Remember the good ole days when hot air solder level was the go-to surface finish for almost all applications? The decision about surface finish was an easy one. The primary function of the surface finish was to protect the copper from oxidation prior to assembly. Wow, have things changed! Today’s expectations include: superior solderability; contact performance; wire bondability; corrosion and thermal resistance; extended end-use life; and of course, all at a low cost.
Common surface finishes now include HASL, both leaded and lead-free, OSP, immersion tin, immersion silver, ENIG and ENEPIG. Unfortunately, there is no one-size-fits-all surface finish that fulfills all the requirements in the industry; the decision really depends on your specific application and design.
Recently, Elizabeth Foradori and I sat down with Robyn Hanson from MacDermid Electronic Solutions to learn about the key considerations for final surface finish choice and the cautions of each from the OEM or assembly perspective. To listen to the discussion, click here. For a concise list of the pros and cons of each finish, click here. Following are some of the highlights.
Considerations for Surface Finish Choice: Does the application require lead or lead-free assembly? Will the end environment have extreme temperatures or humidity concerns? What shelf life is needed, and will it be months or years? Does the design have fine-pitch components? Is this an RF or high-frequency application? Will probe-ability be required for testing? Is thermal resistance or shock and drop resistance required?
Once these questions are answered, the surface finish options can be reviewed to find the best fit.
HASL—Hot Air Solder Leveling:
- The oldest surface finish
- Lead and Lead-free versions are available
- Leaded 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
HASL is blown from the PCB surface to remove excess solder; this can create non-uniform coverage which makes component placement of tight pitch components difficult. The hot temperatures of lead-free HASL can cause warpage and soldermask embrittlement. The plated-through-hole may be plugged or reduced.
OSP—Organic Solderability Preservative:
- Highest volume surface finish, worldwide
- Applications range from low end to high-frequency server boards, also used in selective finishing
- Latest versions are copper selective and more thermally resistant for high-temperature, no-lead applications
- Applied through chemical absorption on the copper surface; no metal-to-metal displacement
- Inexpensive surface finish
- Limited shelf life
OSP does have implications at the assembly level. Older versions of this finish are not thermally resistant and couldn’t resist more than one reflow. The coating hardens with reflow exposure and becomes more difficult to solder. Material transfers onto the probe tip (during electrical test) can result in false readings and will require more frequent probe maintenance or a special probe style. Higher OSP thicknesses are detrimental to solder paste flow and hole fill.
Immersion Tin:
- 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
- Aggressive on soldermask
Cautions at the assembly level include 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.
Immersion Silver:
- 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
The predominant issue seen at the OEM level is micro-voiding. Small voids occurring at the intermetallic layer of the solder joint could cause solder joint fracture. This defect manifests itself preferentially on solder mask defined pads which are more difficult to develop properly.
ENIG—Electroless Nickel Immersion Gold:
- Highest revenue surface finish
- Applications associated with high reliability
- Used often in the flex market
- Aluminum wire-bondable
- No degradation between reflow cycles, can be held mid-assembly for extended times
- New deposit thickness specifications for gold are under review to address the high cost of gold and hyper corrosion/black pad issues with extended dwell times for the gold
This chemistry requires tight process control. Proper plating conditions and control over the entire process are critical to performance. Proper chemical add-backs and numerous chemical analyses are required during start up and during plating. Layer thickness is also critical. Low nickel thickness will result in poor corrosion and thermal resistance in end use. Low gold thickness will result in less resistance to thermal conditioning during assembly and high gold thickness can promote nickel corrosion or black pad. Too much or not enough metal area in the plating bath will affect plating performance.
ENEPIG—Electroless Nickel Electroless Palladium Immersion Gold:
- 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
The primary caution at the assembly level is palladium thickness. Palladium that is too thick reduces the solderability performance. This will be slower to wet and have potentially palladium-rich areas in the solder joint. Palladium does not readily solubilize into the solder joint like silver or gold.
Surface Finish Breakdown by Market Sector:
- Automotive: Silver, OSP, immersion tin
- Data/Telecom: silver, OSP, ENIG
- High end consumer: ENIG, silver, OSP
- Low End Consumer: HASL, OSP
- Aerospace, defense and high-performance electronics: HASL, immersion tin, ENIG, ENEPIG
- Medical: ENIG, ENEPIG, silver
Regardless of whether your application is automotive, medical or military, there are many factors to consider when selecting a final surface finish. Cost, lead or lead-free requirements, end environment, shelf life, fine-pitch components, RF applications, probe-ability, thermal resistance and shock and drop resistance, to name a few. There is not a one-size-fits-all finish. Understanding the advantages and disadvantages of each surface finish allows the designer to select the finish that best fits each particular application.
Please contact us with any questions or for additional information! www.omnipcb.com
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