Sn基钎料与Cu和Al基板的润湿性比较研究

采用润湿平衡法测量了四种Sn基钎料(Sn-37Pb、Sn-3.OAg-0.5Cu、Sn-0.7Cu与Sn-9Zn)分别在250,260和270℃与Cu、Al两种基板的润湿性能.结果表明:钎料与Al基板的润湿时间均比Cu基板长,除Sn-9Zn外,其他三种钎料与Cu基板的润湿力比Al基板大,并且随着温度升高,润湿性能提高,...     

Effect of Zn content on the vibration fracture behavior of Sn-Zn and Sn-Zn-Bi solders

This study investigated the microstructure and vibration properties of Sn-Zn and Sn-Zn-Bi alloys with different Zn contents. Experimental results show that the hypoeutectic Sn-Zn-Bi alloy (with a Zn content of 5 wt.%) has the poorest damping capacity and the lowest critical vibration cycles to failure due to a hardening effect by Bi and intergranular fracturing. On the other hand, since the Zn/Sn interfaces at which internal friction may occur during vibration contribute to the dissipation of vibration energy, the hypereutectic Sn-13Zn samples with numerous massive primary Zn needles possess superior damping capacity and vibration life under constant vibration force conditions.     

钎料稀土相表面Sn晶须生长的研究

为了解对钎料可靠性影响极大的Sn晶须的生长机理,系统研究了钎料稀土相CeSn3、LaSn3及ErSn3表面Sn晶须生长的影响因素.结果表明:稀土相的氧化倾向与时效温度共同影响其表面Sn晶须的生长.室温时效条件下,在稀土相CeSn3与LaSn3表面易出现小尺寸的线状Sn晶须,直径为0.1～0.2 μ.m,而在稀土相ErS...     

Suppression of tin whisker formation on fine pitch connectors by surface roughening

In the electronics industry, lead-free solder processes such as the terminal plating of electronic components, fine pitch connectors, and flexible printed circuits (FPCs) are invariably hampered by the serious problem of tin whisker formation. Here, a new and simple method, the JVC Micro Island (JMI) process, is proposed for the prevention of tin whisker formation in fine pitch connectors. Briefly, the base copper terminal was acid etched to afford a roughened surface, which was then tin plated. The contact test with Knoop indentation proved the effectiveness of the present process. The maximum length of the as-formed tin whiskers was less than 50 μm. The solderability of the JMI FPCs was not influenced by the present process. Thus, the new JMI process is shown to have a great advantage for the prevention of tin whisker formation in fine pitch connectors.     

Interfacial reactions during soldering with lead-tin eutectic and lead (Pb)-free,tin-rich solders

The use of lead (Pb)-containing solders for the interconnections of microelectronic subsystem assembly and packaging has become an environmental issue. Extensive research and development activities for replacing Pb-containing solders with environmentally safe Pb-free solders are in progress in electronic industries, universities, and national laboratories. One key technical issue recognized with the Pb-free, Sn-rich solders is a need to develop a good barrier metallurgy to control the interfacial reactions, namely, dissolution of the base metal(s) and concurrent formation of intermetallics at the soldering interfaces. In this study, the interfacial reactions of Cu and Ni metallizaton with several Pb-free and Pb-containing solders are investigated. The dissolution kinetics of the base metal(s) as well as the growth kinetics of the intermetallics are discussed.     

Modeling the atomic transport kinetics in high-lead solders

The atomic mobilities of Au, In, Pb, and Sn in face-centered cubic Pb-Sn and Pb-In alloys are modeled using the available literature data. A set of parameters describing the composition and temperature dependence of diffusional mobility are provided. The calculated tracer diffusivities of Pb in ternary Pb-Sn-In and Pb-Sn-Au alloys are in very good agreement with the experimental data. Using the model parameters, both tracer and chemical diffusivities can be calculated in the composition and temperature ranges where experimental data is not available. On the assumption of local equilibrium, the simulation of dissolution kinetics of Pd in liquid Pb is demonstrated. The major source of discrepancy between the calculated and experimental diffusion profiles is the uncertainty of the atomic transport kinetics data in the liquid phase. The implications of current kinetic modeling are discussed briefly.     

Study of immersion silver and tin printed-circuit-board surface finishes in lead-free solder applications

The wetting of I-Ag (immersion silver) and I-Sn (immersion tin) printed-circuit-board (PCB) finishes by Sn/Ag/Cu and eutectic Sn/Pb solders was studied in this work with Ni/Au (electroless nickel/immersion gold) and organic solderability preservative (OSP) finishes as baselines. Wetting tests were performed on fresh boards and boards subjected to different preconditioning treatments that simulated the effects of aging, storage, and multiple reflow cycles. When the boards are fresh, the wetting of the I-Sn and Ni/Au finishes is better than that on the I-Ag and OSP finishes. However, after the preconditioning treatments, the wetting of the I-Sn finish degrades the fastest, whereas the wetting of the I-Ag and OSP finishes degrade less through the different preconditioning treatments. The wetting of the Ni/Au finish remains excellent through all the preconditioning treatments. The chemical and microstructural changes in the finishes during aging treatments were evaluated using electron spectroscopy chemical analysis (ESCA), x-ray diffractometry (XRD), and cross-sectioning followed by scanning electron microscopy (SEM). The results indicate that a single lead-free reflow cycle consumes the I-Sn layer faster than a Sn/Pb reflow cycle because of the formation of the Sn/Cu intermetallic compound (IMC). Consequently, I-Sn finished boards having an original Sn thickness of ∼1 μm will not withstand multiple lead-free reflow cycles without significant degradation in wetting but up to two Sn/Pb reflow cycles are still feasible. The minimum thickness of I-Sn required for adequate wetting was evaluated by comparing the wetting after different aging treatments. The exposure of I-Sn samples to 85°C/85% relative humidity (RH) conditions increases the thickness of the Sn-oxide layer, which, above a certain thickness, can degrade wetting. Oxidized copper areas formed on top of the I-Ag surface after exposure to 85°C/85% RH treatment, and this was considered a major factor influencing wetting. The formation of sulfides on I-Ag was detected, but their overall quantity remained too small to have a detectable impact on the wetting.     

无铅喷锡(HASL)润湿不良问题及对策

针对东莞康佳电子有限公司生产无铅喷锡(HASL)PCB板时所遇到的焊盘润湿不良问题,采用了正常PCB板材与异常PCB板材对比,对smt生产制程条件进行内检等方法措施,以及最终对焊盘异常的PCB送国家级实验室5所分析结论确认焊盘润湿不良问题的主要表现为锡膏对PCB焊盘润湿不良,造成不良的主要原因与PCB焊盘HASL表面不平整以及焊盘已发生合金化降低其可焊性有关。并在批量生产中采取烘烤箱使用105±5℃,烘烤4小时烘烤PCB和使用酒精擦洗PCB焊盘来减少润湿不良的方法措施保证生产。     

Interfacial reactions in the pb-free composite solders with indium layers

A Pb-free composite solder is prepared with a Pb-free solder substrate and a plated-indium layer. The indium layer melts during the soldering process, wets the substrates, and forms a sound solder joint. Since the melting temperature of indium is 156.6°C, lower than that of the eutectic Sn-Pb, which is at 183°C, the soldering process can be carried out at a temperature lower than that of the conventional soldering process. Composite solder joints with three different Pb-free solders, Sn, Sn-3.5 wt.% Ag, and Sn-3.5 wt.% Ag-0.5 wt.% Cu, and two substrates, Ni and Cu, are prepared. The interfaces between the indium layer, Pb-free solder, and Ni and Cu substrate are examined. A good solder joint is formed after a 2-min reflow at 170°C. A very thick reaction zone at the indium/Pb-free solder interface and a thin reaction layer at the indium/substrate interface are observed.     

Interfacial reactions between Ni substrate and the component Bi in solders

The reactions between Ni and liquid Bi at 300, 360, 420, and 480°C were studied. Bismuth is an important element in many electronic solders, while Ni is used in many printed circuit board surface finishes. It was found that the only intermetallic compound formed was NiBi₃. The other intermetallic compound NiBi, which is therm odynamically stable at these temperatures, did not form. Reaction at 300°C produced a thick reaction zone, which is a two-phase mixture of NiBi₃ needles dispersed in Bi matrix. The thickness of the reaction zone increased rapidly with reaction time, reaching 400 μm after 360 min. Reactions at 360 and 420°C produced very thin reaction zones, and the major interaction was the dissolution of Ni into liquid Bi. Reaction at 480°C produced extremely thin reaction zone, and the dissolution of Ni into liquid Bi was very fast and was the major interaction. It is proposed that the formation of the reaction zone is controlled by two factors: the solubility limit and the diffusivity of Ni in liquid Bi. Small diffusivity and small solubility limit, i.e., lower temperature, tend to favor the formation of a thick reaction zone. In addition to the NiBi₃ formed within the reaction zone, NiBi₃ also formed outside the reaction zone in the form of long needles with hexagonal cross section. The dissolution rate of Ni into Bi is comparable to that of Ni into Sn at the same temperature, and is much slower than the dissolution rates for Au, Ag, Cu, and Pd into Sn.     

Effect of microelements addition on the interfacial reaction between Sn-Ag-Cu solders and the Cu substrate

Three kinds of Sn-Ag-based lead-free solders, Sn-3.5Ag-0.7Cu, Sn-3.5Ag-0.5Cu-0.07Ni-0.01Ge, and Sn-3.5Ag-0.07Ni (in wt.%), were selected to explore the effect of microelements (Ni and Ge) on the interfacial reaction between the solder and the Cu substrate. The thickness of the interfacial intermetallics formed with the Sn-3.5Ag-0.5Cu-0.07Ni-0.01Ge and Sn-3.5Ag-0.07Ni solders is several times that of the Sn-3.5Ag-0.7Cu solder. The added microelements converted the feature of interfacial intermetallics from pebble shape to worm shape. However, the results of x-ray diffraction (XRD) analysis suggest that the interfacial intermetallics formed with both solders have the same crystal structure. The results of energy dispersive spectroscopy (EDS) analysis show that the major interfacial intermetallic formed with the Sn-3.5Ag-0.7Cu solder is Cu₆Sn₅, while it is (Cu_x,Ni_1−x)₆Sn₅ with Sn-3.5Ag-0.5Cu-0.07Ni-0.01Ge. Ni influences the interfacial intermetallics and plays the influential role on the difference of interfacial reaction rate between liquid solder and solid Cu and the morphology of interfacial intermetallics. Additionally, the growth kinetics of the interfacial intermetallic compounds (IMCs) formed in the systems of Cu/Sn-3.5Ag-0.7Cu and Cu/Sn-3.5Ag-0.07Ni at high-temperature storage was also explored.     

Characterization of the melting and wetting of Sn-Ag-X solders

There is tremendous interest at present with Pb-free solder assembly in the surface mount assembly industry in response to recent Japanese and European initiatives and proposed governmental restrictions regarding Pb usage and disposal. Many different solder alloys have been proposed as potential Pb-free solder replacements and the most promising of these fall into the general alloy families of tin-silver (Sn-Ag), tin-silver-copper (Sn-Ag-Cu) and tin-silver-bismuth (Sn-Ag-Bi). Published melting point data on some of these alloys indicates that they should be capable of reduced reflow temperatures relative to the commonly available Sn-3.5Ag alloy, which melts at 221°C. Differential scanning calorimetry (DSC) and reflow visualization was used to characterize the melting and wetting of the Pb-free alloys and generate the practical reflow temperature requirements. This was compared to the DSC data to gain insight on the meaning of the DSC melting data for surface mount applications. The results show that, in general, the wetting performance of the Sn-Ag-Bi alloys are more similar to Sn-Ag and Sn-Ag-Cu than would be predicted by the major onset melting temperature data as measured by the DSC     

The influence of temperature and humidity on printed wiring board surface finishes: Immersion tin vs organic azoles

Substitution of lead-free solders in electronic assemblies requires changes in the conventional Sn:Pb finishes on substrates and component leads to prevent contamination of the candidate lead-free solder. Options for solderability preservative coatings on the printed wiring board include organic (azole or rosin/resin based) films and tin-based plated metallic coatings. This paper compares the solderability performance and corrosion protection effectiveness of electroless tin coatings vs organic azole films after exposure to a series of humidity and thermal cycling conditions. The solderability of immersion tin is directly related to the tin oxide growth on the surface and is not affected by the formation of SnCu intermetallic phases as long as the intermetallic phase is underneath a protective Sn layer. Thin azole films decompose upon heating in the presence of oxygen and lead to solderability degradation. Evaluations of lead-free solder pastes for surface mount assembly applications indicate that immersion tin significantly improves the spreading of Sn:Ag and Sn:Bi alloys as compared to azole surface finishes.     

Wetting Kinetics of Eutectic Lead and Lead-Free Solders: Spreading over the Cu Surface

Wetting kinetics of Sn, eutectic Sn-Ag, eutectic Sn-Cu, and eutectic Pb-Sn was studied using real-time in situ monitoring of the triple-line movement, facilitated by a hot-stage microscopy system under a controlled atmosphere. Significantly different kinetics of lead versus lead-free solders is documented. In case of the eutectic lead solder, four characteristic spreading stages were identified. Spreading of lead-free solders features two stages with a sharp change of the spreading rate at the early stages of rather insignificant spreading. Scanning electron microscopy and energy-dispersive x-ray spectroscopy analysis of the resolidified solder surface within a halo region is discussed.     

Mechanical strength of Sn-3.5Ag-based solders and related bondings

The tensile strengths of bulk solders and joint couples of Sn-3.5Ag-0.5Cu, Sn-3.5Ag-0.07Ni, and Sn-3.5Ag-0.5Cu-0.07Ni-0.01Ge solders and the shear strengths of ball grid array (BGA) specimens, solder-ball-attached Cu/Ni/Au metallized substrates were investigated. The tensile strength of the bulk is degraded by thermal aging. The Ni-containing solder exhibits lower tensile strength than Sn-3.5Ag-0.5Cu after thermal aging. However, the Ni-containing solder joints show greater tensile strength than the Cu/Sn-3.5Ag-0.5Cu/Cu joint. Fracture of the solder joint occurs between the intermetallic compound (IMC) and the solder. The shear strength and fracture mechanism of BGA specimens are the same regardless of solder composition.     

Analysis of ring and plug shear strengths for comparison of lead-free solders

The drive to replace the use of toxic lead metal and its alloys has spurred the development of many new lead-free solder alloys. Moreover, current leaded solders lack shear strength, resistance to creep and to thermal-mechanical fatigue. Solder that exhibits enhancements of these properties and retains solderability is crucial in applications where the solder joints are subjected to thermal cycling, severe vibrations, and temperatures of up to 125°C. Modified ring and plug joints were made with 18 selected lead-free solders and three well characterized lead-containing solders. Analysis of the results provides a guide for the design of additional testing.     

Effects of reflow on wettability, microstructure and mechanical properties in lead-free solders

Solder joints used in electronic applications undergo reflow operations. Such operations can affect the solderability, interface intermetallic layer formation and the resultant solder joint microstructure. These in turn can affect the overall mechanical behavior of such joints. In this study the effects of reflow on solderability and mechanical properties were studied. Nanoindentation testing (NIT) was used to obtain mechanical properties from the non-reflow (as-melted) and multiple reflowed solder materials. These studies were carried out with eutectic Sn-3.5Ag solders, with or without mechanically added Cu or Ag reinforcements, using Cu substrates. Microstructural analysis was carried out on solder joints made with the same solders using copper substrate.     

Effects of solid-state annealing on the interfacial intermetallics between tin-lead solders and copper

The interfacial intermetallics between Cu and solder were studied for four Sn-Pb compositions at the annealing temperatures of 125°C, 150°C, and 175°C for up to 30 days. The η-phase (Cu₆Sn₅) layer formed during reflow continues to grow during annealing. An additional layer of ɛ-phase (Cu₃Sn) forms at the η/Cu interface after an incubation annealing time. The thickness results fit a power-law relationship against time with average exponents 0.69 and 0.44 for the η phase and the ɛ phase, respectively. On prolonged annealing, the proportions of the individual phases in the total layer reach a steady state.     

Solderability testing of Sn-Ag-XCu Pb-free solders on copper and Au-Ni-plated kovar substrates

Solderability was evaluated for four Pb-free alloys: 95.5Sn-4.3Ag-0.2Cu (wt.%), 95.5Sn-4.0Ag-0.5Cu, 95.5Sn-3.9Ag-0.6Cu, and 95.5Sn-3.8Ag-0.7Cu on oxygen-free electronic grade (OFE) Cu and Au-Ni plated Kovar substrates. The solderability metric was the contact angle, θ_c, as determined by the meniscometer/wetting balance technique. Tests were performed at 230°C, 245°C, and 260°C using rosin-based, mildly activated (RMA) flux, a rosin-based (R) flux, and a low-solids (LS) flux. The Pb-free solders exhibited acceptable to poor solderability (35°<θ_c<60°) on Cu with the RMA flux. Nonwetting occurred in most tests using the R flux. Wetting was observed with the LS flux, but only at 245°C and 260°C and with high contact angles. The solderability of the Pb-free solders improved at all test temperatures on the Au-Ni plated Kovar substrate when using the RMA flux (30°<θ_c<50°). Wetting was observed with the R flux (35°<θ_c<60°) and LS flux (50°<θ_c<85°) for all temperatures. The Pb-free solders had generally lower wetting rates and longer wetting times on Cu than the 63Sn-37Pb solder. The wetting rate and wetting time data were superior on the Au-Ni plated Kovar substrates. In general, solderability, as measured by θ_c along with the wetting rate and wetting time, did not exhibit a consistent dependence on the composition of the Sn-Ag-XCu (X=0.2, 0.5, 0.6, and 0.7) alloys. The better performers were 95.5Sn-3.9Ag-0.6Cu alloy with the RMA flux (both Cu and Au-Ni plated Kovar) and 95.5Sn-3.8Ag-0.7Cu with the R and LS fluxes (Au-Ni-Kovar, only). The solder-flux interfacial tension, γ_LF, had a significant impact on the θ_c values. The magnitudes of the contact angle θ_c suggested that the four Pb-free solders would experience higher solderability defect counts at the printed wiring assembly level.     

Effect of alloying elements on properties and microstructures of SnAgCu solders

Recent years, the SnAgCu family of alloys has been found a widely application as a replacement for the conventional SnPb solders in electronic industry. In order to further enhance the properties of SnAgCu solder alloys, alloying elements such as rare earth, Bi, Sb, Fe, Co, Mn, Ti, In, Ni, Ge and nano-particles were selected by lots of researchers as alloys addition into these alloys. Rare earth (RE) elements have been called the ‘‘vitamin” of metals, which means that a small amount of RE elements can greatly enhance the properties of metals, such as microstructure refinement, alloying and purification of materials and metamorphosis of inclusions. In addition, a small amount of Zn addition has the ability to reduce undercooling efficiently and suppress the formation of massive primary Ag₃Sn plates, and Bi/Ga has the ability to enhance the wettability of SnAgCu alloys as well as Ni. Moreover, adding Co/Fe/Ge can effectively refine microstructure, modify interfacial Cu-Sn compounds and increase the shear strength of joints with Cu. This paper summarizes the effects of alloying elements on the wettability, mechanical properties, creep behavior and microstructures of SnAgCu lead-free solder alloys.