真空环境下的共晶焊接

共晶焊接是微电子组装中一种重要的焊接工艺。文章简要介绍了共晶焊接的工作原理以及共晶焊料如何选用和常用共晶焊料的性能特性。然后比较了几种共晶焊接设备的优缺点,得出用真空可控气氛共晶炉在真空环境下完成共晶焊接能有效防止共晶焊接过程中氧化物的产生,大大降低空洞率,从而提高焊接质量。它同样适用于多芯片组件的一次共晶。对真空环境下影响共晶焊接质量的真空度、保护性气氛、焊接过程中的温度曲线、焊接时的压力等条件做了探讨,得出了几种最优的工艺方案,能适用于大部分的共晶焊接工艺。     

In-Au复合焊料研究

大功率半导体激光器封装过程中,为降低封装引入的应力,踊般用In焊料进行焊接。In焊料具有易氧化、易"攀爬"的特性,因而导致封装成品率很低。提出了一种新型焊料——In-Au复合焊料,使用此焊料进行封装,很好地解决了上述问题。通过理论分析及实验摸索,确定了In-Au复合焊料蒸发条件,分别利用纯In焊料和In-Au复合焊料封装了一批激光器样品,通过对比这两组样品的焊料浸润性、电光参数、剪切强度等,发现利用In-Au复合焊料封装的样品优于纯In焊料封装的样品。     

Microstructures and degradation of heat sink very-thin quad flat package no-leads Pb-free Sn-Ag-Cu solder joints after thermal aging

Heat sink very-thin quad flat package no-leads (HVQFN) packages soldered with Sn-3.8Ag-0.7Cu on metallized laminate substrates have been put to thermal aging. Temperatures from 140°C to 200°C for times up to 30 weeks were applied. The solder joint microstructure develops intermetallic compound layers and voids within the solder. Due to this, the mechanical reliability of the HVQFN inner lead solder joints is degraded. The intermetallic layers are of the type (Cu, Y)₆Sn₅, with Y=Ni, Au or Ni+Au, as well as Cu₃Sn, and follow a power law with aging time: X=C·tⁿ, where n=0.4 to 1.9 depending on temperature. The voids within the solder are attributed to Sn depletion of the solder in favor of the growth of (Cu,Ni)₆Sn₅. They are more pronounced the less the solder volume is in proportion to the intermetallic diffusion area. The amount of voids is quantified as a percentage of the residual solder. The time to reach the failure criterion of 50%, i.e., t_50%, is related to the absolute temperature according to an Arrhenius equation with an activation energy E_a=0.95 eV. This equation is used for determination of the maximum allowable temperature at a certain required operating lifetime.     

Magnetocontrollable Droplet and Bubble Manipulation on a Stable Amphibious Slippery Gel Surface

Smart manipulation of liquid/bubble transport has garnered widespread attention due to its potential applications in many fields. Designing a responsive surface has emerged as an effective strategy for achieving controllable transport of liquids/bubbles. However, it is still challenging to fabricate stable amphibious responsive surfaces that can be used for the smart manipulation of liquid in air and bubbles underwater. Here, amphibious slippery surfaces are fabricated using magnetically responsive soft poly(dimethylsiloxane) doped with iron powder and silicone oil. The slippery gel surface retains its magnetic responsiveness and demonstrates strong affinity for bubbles underwater but shows small and switching resistance forces with the water droplets in air and bubbles underwater, which is the key factor for achieving the controllable transport of liquids/bubbles. On the slippery gel surface, the sliding behaviors of water droplets and bubbles can be reversibly controlled by alternately applying/removing an external magnetic field. Notably, compared with slippery liquid‐infused porous surfaces, the slippery gel surface demonstrates outstanding stability, whether in air or underwater, even after 100 cycles of alternately applying/removing the magnetic field. This surface shows potential applications in gas/liquid microreactors, gas–liquid mixed fluid transportation, bubble/droplet manipulation, etc.     

Study of interfacial reactions between Sn3.5Ag0.5Cu composite alloys and Cu substrate

For development of a lead-free composite solder for advance electrical components, lead-free Sn3.5Ag0.5Cu solder was produced by mechanically mixing 0.5 wt.% TiO₂ nanopowder with Sn3.5Ag0.5Cu solder. The morphology and growth kinetics of the intermetallic compounds that formed during the soldering reactions between Sn3.5Ag0.5Cu solder with intermixed TiO₂ nanopowder and Cu substrates at various temperatures ranging from 250 to 325 °C were investigated. A scanning electron microscope (SEM) was used to quantify the interfacial microstructure at each processing condition. The thickness of interfacial intermetallic layers was quantitatively evaluated from SEM micrographs using imaging software. Experimental results show that a discontinuous layer of scallop-shaped Cu-Sn intermetallic compounds formed during the soldering. Kinetics analysis shows that the growth of such interfacial Cu-Sn intermetallic compounds is diffusion controlled with an activation energy of 67.72 kJ/mol.     

Plastic constraint of large aspect ratio solder joints

The aspect ratio (joint area/joint thickness) of thin (0.001-0.006 in.) surface mount solder (60S-40Pb) joints plays an important role in determining the mechanical properties and fracture behavior of the joints. This study demon-strates that plastic constraint of a large aspect ratio 60Sn-40Pb solder joint can develop triaxial (hydrostatic) stresses several times greater than the average tensile strength of the bulk solder material. A four to sixfold increase in average joint stress and up to a tenfold increase in peak stress was measured on joints with aspect ratios ranging from 400 to 1000. Although a direct relationship of the aspect ratio to the average tensile stress is shown, as the Friction Hill model predicts, the observed stress increase is not nearly as high but proportional to the classical prediction. This is attributed to the existence of internal defects (oxide particles and micro-voids) and transverse grain boundaries which fail producing internal free surfaces. Thus, the actual aspect ratio is thickness/d², where d equals the distance between internal surfaces. The fracture of these constrained joints was brittle, with the separation occurring between a tin-rich copper tin intermetallic at the interface and the solder matrix. Voids within the solder joint are shown to relieve the plastic constraint and lower the average tensile stress of the joint. The Friction Hill model may play an important role in explaining the small percentage of atypical solder joint failures which sometimes occur on electronic assemblies. In particular, the sudden failure of a thin joint in a strain controlled environment may be attributed to the development of a large hydrostatic stress component. Therefore, a flaw free, plastically constrained joint which develops a high stress state will be a high risk candidate for failure.     

Reactivity of no-clean pastes and fluxes for the surface mount technology process—part II: Corrosion risk measurements for printed circuit boards and solder joints

The introduction of no-clean pastes and fluxes for the surface mount technology process needs testing methods of high sensitivity and reliability to evaluate the activity of the residues after the reflow process. An electrochemical method is proposed, suitable to ascertain the reactivity and the corrosion risk of these residues. Products of normal use are tested, and their behavior described and compared. Surface morphologies and compositions are examined after reflow and after testing. Local surface oxides are investigated with x-ray photoelectron spectroscopy. Surface analytical results are interpreted and related to the electrochemical behavior of solder joints. Tin is shown to have the most important role toward the onset of possible localized corrosion.     

Effect of Ionization Characteristics on Electrochemical Migration Lifetimes of Sn-3.0Ag-0.5Cu Solder in NaCl and Na<Subscript>2</Subscript>SO<Subscript>4</Subscript> Solutions

Anodic dissolution characteristics and electrochemical migration (ECM) behavior of Sn-3.0Ag-0.5Cu solder in NaCl and Na₂SO₄ solutions were investigated using anodic polarization tests and water drop tests (WDT). The ECM lifetime of Sn-3.0Ag-0.5Cu solder in NaCl solution (42.4 s) was longer than that in Na₂SO₄ solution (34.8 s). The pitting potential of Sn-3.0Ag-0.5Cu solder in NaCl solution (135 mV, SCE) was higher than that in Na₂SO₄ solution (−367 mV, SCE). The passivity film (SnO₂) formed on Sn-3.0Ag-0.5Cu solder during WDTs in NaCl solution was thicker than that formed in Na₂SO₄ solution. Therefore, the longer ECM lifetime of Sn-3.0Ag-0.5Cu solder in NaCl solution than in Na₂SO₄ solution can be attributed to the higher pitting potential in the NaCl solution, which is ascribed to the formation of a thicker passivity film (SnO₂) in the former. It was confirmed that microelements such as Ag and Cu do not take part in ECM because they form chemically stable intermetallic compounds with Sn. We believe that Sn is the only element that contributes to ECM, and dissolution of Sn at the anode is possibly the rate-determining step of ECM of Sn-3.0Ag-0.5Cu solder.     

Impression creep characterization of 90Pb-10Sn microelectronic solder balls at subsolvus and supersolvus temperatures

The creep behavior of Pb-10wt.%Sn, a common high-lead solder used in microelectronic packaging, was studied by impression creep testing of ball-gridarray (BGA) solder balls attached to an organic substrate, both above and below the solvus temperature (408 K). Below the solvus temperature, the solder microstructure consists of roughly equiaxed grains of the Pb-rich solid solution α, which contains <5wt.%Sn in solution, with a coarse dispersion of Sn-rich β precipitates. Here, the creep behavior of the solder is controlled by dislocation climb via dislocation core diffusion, yielding n≈4 and Q≈60 kJ/mole. Above the solvus temperature, where the entire 10wt.%Sn is in solution, the creep mechanism becomes controlled by viscous glide of dislocations, limited by solute drag, with n≈3 and Q≈92 kJ/mole. Based on experimental data, creep equations for the as-reflowed solder in the two temperature regimes are given. Comparison of the present data with those available in the literature showed good agreement with the proposed laws.     

废退锡液处理方法进展

目前,废退锡液的处理方法有中和法、化学沉淀法、电解法,或用来制备三水合锡酸钡。而利用扩散渗析-离子膜电沉积组合工艺综合回收废退锡液中的硝酸、金属铜和锡是一种新型的处理方法。