PCB Nickel-Gold Surface Failures: the Good, the Bad and the Ugly Seminar – not an ageing spaghetti western produced by Sergio Leone and starring Clint Eastwood but a state-of-the-art technical webinar on a significant industry issue, produced by Bob Willis and starring leading authorities in the field of printed circuit materials, manufacture, test and trouble-shooting: Nigel White from Atotech, Dennis Price from Merlin Circuit Technology, Dr Chris Hunt from the National Physical Laboratory and Professor Martin Goosey from the Institute of Circuit Technology.

Bob Willis put the topic into context with statistics from SMART Group web surveys indicating that, of the PCB defects reported by users, solderable finish faults accounted for more than 35% of the total. Then, with reference to IPC acceptability specifications IPC-A-600, IPC-A-610, the IPC Test Methods Manual, J-Standards and solderable surface finish standards IPC-4552, IPC-4553 and IPC-4554, he showed a series of photographs illustrating many examples of PCB assembly process problems including surface non-wetting, black tarry pads, solder finish lifting, nickel-gold tape-testing failures and BGA-package-to-PCB separation.

Willis then handed over to Nigel White, who gave an introduction to electroless-nickel-immersion-gold technology from the perspective of the process supplier, beginning by listing the attributes expected by the industry: multiple soldering capability for both tin-lead and lead-free processes, aluminium wire bondability, constant contact resistance for keypads and switches, planarity, a phosphorus content in the range 7-10%, and a 12-month shelf life. ENIG was a two-stage metal deposition process, where the nickel-phosphorus functioned as a diffusion barrier and added strength to plated-through holes and vias. The solder joint was actually formed as a tin-nickel intermetallic. The thin gold layer minimized the interaction of nickel with the environment and was dissolved in the solder. ENIG was a mature process, which had been used since the early 1990s. The reaction mechanism involved the deposition of a nickel-phosphorus on a palladium-sensitised copper surface by autocatalytic reduction, followed by the semi-autocatalytic immersion deposition of gold on the nickel surface. White described the process in detail, including some available options to overcome problems such as the entrapment of chemistry in partially plugged via holes, and some notes on process performance and deposit characteristics.

Concerns and limitations were the cost of precious metal, the potential brittleness of the tin-nickel intermetallic, corrosion of nickel by the immersion gold solution, the tight process control required and the unsuitability of ENIG for gold wire-bonding applications. But on the positive side, ENIG was suitable for multiple lead-free soldering, it had planarity to suit surface mount component placement requirements, and the nickel barrier layer prevented dissolution of copper by solder. It also exhibited good shelf life, good resistance to corrosive environments, presented a good surface for ICT probing and contact switching applications and was suitable for aluminium wire-bonding

Nigel White having described ENIG from the supplier side, Dennis Price gave a user view based on his many years of practical experience at Merlin Circuit Technology in controlling and optimising the performance of the process. He emphasised the importance of proper copper surface preparation prior to the application of solder mask: Merlin's preferred method was low-pressure pumice scrubbing with nylon-bristle brushes. Price gave a word of caution regarding solder mask adhesion failure: in the past, many fabricators had attempted to overcome major adhesion problems by initially only part-curing the solder mask to retain flexibility during ENIG processing, with apparent success but with the consequence that sulphur-containing compounds were leached out and contaminated the nickel bath.

 Price went through Merlin's 23-step process sequence stage-by-stage, with hints, tips and observations, stressing the relevance and importance of meaningful laboratory analysis, data recording and statistical process control. One of the main causes of skip plating was, as Nigel White had commented, entrapment of micro-etch solution in partially-closed via holes, which could bleed out and poison the palladium activator on local features. Extraneous nickel deposits could result from wicking of activator into the cut ends of glass yarns in non-plated holes or the activation of trace residues of un-etched copper metal in the laminate surface. Although it could appear attractive to run electroless nickel baths for many metal turnovers to reduce process costs, it had been shown that the increased concentration of contaminants and by-products could result in nickel deposits which were more prone to surface hyper-corrosion during the plating of immersion gold.

Price also underlined the importance of topping-up of evaporation losses by little-and-often additions, and advocated the use of an automatic controller for the electroless nickel plating bath, which could continuously monitor nickel concentration and made additions of nickel and hypophosphite replenishers in small increments to maintain the chemistry within close working limits with a minimum of operator dependence.

After the comprehensive chemistry lessons came a presentation on PCB failure analysis techniques from Chris Hunt, who described how defects were identified and failure mechanisms studied and interpreted, using the wide range of test and inspection techniques available at NPL, including optical, x-ray and scanning electron microscope examination, microsectioning, x-ray fluorescence spectroscopy, shear testing and hot pull testing, illustrated with many examples. He gave details of the Defect Database on printed board assembly and material issues, managed by the NPL Electronics Interconnection Group as a service to the electronics industry. Any visitor to the database could add defect information and photographs from his own observations, and have free access to the continuously increasing catalogue of reference data and images to assist in implementing corrective actions in processes or designs.

Next to speak was Martin Goosey, who was coordinating a three year EC Framework 7 project known as ASPIS - Advanced Surface Protection for Improved Reliability PCB Systems. The ASPIS project aimed to develop new, more reliable materials, processes and testing procedures in order to address the key issues associated with ENIG finishes. The project had adopted a number of technical approaches ranging from an in-depth study of the fundamental mechanisms that influence the metal deposition and subsequent reliability, to the development of new chemical processes based on both traditional aqueous chemistry and new ionic liquids. In addition, work was being carried out to develop better methods for identifying failure mechanisms and a non-destructive prognostic screening tool that could be used to give warning of latent problems on printed circuit boards in advance of components being soldered on to them. Progress to date included the demonstration of nickel and gold deposition from novel ionic liquid-based chemistries, and a more detailed understanding of the key factors influencing the formation of "black pad" defects.

Bob Willis gave the final presentation "BGA Joint Inspection Using Dye & Pry Testing – How to Do It Yourself". The dye-and-pry technique was useful as a means of detecting subtle open circuits caused by wetting problems on pad surfaces or flexing of the BGA or the PCB during reflow. On ENIG-finish PCBs, as the solder paste fused it would dissolve the gold and apparently wet the pad, but could still fail to form a sound intermetallic bond to the nickel, and this type of defect was very difficult to detect by traditional methods of optical or x-ray inspection or electrical test. Dye penetrants had been used for many years in the testing of welded joints and castings prior to destructive analysis, and Willis explained how they could be used as the basis of a low-cost alternative test to quickly and straightforwardly reveal the location of a failure in a BGA assembly. In practical terms, the procedure was to impregnate the suspect area with a proprietary dye, using vacuum assistance if possible, dry it in an oven then pry off the component with a chisel and examine the remains of the joints for evidence of dye penetration. As usual, he provided an abundance of pictorial illustrations, and explained in detail how to interpret and record the results.

The Good, the Bad and the Ugly Seminar proved to be an informative and educational event. Three hours is a long time to hold peoples' attention but Bob Willis did a very professional job of maintaining the continuity of the programme and moderating the real-time question-and-answer session, which generated some highly interactive dialogue and exchange of ideas and experiences.

Pete Starkey

I-Connect007