Surface treatment

        A printed circuit board (PCB) surface finish has two main functions: To prevent the copper from oxidizing and to provide a solderable surface.  Some PCB surface finishes, however, are more popular and versatile than others. As you review your options, you’ll likely consider durability, cost, and the materials used within the finish, such as tin-lead, lead-free, various gold finishes, and silver. Additionally, you may have concerns about the finish compatibility with the subsequent termination process – such as wire bonding or welding. But how do you know which surface finish is best for your next PCB project?  We’ve compiled a comprehensive list of the nine different surface finishes. In this article, we’ll explain the purpose and common uses of each surface — plus the advantages and disadvantages — so you can find the best option for your next project.

         Electroless Nickel Immersion Gold (ENIG) ENIG is the most popular option and is used on about 80% of all PWBs. This finish provides a thin, gold, solderable layer that protects the copper traces with a nickel barrier between it and the copper. ENIG is a good lead-free option that results in a durable, long-lasting finish.  

         Advantages:  Protects copper from passivation Fast, easy process Lead-free Long-lasting and durable Restriction of Hazardous Substances (RoHS) compliant ;Reasonably low cost Very good solderability

         Hot Air Solder Leveling (HASL) With HASL, the printed circuit board is held vertically by clamps and dipped into a flux bath and then a molten solder hot vat. The finish, which contains tin/lead, is leveled off with hot air knives and the printed circuit board panel is raised out of the molten solder vat.  This ensures a homogenous thickness over the entire board. It was once one of the most popular finishes but is now mainly used only on military and aerospace applications.  

          Advantages:  Meets military requirements Low cost

          Disadvantages:  Cannot be used in High Density Interconnect (HDI) structures Extremely high temperatures can affect other materials in the PWB Process is messy, hot and smelly The resulting topography of the HASL finish is not flat enough for many very dense, high pin-count, tight-pitch components surfaces

         Lead-Free (HASL) The properties and uses of this finish are the same as HASL — except there is virtually no lead in the alloy. Instead, the alloy is a tin and copper, tin and nickel, or tin, copper, nickel and germanium alloy. This finish is not widely used.  

         Advantages:  Contains no Lead RoHS compliant

        Disadvantages:  Cannot be used in HDI applications May not meet military and aerospace requirements Possible formation of “tin whiskers” (thin conductive filaments that can cause short circuits and a variety of other issues)

        Immersion Tin: With this finish, a layer of tin is deposited onto the copper surface using a chemical process. This is frequently used in high-speed circuits. Immersion tin works well to protect underlying copper from oxidization, but copper and tin in such close proximity can create diffusion from one metal into the other, resulting in challenges.  

       Advantages:  No nickel Optimal for faster circuitry Works well to protect underlying copper from oxidization

       Disadvantages:  Not ideal for two-sided assembly Not ideal for long storage periods

       Organic Solderability Preservative (OSP)/Entek: A thin layer of material is applied over the copper to protect it prior to soldering. Though this water-based organic surface finish was once very popular, it is not widely used today.  

       Advantages:  Eliminates metal finishes Protects the copper for soldering Lead-free RoHS compliant

      Disadvantages:  Not commonly used Not a long shelf life

      Hard Gold: Hard gold is plated over the nickel, which holds up well and doesn't break down easily compared to other finishes. Because of its durability, this type of PCB plating is used in applications that involve mechanical contact, such as buttons and keypads.  

      Advantages:  Extremely durable Long-lasting Lead-free RoHS compliant

      Disadvantages:  Poor solderability Expensive

      Immersion Silver: Similar to tin, this PCB finish is used in high-speed signals that don't contain any nickel. Immersion silver, however, does require unique storage as it will tarnish if exposed to air. 

     Advantages:  Nickel-free Faster signal speed than tin RoHS compliant and environmentally friendly

     Disadvantages:  Becomes contaminated easily Not ideal for two-sided assembly More expensive Requires the use of sulfur-free paper Will tarnish if exposed to air Not good for long storage periods

     Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG): This three-layer metallic finish is used for printed circuit boards that also require wire bonding. The electroless nickel protects the copper and prevents it from coming into contact with the other metals. Palladium provides another anti-oxidation insurance layer and prevents the nickel from corroding, and the final gold layers adds more of a barrier against oxidation – plus it completely covers the PCB.  

     Advantages:  Provides a flat pad surface Allows more area to land wire as bond pads are flat Ideal replacement for soft bondable gold Palladium helps provide good solderability RoHS compliant Nickel corrosion – or  “black pad syndrome” – is not an issue

     Disadvantages:  Costs a little more than ENIG

     Soft Bondable Gold: This PCB finish has been used for a long time and has the same wire bonding process as ENEPIG. The pad is a crowned surface, which means there is less area to land wires.  

     Advantages:  Suitable for wire bonding RoHS compliant

     Disadvantages:  Difficult solderability Key Considerations