin-Silver-Copper | Nickel and Gold |
| Flatness | Uneven | Smooth |
| Electrical Conductivity | Regular | High |
| Soldering | Good, suitable for THC | Good, suitable for SMT |
| Compatibility | Not suitable for Fine Pitch SMT | Suitable for Fine Pitch, Hi-Density SMT |
| Thermal Stress | High risk of damage | Low risk of damage |
| Conditions of Use | Not suitable for harsh conditions | Withstands harsh environments |
| Eco-Friendliness | Leaded variant is not eco-friendly | Environmentally safe |
| Shelf-Life | Low, prone to oxidation | Longer, Gold prevents oxidation |
| Cost | Simple, cost-effective process | Involved process, expensive |
CAM Handling of HASL and ENIG Processes
During PCB manufacturing, the process requirements and details for HASL and ENIG surface finishes are different and require careful handling by CAM engineers.
HASL
- For the HASL process, the pad design is dependent on the solder flow and thickness. Therefore, CAM engineers must make pad sizes slightly larger to ensure solder will completely cover the pad.
- Additionally, pad hole size may reduce during the HASL process, and CAM engineers must compensate to achieve an appropriate hole size after the HASL operation. In this connection, the choice of solder type is crucial, as is the final thickness control of the molten solder.
- As thickness control in HASL is primarily by hot air leveling, CAM engineers must perform thermal distribution analysis during hot air leveling to ensure there is a minimum adverse effect on other parts of the PCB, while simultaneously paying special attention to surface flatness.
- For panel design in PCBs using the HASL process, CAM engineers must consider thermal stress distribution, to avoid board deformation or warping due to thermal expansion. Moreover, they must address solder overflow and deburring during edge design.
- For solder mask bridge design, CAM engineers must make the bridge wide enough to prevent solder from flowing under the bridge during leveling and causing shorts.
ENIG
- In the ENIG process, the CAM engineer must consider the uniformity and thickness of the gold layer, ensuring precise dimensions of the pad. As the gold layer does not significantly change the hole dimensions after the ENIG application, CAM engineers must define the hole diameter with high accuracy and tight tolerance.
- As ENIG uses chemical processes for nickel and gold plating, CAM engineers must ensure the files have the requisite listing for the appropriate chemical types and concentrations, along with strict control over the plating thickness for meeting corrosion and electrical performance requirements.
- Verification requires CAM engineers to conduct current distribution analysis to ensure uniform plating. They must also mark critical areas in the files to achieve a smooth gold layer.
- For panel design in PCBs using the ENIG process, CAM engineers must ensure uniform electroplating and good circulation of the plating solution to avoid trapped bubbles or uneven plating thickness.
- As there is no solder flow during the ENIG process, the solder mask bridge can be narrower. The bridge spacing can thus be tighter, thereby improving integration and PCB density.
Factors in Selecting the Right Surface Finish
As selecting the right surface finish for your project is critical to its long-term success, you must consider the specific requirements of your application. Major factors that require a careful assessment for a selection include:
- Desired shelf-life
- Solderability
- Dimensional tolerances
- Environment resilience
- Component compatibility
- Budgetary constraints
Broad considerations in deciding on the right surface finish can be:
HASL
A simple process option with built-in cost-effectiveness. It is suitable for projects with an emphasis on availability and cost savings. HASL is a good choice for applications that use through-hole components, require non-critical longevity, and are best for prototypes.
ENIG
An option for long shelf-life, excellent solderability, and oxidation resistance. The smooth thin coating of gold over nickel is eminently suitable for fine-pitch SMT components and wire bonding. ENIG offers reliability in harsh environmental conditions.
How to Choose the Best PCB Surface Finish?
If you are looking for the one major deciding factor between HASL and ENIG, it would be surface smoothness, as this decides the component compatibility, vital for your project needs.
Although HASL is cost-effective, the nature of the hot air solder leveling process does not offer the smooth surface necessary for advanced assembly such as that required by fine-pitch, closely packed SMT components.
On the other hand, ENIG offers the smooth flat surface vital for the assembly of fine-pitch SMT components, suitable for high-precision applications. In addition, the gold layer, apart from providing excellent solderability, also enhances the longevity and reliability of the board, ensuring it can endure harsh environmental conditions and repeated thermal cycling.
The second crucial factor in deciding between HASL and ENIG is compliance with industry standards. While HASL is effective for soldering, the presence of lead, in lead-based HASL, is non-compliant with RoHS regulations. Although lead-free HASL is compatible with RoHS standards, its limitations in dimensional tolerances may make it non-compliant with industry standards for applications requiring close dimensional tolerances. ENIG has no such limitations.
Compliance with environmental standards is another area where ENIG is superior to HASL. The two layers of nickel and gold in ENIG offer superior corrosion resistance, offering better long-term performance in applications that prioritize environmental compliance and sustainability. The medical industry and automotive electronics, therefore, prefer ENIG over HASL.
Conclusion
Whether to use HASL or ENIG surface finish depends on your specific application requirements and budgetary constraints. While HASL is suitable for many standard applications, as it is a cost-effective and widely available option, ENIG offers a reliable, flat, and durable finish, specifically suitable for long-term, high-density, and complex assembly. For most applications, ENIG may be a better choice despite its higher cost. To make an informed decision and to best meet your PCB design and manufacturing needs, it is necessary to understand the advantages and disadvantages of each surface finish.
FAQs
Which surface finish is better for PCBs?
The surface finish for a PCB depends on its intended application, environmental considerations, and budgetary constraints. You can use any surface finish as long as it meets your requirements, as each of them has its own advantages, suitable for different applications.
How thick should be the surface finish?
The thickness of the surface finish on a PCB varies depending on the specific requirement and the type of finish. For instance, the nickel layer in an ENIG finish is between 120 and 240 micro-inches, while the gold layer is typically between 3 and 8 micro-inches. The thickness of a HASL finish can vary depending on the design of the board and the temperature of the solder bath.
Is the ENIG surface finish worth it?
ENIG is necessary for projects using advanced assembly, especially for those using fine-pitched SMT components like BGAs. In addition, it offers excellent oxidation protection while providing a reliable soldering surface. Overall, ENIG is more expensive compared to simpler finishes like HASL.
Does ENIG have any disadvantages?
One of the disadvantages of ENIG is its higher cost, which may affect cost-sensitive projects. Another issue is the potential occurrence of black pad syndrome that affects solder joint reliability. ENIG is also not reworkable once it has been applied.
Why do people still use HASL surface finish?
A. The HASL process is simple and cost-effective. It involves dipping the board into a molten solder bath and then using hot air to remove the excess solder from the board’s surface. This results in a smooth and adequately even finish, mostly suitable for through-hole soldering.