Over the past ten years composite coatings of electroless nickel-phosphorous 3.0-5.0 mm (120-200 m inches) and immersion gold 0.05-0.10 mm (2.0-4.0 m inches) have become established as preferred solderable surface finishes for high reliability applications involving complex circuit designs. Commonly referred to as ENIG, the electroless nickel immersion gold finish has gained market share due to its versatility in a wide range of component assembly methods including solder fusing, wave soldering, and wire bonding.

Table 1. Surface Finish Percent Market Share (TMRC Data).

The ENIG finish provides a highly solderable flat surface that does not tarnish or discolor. It has a long shelf-life, and the precious metal topcoat provides excellent electrical continuity. The nickel serves as a barrier against copper diffusion and prevents copper contamination of the solder during wave soldering and rework operations. The finish also provides EMI shielding properties to the assembly.

It is somewhat paradoxical that a finish initially selected for its high reliability has become the subject of great scrutiny over the last two years due to low levels of interconnection failures being discovered after final assembly. The failure mode is associated with a poorly formed joint at the solder/nickel interface. When the suspect joint is stressed, the connection is easily broken, leaving an open circuit.

In some cases, BGA assemblies for example, the interfacial failure has been attributed to an accumulation of the brittle gold-tin intermetallic at the interface causing joint fracture1. In others, the problem is that a true metallurgical bond never formed at the solder/nickel interface resulting in an inherently weak joint. When this latter failure is traced back to the ENIG finish, the nickel under the gold in the failure area is found to be black in color and a surface analysis typically shows abnormally high phosphorous concentrations. This led to an early theory that the problem was caused by high phosphorous co-deposition during electroless nickel plating.

Figure 1. "Black nickel" or "Black Pad".

However, subsequent investigations have shown that excess nickel corrosion during the immersion gold deposition causes this condition, now commonly referred to as "black nickel" or "black pad" (Fig.1). The black pad defect is particularly insidious since it is initially hidden by the immersion gold topcoat. The ensuing solderability failure is only discovered after assembly when parts have already been attached to the board. Apart from the dramatic increase in material costs involved at this stage, other issues such as lost time, missed schedules, and potential field failures add to the overall seriousness of the problem.

This has led to a great deal of effort being extended by suppliers, fabricators, assemblers, and OEMs, both independently and in conjunction with ITRI, to determine the root cause of the problem and eliminate it. ITRI undertook a major first round initiative to investigate the effects of operating parameters in the Electroless Nickel and the Immersion Gold processes on the occurrence of the problem2. In a second round, board design was also added as a variable. To date, the results of these investigations have failed to determine a single root cause, and it is now generally accepted that any factor that leads to an acceleration in the corrosive attack of the nickel during the immersion gold deposition increases the risk of black pad defects^3,4.

Major Factor Effects

Factors that have been shown to influence the rate of nickel corrosion include:

• The Structure of the Nickel Deposit
  
• The Phosphorous Content of the Nickel Deposit
  
• The Uniformity of the Nickel and Gold Coatings
  
• The Corrosion Rate of the Immersion Gold

Structure of the Nickel Deposit

Figure 2.

Figure 2 shows a typical surface structure of the electroless nickel deposit produced on a catalyzed copper PWB substrate. It is characterized by a spherical topography reminiscent of pebbles on a beach or the bubbly froth produced on the surface of liquids when they are aerated. Since it is well known that discontinuities in metal structures such as grain boundaries and crevices are typical initiation sites for corrosion, the size and shape of the spherical nickel domains are critical issues with respect to the accelerated corrosion phenomena during the immersion gold deposition.

Figure 3

Generally the more nodular the deposit, the deeper the crevices (Fig 3.) and the more tendency for excessive corrosion of the nickel surface. It is theorized that this may be due to the depletion of the gold during the immersion reaction in the deep inter-nodular crevices, setting up galvanic cells between the mismatched solution concentrations in the crevice and on the surface.

Phosphorus Content of the Nickel Deposit

The concentration of co-deposited phosphorus has a major effect on both the deposit structure and degree of corrosion resistance of the coating. At co-deposited phosphorous concentrations below 7% electroless nickel deposits are microcrystalline. Above 7% phosphorous, the structure becomes amorphous and deposits become more corrosion resistant. As the phosphorous content is increased from 7 to 12%, the corrosion resistance of the deposit increases due to increased natural passivity.

All electroless immersion processes, also referred to as displacement or replacement processes, depend on the oxidation of the less noble metal surface to supply electrons for the reduction of the more noble metal from solution. As such, all immersion processes can be considered as a combination of corrosion of the one metal with balanced reduction of another driven by the thermodynamic potential couple between them. The reaction continues until a uniform coating of the more noble metal is produced on the substrate surface, sealing off the corrosion sites and neutralizing the potential difference between them. In the case of electroless nickel/immersion gold, the nickel deposit is oxidized and dissolves into solution while the gold in solution is reduced to metal on the nickel surface.

In other words, the immersion gold is a controlled corrosion process and the balance between activity and passivity of the electroless nickel coating as determined by the phosphorous content is an important control tool in protecting against excessive corrosion while assuring the formation of a cohesive and protective gold deposit. From a black pad prevention standpoint, running at the parameters that will produce the highest phosphorus content without compromising gold coverage and adhesion is recommended.

Uniformity of Nickel and Gold Coatings

As described above, the uniformity of the redox reaction between the nickel and the gold is a primary control on the self-sealing solderability preserving gold coating. It is therefore particularly important that the nickel surfaces presented to the gold process are as uniform as possible. Any physical defects (for example edge effects such as skip plate) and/or irregular surface conditions (for example areas of lower or higher passivity) will tend to promote localized excess corrosion of the nickel increasing the risk of black pad.

Immersion Gold Corrosion Rate

As one would expect, the corrosion medium itself is an important factor in controlling the rate and uniformity of the immersion gold reaction. Conditions that promote an over-aggressive attack of the nickel must be recognized and avoided. Research and development efforts continue in this area to produce an immersion gold with built-in protection against over aggressive attack of the nickel.

Influence of Process Control

As highlighted above, good process control, coupled with consistency and repeatability, are important to manufacturing a defect-free ENIG coating.

The following section follows the process sequence, from material coming to the ENIG line through shipping, with emphasis on the role and importance of each step and its impact on the end product.

Incoming Material

Most manufacturers today use tin or tin/lead as an etch resist. This is eventually stripped after the circuitization process. It is important that the tin or tin/lead stripping is complete and that no copper-tin intermetallics are left which could interfere with subsequent processing. Good rinsing and drying of the stripper chemistry is important to avoid localized corrosion of the copper caused by stripper residues left on the surface.

The boards introduced to the ENIG line usually have soldermask already applied to the surface. Soldermask should be well developed and properly cured. Poor developing results in soldermask residues on the surface and improper curing increases the tendency for the mask to leach during processing.

Intermetallics, corroded copper surfaces, and soldermask residues can all result in non-uniform coatings and contribute to poor morphology of the plated nickel.

Pretreatment

Surface preparation using a combination of cleaner and microetch is a fundamental requirement to provide both a clean surface for the initial catalyzation of the copper and the correct morphology for the subsequent nickel and gold coatings.

Acid cleaners are commonly used to remove all traces of organic contaminants and light oxides from the copper surfaces to be plated. This step should be optimized according to vendor specifications. Temperature, chemical strength, and dwell times are all critical to ensure adequate cleaning. Agitation and loading factors must also be matched correctly. Good rinsing should follow this step.

The microetch step removes a surface layer of copper to produce a pristine and active copper surface for uniform catalyzation and a micro-roughened topography to promote good adhesion of nickel to copper. The degree of micro roughening will have a significant effect on the nickel topography.

Thorough rinsing following the microetch is required to avoid any dragover of oxidant, which has been shown to interfere with the reduction of palladium in the catalyst step.

Catalyst

Catalyzation of copper is required to initiate electroless nickel deposition. Palladium is the most common choice of catalyst, although ruthenium has also been used successfully. The reaction mechanism is that of a standard immersion or displacement reaction, resulting in a catalytic palladium metal coating on the copper surfaces. Neither the laminate nor soldermask surfaces are catalyzed. Uniformity of palladium coverage ensures simultaneous uniform nickel initiation across the circuitry. Non-uniform catalyzation due to contaminants will result in uneven nickel growth and undesirable nickel morphology. Poor catalyst uptake can cause skip plating in the nickel.

Good rinsing is required after catalyst to minimize drag-over of palladium, which will destabilize the nickel solution.

Electroless Nickel

Control of the nickel process is essential to produce a controlled and uniform growth of nickel with consistent phosphorus content.

Rate of deposition is a key element in controlling the structure and determining the resultant topography. To avoid deep intergranular crevices at the domain boundaries, a slower rate of deposition is recommended. This is achieved through the control of the pH and the temperature. The pH controller must be capable of maintaining the pH within I0.1. Since the solution is operated at relatively high temperatures (180-195°F; 82-90°C) and should be controlled within I2°F (I1°C) of optimum, the heating system must be capable of fast recovery especially after the introduction of the work rack. The solution requires continuous replenishment of the nickel and the reducing agent. Auto-dosing by using a calibrated controller is essential for proper maintenance of the solution. The objective here is to minimize fluctuation in the chemical concentration of the bath constituents throughout the deposition cycle and the life of the bath.

Stabilizers are essential to the control of an electroless nickel solution. Their primary function is to help maintain overall solution stability by preventing plate-out on the tank surfaces and spontaneous nickel reduction in the solution. They also help ensure uniformity of coating, and are particularly important in this application to promote good edge coverage. The control of the stabilizer and a method of measuring and monitoring its effectiveness must be part of the routine maintenance of an electroless nickel bath. Out of control stabilizer concentrations can result in skip plating and edge pull back at sharp edges or corners. Solution agitation increases the rate of diffusion of the stabilizer to the surface and must be properly matched to the stabilizer concentration.

Immersion Gold

Temperature of operation and gold concentration are key factors in controlling the rate and uniformity of immersion gold deposition. Too high a temperature results in uneven deposition and over aggressive attack of the nickel. Too low a temperature could shut down the immersion reaction.

Low gold concentration will result in slow immersion, uneven coverage and increased corrosion of the nickel surface before complete coverage.

Theoretically, an immersion reaction should be self-limiting, but experience has shown that deposition at a slower rate will continue, with an increased risk of detrimental local corrosion of nickel when dwell times are extended.

Dwell time should be only long enough to ensure sufficient coverage to preserve the solderability of the nickel surface and/or to meet the minimum thickness specification.

Quality checks instituted after the immersion gold process, such as tape testing and gold stripping, have proved useful in detecting the problem prior to shipment. These tests are targeted at areas of high density circuitry that are either known or projected to be prone to the problem. Test coupons and/or test areas on board with circuitry specifically designed to highlight any control problems are now being introduced as additional control tools.

Post Gold

Any contaminants, organic or inorganic, remaining on the gold surface will interfere with solderability. With the increased use of "no clean" fluxes, surface cleanliness is particularly important.

A final warm deionized water rinse followed by efficient drying is an absolute requirement. Horizontal conveyorized equipment has been used effectively to improve post rinsing and in some cases the boards are sent through a final conveyorized acid clean, rinse, and dry as a final step prior to packaging the boards for shipment.

Conclusion

As new board designs challenge the industry, new solutions in surface finishes will be needed to satisfy board assembly requirements. Today, ENIG fills a specific niche as a surface finish, particularly in high reliability applications involving complex circuit designs. With a good understanding of the critical parameters that influence solderability and solder joint reliability, and the implementation of effective process control procedures, it is expected that the electroless nickel/immersion gold finish will continue to be specified, and its use will grow as an important part of the final finishes portfolio.

Acknowledgements
The authors wish to acknowledge other members of the Shipley Ronal ENIG team: J. Moszczynski, M. Toben, M. Kanzler, E. Huenger, R. DeHoog, and G. Douglas, who have been directly involved with this effort over the last two years.
Special thanks to M. Toben and K.J. Whitlaw for their help in editing the article.