Lead-free Solder Paste Research Articles

Comparative Modeling and Analysis of Lead-Free Solder Extrusion for the Design of Reliability

Abstract Due to rapid growth of the microelectronics industry, packaged devices with small form factors, low costs, high power performance, and increased efficiency have become of high demand in the market. To realize the current market development trend, flip chip interconnection and System-in-Package (SiP) are some of the promising packaging solutions developed. However, a surprising amount of surface mount technology (SMT) defects are associated with the use of lead-free solder paste and methods by which the paste is applied. Two such defects are solder extrusion and tombstoning. Through the use of design of experiments (DOE), lead-free solder defect causes can be better understood and subsequently reduced or eliminated. This paper will examine the failure modes of solder extrusion and tombstoning that occurred when two different types of lead-free solders, Sn-Ag-Cu (SAC) and BiAgX were used within a SiP for attachment of surface mount devices (SMD) chip components. The systematic investigation will include the comprehensive failure analysis of the SMD components and compare the modeling and analysis of the two different solder types utilizing the design of experiments methods.

Employment of roll-offset printing for fabrication of solder bump arrays: Harnessing the rheological properties of lead-free solder pastes using particle size distribution

This study aims to enable roll-offset (RO) printing for fabrication of solder bump arrays for microelectronic packaging by developing lead-free solder paste optimized for RO printing. Using lead-free solder pastes blended with particles of two different sizes (microparticles and nanoparticles, 2μm and 100nm in average diameter, respectively) in various proportions, we carefully characterized the effects of the particle size distribution on the rheological properties (viscosity, power law index, and loss/ storage shear moduli (G′ and G″)) of solder pastes. The solder paste formulated with only micro-particles showed a liquid-like viscous behavior dominant with a low viscosity and high power law index (n). Raising the nanoparticle proportion effectively increased the viscosity, the cross-over stress of G′ and G″ of solder pastes. Such a strong visco-elasticity imparted from the addition of nanoparticles did not originated only from the increase of the effective volume fraction by the particle size reduction, but also the microstructural effect of nanoparticle aggregation. RO printed solder bump arrays with a thickness of 8 ~ 38μm are fabricated using the solder pastes having power law index under 0.7 and viscosity below several tens of Pa·s.

Robust parameter design for the micro-BGA stencil printing process using a fuzzy logic-based Taguchi method

Solder paste is the main soldering material used to form strong solder joints between printed circuit boards (PCB) and surface mount devices in the surface mount assembly (SMA). On average 60% of end-of-line soldering defects can be attributed to inadequate performance of solder paste stencil printing. Recently, lead-free solder paste has been adopted by electronics manufacturers in compliance with the RoHS directive. However, soldering defects in the ball grid array (BGA) packages used in lead-free SMA have become more prevalent and are difficult to detect. In this study, a fuzzy logic-based Taguchi method is proposed to optimize the fine-pitch stencil printing process with multiple quality characteristics for the micro ball grid array (micro-BGA) packages using a lead-free solder paste. A structured data set is first collected from an L18 (21×37) fractional factorial design experiment, followed by multi-response optimizations and analysis of variance (ANOVA) for identifying significant factors. The optimization performance gained by the proposed fuzzy logic-based Taguchi method is compared with the results of other two hybrid methods including a combination of neural networks and genetic algorithms, and the integration of the response surface methodology with a desirability function. The confirmation experiments show that the proposed fuzzy logic-based Taguchi method outperforms the other two methods in terms of the signal-to-noise ratios and process capability index.

Nanoindentation of Sn3.0Ag0.5Cu/ENIG Solder Joint after High Temperature Storage

This study used nanoindentation technique in order to examine the micromechanical properties of Sn3.0Ag0.5Cu (SAC305) on Electroless Nickel Immersion Gold (ENIG) surface finished Cu substrate subjected to high temperature storage. Lead free solder paste of SAC305 were soldered on ENIG substrate by reflow soldering at 215 °C for 8 second. The soldered samples were exposed to high temperature storage at 180 °C for 0, 200, 400, 600, 800 and 1000 hours. Micromechanical properties show that the solder hardness is decreasing with the HTS time from 239.13 MPa for 0 hour to 178.96 MPa for 1000 hours while the reduce modulus results has increased from 62. 16 x 103 MPa for 0 hour to 82.13 x 103 MPa for 1000 hours. The value of hardness and reduced modulus from nanoindentation approach indicate the occurrence of plastic and elastic deformation throughout the test.

Thermal analysis of chip-on-flexible LED packages with Cu heat sinks by SnBi soldering

Purpose – The purpose of this paper is to discuss the possibility of using soldering process for the bonding of chip-on-flexible (COF) light-emitting diode (LED) packages to heat sinks. The common bonding materials are thermal conductive adhesives. For thermal performance and reliability concerns, Tin-Bismuth (SnBi) lead-free solder paste was used for the connection of the COF packages and the Cu heat sinks by a soldering process in this study. Meanwhile, the geometrical effect of the SnBi solder layer on the thermal performance was also investigated. Design/methodology/approach – The effects of the bonding materials and the area of the solder layers on the thermal performance of the LED modules were investigated by finite element simulation and experimental tests. Findings – The SnBi soldered modules show much lower thermal resistance at the bonding layers than the adhesive-bonded LED module. Vertical heat transfer from the LED chips to the heat sinks is the primary heat dissipation mode for the SnBi soldered modules. Thus, the LED module with local solder layer shows similar LED thermal performance with the full-area soldered module. Meanwhile, the local soldering process decreases the possibility to form randomly distributed defects such as the large area voids and residue flux in the solder layers. Research limitations/implications – The research is still in progress. Further studies mainly focus on the reliability of the samples with different bonding materials. Practical implications – COF package is a new structure for LED packages. This study provides a comparison between SnBi solder and adhesive material on the thermal performance of the LED. Meanwhile, the authors optimized the geometrical design for the solder layer. The study provides a feasible bonding process for COF packages onto heat sinks. Originality/value – This study provides a soldering process for the COF LED packages. The thermal performance of the LED light source was improved significantly by the new process.

A 3D-Capable, Flexible, Hybrid �ECoG Optrode

A 3D-Capable, Flexible, Hybrid �ECoG Optrode

Wettability study of lead free solder paste and its effect towards multiple reflow

Nowadays, wafer bumping using solder paste has come into focus as it provides a low cost method. However, since the industries are moving towards lead-free electronic packaging, a new type of no-clean flux was produced specifically for lead-free solder paste. Therefore, this study is used to evaluate the wettability of two different types of no-clean flux onto copper substrate. Besides, its effect towards multiple reflow was also studied. Reflow soldering was conducted for both types of solder paste that contained different type of no-clean flux for up to double reflow. Two different reflow profile was used. The results showed that the Flux A exhibit better soldering performance after first and second reflow soldering. In addition, type of intermetallic compound (IMC) found after first reflow remain the same even after second reflow which was Cu-Sn based. This is shows that Flux A manage to control the diffusion process which will finally leads to a better solder joint performance. Nevertheless, mechanical testing should be carried out in order to evaluate the solder joint strength.

Quantitative Evaluation of Voids in Lead Free Solder Joints

Voiding in solder joints poses a serious reliability concern for electronic products. The aim of this research was to quantify the void formation in lead-free solder joints through X-ray inspections. Experiments were designed to investigate how void formation is affected by solder bump size and shape, differences in reflow time and temperature, and differences in solder paste formulation. Four different lead-free solder paste samples were used to produce solder bumps on a number of test boards, using surface mount reflow soldering process. Using an advanced X-ray inspection system void percentages were measured for three different size and shape solder bumps. Results indicate that the voiding in solder joint is strongly influenced by solder bump size and shape, with voids found to have increased when bump size decreased. A longer soaking period during reflow stage has negatively affectedsolder voids. Voiding was also accelerated with smaller solder particles in solder paste.

Effect of SiC Particles Addition on the Wettability and Intermetallic Compound Layer Formation of Sn-Cu-Ni (SN100C) Solder Paste

The effects of SiC on wettability and intermetallic compound (IMC) formation of Sn-Cu-Ni solder paste composite were systematically investigated. Lead-free solder paste composite was produced by mixing silicon carbide (SiC) particle with Sn-Cu-Ni (SN100C) solder paste. The wettability of composite solder was studied by observing the contact angle between solder and copper substrate. The IMC phase formation on copper substrate interface was identified using X-ray diffraction (XRD). The phase as detected in the composite solder is Cu6Sn5.The wettability of composite solder was observed through contact angle between solder and copper substrate and Sn-Cu-Ni + 1.0 wt.% SiC shows improvements in wetting angle and suppresses the IMCs formation.

Investigation of Wettability of Lead Free-Solder on Bare Copper and Organic Solder Preservatives Surface Finishes

Wettability plays an important role in the integrity of solder joints especially in critical safety operations. It helps in attaining successful soldering and reliable solder joints. The angle of wettability determines the degree of solder contact and strength of the solder joint. Moreover, the contact angle between the solid substrate and the molten solder determines the interconnection reliability. This paper focuses on the wettability of lead free solder on OSP and bare copper substrate surface finishes. The wetting behaviour of lead free solder pastes on bare copper board and Organic Solderability Preservatives (OSPs) substrates were investigated. The test vehicles consist of FR4 PCB single sided 100% copper clad with a thick film metallization and FR4 PCB single sided OSP. The substrate dimension was 100 mm×160 mm×1.6 mm. A total of 480 bumps of Lead-free solder pastes (96Sn-3.8Ag-0.5Cu and 96.5Sn-3.0Ag-0.7Cu alloy composition) were deposited for the investigation. The printing parameters and their values for the experiments include print gap of 0 mm, squeegee speed of 20 mm/s, pressure of 8kg and separation speed of 3 mm/sec. The pastes are screen printed on the substrates using 0.125mm thick stencil mounted on DEK 260 series printing machine. After the stencil printing process, the solder bumps deposited on the substrates were reflowed using a Novostar 2000HT horizontal convention reflow oven with 4-stage (preheat-soak-reflow-cooling). The FTA188 Analyser was employed for measuring the angle of contact between the substrates and the bumps. The results showed that the wetting contact angle of copper surface finish is lower as compared to the organic solderability preservatives surface finish and the wetting angle decreases as the contact surface area increases.

Effects of Fe2NiO4 nanoparticles addition into lead free Sn–3.0Ag–0.5Cu solder pastes on microstructure and mechanical properties after reflow soldering process

This study investigates the effects of the addition of Fe2NiO4 nanoparticles into a SAC-305 lead-free solder paste. Iron, nickel, and oxide nano-elements were mixed with Pb-free solder alloying elements to produce a new form of nanocomposite solder paste, which can be a promising material in electronic packaging. The SAC-305 was mechanically added with 0.5, 1.5, and 2.5wt.% of Fe2NiO4 nanoparticles. The migration of nanoparticles in the nanocomposite solder paste to the outermost surface was clarified using the copper ‘sandwich’ method, which was performed after the reflow soldering process. Varying amounts of nanoparticles in the SAC-305 affected the IMC thickness and mechanical properties of the nanocomposite solder paste. The IMC thickness was reduced by 29.15%, 42.37%, and 59.00% after adding 0.5, 1.5, and 2.5wt.% of Fe2NiO4 nanoparticles in the SAC-305, respectively. However, via nanoindentation method, the hardness of the nanocomposite solder was improved by 44.07% and 56.82% after adding 0.5 and 1.5wt.% of Fe2NiO4 nanoparticles, respectively. If the addition of Fe2NiO4 nanoparticle exceeded 1.5wt.%, the hardness increased infinitely.

Photonic Flash Soldering of Thin Chips and SMD Components on Foils for Flexible Electronics

Ultrathin bare die chips and small-size surface mount device components were successfully soldered using a novel roll-to-roll compatible soldering technology. A high-power xenon light flash was used to successfully solder the components to copper tracks on polyimide (PI) and polyethylene terephthalate (PET) flex foils by using a lead-free solder paste. Results are compared with oven-reflowed solder joints on PI substrates. The delicate PET foil substrates were not damaged owing to the selectivity of light absorption, leading to a limited temperature increase in the PET foil while the chip and copper tracks were heated to a temperature high enough to initiate soldering. The microstructure of the soldered joints was investigated and found to be dependent on the photonic flash intensity. Reliability of the photonically soldered joints during damp heat testing and dynamic flexing testing was comparable with the reflowed benchmark and showed increased reliability compared with anisotropic conductive adhesives bonded on PET foils.

Methods for predicting corrosion on electronic products

Surface insulation resistance (SIR) measurements have become necessary to perform, especially due to the introduction of lead free solders. It has been reported that SIR and other test methods have to be used for evaluating the reliability of printed wiring boards. This paper presents an investigation on how the SIR test environment influences the test results. The factors varied were temperature and humidity. The temperature was varied between 40 and 85°C, and the relative humidity was varied between 60 and 85%. Furthermore, the influence of different types of process chemicals on SIR was evaluated. Seven lead free solder pastes, of which five were no-clean and two were water soluble, were compared. The influence of using conformal coating was also studied. These solder pastes were used to verify the developed method. A proper test method for SIR measurements is suggested in the paper, as well as a recommendation for use of conformal coating.

Shear strengths of CBGA/PBGA solder ball joints with lead-free solder pastes

The present study is carried out to characterise the ball shear strengths of ceramic ball grid array (CBGA) and plastic ball grid array (PBGA) packages with lead-free solder pastes. Three commercially available Sn/Ag/Cu (SAC) solder pastes such as Sn 96.5/Ag3.0/Cu0.5 and 95.5/Ag3.8/Cu0.7 with a no-clean flux are employed. The effects of temperature profiles during reflow on the strength of solder joints are studied. Their printing abilities are also observed. Of particular concern for ceramic ball grid array packages is the interaction between 90/10 Pb/Sn compound solder balls and SAC solder pastes. The composition of the solder paste region between the I/O pad and the solder ball is analysed using the energy dispersive X-ray (EDX) machine.

Investigation of the Mechanical Properties of Lead-Free Solder Materials

Reliability and life-time of electric/electronic products is still a highly actual and discussed topic. Every electric/electronic product is heterogeneous system which consists not only from components and constructional parts, but also from many interconnections. Basic segments of interconnection are solder joints, recently created by lead-free materials. The passing from well-proven lead solders to lead free compositions brought many new aspects in reliability and life-time of electronic systems. Some of the negative factors are their poor electro-mechanical properties, which are based on complicated microstructure of interlayer interface. This fact causes defects in solder joints and consequential failure of the function. This article is focused on an investigation of mechanical properties of lead-free solder joints. There are investigated various properties of common lead-free solder paste depending on various concentrations of O2 by reflow process and on various components size. Next, there are shown microstructures by micro sections for various concentrations of O2. Mechanical properties (mechanical strength) are tested by shear test equipment DAGE 2400.

A Drop-in Die-Attach Solution for the High Temperature Lead-Free BiAgX Solder Paste System

In the current work, a mixed powder BiAgX solder paste system with the melting temperature above 260°C and comparable, or better, reliability to the high lead-containing solders has been studied. The mixed powder solder paste system is composed of a high-melting first alloy solder powder as a majority and the additive solder powder as a minority. The additive solder is designed to react aggressively with various surface finish materials before, or together with, the melting of the majority solder to form a controllable IMC layer. The IMC layer of the mixed powder system is controllable by the species and quantity of the additive solder, and it is observed to be insensitive to thermal aging and thermal cycling in current tests, while the high lead-containing solders show a considerable increase in IMC layer thickness. Microstructure investigation shows that the fishbone shaped IMC layer interlocks with the bonding interface between solder and components. Both micron-sized and nano-sized Ag-rich precipitations in the joints have been observed to be well distributed in the joint. The exposed Ag-rich particles and the surrounding stepwise pattern in the Bi matrix on the fracture surface indicate that these Ag-rich particles constrain the dislocation movement in Bi matrix, enhancing the strength and the ductility of the joint. Under thermal aging and thermal cycling, both the micron-sized and nano-sized Ag-rich precipitations exhibit only discernible and localized coarsening. The stable interfacial IMC together with the existence of the well-dispersed Ag-rich particles are attributed to the promising reliability in the BiAgX solder paste system.

Analysis of Pin in Paste Soldering on Printed Circuits Boards Assembly Using Lead Free Solder Paste

The THC (Through Hole Components) assembly in a SMT board using Pin-in-Paste (PIP) process was studied and implemented using SAC lead-free solder paste. The main idea of this process is to solder in the same thermal step THCs and SMDs (Surface Mount Devices), eliminating the step of wave soldering which is the most used in electronics assembly today for THCs. As results one could achieve reduction on production time and costs, as well as improvements in soldering quality and also environmental aspects. The experiment was conducted to identify the best PCB and stencil design parameters. Printing and reflow parameters were also considered to reach the best solder joints. Some problems could be found with solder excess but the solder quality of the reflowed boards was acceptable for almost all components.

Evaluation of rheological properties of lead-free solder pastes and their relationship with transfer efficiency during stencil printing process

Although stencil printing is widely used in surface mount technology, it is believed to be the main source of majority of defects in the final assembly. Such defects, which could lead to major reliability issues, can be controlled and/or minimised through proper understanding and control of the flow and deformation behaviour of solder pastes. This study concerns the characterisation of three different Pb-free solder pastes used for ultra-fine pitch assembly applications. We also investigate the paste transfer efficiency through linearly varying stencil apertures sizes, and correlate the paste rheological properties with their transfer efficiency – to provide further understanding of the effects and interactions of stencil printing process variables on the paste transfer efficiency. Three rheological tests, namely: the viscosity, thixotropic and oscillatory amplitude sweep were employed in the characterisation. The paste samples were printed on Cu substrates using stencil printing (with varying aperture cavity dimensions in the range 0.79 mm 3–1.62 mm 3). A three-level design on two factors experiment, 3 2, was used to determine significant level of parameters in terms of main effect and interactions. Our results show that the paste type and volume of stencil aperture interact during printing and that pastes with unique rheological properties produced distinctive transfer pattern. The results also show that the printing process variables and their interactions were significant on transfer efficiency of pastes. These results will be of interest to R&D staff involved in formulation of new Pb-free pastes and the design of stencils for ultra-fine pitch assembly applications.

Oscillatory and Creep Recovery Test on Nano-Composite Lead-Free Solder Pastes (Nickel and Platinum Powder)

This study investigates the relationship of rheological properties of nanoparticles added to lead-free solders paste. For lead-free solder paste, a large number of combinations can be made from different elements. Every ingredient plays a role of the paste and the rheological properties. The effect of filler concentration was studied in this present investigation. Creep recovery, stress is applied on samples for a period of time and later release in order for it to recovery itself. In the other words, creep recovery involves a tensile specimen under constant load maintained at a constant temperature. Oscillatory tests were carried out to study the visco-elastic behaviour and paste stability. The visco-elastic behaviour of the paste lies between solid and liquid characteristic of the paste, which could be used to study the flow behaviour of paste during stencil printing process. From this study, it was found that the solid characteristic, G' is higher than liquid characteristic, G'' for all the samples. In addition, the results from the study showed that the 0.06 volume fraction sample system with a large yield point which G'= G'' has a higher cohesiveness and hence resulting in poor withdrawal of the paste during stencil printing process. The phase angle, δ was used to correlate the quality of the dense suspensions to the formulation of solder paste materials. Lower value of phase angle for 0.06 volume fraction sample system may indicating that the paste is very tacky compare to the other samples. As the demand for lead-free pastes increases rheological measurements can assist with the formulation of new pastes.

Rheology and Processing of Ni Based Nano-Composite Pastes for Electronic Assembly

Lead-free solder paste printing process accounts for majority of the assembly defects in the electronic manufacturing industry. In the stencil printing process, the solder paste must be able to withstand low and high shear rates, which results in continuous structural breakdown and build-up. The study investigates the effect of nickel additional to the thixotropic behaviour of lead-free Sn/Ag/Cu solder pastes using the structural kinetic model. A hysteresis loop test and constant shear test is utilized to investigate the thixotropic behaviour of the pastes using parallel plate rheometry in temperature 30 oC. In this study, the shear rates were increased from 0.01 s-1 to 10s-1 and the second curve was a result of decreasing shear rate from 10s-1 to 0.01s-1. For the constant shear test, the samples were subjected to two constant shear rates of 0.1s-1 and 1s-1. The constant shear rate test was designed to study the structural breakdown and build-up of the paste materials. From this investigation, hysteresis loop test was shown to be an effective test method to differentiate the extent of structural recovery in the solder pastes. The results of the structural parameter values obtained from the kinetic model showed a good correlation to the breakdown and build-up of the paste structure.