Flex Talk: Final Surface Finish—How Do You Choose?

There are so many final surface finish options to choose from today. How do you decide which is best? HASL—both tin-lead and lead-free—immersion tin, immersion silver, ENIG, OSP, and ENIPIG are the primary finishes used in PCB fabrication. Fabricators and assemblers generally work with the majority of these surface finishes to support their customers’ requirements. So the question is, with all of these available, how do OEMs select their preferred surface finish?

In the past, the primary function of the surface finish was to protect the copper from oxidation prior to the soldering of components. Today’s expectations also include: superior solderability, contact performance, wire bondability, corrosion and thermal resistance, and extended end use life. Designs have changed. Lines and spaces are reduced, solder types and flux chemistries are different due to no-lead requirements, the number of assembly cycles has increased, and the product may need to carry highfrequency signals.

Things to think about when selecting a final surface finish: 

  • Does the application require tin-lead or lead-free assembly?
  • Will the end environment have extreme temperatures or humidity concerns?
  • What shelf life is needed? Will it be months or years?
  • Volume and throughput
  • Does the design have fine-pitch components?
  • How many assembly cycles will be required?
  • Is this an RF or high-frequency application?
  • Will probe-ability be required for testing?
  • Is thermal resistance required?

Once the project requirements have been identified, the surface finish options can be reviewed to find the best fit.

HASL—Hot Air Solder Leveling

Let’s start with HASL. Fifteen or 20 years ago, HASL was the universal go-to surface finish. Today, that is not at all the case. A couple of things greatly influenced this change. The first was RoHS and lead-free requirements. The second is miniaturization and the need for tightpitch components. HASL is blown from the PCB surface to remove excess; this can create uneven coverage, which makes placement of these tight-pitch components difficult at assembly.

This finish is used in aerospace, defense and high-performance electronics as well as lowerend consumer markets.

Things to keep in mind: 

  • The oldest surface finish
  • Tin-lead and lead-free versions are available
  • Tin-lead HASL currently in limited use due to RoHS and WEEE initiatives – Currently exempt: industrial vehicles, military, aerospace and defense, high-performance electronics
  • Leaded versions are harder to source
  • Long shelf life
  • Not suited for fine pitch 

OSP—Organic Solderability Preservative

OSP is the highest volume surface finish worldwide, with applications spanning data/telecom, automotive and both low-end and high-end consumer products. Older versions of this chemistry were not thermally resistant and were not able to resist more than one reflow cycle. Improvements have been made to allow higher temperatures and multiple reflows without degrading. This finish does well as a selective finish. For example, when ENIG is applied as a surface finish and OSP is used selectively, it will not adhere to or stain any of the gold surfaces, so there is no need to plasma clean.

Things to keep in mind: 

  • Highest volume surface finish worldwide
  • Applications range from low end to high-frequency server boards; also used in selective finishing
  • The latest versions are copper selective and more thermally resistant for high-temp, no-lead applications
  • OSP is applied through chemical absorption on the copper surface; there is no metal-to-metal displacement
  • Inexpensive surface finish
  • Limited shelf life

Immersion Tin

Applications for immersion tin are predominantly in automotive, U.S. military and aerospace. One caution at the assembly level is the fact that pure tin thickness is lost to the copper intermetallic with time and temperature. Loss of pure tin will degrade solder performance. The first reflow exposure will dramatically reduce the pure tin thickness and deposit stress could result in tin whiskers. This is a naturally occurring characteristic of tin in direct contact with copper.

Things to keep in mind: 

  • Applications are predominately automotive, U.S. military and aerospace
  • Excellent for press-fit applications (i.e., large back panels)
  • All contain anti-whiskering additives, but tin whisker elimination is not guaranteed
  • Low-cost, flat and suited for fine-pitch use
  • Aggressive on soldermask 

Immersion Silver

Immersion silver is well-suited for high-frequency applications. It has the greatest conductivity of all the surface finishes and it is flat. The signal travels to the top of the circuit reducing signal loss. This finish is often used in the data/telecom, automotive, high- and low-end consumer and medical markets.

Things to keep in mind: 

  • Greatest conductivity of all the surface finishes; well-suited for high-frequency applications
  • Applications range from low-end to high-reliability product
  • Topcoats have been formulated to overcome tarnish and corrosion issues in aggressive environments
  • Flat—suited for fine-pitch with excellent solderability
  • Easily scratched; sliding connector limitations
  • Microvoiding is something to be aware of with soldermask defined pads

ENIG—Electroless Nickel/Immersion Gold

ENIG has become one of the most common surface finishes and is often seen in aerospace and defense, medical, and other high performance markets. It is also predominant in the flex market. While this process requires many processing steps and numerous chemical analyses, fabricators run this process day in and day out with very little issue.

Things to keep in mind: 

  • Applications associated with high reliability
  • Used often in the flex market
  • High corrosion resistance due to nickel barrier
  • Aluminum wire bondable
  • No degradation between reflow cycles; can be held mid-assembly for extended times
  • Potential for nickel corrosion (aka black pad) if time in gold bath is excessive 

ENEPIG—Electroless Nickel/Electroless Palladium/Immersion Gold

ENEPIG is the new kid on the block. A significant advantage to this finish is that it is gold wire bondable. Typical applications are in the medical and the U.S. military markets. This finish is expensive to process and is still relatively low-volume in the market. Fabricators are slowly bringing this process in-house as volume makes outsourcing less economical.

Things to keep in mind: 

  • Gold and aluminum wire bonding
  • Applications include medical and U.S. military
  • Excellent solderability
  • Mitigation of black pad
  • Gaining interest and acceptance in the market 

There are many factors to consider when selecting a final surface finish and unfortunately, there is not a universal finish that works best for all applications. Understanding the advantages and disadvantages of each surface finish allows the designer to select the surface finish that best fits each application. Chemistry suppliers, fabricators and assemblers are all happy to offer suggestions based on their experience, take advantage of the resources available if you have questions or need assistance. PCB

Tara Dunn is the president of Omni PCB. To read past columns or to contact her, click here

Editor's Note: This article originally appeared in the January 2017 issue of The PCB Magazine.



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