Estimated reading time: 5 minutes

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 

Page 1 of 2

• Next Page >