Solder Assembly Research Articles

Fatigue analysis of miniaturized lead-free solder contacts based on a novel test concept

The paper presents the method of generating lifetime-prediction-laws on special prepared very stiff specimen. The combination of thin- and thick-film technology allows building up test samples on ceramic very similar to electronic packages including the measurement issues. Influences of pad surface metallurgy, microstructure of solder, ineutectic solder alloys and assembly process parameter are regarded now. The investigation objects provide monitoring of electrical and mechanical damage process of SnAgCu solder bump. Different thermo-mechanical loads will be applied in temperature ranges of 0 to +80 °C, −40 to +125 °C and −50 to +150 °C, where the temperature gradient and cycle frequency also vary. A Variation of four different chip sizes allows the determination of fatigue laws for each temperature profile, to be able to compare in between them. The results of these tests will give universal lifetime-prediction laws for SnAgCu base solder joints. Main goals are to find coefficients for lifetime prediction models such as Coffin–Manson- or Norris–Landzberg-relation, which are transferable in between different electronic packages.

Re‐calibration of Engelmaier's Model for Leadless, Lead‐free Solder Attachments

AbstractIn this paper, the solder attachment fatigue model created by Werner Engelmaier is re‐calibrated in order to make it applicable in conjunction with leadless, lead‐free solder attachments. Sn3.8Ag0.7Cu solder attached ball‐grid‐array components are addressed to three thermal cycling test profiles. Based on the results, both physical and statistical parameters are obtained and compared with the values relevant to tin–lead solder assemblies. The validity of the statistical distribution selection (two‐parameter Weibull) is studied. Acceleration factors correlating different test profiles are obtained, and they are found to be only weakly dependent on the test vehicle type. Copyright © 2006 John Wiley & Sons, Ltd.

Lifetime of solid metals in contact with liquid solders for high-temperature liquid solder assemblies

Abstract The rates of barrier layer thickness losses for Ni, Cu, Pt, Ti, V, Nb, Ta and W metallizations in contact with the molten 52In–48Sn solder at 200 °C have been studied. Despite the expected complexity of the kinetics of dissolution and intermetallic compound growth, we demonstrate simple empirical relationships between the lifetimes of 1 μm thick barrier layers and thermodynamic properties such as their melting point, specific heat and latent heat of fusion.

Integrating nanowires with substrates using directed assembly and nanoscale soldering

This paper describes a new methodology for integrating nanowires with micropatterned substrates using directed assembly and nanoscale soldering. Nanowires containing ferromagnetic nickel segments were fabricated by electrodeposition in nanoporous membranes. The nanowires were released by dissolution of the membrane and subsequently aligned relative to micropatterned substrates using magnetic field-directed assembly. After assembly, the wires were permanently bonded to the substrates using solder reflow to form low-resistance electrical contacts. This is the first demonstration of the use of nanoscale solder reflow to form low-resistance electrical interconnects between nanowires and substrates, and we demonstrated the utility of the strategy by fabricating a nanowire-based functional analog integrator.

Lead-Free Chip Scale Packages: Assembly and Drop Test Reliability

Three underfill options compatible with lead-free assembly have been evaluated: capillary underfill, fluxing underfill, and corner bond underfill. Chip scale packages (CSPs) with eutectic Sn/Pb solder were used for control samples. Without underfill, lead-free and Sn/Pb eutectic drop test results were comparable. Capillary flow underfills, dispensed and cured after reflow, are commonly used in CSP assembly with eutectic Sn/Pb solder. With capillary flow underfill, the drop test results were significantly better with lead-free solder assembly than with Sn/Pb eutectic. Fluxing underfill is dispensed at the CSP site prior to CSP placement. No solder paste is printed at the site. The CSP is placed and reflowed in a standard reflow cycle. A new fluxing underfill developed for compatibility with the higher lead-free solder reflow profiles was investigated. The fluxing underfill with lead-free solder yielded the best drop test results. Corner bond underfill is dispensed as four dots corresponding to the four corners of the CSP after solder paste print, but before CSP placement. The corner bond material cures during the reflow cycle. It is a simpler process compared to capillary or fluxing underfill. The drop test results with corner bond were intermediate between no underfill and capillary underfill and similar for both lead-free and Sn/Pb eutectic solder assembly. The effect of aging on the drop test results with lead-free solder and either no underfill or corner bond underfill was studied. Tin/lead solder with no underfill was used for control. This test was to simulate drop performance after the product has been placed in service for some period of time. There was degradation in the drop test results in all cases after 100 and 250 h of storage at 125/spl deg/C prior to the drop test. The worst degradation occurred with the lead-free solder with no underfill.

Lead-free 0201 manufacturing, assembly and reliability test results

The trend towards smaller, faster and cheaper electronic devices has led to an increase in the use of 0201 ( L ∼ 0.02 in.; W ∼ 0.01 in.) and even smaller sized passive components. The size advantages of the 0201 component make it a popular choice among design engineers but not among manufacturing engineers. From a manufacturing perspective, the size of the 0201 package poses significant challenges to the printed circuit board (PCB) assembly process. The many challenges with 0201 assembly can be attributed to the solder paste volume, pad design, aperture design, board finish, type of solder paste, pick-and-place and reflow profile. If these factors are not optimized, they will introduce undesirable manufacturing defects. The small size of 0201 packages and undetected manufacturing defects will also raise concerns about their second level interconnect reliability, especially for lead-free solder alloys and surface finishes, with new processes and higher reflow requirements. To determine the optimum conditions, a design-of-experiment (DOE) study was carried out to investigate the effects of these parameters on assembly defects and solder joint reliability. This paper presents the test results and comparative literature data on the influence of a few key manufacturing parameters and defects associated with the 0201 component using lead-free and tin–lead solder alloys. Data pertaining to component shear strength before and after isothermal aging at 150 °C and intermetallic growth up to 500 h of aging are presented. A number of test vehicles were also subjected to thermal cycling (1500 cycles) in the range of −55/100 °C to determine the solder fatigue behavior. Shear test results for test vehicles subjected to thermal cycling is also presented. In addition, optical microscopy analysis of solder joint behavior during thermal cycling showing the progress of the solder damage and cross-sectional photos taken at 1500 cycles is included.

Deformation analysis of lap-shear testing of solder joints

A combined numerical and experimental study of lap-shear testing of solder joints was carried out. The primary objective was to provide a quantitative evaluation of shear deformation experienced by the solder. The numerical analysis was based on finite element modeling of deformation in the substrate–solder assembly. The calculated shear response, utilizing both the commonly adopted far-field measurements and the actual shear strain in solder, was found to differ significantly. The geometric and material parameters during lap shear were explored to provide physical insight into the problem. The nature of deformation of the substrate was seen to greatly influence the shear behavior of the solder. The “transmission” of shear strain into the solder is very ineffective when the solder is in the elastic state, resulting in smaller shear strains in the solder than the nominal values. This effect is also true even when the solder is deforming plastically. Experimental measurements of solder shear using direct optical measurements confirmed the numerical findings. Subtraction of the average deformation of the Cu substrate provides a reasonable, albeit not complete, approximation of the solder strain.

Electrostatically Actuated Micromirror Array Assembled by Using Solder Flip Chip Bonding and Electro-Thermal Fuse-Away Tethers

We propose a new electrostatically-actuated micromirror array. Using solder flip chip bonding technology, the desired electrode gap and the mirror rotation range can be easily created and enlarged. The use of solder assembly technology also provides us not only design flexibility but also precise gap height and mirror position control. In the mirror assembly process, a new MEMS transfer method using temporary fuse-away tethers is used to achieve robust and clean batch assembly. The mirror array is designed through FEM analysis and design optimization using a surface response method. The driving power of the optimized mirror system is reduced to one-third that of the initial design. The testing results of the fabricated mirror device show consistency with the predicted mirror performance and assembly precision.

The Alignment Shift Formation Mechanism of Thin Film Based DWDM Module With Solder Assembly Packaging

Thermal residual plastic deformation of bond joints on thin film based DWDM module under thermal cycles loading was investigated using three-dimensional finite element analysis. Finite element simulations were carried out to investigate the effect of the 4 and 6 solder-joint designs for the metal tube with 0.1 mm alignment offset. It was found that 6-solder joint requires more thermal cycles to arrive a stable tilt angle. The transverse movement of metal tube under thermal loading was also examined to determine the influence of solder volume imperfections and solder materials. Favorable results were obtained for 80Au20Sn solder as compared to 63Sn37Pb solder. From the thermal-elasto-plastic analysis, the solder volume control has great impact on the metal tube movement and tilt angle during thermal loadings. If adequate solder volume is provided in four solder joints, the minimum alignment shift of the metal tube is projected to be comparable to an optimally designed bond joint geometry.

A new phase for vapour‐phase – an overview of the current status of vapour phase technology for soldering and related applications

This paper focuses on the renewed interest in the use of condensation, or vapour‐phase, soldering in lead‐free surface mount assembly soldering and notes also the corresponding legislation, which will preclude the use of lead‐based solders in most applications by 2006. The process is once again becoming popular, because of the absolute control of soldering temperature, and because high‐quality batch and in‐line equipment is now available. It notes that condensation heating with perfluorocarbon liquids and present well‐designed equipment can be used not only in the mass‐assembly of electronic circuit boards, but also for a large variety of other heating tasks in many industries.

Properties of lead-free solder alloys with rare earth element additions

Due to the inherent toxicity of lead (Pb), environmental regulations around the world have been targeted to eliminate the usage of Pb-bearing solders in electronic assemblies. This has prompted the development of “Pb-free” solders, and has enhanced the research activities in this field. In order to become a successful solder material, Pb-free alloys need to be reliable over long term use. Although many of these alloys possess higher strength than the traditional Sn–Pb ones, there still exist reliability problems such as electromigration and creep. Also, the solderability of many Pb-free alloys is inferior to that of Sn–Pb and any improvement or replacement will be welcomed by industry. In order to develop new Pb-free solders with better properties, trace amounts of rare earth (RE) elements were selected by some researchers as alloying additions into Sn-based solders. These solder alloys are mainly Sn–Ag, Sn–Cu, Sn–Zn and Sn–Ag–Cu. In general, the resulting RE-doped solders are found to have better performances than their original ones. The improvements include better wettability, creep strength and tensile strength. In particular, the increase in creep resistance in some RE-doped alloys gives creep rupture time increases by over four times for Sn–Ag and seven times for Sn–Cu and Sn–Ag–Cu. Like other Sn-based alloys, their creep rates are controlled by dislocation pipe diffusion in the Sn matrix. Also, it was found that the creep rate of these Sn-based alloys can be represented by a single empirical equation. With the addition of RE elements, solders for bonding on difficult substrates such as on semiconductors, diamond, and optical materials have also been developed. This report summarizes the effect of RE elements on the microstructure, mechanical properties, wetting behavior of certain Pb-free solder alloys. As an illustration of the advantage of RE doping, interfacial studies were carried out for electronic interconnections with RE-doped Pb-free alloys. It was found that the intermetallic compound (IMC) layer thickness and the amount of interfacial reaction were reduced in a Ball Grid Array (BGA) package. These results indicate that RE elements would play an important role in providing better electronic interconnections.

Numerical simulation of shear stress distribution in as soldered Al-Al2O3assembly

AbstractThe shear stress distribution in an Al-Al2O3 soldering assembly after the cooling stage has been analysed via finite element numerical simulation. The effects of the coefficient of thermal expansion of the solder alloy and the fillet geometry have been investigated. The calculation results showed that, to achieve a minimal shear stress response, the coefficient of thermal expansion of the solder alloy should match that of the Al base metal and the geometry of the soldering fillet should be a concave shape having an extruding length somewhat less than the fillet height.

Open Defects in PBGA Assembly Solder Joints

PBGA has the advantage of being compatible with existing surface-mount process. The eutectic solder balls collapse during reflow accommodates the lack of co-planarity, the surface tension between solder ball and associated pad creates a strong self-centering property to compensate for any placement offsets. However, some problems emerged in practice due to its special features. The major challenges for manufacturer are the inspection of solder joint and to perform touch-up. In most cases, the defect of solder joint cannot be found until in-circuit or functional testing is performed. At this stage, it becomes difficult to determine whether the defect is due to a solder joint or the component itself. Hence controlling the open defect is very important in the PBGA assembling process. It was found that three kinds of defective modes are responsible for the open defect; insufficient heating in the solder melting phase, poor thermal stability of PCB and PBGA and insufficient amount of printing solder paste. Based on the defect mechanism analysis, by optimizing the soldering process, very high assembly yield was achieved.

A multi-component solder self-assembled micromirror

Solder self-assembled micromirrors have the advantages of rigid electrical and structural connections to the substrate as well as compact assembly mechanisms. In addition solder assembly allows a structure to be rotated to any angle desired. This work presents a design process that uses these advantages to produce a novel electrostatic micromirror that can stably rotate ±10° from its assembled position in a rotation range that does not include the plane of the substrate. The compact solder mechanism allows each of three mirror components to be assembled independently to individual angles. A lumped parameter model based on a torque balance between the electrostatic force on the mirror and the restoring torque of the torsion beams is used to create the mirror design. After fabrication and assembly the design is statically tested and compared to the predicted model performance.

Development of the micro rotary fan

Abstract This paper reports on the design and experimental evaluation of novel MEMS based rotary fans. Both flip-chip and solder self-assembled designs are demonstrated — each utilizing a scratch drive actuator (SDA) rotary motor with forward and reverse control at speeds up to 180 rpm. The SDA motor is powered using a sinusoidal 30–150 V0-P ac signal at frequencies from 2–15 kHz. Resonant mode vibration at 3 kHz allowed the fan motor to be driven using ac signals of less than 30 V0-P. The fan blades are rotated to a specific angle-of-attack using gold coated polysilicon bimorph structures or solder assembly mechanisms. The advantages to each assembly option are discussed with solutions applicable to many electromechanically coupled micro systems. The fabrication and assembly techniques used to create the MEMS fans are well suited for mass production for commercial applications. A versatile flow visualization technique for MEMS based gas flow actuators is also demonstrated using laser illuminated smoke particles introduced into a macro scale chamber containing the fan.

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/spl deg/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.

Characterization and Soldering of Organic Modified Aluminum Thin Films and Bond Pads

ABSTRACTAn immersion plating process using organic solutions was used to deposit a seed layer of copper and to activate the surface of thin film aluminum surfaces and bond pads. The seeded surfaces were subsequently electroless copper plated and tested for compatibility with soldering assembly. It was demonstrated that the copper particles deposited from the organic solution onto sputter deposited aluminum thin films were able to act as nucleation sites for the subsequent build up of adherent, continuous copper films from conventional electroless plating baths. In addition, the metal deposition from organic solution process was found to be very selective on patterned test vehicles and integrated circuits as metal deposition occurred only on the exposed aluminum bond pads and not on any passivated surfaces. Tests were conducted to determine the solderability of the samples after only seeding in the organic solution as well as the combination of seeding plus electroless copper deposition. In these tests, all attempts to directly solder without any pre-treatment were unsuccessful. Results on solder adherence varied with different substrates. Nevertheless, the concept of depositing metal seed layers in these applications is feasible. Additional work is needed, however, to optimize the deposition process for each particular surface to be soldered.

Wafer level and flip chip design through solder prediction models and validation

In flip chip package applications, bumped dies are flip-chip assembled to substrate metal pads creating joints that serve electrically and mechanically. Resulting solder joint profiles are defined by the solder bump volume, the under bump metallurgy (LTBM) area, and the substrate metal pad size and shape. Solder bump height and diameter was predicted by the geometrical truncated sphere model and surface evolver model at the wafer level, using the known solder volume deposited by stencil printing method. The surface evolver model was used to predict the assembled solder joint height, gap height, collapse height, and maximum bump diameter of flip chip assemblies. In turn, substrate pads were fine-tuned to achieve required gap heights. Collapse heights provided the means to develop assembly tolerances and relative risk of bridging was determined from knowledge of resulting bump diameters. Through validated design of the stencil printing technology and prediction of realistic bump and assembly solder geometries, the results are improved processes and die level design and assembly. Optimized design parameters are incorporated and accurately represented in simulation and experimentally validated with assemblies.

Pb-free surface-finishing on electronic components’ terminals for Pb-free soldering assembly

Pb-free solderable surface finishing is essential to implement Pb-free solder assembly in order to meet with the growing demand of environmental consciousness to eliminate Pb from electronic products. Two types of widely applicable Pb-free surface finishing technologies are developed. One is the multilayer-system including Pd with Ni undercoat. Heat-resistance of Pd enables whole-surface-plating on to leadframe before IC-assembling process. The other is the double-layer-system with low-melting-point-materials, for example, thicker Sn underlayer and thinner Sn-Bi alloy overlayer, dilutes Sn-Bi alloy’s defects of harmful reactivity along with substrate metal and mechanical brittleness with keeping its advantages of solder-wettability and no whisker.

Yield prediction for flip-chip solder assemblies based on solder shape modeling

A model has been developed to predict the yield of flip-chip solder assemblies based on calculated solder shapes. It is a physical model considering a limited number of physical factors such as the mean and the standard deviation of the solder volume distribution, the assembly warpage, the die size, and the number of inputs/outputs (I/O's). The main features of the model are: (1) the quick calculation of the solder height of every solder joint out of hundreds or thousands of connections; (2) the definition of failure criteria to determine the assembly yield. The quick calculation is accomplished by the use of a force model to estimate molten solder's normal reaction force corresponding to the solder's height, volume, circular pad diameter and surface tension coefficient. The failure criteria are height limits determined using surface evolver, which is a public tool for solder shape estimation. Outside the height limits, it is difficult to achieve convergent solutions; therefore, the limits are chosen as failure criteria since they implied large potential manufacturing variations in solder shapes. To illustrate the modeling capability, a case with 1 cm/spl times/1 cm chip, 1024 I/O's is studied. The model shows that a standard deviation of 25% for 4.48 e/sup -7/ cm/sup 3/ mean solder volume produces low yields. The variance of 19% for 4.48 e/sup -7/ cm/sup 3/ is required for a 100% yield.