Surface finish is a critical component in PCB design and functionality. Surface finishes form the interface between the component and the circuitry. As its most essential function, the final finish process is intended to provide exposed copper circuitry with a protective coating to preserve solderability. Select surface finishes are also used for wire bonding or as an electrical contacting surface. Perhaps no other step in the PCB manufacturing process has undergone more change in the era of surface mount manufacturing than the final finish chemical process.
Electrical signals are transmitted through copper circuitry connecting the different components throughout the finished board. Copper is a highly conductive metal but one of its shortcomings is that it oxidizes when exposed to ambient air. An oxidized copper surface will not solder and is a poor conductor with no possibility of wire-bonding. Surface finish is applied to ensure no exposed copper is in the final product. Soldering, wire bonding, and contacting are all made with the finished surface.
Today there is a wide variety of finishes that board designers can specify to meet the desired functionality and intended use of their circuit boards:
- A reliable solder joint with leaded and lead-free solder interface
- Bonding surface Al, Au, Cu, Cu/Pd
- Contacting surface insertion
- A finish that will not impede RF signal transmission
When soldering of through-holes was the only desired attribute, three SFs dominated in manufacturing facilities. These were hot air solder leveling (HASL) and reflowed tin/lead for surface contacting (insertion) and electrolytic tab plating of nickel/gold as needed. As solder mask over bare copper (SMOBC) made its debut, reflowed tin/lead fell out of favor.
In the next generation (lighter, smaller, and faster), two major manufacturing developments dominated PCB designs, namely surface mount technology (SMT) and ball grid array (BGA) to meet the requirements of newer designs.
SMT and BGA created challenges at assembly. Coplanarity was an absolute must. HASL formed a meniscus that interfered with the application of solder paste on surface mount pads.
This requirement made organic solderability preservative (OSP) and electroless nickel/immersion gold (ENIG) come to the forefront for these applications.
The next major evolution in PWB manufacturing was the elimination of lead from solder. A new generation of lead-free (LF) solder—tin/silver/copper (SAC) alloys—were the prevailing replacement for tin/lead. The SAC family of alloys have a melting point of 217–219oC, with a peak liquidus temperature of 240oC for complete wetting and for forming a consistent intermetallic compound (IMC), as compared to tin/lead which averaged 187oC for a melting point and a peak soldering temperature of 215oC.
To accommodate the elimination of lead from solder, equipment makers made design changes to accommodate the higher reflow temperature of LF alloys. Although HASL only provides a soldering surface, it remains a viable surface finish today for products that have adequate spacing between pads and do not require contacting or bonding. In the same way, OSP suppliers were able to produce the next generation, namely OSP-HT (high temperature). OSP and OSP-HT are widely used worldwide.
For designs that require high temperature soldering and coplanarity, immersion silver and immersion tin filled that need. However, neither could meet all the demands of solder joint reliability and long shelf life. Silver is susceptible to tarnishing and creep corrosion, and tin requires a thick immersion coating to retain its solderability. Over time, copper will diffuse into the immersion tin, forming a non-solderable Cu/Sn IMC. Both immersion silver and immersion tin remain viable surface finishes with allowances made to overcome their shortcomings.
ENIG, although it is a more complex and more costly process compared to immersion silver and immersion tin, was successful in filling the need for a surface finish that is solderable with LF solder, aluminum wire bondable, and a good contacting surface with an extended shelf life. ENIG had a challenging start when it was first introduced; there were incidents of nickel corrosion under the immersion gold. The corrosion, if excessive, would interfere with IMC formation and the affected part would fail to form a reliable solder joint.
IPC ENIG Specification 4552 Rev B, issued in 2021, spelled out a method to evaluate and measure the extent of ENIG nickel corrosion. Now that there is a method to measure and quantify nickel corrosion, the defect is on its way to being eliminated. “You can’t fix what you can’t measure.”
ENIG remained a popular finish for parts that required its attributes. The next challenge was the need for an additional attribute and that was gold (Au) wire bonding. ENIG is not Au wire bondable because of the possible diffusion of nickel into the thin immersion gold layer. A diffusion barrier was needed to prevent the nickel from reaching the surface. Electroless palladium was the answer and ENEPIG (electroless nickel/electroless palladium/immersion gold) was the right finish when the desired attributes included Au wire bonding.
As military and aerospace boards continued to advance into RF signal propagation, there was a need for a new class of finishes that did not include electroless nickel. RF signals travel along the surface of the trace and the presence of electroless nickel would interfere with high frequency (RF) signal propagation.
One way to achieve this was to dramatically reduce the thickness of the nickel layer in ENEPIG to below 0.1 micron. Other finishes, including EPIG or EPAG (electroless palladium/immersion gold or electroless palladium/autocatalytic gold), eliminated the nickel completely. These finishes relied on catalyzing the copper surface with immersion palladium; a more advanced system to achieve the same goal is using immersion gold to catalyze the copper surface. IGEPIG (immersion gold/electroless palladium/immersion gold) was shown to provide a more reliable solder joint.
Still another option was to also eliminate the palladium and immerse gold directly on copper, immersion gold (DIG). A new development in DIG is reduction-assisted immersion gold. RAIG deposits a thicker gold layer (6-8 µins) that prevents the diffusion of copper into the gold wire bonding surface.
Newer and advanced surface finishes are constantly being researched, tested, and implemented. It is important to keep in mind that all the SFs mentioned above are presently used in board fabrication. More advanced finishes are beginning to make headway as the higher technology boards continue to use high RF signals in their designs. Stay tuned.
This column originally appeared in the October 2022 issue of PCB007 Magazine.
