OSP's come in thin and thicker versions. These are relatively inexpensive and easily applied but are limited in the number of heat cycles that can be tolerated in subsequent assembly. Some OSPs require a nitrogen atmosphere at assembly. They are also not suitable for wire bonding or as a contacting surface.
Immersion Tins have had some early success as a solderable surface but may have a limited future. The main weaknesses are the use of a carcinogenic ingredient (thiourea) and the evidence of occasional whiskering as well as intermetallic formation. Whiskers are especially a concern, specially with fine line and spaces, where such whiskers could be knocked out of a hole during part insertion, thus raising the possibility of a subsequent electrical short. Copper/Tin intermetallic forms during deposition and continues to grow thus 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 that it is almost as expensive as electroless nickel/immersion gold – without the additional benefits.
Immersion Silver is easy to apply to the boards, relatively inexpensive, and usually performs well. Like the OSP's, thin (2-5 μin) and thicker (8 – 12μin) deposit versions have been sold but the preference seems to be towards the thicker products. To prevent tarnishing, the processes have included an anti-tarnish 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 (holes of various sizes and depths that don't completely penetrate multi-layer boards). Other process concerns are the possible inclusion of voids in the solder joint, and better thickness uniformity per part. Some manufacturers have complained about issues with corrosion of the copper surface near holes. If severe enough, this could lead to shorts (thus failure of the board).
Electroless Nickel / Immersion Gold ("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 (Cleaner / Microetch, Activator--usually a palladium catalyst, Electroless nickel and Immersion gold). Parts must be clean and have a smooth copper surface on which to build.
The electroless nickel is an auto-catalytic 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 – 8% P, too low would easily corrode/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 such time as it is soldered to. The recommended gold thickness is 2 – 4 μin. As the purpose of the gold layer is to maintain the solderability of the nickel surface, it is necessary that it be thin (two to four microinches is preferred) and pore-free.
ENIG is by its nature a sophisticated chemical system that requires knowledgeable plating personnel and lab maintenance.
The most successful ENIG chemistries have several design strengths: low palladium/no chloride activators for solder mask compatibility and to avoid background and skip plating; a mid-phos nickel that runs at lower temperatures, doesn't need frequent dummy plating, holds its ability to perform even after repeated heat-ups, and gives level plating around the shoulders of pads and lines; an immersion gold that has gentle chemistry and is self limiting in thickness.
To enhance the performance of the nickels, sophisticated controllers are used to make additions and also provide history to the quality department. To maintain steady state in the nickel chemistry, the better controllers bleed in the ingredients in measured amounts versus major ads made periodically.
Newer combination immersion golds are being used where the first microinch is the traditional replacement/immersion type. The next microinch or two are attached through an autocatalytic procedure that allow gold to attach itself to gold in a non-replacing build-up. This gives rise to a non-corrosive bath that produces a pore free gold surface.
Direct gold over copper (also called DIG — “direct immersion gold” — although that’s somewhat of a misnomer as this is more than an immersion process) was developed specifically for parts where nickel could create RF interference. The planarity is exceptional – again dependent 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. Normal immersion golds will have trouble properly attaching to the copper surface and providing the necessary pore-free layer.
One last option has been developed for more sophisticated boards intended for gold wire bonding. A neutral pH, autocatalytic electroless gold is in use that allows thicker gold deposits on top of ENIG without harming the solder mask. The more traditional, high pH electroless gold is great for non-solder masked parts.
Electroless Palladiums have also been created but have not grown in use due to the higher, more volatile cost of palladium metal. The additional hardness benefits will provide some market use going forward as these chemistries are mastered and get accepted.
Constant change will continue to face all fields. The successful companies will be open to it while also, hopefully, making wise choices. Not all chemistries are created equal, and sophistication demands wiser choices in buying the process designed for the specific product's end use.