Black Pad Research Articles

Identification and prevention of “Black Pad” in Sn/Pb soldering

PurposeThe 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.Design/methodology/approachScanning 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.FindingsGrain 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.Research limitations/implicationsOnly 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.Practical implicationsThe 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.Originality/valueThe 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.

The Black Pad with Graphite Impregnation for CMP

CMP has been practiced by polishing wafers with white polyurethane pads that contain pores. These pores may reduce the contact areas at the interface so the polishing rate is increased by enhanced pressure. Moreover, the pores also serve as the reservoir for storing excess slurry that can replenish the consumed abrasive particles at the polishing site from time to time. A new design of black pads has eliminated the need of built in pores. Instead, submicron graphite particles are impregnated in polyurethane matrix. The graphite particles can be plucked off at the polishing site so the contact pressure at the polishing site is increased as if pores are present. Furthermore, graphite particles are hydrophilic so slurry can be retained better than that in water repelling white pad. The wetting can also lubricate the polishing site and allow faster polishing of the wafer without damaging the local circuitry. This reports summarizes various features of the black pads including their CMP performance compared to conventional white pads.

A corrosion couple experiment reproducing the black pad phenomenon found after the electroless nickel immersion gold process

The “black pad” phenomenon, which rarely occurs in electrolessly plated nickel–phosphorous (Ni–P) films, was reproduced. This work reports that nodular variation of the P content induces potential differences large enough to drive galvanic corrosion when the film is exposed to an electrolyte containing gold cyanide. Subsequent corrosion couple experiments using Ni–P films with different P contents demonstrated that preferential corrosion occurred at the low P side, and the propensity of forming black pad increased with the difference in P content (ΔP).

Is “black pad” still an issue for ENIG?

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

A Study on the Mechanism of Black Pad Formation during Electroless Nickel Immersion Gold process

The “Black Pad” phenomenon referring to blackening of electrolessly plated nickel-phosphorous (Ni(P)) films after the immersion gold (IG) process was reproduced using pure chemicals for the first time. Chemical composition of the electrolyte most suitable for the black pad reproduction was proposed, and it was found that submicron scale nodular variation of the P content of the Ni(P) film induced potential differences large enough to drive galvanic corrosion when exposed to the electrolyte; gold cyanide solution in this case. Instead of conventional potential measurement techniques, corrosion couple experiments using Ni(P) films with different P content were performed to substantiate preferential corrosion, which occurred at the lower P side with the propensity of forming black pad getting more severe with the P content difference (ΔP) between films. Subsequent AFM analysis showed that variations of nodular shapes and curvatures among nodules led to submicron scale variations of the P content in the film, which could promote galvanic corrosion.

Characterization of Black Pad Defect on Electroless Nickel- Immersion Gold (ENIG) Plated Circuits

The advent of the RoHS legislation, which mandated lead-free solders, and the rise in popularity of the BGA package, have resulted in ENIG becoming a popular choice for surface finish due to its excellent long term solderability and surface planarity. One of the pitfalls of this surface finish is the possibility of creating a layer of overly elevated phosphorous content between the electroless nickel and immersion gold, known as black pad defect. Most of the available literature suggests black pad defect is caused by accelerated reduction (corrosion) of the nickel in the nickel- phosphorus (Ni-P) layer during the immersion gold step of the ENIG process. Black pad defect can manifest as corrosion spikes at the nodule boundaries in the Ni-P and can progress to an abnormally thick high phosphorus region at the top of the Ni-P. One failure mechanism associated with black pad defect is the brittle failure of well wetted solder joints from the underlying Ni-P layer due to the presence of the high phosphorus region. In severe cases, black pad defect can cause solderability issues and retard the formation of tin-nickel intermetallic preventing good wetting of the solder joint. We have had the opportunity to examine many different cases of black pad, from severe to mild, and have a large amount of knowledge to share. This paper will give the reader a basic understanding of how to recognize black pad and subsequently determine its severity.

ENEP – A Cost-Effective Alternative for High Reliability Soldering Applications

Electroless nickel / immersion gold (ENIG) has been recognized within the electronics packaging industry as a reliable surface finish for soldering applications. Unlike surface finishes, such as OSP, immersion silver or immersion tin, which produce a Cu-Sn intermetallic upon soldering, the nickel layer of ENIG acts as a barrier to minimize copper dissolution during the soldering application. This function is especially important for assemblies that require multiple reflow steps. However, if not properly controlled, excessive corrosion of the nickel by the immersion gold step can also cause the well documented “black pad” effect, often resulting in solder joint reliability issues, including brittle fracture. As an alternative to ENIG, an electroless pure palladium layer over the electroless nickel (ENEP) provides a solderable surface without the noted risk of damage to the underling nickel layer. By eliminating the corrosive attack of the gold bath, the final layer stack would be suitable for high reliability soldering involving both IC substrate and PWB applications. In addition, because of the current high price of gold, replacing it with a pure palladium deposit can offer measureable cost savings for the ENEP process in comparison to ENIG. The paper summarizes an in-depth study of the effect on solder joint reliability caused by replacing the immersion gold by an electroless pure palladium layer. The study includes evaluations for both Pb-free (Sn-3.0Ag-0.5Cu) and eutectic SnPb (63Sn-37Pb) soldering applications. Results of investigations include: (1) cold ball pull testing to evaluate solder joint integrity, (2) SEM examinations of the underlying nickel surface, (3) IMC examinations to quantify nickel thickness degradation after multiple solder reflow cycles and (4) surface wetting through solder spread examinations. The paper discusses the relatively simple surface finish that, if proven effective in large-scale fabrication, may offer measureable performance and cost benefits in comparison to the more traditional ENIG finish.

A New Species ofParatelmatobius(Amphibia, Anura, Leptodactylidae) from Atlantic Rain Forest of Southeastern Brazil

ABSTRACT. Anew species of Paratelmatobius is described from Parque das Neblinas at Serra do Mar in Bertioga Municipality, State of Sao Paulo, Brazil. The new species, included in the P. cardosoi species group, is diagnosed by its larger size among the species of the group; tympanum visible externally; vocal slits present in males; presence of tubercles on the upper eyelid; well developed tubercle at base of mandible; dorsolateral fold developed; large black spinous nuptial pad on thumb in males; first finger longer than second; belly, ventral surface of arm and forearm, and external margin of throat blotched with orange-reddish spots.

The root causes of the “Black pad” phenomenon and avoidance tactics

The “black pad” phenomenon of Ni-P electroless plating has been one of the serious electronics failures in the market although this plating is known to be a useful barrier coating against oxidation. To understand the mechanism of this phenomenon, the weak interfaces of Ni-P plating soldered with leaded and lead-free solders have been analyzed with the aid of transmission-electron microscopy. Two causes of degradation are described in this paper. One is the initial degraded Ni-P plating covered with gold thin plating. In this case, the surface of the Ni-P plating was locally corroded to form a sparse amorphous oxide structure beneath the gold plating. The other is the extreme interface reaction between the Ni-P plating and solders. Based on the findings, it was concluded that the quality of the Au/Ni-P plating and the process conditions of soldering are to be carefully controlled.

Origin of Surface Defects in PCB Final Finishes by the Electroless Nickel Immersion Gold Process

We suggest a unique mechanism for surface defect generation causing solder joint or bonding failures in printed circuit boards (PCBs). Surface defects can be defined as corroded holes or spikes of the Ni-P layers on the soldering or wire bonding pads of PCBs. The typical defects are the black pad or pinhole pad defects generated after final finishing by the electroless nickel immersion gold (ENIG) process. Once corroded voids or spikes are plentifully created in nickel/gold interfaces, the bonding strength of solder or wire bonding joints is reduced. Therefore, it is important to characterize the details of these surface defects. In this paper, the defect microstructures and the P content variation with the ENIG processes are investigated. The surface defect selectivity with pad size and pad connectivity is suggested based on the key findings of P content variation. An overall mechanism is proposed based on a mixed mode of concentration cell corrosion and galvanic cell corrosion. Based on these results, more reasonable root causes are suggested.

ENIG 표면처리 공정 및 특성에 관한 연구

Ni coating layers were formed using a newly developed electroless Ni plating solution. The properties of Ni coating layer such as internal stress, hardness, surface roughness, crystallinity, solderability and surface morphology were investigated using various tools. Results revealed that internal stress decreased with plating time and reached at 20 minutes of the plating time. Hardness increased with increasing P content and thickness. Surface roughness of the pad decreased with Ni and Ni/Au plating. Crystallinity decreased with increasing P content. Solderability based on wettability decreased with Ni and Ni/Au plating. Based on surface morphology, it is expected that Ni coating layer formed using a newly developed electroless Ni plating solution is lower than that formed using a commercial electroless Ni plating solution in possibility of black pad occurrence.

The root cause of black pad failure of solder joints with electroless Ni/Immersion gold plating

This paper reports on a study of the reaction of solder with the electrolessnickel with immersion gold (ENIG) plating system, and the resulting interfacial structures. Aforcused-ion beam (FIB) was used to polish the cross sections to reveal details of the microstructure of the ENIG-plated pad with and without soldering. High-speed pull testing of solder joints was performed to expose the pad surface. Results of scanning-electron microscopy/energy-dispersive x-ray analysis of the cross sections and fractured pad surfaces support the suggestion that black pad is the result of galvanic hyper-corosion of the plated electroless nickel by the gold plating bath. Criteria are proposed for diagnosing black pad of ENIG plating.

Failure mechanisms of lead-free chip scale package interconnections under fast mechanical loading

The reliability of chip scale package (CSP) components against mechanical shocks has been studied by employing statistical, fractographic, and microstructural research methods. The components having high tin (Sn0.2Ag0.4Cu) solder bumps were reflow soldered with the Sn3.8Ag0.7Cu (wt.%) solder paste on Ni(P)|Au- and organic solderability preservative (OSP)-coated multilayer printed wiring boards (PWBs), and the assemblies were subjected to the standard drop test procedure. The statistically significant difference in the reliability performance was observed: the components soldered on Cu|OSP were more reliable than those soldered on Ni(P)|Au. Solder interconnections on the Cu|OSP boards failed at the component side, where cracks propagated through the (Cu,Ni)6Sn5 reaction layer, whereas interconnections on the Ni(P)|Au boards failed at the PWB side exhibiting the brittle fracture known also as “black pad.” In the first failure mode, which is not normally observed in thermally cycled assemblies, cracks propagate along the intermetallic layers due to the strong strain-rate hardening of the solder interconnections in drop tests. Owing to strain-rate hardening, the stresses in the solder interconnections increase very rapidly in the corner regions of the interconnections above the fracture strength of the ternary (Cu,Ni)6Sn5 phase leading to intermetallic fracture. In addition, because of strain-rate hardening, the recrystallization of the as-soldered microstructure is hindered, and therefore the network of grain boundaries is not available in the bulk solder for cracks to propagate, as occurs during thermal cycling. In the black pad failure mode, cracks nucleate and propagate in the porous NiSnP layer between the columnar two-phase (Ni3P+Sn) layer and the (Cu,Ni)6Sn5 intermetallic layer. The fact that the Ni(P)|Au interconnections fail at the PWB side, even though higher stresses are generated on the component side, underlines the brittle nature of the reaction layer.

Kirkendall void formation in eutectic SnPb solder joints on bare Cu and its effect on joint reliability

The electronic packaging industry has been using electroless Ni(P)∕immersion Au as bonding pads for solder joints. Because of the persistence of the black pad defect, which is due to cracks in the pad surface, the industry is looking for a replacement of the Ni(P) plating. Several Cu-based candidates have been suggested, but most of them will lead to the direct contact of solder with Cu in soldering. The fast reaction of solder with Cu, especially during solid state aging, may be a concern for the solder joint reliability if the package will be used in a high temperature environment and is highly stressed. In this work, the reaction of eutectic SnPb solder with electrodeposited laminate Cu is studied. Emphasis is given to the evolution of the microstructure in the interfacial region during solid state aging and its effect on solder joint reliability. A large number of Kirkendall voids were observed at the interface between Cu3Sn and Cu. The void formation resulted in weak bonding between solder and Cu and led to brittle fracture at the interface in the ball shear and pull tests. The experimental results indicate that a barrier for Cu diffusion may be needed between the solder and the type of Cu used in the test vehicle for the packages that will experience high temperature (>100°C) and high stress.

Package to Board Interconnection Shear Strength (PBISS): Effect of Surface Finish, PWB Build-Up Layer and Chip Scale Package Structure

The quality of the interconnection in a fine pitch, area array chip scale package (CSP) is evaluated at the system level using a new test method, the package to board interconnection shear strength (PBISS) technique. The influence of printed wiring board (PWB) sample finish, build-up layer and package structure are quantified after surface mount assembly. Clear differences were evident in the shear strength and fracture location data indicating that the PBISS method is sensitive to the presence of black pad, weak build-up layer and package stiffness. The PBISS value of the CSPs with OSP/RCC-FR4 pad/build-up layer combination were measured to be 37 /spl plusmn/ 6 MPa and 38 /spl plusmn/ 2 MPa for the two different structures investigated. The PBISS method is demonstrated to be a viable candidate technique to quantify interconnection quality at the system level due to issues such as black pad, etc. and this method can help identify weaknesses in the interconnection chain for effective assessment of supplier or product quality.

Does living in streams with fish involve a cost of induced morphological defences?

Previous studies have shown that chemical cues from brook trout (Salvelinus fontinalis) induce relatively longer caudal filaments and heavier exoskeletons in the mayfly Drunella coloradensis. These characters constitute morphological defences that reduce larval mortality from brook trout predation. There is also a potential fitness cost of living in streams with trout, as D. coloradensis females emerge at smaller sizes from streams with fish compared with females in streams without fish. In this study, we obtained additional data to evaluate the hypothesis that these costs of living in streams with fish could be attributed to inducible defences. A field survey of seven different streams showed that mature (black wing pad) female larvae from streams with fish invested a smaller proportion of their body mass in eggs than females maturing in streams without fish. Furthermore, a negative relationship between female allocation to eggs and to morphological defence characters (relative length of the caudal filament) provides evidence of a cost of inducible defences in this species.

Factors influencing the formation of “black pad” in electroless nickel‐immersion gold solderable finishes—a processing perspective

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.

Black Pad Metallography in Electronics

Black Pad Metallography in Electronics

Electroless Ni/Immersion Au interconnects: Investigation of black pad in wire bonds and solder joints

Black Pad was observed on Electroless Ni/Immersion Au (ENIG) wire bond pads. Thick immersion Au on highly corroded electroless Ni was detected. It was determined that the pads were electrically connected to the Cu ground plane due to a Ni bridge formed inside normally open photovias. The mechanism of the bridge formation was verified and preventative actions were taken; it was demonstrated that formation of Black Pad could be switched on and off. The mechanism of Black Pad formation is proposed to be defective ENIG plating involving variation of both the electroless Ni and immersion Au plating processes. The intermetallic structures of solder joints on the above pads were studied. The study was conducted on both defective and non-defective pads to show differences in intermetallic structure and composition. Me2Sn4 and Me2Sn2 (Me=Cu, Ni, and Au) intermetallics were formed on non-defective pads, which nucleated on the Ni layer and grew inside the molten solder. However, only the Me3Sn intermetallic was formed on defective pads inside the corroded Ni Layer. Both mechanisms of intermetallic formation were found on pads with mildly corroded Ni and intermediate Au thickness (4.5–7 in).