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

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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