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Pros and cons of the newest alternative finishes.

The choice of a "final finish" used to be simple: hot air solder leveling (HASL) or electrolytic nickel/gold. Technological improvements made necessary more sophisticated surface finishes. The main driver was the need for pads with better planarity, as well as finished pads for wire bonding and pressfit connectors. The surface finish is also called upon to serve as a contacting surface. Now legal influences (specifically RoHS) are driving board shops to replace HASL with an alternative as quickly as possible.

As boards to shrink in size, surface geography becomes more valuable, making bussing for electrolytic processes virtually impossible. This drove the industry to electroless processes. There is also a perceived environmental benefit when eliminating lead in the HASL process.

Recently, HASL was used on over 70% of boards in North America, but after July 1 it will not be permitted on products bound for Europe and other markets. Many options have been brought to market for coverage of the exposed copper surfaces. The industry has tried various organic solderability preservatives (OSPs); immersion tin; immersion silver; electroless nickel/immersion gold (ENIG); ENIG with an optional layer of thicker electroless gold (ENEG); electroless nickel/electroless palladium/immersion gold (ENEPIG); and a new gold direct on copper. Each has benefits and potential weaknesses.

OSPs come in thin and thicker versions, with high-temperature for Pb-free solders. These are relatively inexpensive and are easily applied but limited in the number of heat cycles that can be endured at assembly. Some OSPs require a nitrogen atmosphere at assembly. They are also not suitable for wire bonding or as a contacting surface. Concerns about solder voids with high-temperature OSPs are now being evaluated.

Immersion tin has shown early success as a solderable surface but may have a limited future. The main weaknesses are the use of the carcinogenic ingredient thiourea and the evidence of occasional whiskering, as well as intermetallic formation. Whiskers are especially a concern with fine lines and spaces where they could be knocked out of a hole during part insertion, thus increasing the possibility of a subsequent electrical short. A copper/tin intermetallic can form during deposition and continue to grow, limiting the useful shelf life of the stored parts. In this age of cost awareness, another factor that has limited this product's growth is cost; it is almost as expensive as ENIG but without the additional benefits.

Immersion silver seems to have a bright future. It is easy to apply to boards, relatively inexpensive and usually performs well. Like OSPs, thin (2 to 5 µ in.) and thicker (8 to12 µ in.) deposit versions have been sold but the preference seems to be toward thicker products. To prevent tarnishing, the processes have included anti-tarnish either as an ingredient within the silver bath or applied in a subsequent step. Current testing is looking for methods to provide complete coverage on the walls of through-holes and into blind vias. Other process concerns are the possible inclusion of voids in the solder joint, and the desire for better thickness uniformity per part. Some manufacturers have complained of corrosion of the copper surface near holes. If severe enough, this could lead to shorts. Another complaint was recently named the "Red Plague." This refers to a discolored surface from copper contamination on the silver surface.

ENIG has been growing steadily in use. It is the most expensive of the final finishes but offers the most benefits. This process also requires the most steps. Parts must be clean and have a smooth copper surface on which to build. The electroless nickel is an autocatalytic process that deposits nickel on the palladium-catalysed copper surface. The process requires continuous replenishment of the nickel ion and the reducing agent. Good process control (constituent concentration, temperature and pH) is the key to a consistent reproducible deposit. It is very important that the nickel be able to plate a surface with consistent phosphorus levels. Most prefer a middle range of 6 to 8% phosphorus - too low would easily corrode, but too high makes subsequent soldering of parts more difficult.

Immersion golds are replacement chemistries. This means that they attach themselves to the nickel by replacing atoms of nickel with atoms of gold. The purpose of the immersion gold layer is to protect the nickel surface until it is soldered. The recommended gold thickness is 2 to 4 µ in. As the purpose of the gold layer is to maintain the solderability of the nickel surface, it is necessary that it be thin (2 to 4 µ in. are preferred) and pore-free.

Immersion gold has encountered various issues. Early problems included:

• Background plating (speckled areas on the soldermask not intended to be plated) and bridging or "Ni foot" (extraneous plating between lines thus causing shorts).

• Incompatibility with soldermasks.

• Skip plating (where some pads are not plated with ENIG, though sometimes gold does adhere to the copper) often due to surface contamination on the copper or a static charge.

• Black nickel, such that the nickel surface has become passivated or corroded, even if mostly still covered by the immersion gold layer. Black nickel is hard to detect. It results from the combination of a compromised nickel deposit coupled with prolonged dwell in an aggressive immersion gold bath. The compromised nickel deposit may be due to tin or soldermask residues on the incoming copper surface, or a poorly maintained or poorly designed nickel bath. Black nickel is usually just the scapegoat (especially with untrained operators using Pb-free solders), or poor plating practice.

Direct gold over copper (also called DIG - direct immersion gold) was developed specifically for parts where nickel could create RF interference. The planarity is exceptional, again, depending on the quality of the copper surface. To avoid the inherent problems of copper migration through the thin gold surface, it is necessary for these parts to go to final assembly within four months. Even that period is only possible with a combined replacement/autocatalytic process. Conventional immersion gold will have trouble properly attaching to the copper surface and providing the necessary pore-free layer.

An option for boards intended to be gold wire-bonded is ENEG, a neutral pH, autocatalytic electroless gold that permits thicker gold deposits on top of ENIG without harming the soldermask. The more traditional, high-pH electroless gold is great for non-soldermasked parts.

Electroless palladiums have also been created but have not grown in use due to the higher, more volatile cost of palladium metal. ENEPIG is a very versatile surface finish with gold wire-bonding capabilities. Recent studies also show that ENEPIG may prove to be an ideal soldering surface for SAC alloys.

Not all chemistries are created equal, and sophistication demands wiser choices in specifying the process designed for the specific product's end use. Final finishes work well only on boards that have been properly manufactured prior to the final application.

Don Walsh is national sales manager, George Milad is national accounts manager for technology, and Don Gudeczauskas is technical director at Uyemura International (uyemura.com); dwalsh@uyemura.com.

Ed.: This article first appeared in Printed Circuit Design & Manufacture in February 2006, and is reprinted here with permission.

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