Electroless Palladium Immersion Gold Research Articles

Evaluation of the Bondability of the Epoxy-Enhanced Sn-58Bi Solder with ENIG and ENEPIG Surface Finishes

The effect of different surface finishes, electroless nickel immersion gold (ENIG) and electroless nickel electroless palladium immersion gold (ENEPIG), on the mechanical properties of Sn-58Bi bumps made with solder paste enhanced with epoxy were investigated. The microstructure and fracture surfaces were observed with scanning electron microscopy, and the compositions of the IMC and solder were measured using energy dispersive spectrometry and an electron probe micro-analyzer (EPMA). To evaluate the mechanical properties, low-speed shear tests and board-level drop tests were performed. The result of the shear tests showed that the bonding strength of the epoxy-enhanced Sn-58Bi solder bumps was higher than that of Sn-58Bi solder for all surface finishes, because of the epoxy surrounding the solder, and the fracture surfaces of epoxy-enhanced Sn-58Bi indicated ductile fracture in the solder joint. However, the result of the drop tests showed that samples with the ENIG and ENEPIG surface finishes had lower drop numbers compared to the sample without these surface finishes. The lower performance resulted from insufficient ejection of epoxy from the ENIG and ENEPIG surface finishes during reflow, which reduced the interfacial bonding area.

Effect of Electromigration on the Type of Drop Failure of Sn–3.0Ag–0.5Cu Solder Joints in PBGA Packages

Board-level drop tests of plastic ball grid array (PBGA) packages were performed in accordance with the Joint Electron Devices Engineering Council standard to investigate the effect of electromigration (EM) on the drop reliability of Sn–3.0Ag–0.5Cu solder joints with two substrate surface finishes, organic solderability preservative (OSP) and electroless nickel electroless palladium immersion gold (ENEPIG). In the as-soldered state, drop failures occurred at the substrate sides only, with cracks propagating within the interfacial intermetallic compound (IMC) layer for OSP solder joints and along the IMC/Ni–P interface for ENEPIG solder joints. The drop lifetime of OSP solder joints was approximately twice that of ENEPIG joints. EM had an important effect on crack formation and drop lifetime of the PBGA solder joints. ENEPIG solder joints performed better in drop reliability tests after EM, that is, the drop lifetime of ENEPIG joints decreased by 43% whereas that of OSP solder joints decreased by 91%, compared with the as-soldered cases. The more serious polarity effect, i.e., excessive growth of the interfacial IMC at the anode, was responsible for the sharper decrease in drop lifetime. The different types of drop failure of PBGA solder joints before and after EM, including the position of initiation and the propagation path of cracks, are discussed on the basis of the growth behavior of interfacial IMC.

Solder Size Effect on Early Stage Interfacial Intermetallic Compound Evolution in Wetting Reaction of Sn3.0Ag0.5Cu/ENEPIG Joints

Solder size effect on early stage interfacial intermetallic compound (IMC) evolution in wetting reaction between Sn–3.0Ag–0.5Cu solder balls and electroless nickel electroless palladium immersion gold (ENEPIG) pads at 250 °C was investigated. The interfacial IMCs transformed from initial needle- and rod-type (Cu,Ni)6Sn5 to dodecahedron-type (Cu,Ni)6Sn5 and then to needle-type (Ni,Cu)3Sn4 at the early interfacial reaction stage. Moreover, these IMC transformations occurred earlier in the smaller solder joints, where the decreasing rate of Cu concentration was faster due to the Cu consumption by the formation of interfacial (Cu,Ni)6Sn5. On thermodynamics, the decrease of Cu concentration in liquid solder changed the phase equilibrium at the interface and thus resulted in the evolution of interfacial IMCs; on kinetics, larger solder joints had sufficient Cu flux toward the interface to feed the (Cu,Ni)6Sn5 growth in contrast to smaller solder joints, thus resulted in the delayed IMC transformation and the formation of larger dodecahedron-type (Cu,Ni)6Sn5 grains. In smaller solders, no spalling but the consumption of (Cu,Ni)6Sn5 grains by the formation of (Ni,Cu)3Sn4 grains occurred where smaller discrete (Cu,Ni)6Sn5 grains formed at the interface.

Interfacial reaction and intermetallic compound formation of Sn–1Ag/ENIG and Sn–1Ag/ENEPIG solder joints

Interfacial reactions and intermetallic compound (IMC) formation of Sn–1wt.%Ag solder with two different surface finishes, electroless nickel–immersion gold (ENIG) and electroless nickel–electroless palladium–immersion gold (ENEPIG), were evaluated during a reflow process. We first compared the interfacial reactions of the two solder joints and identified a relationship between the interfacial reaction behavior and solder/substrate compatibility. The Sn–1Ag/ENIG joint exhibited stable IMC growth and superior interfacial stability compared with the Sn–1Ag/ENEPIG joint. The growth rate of the interfacial IMC layer and consumption rate of the Ni(P) layer were closely related to the interfacial microstructure and morphology of the IMCs. The layer-type well-attached interfacial IMCs had advantages with respect to IMC growth rate and Ni(P) consumption rate. Compared to the previous Sn–3.0Ag–0.5Cu study, ENEPIG performed very well with Sn–Ag–Cu solder alloy. On the other hand, ENEPIG proved not to be a suitable surface finish for Sn–Ag solder alloy in this study.

Interfacial reaction and mechanical properties between low melting temperature Sn–58Bi solder and various surface finishes during reflow reactions

Interfacial reactions and joint mechanical reliability of a representative low melting temperature Sn–58 wt%Bi solder with three different surface finishes were evaluated during a reflow process. The surface materials consisted of organic solderability preservative (OSP), electroless nickel–immersion gold (ENIG), and electroless nickel–electroless palladium–immersion gold (ENEPIG). The relationships between the interfacial reaction, surface finish, shear speed, and shear force were elucidated in this study. The interfacial intermetallic compounds (IMCs) on the ENEPIG substrate were sequentially changed during reflowing at 180 °C in the following order: (Pd,Ni)Sn4, (Pd,Ni)Sn4 + Ni3Sn4, and Ni3Sn4. Compared to the Cu6Sn5 IMC on the OSP-finished Cu substrate, the Ni3Sn4 IMCs on the ENIG and ENEPIG substrates were much thinner. The IMCs of each of the different surface finishes grew as the reflow time increased; however, the morphologies of the IMCs were different each other. Layer-type interfacial IMCs created a stable metallurgical interfacial structure during the reflow reaction, resulting in reduced rates of IMC growth and Ni–P consumption. High-speed shear tests were conducted to evaluate the effects of interfacial reactions on the mechanical properties of three Sn–58Bi solder joints. In spite of the different interfacial reactions and IMC formations, there were no large differences among shear forces in the three Sn–58Bi solder joints. Further, the type of surface finish material did not significantly affect the shear force of the Sn–58Bi solder joints under high-speed shear loading.

Stress relaxation and failure behavior of Sn–3.0Ag–0.5Cu flip-chip solder bumps undergoing electromigration

Abstract

Review of Capabilities of the ENEPIG Surface Finish

Surface finishes are used to protect exposed copper metallization in printed circuit boards from oxidation and to provide a solderable surface on which to mount electronic components. While it is true that some people have called electroless nickel electroless palladium immersion gold (ENEPIG) a “universal finish” for a wide range of applications from wire bonding to solder interconnects, this paper provides a review of the current literature on ENEPIG and assesses its overall capabilities compared to other surface finishes. Gaps in understanding the performance of ENEPIG as a printed wiring board surface finish are identified and further testing is recommended.

무전해 니켈·팔라듐·금도금 표면처리 공정의 도금 번짐 불량 및 개선

무전해 니켈 팔라듐 금 표면처리 공정의 도금 번짐 불량의 근본적인 원인을 이해하고 이를 해결하기 위한 방법을 제시하였다. 이에 계산화학을 이용하여 공정을 정성적으로 설명하고 이를 바탕으로 가정을 검증하기 위한 실험을 계획하였다. 도금 번짐으로 발전되는 고분자 레진 위의 팔라듐 시드의 핵 생성을 막는 것에 초점을 맞추어 고분자 레진과 화학적으로 결합력이 약한 <TEX>$PdCl_2$</TEX> 팔라듐 촉매를 도입하였으며 이 촉매가 수용액 중에서 추가적인 가수분해 반응으로 더 안정한 <TEX>$Pd(OH)_2$</TEX>를 형성함으로써 고분자 레진 위에 팔라듐 시드의 원천으로 작용하지 않도록 염산(HCl)의 농도를 높이거나 팔라듐 활성화 공정의 온도를 낮추어 보았다. 계산화학은 매 단계 실험의 이론적인 근거를 제시해 주었으며 실험 결과를 해석하는 데 큰 도움을 주었다. 이와 같이 실험과 이론을 접목시킨 본 연구의 접근법은 관련 공정에서 매우 유용하게 활용될 수 있을 것으로 기대된다. The running blots between patterns during electroless nickel electroless palladium immersion gold (ENEPIG) surface finish of printed circuit board (PCB) are investigated and a proper solution is presented. Computational chemistry is first employed to understand the process and experiments are then designed to verify the proposed ideas. A <TEX>$PdCl_2$</TEX> activator which has relatively weak chemical bonding to the epoxy resin is introduced to prevent the formation of palladium seeds on the epoxy resin and a couple of operational measures such as increasing HCl concentration and lowering the temperature of Pd activation process are executed to prevent a further hydrolysis of <TEX>$PdCl_2$</TEX> to more stable <TEX>$Pd(OH)_2$</TEX> in aqueous solution. Computational chemistry provides thermodynamic backgrounds for experiments and their results. This combined approach is expected to be very useful in the research of relevant processes.

High-Speed Shear Test for Low Alpha Sn-1.0%Ag-0.5%Cu (SAC-105) Solder Ball of Sub-100-$\mu{\rm m}$ Dimension for Wafer Level Packaging

The issue of soft error in microelectronics packaging has necessitated the development of low alpha activity solders, as solders are found to be one of the major sources of radiation in electronic devices that causes soft error. Low alpha ray emitter Sn-1.0Ag-0.5Cu (SAC) solders were prepared and their alpha activity was measured using ultra-low background alpha particle counting systems. The solders are confirmed to be low alpha solders having activity less than 0.005 α h-1 cm-2. High-speed shear test is performed to assess the strength of low alpha SAC solder balls of sub-100-μm diameter for different pad finishes. Two types of fluxes, one rosin mildly activated (RMA) and the other water soluble (WS) types are used during soldering to investigate the effect of flux on solder joint strength. Fracture energy has proved to be better index than maximum shear force in interpreting the test results as the former correlates with the fracture mode better. The performance of WS flux is better than RMA flux across all pad finish and speed conditions and among the pad finish electroless nickel electroless palladium immersion gold (ENEPIG) is superior to electroless nickel immersion gold, which in turn is better than organic solder preservative. Clearly the best choice is ENEPIG pad finish with WS flux. Intermetallic compound composition and morphology has been found to have significant effect on strength of solder joint. The shear strength of low alpha SAC105 is found to be lower than the high-Ag containing SAC305 and SAC405 solders. However, the effect of removing radioactive impurities from SAC105 on its shear strength and fracture energy remains inconclusive and needs further investigations.

Experimental study of bump void formation according to process conditions

Experimental studies of void formation were performed using a solder paste with a small particle size and a thin substrate with a small solder resist opening (SRO) size. Two kinds of Sn–Ag–Cu solder paste, screen printing mask opening (MO) size and pad finishes, and three kinds of SRO sizes were used as process variables. In this study, paste type and MO size had little influence on the formation of voids. In general, fewer voids were formed using the electroless nickel electroless palladium immersion gold (ENEPIG) pad finish than the organic solderability preservative (OSP) pad finish. However, the suitability of the pad finish was different depending on the type of paste. Void formation reduced with decreasing SRO. Both focused ion beam cross-sectioning and a thermal video system were used to ascertain the mechanism of void formation when using the paste with a small particle size, and the entrapment of flux was identified as the main cause for the formation of voids.

Effect of Isothermal Aging 2000 Hours on Intermetallics Formed between Ni-Pd-Au with Sn-4Ag-0.5Cu Solders

The present study investigated the effect of isothermal aging up to 2000 hours on the intermetallics formed between Sn-4Ag-0.5Cu lead free solder on electroless nickel electroless palladium immersion gold surface finish (Ni-Pd-Au). For all parameters, aging have an effect of changing the intermetallic morphology to coarser and dense structure. The intermetallic compound formed for the interconnection of the lead free solder changes with increased aging time from (Cu,Ni)6Sn5 compound to (Ni,Cu)3Sn4. At the end of the 2000 hours aging time, it changes to Ni3Sn4. This is the effect of Cu element availability during the intermetallics growth process. Starting from as reflow process, (Pd, Ni)Sn4 intermetallics formed near the interface of the solder joint. The formation of the (Pd, Ni)Sn4 intermetallics act like a diffusion barrier to slow down the growth of interface intermetallics. Lastly, Au element was detected in the Pd-Sn based intermetallic after aging more than 1000 hours.

Evaluation of ENEPIG Surface Treatment for High-reliability PCB in Mobile Module

We evaluated characteristics of ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold) surface treatment for mobile equipment that requires high reliability, in addition to investigating surface treatment processes for semiconductor boards that require high reliability such as regular PCB-package systems, board-on-chip, chip-scaled package (CSP), etc and application for semiconductor package board of SIP, BOC. As a result, it appeared that ENEPIG has superior properties compared to ENIG surface treatment in corrosion resistance, solder junction, wetting, etc. We anticipate that these results will be able to lend credibility to ENEPIG as a low-cost alternative for producing mobile devices such as the cell phones, especially when applied to mass production.