Citation
(2011), "ASPIS – a new multi-partner research project to enhance the performance of nickel-gold solderable finishes", Soldering & Surface Mount Technology, Vol. 23 No. 2. https://doi.org/10.1108/ssmt.2011.21923bab.003
Publisher
:Emerald Group Publishing Limited
Copyright © 2011, Emerald Group Publishing Limited
ASPIS – a new multi-partner research project to enhance the performance of nickel-gold solderable finishes
Article Type: Industry news From: Soldering & Surface Mount Technology, Volume 23, Issue 2
A new European research project has recently commenced in order to undertake a multi-faceted approach for developing novel and improved nickel-gold solderable finishes. The three-year project is known as ASPIS, which is an abbreviation of Advanced Surface Protection for Improved Reliability PCB Systems, and it is being supported via the European Commission’s seventh Framework programme under the FP7-SME AG-2008-2 call. It is a “research for SME associations” project that has 12 partners from across Europe, including four key research organisations who will undertake much of the research programme on behalf of the project’s SME members. There are two UK-based research providers, namely ITRI Ltd and the University of Leicester, along with TNO from The Netherlands and the Center for Physical Sciences and Technology in Lithuania. The other partners represent a wide cross-section of organisations from the PCB sector that cover the whole of the requisite industry supply chain. These organisations are Atotech GmbH, Graphic Plc, Merlin Circuits Technology Ltd, Scionix Ltd, Somacis S.p.a and Global Interconnect Services. In addition, the European Institute of Printed Circuits and the Institute of Circuit Technology (ICT) are partners in the project and they are undertaking the dissemination activities to bring the technology developed to the attention of the PCB industry in the UK, the rest of Europe and further afield. The project is being managed by the ICT.
The ASPIS project has a focus on nickel-gold (ENIG) solderable finishes for PCBs, and will develop new, more reliable materials and processes in order to address the key issues such as “black pad” that have been a cause of concern for many years to both PCB fabricators and end-users. One of the key factors impacting the overall reliability of electronic assemblies is the quality of the solder joints that connect components to a circuit board and a critical factor in determining this reliability is the solderable finish that is applied to the board. The de facto standard choice of PCB solderable finish for high reliability, high-value electronics is nickel-gold. This utilises an undercoat of electroless nickel which is deposited on to the copper of the PCB and on top of which is a thin coating of immersion-plated gold. Globally, the market share of ENIG in the PCB industry as a whole is estimated at 15 per cent by surface area of PCB manufactured. However, in terms of the value of the resulting products, the market share is considerably higher. For the EU’s electronics manufacturers, which are increasingly producing high value and high-reliability products, the importance of ENIG is much greater. ENIG has several key advantages over other types of PCB solderable finishes. These advantages are highly significant as, despite the higher price of ENIG over other alternatives, it is the preferred choice of finish for many applications. A key advantage of ENIG is that it offers excellent solderability and can retain this during prolonged storage prior to soldering assembly and during the multiple soldering operations encountered when assembling complex, high-value electronics. While it was predicted that ENIG’s market share would increase significantly after implementation of the restriction of certain hazardous substances directive (RoHS) in 2006, the expected increase has not materialised due to a combination of cost, the emergence of some lead-free HASL alternatives and because of a fear of ENIG-specific problems which can lead to disastrous yields and product failures if they occur. Another advantage of ENIG finishes is that they have excellent planarity and this is crucial when assembling and soldering small fine-pitched surface mount components. These are often of high value and placement accuracy is critical for achieving the required assembly yields. Unlike other coating technologies, which are applied solely to facilitate storage and solderability during soldering assembly, ENIG is a functional coating that utilises the solid-state properties of the nickel to stabilise the soldered interface, in effect, acting as a barrier layer to prevent the growth of tin-copper intermetallic compounds, which can embrittle the soldered joints during the service-life of the product. This greatly enhances the reliability of ENIG-coated PCB assemblies, especially during prolonged use at elevated temperatures.
However, while ENIG coatings have a good reputation for excellent solderability, there are a number of technical and economic factors that can cause problems for PCB fabricators and their customers. The most widely recognised of these is referred to as “black pad” which is thought to be attributable to excessive corrosion of the nickel coating during the subsequent immersion gold deposition process. Despite having been identified more than ten years ago, the mechanisms that cause the effect (and the contributing factors) are still poorly understood (Figure 1).
Moreover, no design rules, non-destructive testing methods, process optimisation/mitigation steps or viable alternatives currently exist. Other problematic failure mechanisms include insufficient gold coating thickness, gold coating porosity, nickel migration, excessive phosphorus levels in the nickel layer, poor plating quality, excessive coating stress in the nickel and general solderability degradation. When ENIG-related problems occur they usually affect an entire product design or batch and the problems are often only identified after assembly, during which expensive components have been soldered to the PCBs. Many ENIG-related problems are only identified later via field-failures and when this occurs, large sums of money are spent identifying and resolving the problem, usually by several members of the electronics supply chain. In some circumstances, especially if liabilities and compensation claims are pursued, the total cost of an ENIG-related problem could be orders of magnitude higher than the cost of the PCBs supplied by the fabricator. It is probable that any ENIG user who has been using the finish for a substantial length of time will have experienced some of these problems.
One of the ASPIS project partners, ITRI, has extensive experience in diagnosing and troubleshooting ENIG-related problems. Such problems are highly sensitive commercially and are virtually never revealed publicly due to the potential financial ramifications and loss of reputation. In one investigation, ITRI conducted a particular design of mobile phone was experiencing failure rates in excess of 50 per cent in post-assembly drop tests. Three PCB suppliers’ boards had been used for the product, of which two suffered high levels of failures. Laboratory testing showed that one of the suppliers’ boards was experiencing a “black pad” corrosion issue, whereas the other’s board was affected by excessive gold coating porosity which had led to oxidation of the underlying nickel and poor solderability (Figure 2). Another “black pad” problem occurred with two different avionics systems in a single year. One was an engine controller board and the other an auxiliary system board. Both problems required immediate servicing to replace potentially affected boards from aircraft and replacement stocks. Emergency manufacturing of replacements had to take place. As well as the direct costs of replacement and troubleshooting the problem, significant disruption was caused to flight operations, seriously impacting the end-users’ day-to-day operations. Discussions with large European electronics manufacturers have confirmed their concerns about nickel-gold finishes.
The ASPIS project aims to help both the European PCB fabrication and broader electronics industries by developing new chemical processes, as well as methods for predicting, avoiding and detecting ENIG-related issues. Currently, the failure mechanisms are not fully understood and, therefore, the factors which could contribute to them are not all known. This prevents fabricators and assemblers from properly optimising their processes and users from predicting or even identifying which PCBs might be affected or by which type of mode products have failed. The ASPIS project also aims to strengthen the European electronics industry supply chain, from material suppliers to end producers, by helping ENIG users and by offering enhanced coating technologies with similar solderability and reliability benefits. It is important to support ENIG users because PCB fabricators’ customers will continue to specify it for their products, regardless of whether new alternatives emerge. This will be due to familiarity, the lack of historical data of field reliability and because purchase specifications exist which cannot be changed without requalification; a long and expensive process. For these people, the fundamental knowledge developed in ASPIS will help them to identify and avoid design and process-related factors which can increase the risk of failures occurring. As ENIG will continue to be used on existing boards, which obviously cannot retroactively be subject to these precautions, the knowledge gained will also enable engineers to identify key areas to investigate in order to validate the quality of the ENIG-coated boards. The ASPIS project will also reduce the likelihood of ENIG-related problems occurring by improving the coating deposition technologies. Unlike current developments, which have largely been undertaken empirically, the fundamental knowledge developed will be used to intelligently target specific improvements. For example, one important area to be addressed will be the thickness of the gold deposited. Sufficient gold is required to provide a continuous non-porous film but, as this thickness increases, so does the cost and there can also be a negative impact on solder joint reliability.
Through specific work packages focussing on plating development, the ASPIS project aims to develop new or enhanced processes that will enable thinner coatings to be used, thus reducing the cost of using ENIG while offering longer term reliability enhancements. Another key goal is to develop methods for identifying potential problems related to the use of ENIG on PCBs and two key approaches will be investigated. The first will be to develop laboratory methods for identifying which failure mechanism caused a particular problem. The second method is to develop a non-destructive screening method that can be used at the PCB fabricator or electronics assembler in order to identify problems on boards before components are soldered onto them. Therefore, another key aim of the ASPIS project is to build a prototype instrument for demonstration and validation purposes. One way to prevent ENIG-related problems is to avoid them altogether by using different coating technologies, either via different deposition methods or by the use of alternative materials or structures. Specific work packages of the ASPIS project will investigate and develop alternative deposition methods from aqueous solution and ionic liquids, respectively. Most, though not all, of the methods for depositing different metals from aqueous solution are well established and are used for other applications. The novelty is applying them to this high reliability, functional, solderable coating technology. Plating from ionic liquids is a more recent development and is not yet used in the PCB fabrication industry. However, it is known that metal films deposited from ionic liquids can exhibit improved properties which may, for example, enable thinner coatings of gold to be used or even alternative barrier layers to be deposited.
The current lack of in-depth understanding of the nickel-gold process means that no design rules exist for avoiding problems such as “black pad,” nor are there standards in place that can be used to minimise the risks. The most widely applied industry standard, the IPC 6010 series, merely specifies a minimum coating thickness for the nickel and gold layers of 3.00 and 0.05 μm, respectively, for coatings on rigid boards. The knowledge, data and experiences gained and accumulated in ASPIS will be used to improve and expand the industry standards and purchasing specification requirements. Improved ENIG or alternative coating technologies will, likewise, require applicable standards in order to get industry acceptance.
The new information and methodologies being developed by the ASPIS project will help reduce the number of ENIG-related problems reaching production and also accelerate investigations and appropriate response times. This will reduce associated costs and increase customer confidence in the ENIG process. The development of an assembly line tool for identifying problematic PCBs will reduce lost production costs and lower the risk of field-failures and their potential consequences. The development of improved and alternative coatings technologies will also enable European PCB fabricators and their customers to produce more competitive products via both increased process and assembly yields at subsequent higher levels of quality in finished products. Reducing the quantities of nickel and gold used in the solderable coatings by improvement or replacement will also reduce the overall cost of the PCB finish and also help the European electronics industry to adopt more sustainable approaches to materials use.
Further information on the ASPIS project will soon be available from a dedicated web site and there will also be a wide range of dissemination activities throughout the life of the project. These will include ongoing updates in this journal. In the meantime, further information can be obtained directly from the project co-ordinator at: martingoosey@aol.com
Martin GooseyASPIS Project Coordinator