Difficulty of choice is a problem faced by all the people of the world. Unlike some products where a ‘both-and’ approach can be applied, PCB surface finish requires an ‘either-or’ decision, impacting soldering quality and circuit performance directly. Among the array of technologies, HASL and ENIG stand out with their exceptional performance. However, the question remains – which one should I choose? In addressing this predicament, TechSparks will provide unique insights in this article, comparing aspects such as cost, lifespan, and environmental suitability to guide you in selecting the surface finish technology that best suits your project.
HASL, which stands for Hot Air Solder Leveling, is a traditional surface finish technique. Its principle involves depositing a thin layer of tin on the copper surface of the PCB through chemical processing. Subsequently, hot air is applied to solidify the tin, creating a uniform tin layer that acts as a barrier to the air. With changing demands, there are currently two types of HASL: HASL and Lead-Free HASL. Lead-Free HASL is considered more advantageous, both for people and the environment.
ENIG, or Electroless Nickel Immersion Gold, is an alternative to electroplated gold. It involves depositing a layer of nickel and gold on the copper surface of the PCB through a chemical process to isolate it from the air. However, as nickel is an oxygen-sensitive metal, ENIG may exhibit black pads. Fortunately, for higher precision PCB, an upgraded version of ENIG called ENEPIG can be used, which introduces a palladium layer on between the nickel and gold layers to enhance the overall performance of ENIG.
Similar to other items, the lifespan of surface finishes gradually reaches its limit over time, and the duration is closely related to storage conditions. Therefore, for some overseas projects, vacuum preservation is often used for transportation. In terms of lifespan, HASL performs admirably compared to other technologies under normal usage conditions. However, it still falls slightly short when compared to ENIG.
ENIG utilizes pure gold on its surface, which possesses strong chemical stability, making it almost immune to oxidation or corrosion under normal circumstances. Among various surface finish technologies, ENIG’s lifespan is second only to ENEPIG, reaching 3-5 years.
In contrast, HASL’s surface employs tin, a metal that gradually oxidizes over time when exposed to air, forming tin oxide. This phenomenon becomes more pronounced in high-temperature or humid environments. Additionally, for some high-power applications generating significant heat during operation, without proper heat dissipation design, the accumulated heat on the PCB surface may cause defects such as warping in the HASL coating under thermal stress.
Based on the comparison of ENIG and HASL in terms of lifespan, it can be inferred that ENIG is more suitable for applications with higher reliability and lifespan requirements, such as medical, military, and aerospace, while HASL is effective for general electronic applications. Furthermore, when considering lifespan, it’s important to note that HASL allows for rework. When the HASL coating is damaged (oxidized, scratched), the old coating can be removed and replaced with a new one using a hot air gun and soldering equipment, a process that is challenging with ENIG.
Surface Smoothness Comparison
In PCB surface finish, surface smoothness is a crucial factor influencing solderability. After HASL treatment, a uniform layer of tin can be formed on the copper surface, but it may appear less smooth, and even slightly rough for applications with high density and fine traces. In contrast, ENIG provides an extremely uniform and smooth metal surface, suitable for any precision circuit. However, it is essential to note that ENIG is more challenging to operate and may exhibit the ‘Pavlov’s Effect,’ causing pitting or small holes during gold deposition on nickel.
High-temperature processing in HASL may lead to thermal stress on the PCB, affecting surface smoothness.
Coating uniformity largely depends on the melting and hot air leveling processes.
The chemical reaction in ENIG involves factors like time, temperature, and concentration, impacting the uniformity of deposition and being challenging to control.
The quality of cleaning directly affects surface smoothness.
Price is typically a crucial deciding factor. In the previous comparisons, ENIG excelled over HASL in many aspects, but its higher cost has impacted its market share. Let’s explore why ENIG is more expensive, primarily due to the following reasons:
Firstly, raw material costs play a significant role. ENIG requires the use of gold, nickel, and various chemical solutions, while HASL primarily uses tin and flux. Gold, being a non-renewable resource, experiences significant price fluctuations and is relatively expensive.
Secondly, process complexity also influences the price. In modern electronics manufacturing, automated production is predominant, and ENIG involves more steps, requiring additional equipment and a higher level of process control. This increases the investment costs for manufacturers.
Moreover, the complexity of the ENIG process requires more time and manpower. In highly automated manufacturing environments, this may lead to a decrease in production efficiency, thereby increasing the manufacturing cost per unit.
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