Improving the shear strength of Sn–Ag–Cu–Ni/Cu–Zn solder joints via modifying the microstructure and phase stability of Cu–Sn intermetallic compounds

This study focuses on the correlation between high-speed impact tests and the interfacial reaction in Sn-3.0Ag-0.5Cu-0.1Ni/Cu (wt%) and Sn-3.0Ag-0.5Cu-0.1Ni/Cu-15Zn solder joints. Adding Ni into the Sn–Ag–Cu solder alters the interfacial morphology from scallop type to layer type and exhibits high shear strength after reflow in both solder joints. However, the shear strength of Sn-3.0Ag-0.5Cu-0.1Ni/Cu solder joints degrades significantly after thermal aging at 150 °C for 500 h. It is notable that Sn-3.0Ag-0.5Cu-0.1Ni/Cu-15Zn solder joints still present higher shear strength after aging at 150 °C. The weakened shear strength in Sn-3.0Ag-0.5Cu-0.1Ni/Cu solder joints is due to stress accumulation in the interfacial (Cu,Ni)₆Sn₅ compound induced by the phase transformation from a high-temperature hexagonal structure (η-Cu₆Sn₅) to a low-temperature monoclinic structure (η'-Cu₆Sn₅). However, doping small amounts of Zn into (Cu,Ni)₆(Sn,Zn)₅ can inhibit the phase transformation during thermal aging and maintain strong shear strength. These experiments demonstrate that Sn-3.0Ag-0.5Cu-0.1Ni/Cu-15Zn solder joints can act as a stable connection in the micro-electronic packaging of most electronic products at their average working temperatures.     

Kinetics of intermetallic phase formation at the interface of Sn-Ag-Cu-X (X = Bi, In) solders with Cu substrate

The effects of Bi and In additions on intermetallic phase formation in lead-free solder joints of Sn-3.7Ag-0.7Cu; Sn-1.0Ag-0.5Cu-1.0Bi and Sn-1.5Ag-0.7Cu-9.5In (composition given in weight %) with copper substrate are studied. Soldering of copper plate was conducted at 250 °C for 5 s. The joints were subsequently aged at temperatures of 130-170 °C for 2-16 days in a convection oven. The aged interfaces were analyzed by optical microscopy and energy dispersive X-ray spectroscopy (EDX) microanalysis. Two intermetallic layers are observed at the interface - Cu₃Sn and Cu₆Sn₅. Cu₆Sn₅ is formed during soldering. Cu₃Sn is formed during solid state ageing. Bi and In decrease the growth rate of Cu₃Sn since they appear to inhibit tin diffusion through the grain boundaries. Furthermore, indium was found to produce a new phase - Cu₆(Sn,In)₅ instead of Cu₆Sn₅, with a higher rate constant. The mechanism of the Cu₆(Sn,In)₅ layer growth is discussed and the conclusions for the optimal solder chemical composition are presented.     

Effect of rare earth metal Ce addition to Sn-Ag solder on interfacial reactions with Cu substrate

The effect of adding a small amount of rare earth cerium (Ce) element to low Ag containing Sn-1wt%Ag Pb-free solder on its interfacial reactions with Cu substrate was investigated. The growth of intermetallic compounds (IMCs) between three Sn-1Ag-xCe solders with different Ce contents and a Cu substrate was studied and the results were compared to those obtained for the Ce-free Sn-1Ag/Cu systems. In the solid-state reactions of the Sn-1Ag(-xCe)/Cu solder joints, the two IMC layers, Cu₆Sn₅ and Cu₃Sn, grew as aging time increased. Compared to the Sn-1Ag/Cu joint, the growth of the Cu₆Sn₅ and Cu₃Sn layers was depressed for the Ce-containing Sn-1Ag-xCe/Cu joint. The addition of Ce to the Sn-Ag solder reduced the growth of the interfacial Cu-Sn IMCs and prevented the IMCs from spalling from the interface. The evenly-distributed Ce elements in the solder region blocked the diffusion of Sn atoms to the interface and retarded the growth of the interfacial IMC layer.     

热循环对Sn58Bi(纳米Ti)/Cu焊点界面与性能影响

为了改善Sn-58Bi低温钎料的性能,通过在Sn-58Bi低温钎料中添加质量分数为0.1%的纳米Ti颗粒制备了Sn-58Bi-0.1Ti纳米增强复合钎料。在本文中,研究了纳米Ti颗粒的添加对-55～125 _^oC热循环过程中Sn-58Bi/Cu焊点的界面金属间化合物(IMC)生长行为的影响。研究结果表明：回流焊后,在Sn-58Bi/Cu焊点和Sn-58Bi-0.1Ti/Cu焊点的界面处都形成一层扇贝状的Cu₆Sn₅ IMC层。在热循环300次后,在Cu₆Sn₅/Cu界面处形成了一层Cu₃Sn IMC。Sn-58Bi/Cu焊点和Sn-58Bi-0.1Ti/Cu焊点的IMC层厚度均和热循环时间的平方根呈线性关系。但是,Sn-58Bi-0.1Ti/Cu焊点的IMC层厚度明显低于Sn-58B/Cu焊点,这表明纳米Ti颗粒的添加能有效抑制热循环过程中界面IMC的过度生长。另外计算了这两种焊点的IMC层扩散系数,结果发现Sn-58Bi-0.1Ti/Cu焊点的IMC层扩散系数(整体IMC、Cu₆Sn₅和Cu₃Sn IMC)明显比Sn-58Bi/Cu焊点小,这在一定程度上解释了Ti纳米颗粒对界面IMC层的抑制作用。     

Intermetallic compound growth suppression at high temperature in SAC solders with Zn addition on Cu and Ni-P substrates

The effect of adding 0.5-1.5 wt.% Zn to Sn-3.8Ag-0.7Cu (SAC) solder alloy during reflow and solid state ageing has been investigated. In particular, the role of the Zn addition in suppressing interfacial Intermetallic Compound (IMC) growth on Cu and Ni-P substrates has been determined. Solder-substrate couples were aged at 150 °C and 185 °C for 1000 h. In the case of 0.5-1.0 wt.% Zn on Cu substrate, Cu₃Sn IMC was significantly suppressed and the morphology of Cu₆Sn₅ grains was changed, leading to suppressed Cu₆Sn₅ growth. In the SAC-1.5Zn/Cu substrate system a Cu₅Zn₈ IMC layer nucleated at the interface followed by massive spalling of the layer into the solder, forming a barrier layer limiting Cu₆Sn₅ growth. On Ni-P substrates the (Cu,Ni)₆Sn₅ IMC growth rate was suppressed, the lowest growth rate being found in the SAC-1.5Zn/Ni-P system. In all cases the added Zn segregated to the interfacial IMCs so that Cu₆Sn₅ became (Cu,Zn)₆Sn₅ and (Cu,Ni)₆Sn₅ became (Ni,Cu,Zn)₆Sn₅. The effect of Zn concentration on undercooling, wetting angles and IMC composition changes during ageing are also tabulated, and a method of incorporating Zn into the solder during reflow without compromising solder paste reflow described.     

Microstructural evolution of intermetallic compounds in Sn–3.5Ag–X (X = 0, 0.75Ni, 1.0Zn and 1.5In)/Cu solder joints during liquid aging

In this paper, the microstructural evolution of IMCs in Sn–3.5Ag–X (X = 0, 0.75Ni, 1.0Zn, 1.5In)/Cu solder joints and their growth mechanisms during liquid aging were investigated by microstructural observations and phase analysis. The results show that two-phase (Ni₃Sn₄ and Cu₆Sn) IMC layers formed in Sn–3.5Ag–0.75Ni/Cu solder joints during their initial liquid aging stage (in the first 8 min). While after a long period of liquid aging, due to the phase transformation of the IMC layer (from Ni₃Sn₄ and Cu₆Sn phases to a (Cu, Ni)₆Sn₅ phase), the rate of growth of the IMC layer in Sn–3.5Ag–0.75Ni/Cu solder joints decreased. The two Cu₆Sn₅ and Cu₅Zn₈ phases formed in Sn–3.5Ag–1.0Zn/Cu solder joints during the initial liquid aging stage and the rate of growth of the IMC layers is close to that of the IMC layer in Sn–3.5Ag/Cu solder joints. However, the phase transformation of the two phases into a Cu–Zn–Sn phase speeded up the growth of the IMC layer. The addition of In to Sn–3.5Ag solder alloy resulted in Cu₆(Sn_x,In_1?x)₅ phase which speeded up the growth of the IMC layer in Sn–3.5Ag–1.5In/Cu solder joint.     

Interfacial microstructure and properties of Sn-0.7Cu-0.05Ni/Cu solder joint with rare earth Nd addition

Interfacial microstructure and properties of Sn-0.7Cu-0.05Ni/Cu solder joints with trace amounts Nd additions have been investigated in this paper. The evolution of interfacial morphology of SCN solder joints with and without the presence of Nd under long-term room ambience was also studied. The greatest improvement to the solderability and tensile strength of SCN-xNd are obtained at 0.05 wt.% Nd. Meanwhile, the morphology and growth of interfacial intermetallic (IMC) layer are greatly improved by adding Nd, and the propensity for IMC spalling from the interface of solder joint is definitely reduced with the presence of rare earth Nd, since the sponge-like structure was completely inhibited in the solder joint containing Nd. In addition, a significant amount of Sn whiskers were present on the surface of NdSn₃ phases in the SCN0.15Nd/Cu solder joints, and the reason for this phenomenon has been briefly discussed.     

Stabilisation of Cu6Sn5 by Ni in Sn-0.7Cu-0.05Ni lead-free solder alloys

Cu₆Sn₅ exists at least in two crystal structures with an allotropic transformation from monoclinic η'-Cu₆Sn₅ at temperatures lower than 186 °C to hexagonal η-Cu₆Sn₅. We recently discovered that the hexagonal structure of Cu₆Sn₅ in lead-free solder alloys with trace Ni additions is stable down to room temperature using high resolution TEM/ED/EDS. This report further confirm the phase stabilising effect of Ni by analysing samples of Cu₆Sn₅ extracted from a Sn-0.7wt%Cu-0.05wt%Ni lead-free solder alloy. Techniques used include X-ray diffraction, transmission electron microscopy and differential scanning calorimetry.     

Interfacial Reactions and Bonding Strength of Sn-xAg-0.5Cu/Ni BGA Solder Joints

The present study details the microstructure evolution of the interfacial intermetallic compounds (IMCs) layer formed between the Sn-xAg-0.5Cu (x = 1, 3, and 4 wt.%) solder balls and electroless Ni-P layer, and their bond strength variation during aging. The interfacial IMCs layer in the as-reflowed specimens was only (Cu,Ni)₆Sn₅ for Sn-xAg-0.5Cu solders. The (Ni,Cu)₃Sn₄ IMCs layer formed when Sn-4Ag-0.5Cu and Sn-3Ag-0.5Cu solders were used as aging time increased. However, only (Cu,Ni)₆Sn₅ IMCs formed in Sn-1Ag-0.5Cu solders, when the aging time was extended beyond 1500 h. Two factors are expected to influence bond strength and fracture modes. One of the factors is that the interfacial (Ni,Cu)₃Sn₄ IMCs formed at the interface and the fact that fracture occurs along the interface. The other factor is Ag₃Sn IMCs coarsening in the solder matrix, and fracture reveals the ductility of the solder balls. The above analysis indicates that during aging, the formation of interfacial (Ni,Cu)₃Sn₄ IMCs layers strongly influences the pull strength and the fracture behavior of a solder joint. This fact demonstrates that interfacial layers are key to understanding the changes in bonding strength. Additionally, comparison of the bond strength with various Sn-Ag-Cu lead-free solders for various Ag contents show that the Sn-1Ag-0.5Cu solder joint is not sensitive to extended aging time.     

Interfacial reactions and mechanical properties between Sn-4.0Ag-0.5Cu and Sn-4.0Ag-0.5Cu-0.05Ni-0.01Ge lead-free solders with the Au/Ni/Cu substrate

Sn-4.0Ag-0.5Cu (SAC) and Sn-4.0Ag-0.5Cu-0.05Ni-0.01Ge (SACNG) lead-free solders reacting with the Au/Ni/Cu multi-layer substrate were investigated in this study. All reaction couples were reflowed at 240 and 255 °C for a few minutes and then aged at 150 °C for 100-500 h. The (Cu, Ni, Au)₆Sn₅ phase was formed by reflowing for 3 min at the interface. If the reflowing time was increased to 10 min, both (Cu, Ni, Au)₆Sn₅ and (Ni, Cu, Au)₃Sn₄ phases formed at the interface. The AuSn₄ phase was found in the solder for all reaction couples. An addition of Ni and Ge to the solder does not significantly affect the IMC formation. After a long period of heat-treatment, the thickness of the (Cu, Ni, Au)₆Sn₅ and (Ni, Cu, Au)₃Sn₄ phases increased and the intermetallic compounds (IMCs) growth mechanism obeyed the parabolic law and the IMC growth mechanism was diffusion-controlled. The mechanical strengths for both the soldered joints decreased with increasing thermal aging time. The SACNG/Au/Ni/Cu couple had better mechanical strength than that in the SAC/Au/Ni/Cu couple.     

Cu基板退火处理的Cu/Sn58Bi/Cu钎焊接头界面微结构

研究了铜基板退火处理对Cu/Sn58Bi界面微结构的影响. 结果表明,在回流以及时效24 h后Cu/Sn58Bi/Cu界面只观察到Cu₆Sn₅. 随着时效时间的增加,在界面形成了Cu₆Sn₅和Cu₃Sn的双金属间化合物(IMC)层,并且IMC层厚度也随之增加. 长时间时效过程中,在未退火处理的铜基板界面产生了较多铋偏析,而在退火处理的铜基板界面较少产生铋偏析. 比较退火处理以及未退火处理的铜基板与钎料界面IMC层生长速率常数,发现铜基板退火处理能减缓IMC层生长,主要归因于对铜基板进行退火处理能够有效的消除铜基板的内应力与组织缺陷,从而减缓Cu原子的扩散,起到减缓IMC生长的作用.     

Effect of bismuth on the growth kinetics of intermetallic compounds in Sn-3.5Ag solder joints: A growth kinetic model

The effect on the growth kinetics of the intermetallic compounds (IMCs) in solder/Cu joints, caused by adding Bi to eutectic Sn-3.5Ag solder alloy, was examined at the aging temperatures of 150°C and 180°C. The Cu₆Sn₅ layer growth was significantly enhanced, but the Cu₃Sn layer growth was slightly retarded by the addition of Bi, resulting in significant growth enhancement of the total (Cu₆Sn₅+Cu₃Sn) IMC layer with increasing Bi addition. The IMC layer growth in the Bi-containing solder joints was accompanied by the accumulation of Bi ahead of the Cu₆Sn₅ layer that resulted in the formation of a liquid layer at the Cu₆Sn₅/solder interface. A kinetic model was developed for the planar growth of the Cu₆Sn₅ and Cu₃Sn layers in the solder joints, accounting for the existence of interfacial reaction barriers. Predictions from the kinetic model showed that the experimental results could be well explained by the hypothesis that the formation of a Bi-rich liquid layer at the Cu₆Sn₅/solder interface reduces the interfacial reaction barrier at the interface.     

Pb-free solders for flip-chip interconnects

A variety of lead-free solder alloys were studied for use as flip-chip interconnects including Sn-3.5Ag, Sn-0.7Cu, Sn-3.8Ag-0.7Cu, and eutectic Sn-37Pb as a baseline. The reaction behavior and reliability of these solders were determined in a flip-chip configuration using a variety of under-bump metallurgies (TiW/Cu, electrolytic nickel, and electroless Ni-P/Au). The solder micro-structure and intermetallic reaction products and kinetics were determined. The Sn-0.7Cu solder has a large grain structure and the Sn-3.5Ag and Sn-3.8Ag-0.7Cu have a fine lamellar two-phase structure of tin and Ag₃Sn. The intermetallic compounds were similar for all the lead-free alloys. On Ni, Ni₃Sn₄ formed and on copper, Cu₆Sn₅Cu₃Sn formed. During reflow, the intermetallic growth rate was faster for the lead-free alloys, compared to eutectic tin-lead. In solidstate aging, however, the interfacial intermetallic compounds grew faster with the tinlead solder than for the lead-free alloys. The reliability tests performed included shear strength and thermomechanical fatigue. The lower strength Sn-0.7Cu alloy also had the best thermomechanical fatigue behavior. Failures occurred near the solder/intermetallic interface for all the alloys except Sn-0.7Cu, which deformed by grain sliding and failed in the center of the joint. Based on this study, the optimal solder alloy for flip-chip applications is identified as eutectic Sn-0.7Cu. Editor’s Note: A hypertext-enhanced version of this article can be found at www.tms.org/pubs/journals/JOM/0106/Frear-0106.html For more information, contact D.R. Frear, Interconnect Systems Laboratories, Motorola, Tempe, AZ 85284; (480) 413-6655; fax (480) 413-4511; e-mail darrel.frear@motorola.com.     

Suppressing effect of 0.5 wt.% nano-TiO2 addition into Sn-3.5Ag-0.5Cu solder alloy on the intermetallic growth with Cu substrate during isothermal aging

This study investigated the effects of adding 0.5 wt.% nano-TiO₂ particles into Sn3.5Ag0.5Cu (SAC) lead-free solder alloys on the growth of intermetallic compounds (IMC) with Cu substrates during solid-state isothermal aging at temperatures of 100, 125, 150, and 175 °C for up to 7 days. The results indicate that the morphology of the Cu₆Sn₅ phase transformed from scallop-type to layer-type in both SAC solder/Cu joints and Sn3.5Ag0.5Cu-0.5 wt.% TiO₂ (SAC) composite solder/Cu joints. In the SAC solder/Cu joints, a few coarse Ag₃Sn particles were embedded in the Cu₆Sn₅ surface and grew with prolonged aging time. However, in the SAC composite solder/Cu aging, a great number of nano-Ag₃Sn particles were absorbed in the Cu₆Sn₅ surface. The morphology of adsorption of nano-Ag₃Sn particles changed dramatically from adsorption-type to moss-type, and the size of the particles increased.The apparent activation energies for the growth of overall IMC layers were calculated as 42.48 kJ/mol for SAC solder and 60.31 kJ/mol for SAC composite solder. The reduced diffusion coefficient was confirmed for the SAC composite solder/Cu joints.     

Growth Behavior of Intermetallic Compounds at SnAgCu/Ni and Cu Interfaces

The growth behavior of reaction-formed intermetallic compounds (IMCs) at Sn3.5Ag0.5Cu/Ni and Cu interfaces under thermal-shear cycling conditions was investigated. The results show that the morphology of (Cu _x Ni_1–x )₆Sn₅ and Cu₆Sn₅ IMCs formed both at Sn3.5Ag0.5Cu/Ni and Cu interfaces gradually changed from scallop-like to chunk-like, and different IMC thicknesses developed with increasing thermal-shear cycling time. Furthermore, Cu₆Sn₅ IMC growth rate at the Sn3.5Ag0.5Cu/Cu interface was higher than that of (Cu _x Ni_1–x )₆Sn₅ IMC under thermal-shear cycling. Compared to isothermal aging, thermal-shear cycling led to only one Cu₆Sn₅ layer at the interface between SnAgCu solder and Cu substrate after 720 cycles. Moreover, Ag₃Sn IMC was dispersed uniformly in the solder after reflow. The planar Ag₃Sn formed near the interface changed remarkably and merged together to large platelets with increasing cycles. The mechanism of formation of Cu₆Sn₅, (Cu _x Ni_1–x )₆Sn₅ and Ag₃Sn IMCs during thermal-shear cycling process was investigated.     

Influence of rare earth Ce addition on the microstructure,properties and soldering reaction of pure Sn

The influence of trace rare earth (RE) Ce addition on the microstructure, melting point and wettability of pure Sn as well as on the soldering reactions in Sn-xCe/Cu(Ni) solder joints was investigated. In bulk Sn-xCe solders, large β-Sn grains were observed with the Ce addition less than 0.2 wt%; while the β-Sn grain size decreased markedly when the Ce addition was 0.2 wt%, resulting in a refined microstructure. The addition of trace RE Ce had little effect on the melting temperature of the solders. Smaller wetting angles of Sn-xCe solders on both Cu and Ni substrates were measured when the samples were reflowed at a higher temperature. The Sn-0.2Ce solder owned the best wettability on Cu substrate. Scallop-like Cu₆Sn₅ intermetallic compound (IMC) grains formed at the Sn-xCe/Cu interfaces, while a continuous Ni₃Sn₄ IMC layer formed at each Sn-xCe/Ni interface. With the increase of Ce addition, the interfacial IMC grain size and the interfacial IMC layer thickness on both Cu and Ni substrates decreased gradually. The activity of Sn was lowered with the Ce addition, which depressed the growth of the interfacial IMC. In the current study, the Ce addition of 0.2 wt% exhibits the optimized performance.     

Dissolution and interfacial reactions of thin-film Ti/Ni/Ag metallizations in solder joints

The dissolution and interfacial reactions involving thin-film Ti/Ni/Ag metallizations on two semiconductor devices, diode and metal-oxide-semiconductor field-effect transistor (MOSFET), a Sn–3.0Ag–0.7Cu solder, and a Au-layer on the substrates are studied. To simulate the dissolution kinetics of the Ag-layer in liquid solder during the reflow process, the computational thermodynamics (Thermo-Calc) and kinetics (DICTRA: DIffusion Controlled TRAnsformations) tools are employed in conjunction with the assessed thermochemical and mobility data. The simulated results are found to be consistent with the observed as-reflowed microstructures and the measured Ag contents in the solder. In the as-reflowed joints two different intermetallic compounds (IMC) are found near the diode/solder interface. Both are in the form of particles of different morphologies, not a continuous layer, and are referred to as IMC-I and IMC-II. The former corresponds to Ni₃Sn₄ with Cu atoms residing in the Ni sublattice. It is uncertain whether IMC-II is Cu₆Sn₅ phase with Ni atoms residing in the Cu sublattice or a Cu–Ni–Sn ternary phase. Near the as-reflowed MOSFET/solder interface, both particles and a skeleton-like layer of Ni₃Sn₄ are observed. The primary microstructural dynamics during solid state aging are the coarsening of IMC particles and the reactions involving the unconsumed (after reflow) Ni- and the Ti-layer with Sn and Au. While the reaction with the Ni-layer yields only Ni₃Sn₄ intermetallic, the reaction involving the Ti-layer suggests the formation of Ti–Sn and Au–Sn–Ti intermetallics. The latter is due to the diffusion of Au from the substrate side to the die side. It is postulated that the kinetics of Au–Sn–Ti layer is primarily governed by the diffusion of Au through the Ni₃Sn₄ layer by a grain boundary mechanism.     

Anisotropic thermal expansion of Ni3Sn4, Ag3Sn,Cu3Sn,Cu6Sn5 and βSn

The directional coefficient of thermal expansion (CTE) of intermetallics in electronic interconnections is a key thermophysical property that is required for microstructure-level modelling of solder joint reliability. Here, CTE ellipsoids are measured for key solder intermetallics using synchrotron x-ray diffraction (XRD). The role of the crystal structure used for refinement on the CTE shape and temperature dependence is investigated. The results are used to discuss the βSn-IMC orientation relationships (ORs) that minimise the in-plane CTE mismatch on IMC growth facets, which are measured with electron backscatter diffraction (EBSD) in solder joints on Cu and Ni substrates. The CTE mismatch in fully-intermetallic joints is discussed, and the relationship between the directional CTE of monoclinic and hexagonal polymorphs of Cu₆Sn₅ is explored.     

Nd对Sn-3.8Ag-0.7Cu/Cu焊点高温可靠性的影响

通过研究150℃时效条件下Sn-3.8Ag-0.7Cu-0.05Nd/Cu焊点剪切力变化和界面微观结构演变,探讨稀土元素Nd对焊点高温可靠性的影响及其影响机制.结果表明,不同时效时间后Sn-3.8Ag-0.7Cu-0.05Nd/Cu焊点剪切力明显高于Sn-3.8Ag-0.7Cu/Cu焊点,且时效过程中Sn-3.8Ag-...     

The embrittlement mechanism and improvement of impact strength for lead-free solder joints in BGA packaging using electrolytic Ni/Au plating

Impact strength evaluation and fracture mechanism analysis in board level of Sn–3mass%Ag–0.5mass%Cu solder joints of ball grid arrays (BGA) using electrolytic Ni/Au plating were performed. The cause of impact strength degradation of BGA solder ball joints is the existence of low density defects, which contain organic materials, in the (Cu,Ni)₆Sn₅ intermetallic compound grain boundary formed in the solder joints. These organic materials are taken in by the nickel plating film at the time of nickel plating. To improve the impact strength of the Sn–3mass%Ag–0.5mass%Cu solder joint of the BGA, it is necessary to lower the concentration of these organic materials. The contamination prevention and nickel plating bath sanitization, solder mask material selection (to minimize nickel plating bath contamination) and higher current density of nickel plating are effective to keep a lower concentration of organic materials in nickel plating film.