Solder Assembly Research Articles

Process optimization of SnCuNi soldering material using artificial parametric design

The European Union has implemented the directive restriction of hazardous substances (RoHS) prohibiting the uses of tin-lead solder. SAC305 (Sn96.5/Ag3.0/Cu0.5) has come into widespread use as a candidate soldering material in the electronics manufacturing industry. Nevertheless, the price of silver has increased dramatically in recent years. This study evaluates the feasibility of replacing the commonly used SAC305 with low cost SnCuNi (Sn99.25/Cu0.7/Ni0.05/Ge; SCN) solder alloy in wave soldering for high layer count printed circuit board. However, the melting temperature of SCN alloy is 227, 10 $$^{\circ }\text{ C}$$ higher than SAC305. The objective of this research is to investigate manufacturing issues and propose an optimal process. Process parameters such as soldering temperature and dwell time are determined to achieve the desired quality levels. Multiple quality characteristics, namely assembly yield and solder joint pull strength, are considered. Thus, this study compares two approaches, integration of principal component analysis/grey relational analysis and artificial neural networks (ANN) combined with genetic algorithms (GA), to resolve the problems of multiple quality characteristics. The results of verification test shows that samples prepared with the process scenario suggested by the ANN combined with GA are superior. The process scenario with maximum desirability value is 268.64 $$^{\circ }\text{ C}$$ soldering temperature and 7.42 s dwell time, indicating the recommended manufacturing process.

Induction devices for assembly soldering in electronics

Induction devices with a magnetic core are capable of very accurately maintaining the temperature of the heating of components at the high heat transfer in the process of assembly soldering. The maximum heating rate at the optimal parameters of the process reaches 80°C/s. This allows one to perform the contactless heating of small size components in the inductor’s magnetic core gap.

Sn–Cu–Ni Soldering Process Optimization Using Multivariate Analysis

One lead free solder candidate, Sn-Ag-Cu305 (Sn96.5/Ag3.0/Cu0.5), has come into widespread use as a replacement for traditional tin-lead solder (Sn63/Pb37). However, the price of silver has increased dramatically in recent years. This has increased manufacturing costs, impacting firm competitiveness. This paper evaluates the performance of the low cost Sn-Cu-Ni solder alloy used for wave soldering. Sample products for several applications are used to assess critical factors in the wave soldering process. A principal component analysis was performed in this paper to integrate multiple quality characteristics, namely assembly yield and solder joint strength, for process development. As a result, a parameter combination of 270 soldering temperature and 8 s dwell time is recommended. A test vehicle with daisy-chain circuitry design is used to investigate the performance of samples prepared by the optimal process. Samples are subject to the thermal cycling test. The performance of the recommended process is therefore verified.

Comprehensive Study of Lead-Free Reflow Process for a 3-D Flip Chip on Silicon Package

This paper explores the lead-free (Pb-free) solder reflow process to achieve high assembly yield and solder joint quality for a 3-D silicon-on-silicon flip chip package. Three parameters, which included soak time, peak temperature, and time above liquidus (TAL), were investigated using statistical analysis to identify the nonlinear effect (quadratic effect) of each factor as well as the interactions among the factors. Different reflow ambient gases such as air and nitrogen were investigated to understand the sensitivity of the yield under ambient gases. This paper also revealed that 100% assembly yield can be achieved with a wide process window on the structured silicon-on-silicon flip chip package under nitrogen reflow environment, by investigating 15 reflow profiles designed following the response surface methodology. Within the reflow process window, the solder joint intermetallic compound (IMC) thickness was measured by calculating the average IMC thickness using a mathematical integration method. It was concluded that an increase in TAL results in an increase of IMC thickness.

Conductive filament formation in printed circuit boards: effects of reflow conditions and flame retardants

Conductive filament formation, a major cause of failures in printed circuit boards, is an electrochemical process that involves the transport of a metal through or across a nonmetallic medium under the influence of an applied electric field. With an increasing potential to market “green” electronics, environmental and health legislations, and the advent of lead-free and halogen-free initiatives, newer types of printed circuit board materials are being exposed to ever higher temperatures during solder assembly. The higher temperatures can weaken the glass-fiber bonding, thus enhancing conductive filament formation. The effects of the inclusion of halogen-free flame retardants on conductive filament formation in printed circuit boards are not completely understood. Previous studies, along with analysis and examinations conducted on printed circuit boards with failure sites that were due to conductive filament formation, have shown that the conductive path is typically formed along the delaminated fiber glass and epoxy resin interfaces. This paper is a result of a year-long study on the effects of reflow temperatures, halogen-free flame retardants, glass reinforcement weave style, and conductor spacing on times to failure due to conductive filament formation.

Investigation of a connector electrical failure

AbstractA PC product built by Celestica experienced a high rate of functional failure. Failure was associated with the CPU and CPU connector (known as a zero insertion force, ZIF type) since mechanically flexing the CPU stack induced failure. This problem became particularly serious when the customer adopted an uncontrolled (manual) “press” test, resulting in a large amount of rejected product which may have been functional under normal operating conditions.A failure analysis method involving static loading the CPU and epoxy potting the connector stack was developed to mechanically “trap” the fail condition. Sectioning and electrical probing was used to determine which of the 240 active connector pins have high resistance.Analysis of suspect connector pins and contact surfaces pointed to several possible causes of failure. SEM‐EDX revealed localized damage to the Au plating with exposed Ni and NiO. XPS and TOF‐SIMS with depth profiling confirmed the presence of a ∼100 nm layer of a fluorocarbon on the Au surface. Although it was not possible to clearly locate the individual electrical contact spots (1–10 µm diameter) to identify localized contamination, the presence of a probable insulating coating was sufficient evidence to follow‐up with the connector supplier. The connector spring contacts were found to be coated with an “anti‐flux” agent to prevent wicking of liquid solder onto contact surfaces during the wave solder assembly process. The corrective action was to change the ZIF connector to a type without anti‐flux coating and the failure rate was significantly reduced. Copyright © 2010 John Wiley & Sons, Ltd.

Permanent Attachment of Silicon Structures via Joule Heat Induced Welding

We introduce a new method for the permanent attachment of assembled silicon microstructures using the precise application of current for localized welding. The attachment technique for planar SOI structures was demonstrated using an assembly mechanism that allows the post-release adjustment of micromachined gaps. The concept utilizes a mechanical assembly technique that takes advantage of the motion reduction experienced by the individual beams of a folded suspension ultimately allowing sub-micron position control of released structures. The proposed mechanism was incorporated into the sense mode of a symmetrically decoupled MEMS tuning fork gyroscope in order to demonstrate its potential for the realization of high performance inertial sensors. Two types of mechanisms were implemented resulting in 7.8 and 8.8 % motion reduction allowing capacitive gaps of 7.5 μm to be adjusted to 1.67 μm; the gaps can be further reduced by alternate suspension designs and implemented on arbitrarily thick layers. After assembly, the mobile electrode structure was rigidly attached to the anchor by locally melting the interface via a controlled current. The weld is achieved by applying 15 V with a current limit of 3 mA across the latching mechanism resulting in the permanent attachment of the two structures. Thus, when the desired gap is achieved, the previously released anchor can be fixed to eliminate any potential spurious motion of the electrodes. Our results demonstrate feasibility of the approach, which can be extended beyond planar devices to include out of plane and die-level 3-D assembled structures that typically rely on epoxy bonding or solder assembly to maintain their shape.

Influence of electrochemical properties on electrochemical migration of SnPb and SnBi solders

SnPb solders have been widely used in many fields because of their excellent solderability. To respond to environmental and health concerns regarding SnPb solders, it is necessary to develop lead-free solder for electronic assembly. In this paper, the resistance to the electrochemical migration of eutectic SnPb and SnBi solder alloys was evaluated and analyzed to detect the difference in the electrochemical migration behavior between these solders by their electrochemical properties. Pb and Bi additions to a Sn-base solder improved the resistance to electrochemical migration because of enhanced polarization properties. The resistance was closely related to the cathodic deposition efficiency and the anodic dissolution behavior. In addition, the composition of the dendrite formed by the electrochemical migration was related to the standard electrode potential of the alloying element.

Effect of Mn on Sn–Ag–Cu ternary lead free solder alloy–Cu assembly: a comparative study

In the present investigation, transition joints were produced by soldering Sn–Ag–Cu and Sn–Ag–Cu–Mn alloys to Cu substrate. Microstructure and mechanical properties of the assemblies were determined in reflowed condition and after aging at 100°C for variable time. The shear strength as well as the reaction layer thickness of Sn–Ag–Cu versus Cu is lower in comparison to Sn–Ag–Cu– Mn versus Cu solder assembly in reflowed condition. The reaction zone for Sn–Ag–Cu versus Cu is decorated with Cu3Sn and Cu6Sn5; however, in the case of Sn–Ag–Cu–Mn versus Cu assembly only Cu6Sn5 was observed. Mn is responsible for enhanced width of Cu6Sn5, which act as a barrier layer restricting the diffusion of Sn across the interface and Cu3Sn formation has not been found even after aging of 200 h. Aging treatment leads to growth of intermetallic layer for both the joints and decrement in shear strength of the assemblies. Still Sn–Ag–Cu–Mn versus Cu solder assembly exhibits superior mechanical strength than Sn–Ag–Cu versus Cu transition joint.

Comparison of Printed Circuit Board Property Variations in Response to Simulated Lead-Free Soldering

Use of lead-free solders such as Sn/Ag/Cu results in exposure of printed circuit boards to higher temperatures during assembly compared with eutectic tin-lead solder. If the thermo-mechanical and electrical properties of the laminate materials get affected by exposure to this higher temperature, that may impact the performance and reliability of the circuit board. Variations, if any, in laminate material properties before and after board assembly should be considered in the selection of appropriate laminates. The board and system designers need to be cognizant of such variations and account for them during laminate selection for an application. This paper presents guidelines for laminate selection along with the process used to derive the guidelines. The process includes measurement of key material properties (glass transition temperature, coefficient of thermal expansion, decomposition temperature, time-to-delamination, water absorption, flammability, dielectric constant, and dissipation factor), and their responses to lead-free soldering assembly conditions. A range of commercially available FR-4 printed circuit board laminate materials, classified on the basis of glass transition temperature (high, medium, and low), curing agents (dicyandiamide and phenolic), flame retardants (halogenated and halogen-free), and the presence of fillers, are included in the measurements. The measurements are conducted in accordance with IPC-TM-650 test methods before and after exposure to multiple lead-free soldering profiles. The extent of variations in the properties due to lead-free soldering exposures are reported and analyzed as a function of classification parameters. The causes behind the variations in material properties are investigated by Fourier transform infrared spectroscopy analysis and a conjunctional property analysis. This study also suggests that the preconditioning steps specified in the IPC test methods should address the initial moisture content of the laminate test samples in material property measurement tests, otherwise significant errors can be introduced.

Recent applications of optical and computer-vision methods to research for microelectronics assembly reliability

Material and structure evaluation for reliability is a key research area for advanced microelectronics and miniatures. This paper presents a new experimental mechanics-based approach devoted to the evaluation of critical failure conditions for solder materials and assembly structures. Several optical and computer-vision-based metrological techniques have been adapted and employed for three-dimensional (3D), multi-scale and real-time deformation measurement. Case studies are presented to illustrate how the approach accomplishes an evaluation with high efficiency and low cost. The paper raises the prospect for the in-situ deformation measurement to play a more extensive role in this area.

Validated High Speed Pull and Shear Test Methodologies to Evaluate Pb-Free BGA Mechanical Strength

The conversion to lead free Ball Grid Array (BGA) packages has raised several new assembly and reliability issues. Lead-free solder joints are generally stiffer than tin-lead solder joints, and mechanically induced failures have become more prevalent in lead-free solder assemblies. Traditionally, assembly bend and shock testing is performed to evaluate mechanical assembly reliability. However, bend and shock tests are expensive, cumbersome and not feasible for evaluating lot-to-lot variations in mechanical strength. Consequently, there is a need for a validated, component level test method that can be used as an accurate indicator of assembly level mechanical strength. Several publications have been written to evaluate the efficacy of high speed pull and shear testing as a viable reliability indicator [1 – 10]. In this study, a comprehensive Gauge R&R study was performed to evaluate the accuracy of the test equipment, including high speed video calibration. Then, test studies were performed to compare the accuracy of the results, spread across different package constructions, solder metallurgies, ball pitch and surface finish. In addition, the effect of parameters like multiple reflows and aging on specific metallurgies and surface finishes was studied. The results were generated over more than 2000 test runs. Finally, the study rank orders all critical test parameters and articulates what precise steps can be taken to generate relevant data for standard and custom devices during early evaluation stages and during high volume manufacturing.

Flexible Electronics: Thin Silicon Die on Flexible Substrates

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Silicon thinned to 50 <formula formulatype="inline"><tex Notation="TeX">$\mu{\hbox {m}}$</tex></formula> and less is flexible allowing the fabrication of flexible and conformable electronics. Two techniques have been developed to achieve this goal using thinned die: die flip chip bonded onto flexible substrates [polyimide and liquid crystal polymer (LCP)] and die flip chip laminated onto LCP films. A key to achieving each of these techniques is the thinning of die to a thickness of 50 <formula formulatype="inline"><tex Notation="TeX">$\mu{\hbox {m}}$</tex></formula> or thinner. Conventional grinding and polishing can be used to thin to 50 <formula formulatype="inline"><tex Notation="TeX">$\mu{\hbox {m}}$</tex></formula>. At 50 <formula formulatype="inline"><tex Notation="TeX">$\mu{\hbox {m}}$</tex></formula>, the active die becomes flexible and must be handled by temporarily bonding it to a holder die for assembly. Both reflow solder and thermocompression assembly methods are used. In the case of solder assembly, underfill is used to reinforce the solder joints. With thermocompression bonding of the die to an LCP substrate, the LCP adheres to the die surface, eliminating the need for underfill. </para>

Effect of Lead-Free Soldering on Key Material Properties of FR-4 Printed Circuit Board Laminates

The transition to lead-free soldering of printed circuit boards (PCBs) using solder alloys such as SnAgCu has resulted in higher temperature exposures during assembly compared with eutectic SnPb solders. The knowledge of PCB laminate material properties and their dependence on the material constituents, combined with their possible variations due to lead-free soldering temperature exposures, is an essential input in the laminate selection process. This paper provides laminate selection guidelines that were arrived at by assessing key material properties (glass transition temperature, coefficient of thermal expansion, decomposition temperature, and water absorption), and their responses to lead-free soldering assembly conditions. A range of commercially available FR-4 PCB laminate materials, classified on the basis of glass transition temperature (high, mid, and low), curing agents (dicyandiamide and phenolic), flame retardants (halogenated and halogen-free), and fillers (presence or absence) were studied. The laminate material properties under investigation were measured as per the IPC-TM-650 test methods before and after exposure to multiple lead-free soldering cycles. Combinatorial property analysis was conducted to investigate the causes behind variations in material properties.

Electrochemical composite deposition of Sn–Ag–Cu alloys

The dominant materials used for solders in electronic assemblies over the past 60 years have been Pb–Sn alloys. Increasing pressure from environmental and health authorities has stimulated the development of various Pb-free solders. One of the most promising replacements is eutectic or near-eutectic Sn–Ag–Cu alloys produced by electrodeposition. In this study, simple and “green” Sn–Cu-citrate solutions with suspended Ag particles have been developed and optimized for electrochemical composite deposition of eutectic and near-eutectic Sn–Ag–Cu solder films. Different plating conditions, including solution concentration, current density, agitation and additives, are investigated by evaluating their effects on plating rate, deposit composition and microstructure.

Microstructure, Defects, and Reliability of Mixed Pb-Free/Sn-Pb Assemblies

The results of an intensive reliability study on Pb-free ball grid array (BGA)/Sn-Pb solder assemblies as well as some lessons learnt dealing with mixed assembly production at Celestica are described in this paper. In the reliability study, four types of Pb-free ball grid array components were assembled on test vehicles using the Sn-Pb eutectic solder and typical Sn-Pb reflow profiles with 205°C to 220°C peak temperatures. Accelerated thermal cycling (ATC) was conducted at 0°C to 100°C. The influence of the microstructure on Weibull plot parameters and the failure mode will be shown. Interconnect defects such as nonuniform phase distribution, low-melting structure accumulation, and void formation are discussed. Recommendations on mixed assembly and rework parameters are given.

Low-Level Lead Exposure Among South Korean Lead Workers, and Estimates of Associated Risk of Cardiovascular Diseases

This study investigated the distribution of blood lead (PbB) levels, especially low levels, among Korean lead workers. The authors also estimated the potential effects of PbB on the blood pressure (BP) and cardiovascular diseases using models taken from the published meta-analyses. The PbB data from a total of 13,043 lead workers in 1217 companies throughout Korea were used. The geometric mean PbB level was 6.08 μ g/dL (geometric standard deviation was 2.53), and 56.6% of the workers showed PbB levels greater than 5 μ g/dL. Females accounted for 31.3% of all Korean lead workers in 2003. Considering two factors, such as PbB levels and the number of lead workers, the relatively important industry subclasses were identified as Manufacture of Accumulators; Manufacture of Other Electronic Valves, Tubes, Electronic Components n.e.c.; and Manufacture of Other Parts and Accessories for Motor Vehicles. The industrial processes of relative importance included battery assembly, acid treatment, and other soldering. Although uncertainties exist in the prediction model and associated model parameters, the authors attempted to estimate potential adverse health effects related to the lead exposure. It was estimated that 7383 South Korean lead workers might have increased blood pressure and the health risks due to the lead exposures in 2003. The highest estimated risk of BP increases due to lead exposures was expected in workers of industrial subclasses and processes, such as Other Basic Nonferrous Metal Industries and Maintenance. If the models in this study were applied to the South Korean population, the impact fractions for cardiac disease among lead workers would be estimated at 4.9–12.8 times those of the general population.

Silicon microphones: a commercial success? and what comes next

The first silicon microphone commercial product was introduced to the market in late 2005. As of early 2008 the cumulative production is well over a half billion units and the production rates continue to rise. To date the acoustical performance of these microphones is only as good as that of the of the lowest priced miniature electret microphones. Yet acoustical systems manufacturers often prefer silicon microphones because they are compatible with automated insertion and wave soldering assembly methods. Silicon microphones are also more environmentally stable in sensitivity than are electrets which may be important in matched microphone applications. Current research activities are investigating methods to reduce the internal noise improve matching tolerances, and further reduce manufacturing costs. This paper will review the current state of silicon microphones in commercial manufacture, and will survey some of the areas of current research in an attempt to suggest some possible directions of future market development.

Electrical measurement of a lead-free solder assembly after environmental tests by SEM internal probing

In this study, electrical characterization for lead-free materials in bump technology was developed and joint assemblies were thermally treated under temperature cycling tests. Measurements of sheet resistivity and contact resistance of intermetallic compounds (IMCs) in solder joints were conducted. Quantitative analysis and elemental redistribution of IMCs were obtained by field-emission electron probe microanalyzer. A scanning-electron microscopy internal probing system was introduced to evaluate electrical characteristics in IMCs after thermal treatments. To determine resistivity of IMCs, a novel method incorporating SELA-EM2 and a focused ion beam was developed to prepare the joint sample.

Evolution of mechanical and electrical properties of tin–lead and lead free solder to copper joint interface

Cu–(Sn37Pb) and Cu–(Sn3.5Ag0.5Cu) solder joints were prepared at the same reflow temperature of 230 °C. The microstructural observation of the solder assemblies in scanning and transmission electron microscopes confirmed the presence of η-Cu 6Sn 5 in case of the former, and Cu 3Sn + η-Cu 6Sn 5 for the latter in the reaction zone. The findings are correlated with the electrical and mechanical properties of the joints. Lead free solder-Cu joint exhibited lower reaction zone thickness and improved electrical conductivity (0.28 × 10 6Ω − 1 cm − 1 ) and shear strength ∼ 68MPa compared to conventional lead–tin solder-Cu joint. The latter showed electrical conductivity and shear strength of 0.22 × 10 6Ω − 1 cm − 1 and ∼ 55 MPa, respectively. The difference in reaction zone thickness is explained on the basis of melt superheat, with Sn being the primary diffusing species in the intermetallic layer.