Search results

The use of an electroless nickel/immersion gold (ENIG) surface finish comes with the inherent potential risk of Black Pad failures that can cause fracture embrittlement at the interface between the solder and the metal pad. As yet, there is no conclusive agreed solution to effectively eliminate Black Pad failures. The case studies presented are intended to add to the understanding of the Black Pad failure mechanism and to identify both the plating and the subsequent assembly processes and conditions that can help to prevent the likelihood of Black Pad occurring.

Scanning electron microscope (SEM) analysis of exposed pad surfaces on failed PCBs demonstrated a “mud‐crack” appearance, which is a characteristic of the Black Pad phenomenon. In addition, energy dispersive X‐ray (EDX) analysis was used to identify the elemental composition of the fractured layer between the Ni₃P and Ni₃Sn₄ inter‐metallic compound, confirming the presence of Black Pad.

Grain boundaries or “mud‐cracks” that can be clearly seen in a top view of the failed pad surface and corrosion spikes in the failed pad surface, as evident from the cross‐section sample, should be used as a guideline to confirm Black Pad failures. Maintaining an optimum and well‐controlled EN and immersion gold bath, in addition to good process control prior to nickel‐gold deposition is recommended as the best approach for minimizing the occurrence of Black Pad failures.

Only Sn/Pb soldering processes using ENIG PCBs or package substrates were evaluated and discussed. Thus, the current case studies do not encompass Black Pad failures with lead‐free soldering.

The work reported provides guidelines that can be used to identify Black Pad occurrence. It also proposes relevant approaches for minimizing the possible occurrence of Black Pad.

The findings of these studies provide a basic understanding of the Black Pad failure mechanism. Subsequently, both the plating and the ensuing assembly processes and conditions that can help to prevent the likelihood of Black Pad occurrence were identified.

Nickel–gold planar surface coatings have been increasingly specified over the last ten years as the circuit board solderable finish of choice. Although offering a number of significant advantages over both conventional Hot Air Solder Levelled (HASL) finishes and alternative planar finishes, nickel–gold can, under certain conditions, be associated with a premature brittle interfacial solder joint fracture failure. This failure typically occurs at the interface of the nickel deposit and the intermetallic formed during soldering. The exposed nickel usually exhibits a “blackish” discolouration that has led to the term “black pad” being used to describe such failures. Although black pad usually occurs at very low levels, its incidence can be catastrophic and hence much work has been done by numerous workers to elucidate further the causes and mechanisms of this failure. This paper reviews the current understanding of the black pad failures and details work carried out by Shipley to extend this knowledge and to help users minimise the likelihood of its formation.

A problem exists with electroless nickel/immersion gold (E.Ni/I.Au) board surface finish on some pads, on some boards, that causes the solder joint to separate from the nickel surface, causing an open circuit. The solder joint cracks and separates when put under stress or when it experiences a shock. An ITRI (Interconnect Technology Research Institute) project to investigate this E.Ni/I.Au problem was initiated about a year‐and‐a‐half ago. Since the electroless nickel/immersion gold board finish performs satisfactorily most of the time, a 24 variable experiment was developed to investigate which parts of the chemical matrix are satisfactory to use and which need to be avoided. This paper describes some of the activities that have occurred on the ITRI consortium, from the design of the test vehicle to building hundreds of BGA assemblies, then pulling those BGA assemblies apart and inspecting the results.

The surface finish is an essential step in printed circuit boards design because it provides a solderable surface for electronic components. The purpose of this study to investigate the effects of different surface finishes during the soldering and ageing process.

The solder joints of Sn-4.0Ag-0.5Cu/Cu and Sn-4.0Ag-0.5Cu/electroless nickel/immersion silver (ENImAg) were investigated in terms of intermetallic (IMC) thickness, morphology and shear strength. The microstructure and compositions of solder joints are observed, and analyzed by using scanning electron microscopy (SEM-EDX) and optical microscope (OM).

Compounds of Cu₆Sn₅ and (Cu, Ni)₆Sn₅ IMC were formed in SAC405/Cu and SAC405/ENImAg, respectively, as-reflowed. When the sample was exposed to ageing, new layers of Cu₃Sn and (Ni, Cu)₃Sn₅ were observed at the interface. Analogous growth in the thickness of the IMC layer and increased grains size commensurate with ageing time. The results equally revealed an increase in shear strength of SAC405/ENImAg because of the thin layer of IMC and surface finish used compared to SAC405/Cu. Hence, a ductile fracture was observed at the bulk solder. Overall, the ENImAg surface finish showed excellent performance of solder joints than that of bare Cu.

The novel surface finish (ENImAg) has been developed and optimized. This alternative lead-free surface finish solved the challenges in electroless nickel/immersion gold and reduced cost without affecting the performance.

The aim of this paper is to detail the changes needed to ensure compatibility of printed‐circuit board (PCB) surface finishes with the use of lead‐free solders and in lead‐free assembly processes.

The paper describes the various popular solderable surface finishes that are currently available. It then reviews them in terms of the required adaptations necessary to meet the requirements of the Restriction of Hazardous Substance (RoHS) Directive and to ensure compliance, whilst meeting the performance needs of the product.

Some of the available and popular finishes, such as organic solderability preservatives and tin require modifications while, others including silver, direct immersion gold and electroless nickel immersion gold are transitioning well into the world of lead‐free. Electroless nickel, electroless palladium, immersion gold is one finish that performs better with lead‐free assembly than it did with conventional eutectic solder‐based approaches.

The paper provides a concise overview of the implications for specific surface finishes when making choices for use with lead‐free and RoHS compliant PCB soldering and assembly.

The purpose of this paper is to detail progress on the European Commission supported FP7 ASPIS project that is undertaking a multi‐faceted approach to develop novel and improved nickel‐gold (ENIG) solderable finish chemistries and processes in order to overcome issues such as “black pad” that are known to cause reliability issues.

The ASPIS project has four key and discrete approaches; research into “black pad” formation mechanisms, development of new aqueous chemical deposition methods, formulation of new processes based on ionic liquids and the development of prognostic screening tools to enable early prediction of reliability issues.

Key factors influencing “black pad” formation include immersion gold bath pH value, concentration of citrate and thickness of the immersion gold layer. In addition, copper substrate preparation is also important. Work to develop new metal deposition processes using ionic liquids has also been demonstrated and may provide a viable alternative to more conventional aqueous based chemistries, thereby enabling some of the conditions that lead to “black pad” to be avoided.

This paper summarises the work carried out in the first year of a three‐year project and so the outputs to date are relatively limited. The project is continuing for another two years, when further progress will be made. It is hoped to report this progress in a future update paper.

The ASPIS project has undertaken multiple approaches to the development of new high reliability nickel gold finishes and this combination of approaches should offer synergies over more discrete traditional methodologies. As well as undertaking a detailed analysis of the mechanisms causing reliability problems, radical new formulation and prognostic approaches are also being developed.

The purpose of this paper is to verify the principal conclusions, done during the implementation of FP7 ASPIS project objectives in fundamental research of ENIG‐related failures by investigating real problematic PCB samples of different suppliers.

SEM, EDS and XPS techniques were applied for morphology and composition studies of ENIG coatings of three PCB samples (A, B and C), while electrochemical measurements were used to determine the porosity of EN and IG layers.

The surface morphology analysis of the un‐soldered pads of PCB A disclosed the fact that the surface of substrate was not pre‐treated in a proper manner before EN deposition, which generated structural defects such as cracks and opening pores in the EN layer, which in turn could produce the voids in the solder layer during the soldering process. The results of PCB B analysis confirmed the authors' observation that Au layers deposited on EN substrate from IG solution contaminated with Cu ions are highly porous and loosely adhering to EN coating, which, in addition, undergoes serious corrosion damages and may be the principal reason for the black pad defect occurrence. High porosity of IG deposit and the presence of the intermediate layer between Au and Ni‐P, which was enriched in Cu and O, were the main reasons for the black pad issue in the case of PCB C.

The gained knowledge on the mechanism of ENIG‐related failures, which cause reliability problems in PCB manufacture, makes it possible to elaborate potential non‐destructive techniques for detecting ENIG problems.

Quality of the film set used for PCB manufacture and care in handling are of vital importance. Reference is made to the need for proper nomenclature and the stability, size and registration requirements. Production PCB films can be generated by reduction from oversize master artworks, or directly at one to one scale on a photo‐plotter. Plotted films; cut and strip master films; manual artwork; two colour artwork; single pad, multiple track sheet artworks; and hybrid artworks are discussed. Special attention is given to planes and solder resist films, photo‐reduction, phototools and film processing.

The purpose of this paper is to present a better understanding of nickel corrosion, also known as “black pad” during gold deposition of EN1G.

This paper presents an accumulation of personal experience and observations of the problem over a period of ten years. It incorporates the experience of the Global Trade Association connecting the electronic industries (IPC) plating committee as it set out to write the IPC electroless nickel‐immersion gold (ENIG) specification‐4552.

Understanding how corrosion occurs will go a long way in helping printed circuit board (PCB) manufacturers stay clear of the issue and make high quality ENIG finished PCBs.

The majority of the data presented has been substantiated.

The paper details how, by good understanding of the mechanism of formation of corrosion products, manufacturers can steer clear from the problem.

The purpose of this study is to address two kinds of printed circuit board (PCB) failures with electrolytic Ni/Au as the surface finish. One was the weak bondability of gold wires to Ni/Au pads and the other was “dull gold” and weak solder wettability, which both caused great loss for the PCB manufacturer.

The failure samples were studied and analyzed in terms of macro- and micro-morphology of the surface finish, its element composition and thickness by various characterization techniques, such as three-dimensional stereo microscope, scanning electron microscope, energy dispersive spectroscopy and X-ray fluorescence spectrum.

Then the causes of the two failures were both found to be the inadequate thickness of gold deposit and other surface finish defects, but these causes played different roles in either failure or the mechanisms differ. Finally, their failure mechanisms were discussed and corresponding countermeasures were put forward for prevention.

This study not only addresses a practical failure problem but also provides some clues to a better and further understanding of the effect of PCB process and management on its quality and reliability in manufacturing practice.

It sheds light on how the thickness and quality of surface finish affects its wire bonding and soldering performances.