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The purpose of the present study was to review the thermo-mechanical challenges of reflowed lead-free solder joints in surface mount components (SMCs). The topics of the review include challenges in modelling of the reflow soldering process, optimization and the future challenges in the reflow soldering process. Besides, the numerical approach of lead-free solder reliability is also discussed.

Lead-free reflow soldering is one of the most significant processes in the development of surface mount technology, especially toward the miniaturization of the advanced SMCs package. The challenges lead to more complex thermal responses when the PCB assembly passes through the reflow oven. The virtual modelling tools facilitate the modelling and simulation of the lead-free reflow process, which provide more data and clear visualization on the particular process.

With the growing trend of computer power and software capability, the multidisciplinary simulation, such as the temperature and thermal stress of lead-free SMCs, under the influenced of a specific process atmosphere can be provided. A simulation modelling technique for the thermal response and flow field prediction of a reflow process is cost-effective and has greatly helped the engineer to eliminate guesswork. Besides, simulated-based optimization methods of the reflow process have gained popularity because of them being economical and have reduced time-consumption, and these provide more information compared to the experimental hardware. The advantages and disadvantages of the simulation modelling in the reflow soldering process are also briefly discussed.

This literature review provides the engineers and researchers with a profound understanding of the thermo-mechanical challenges of reflowed lead-free solder joints in SMCs and the challenges of simulation modelling in the reflow process.

The unique challenges in solder joint reliability, and direction of future research in reflow process were identified to clarify the solutions to solve lead-free reliability issues in the electronics manufacturing industry.

The use of Taguchi experimental design techniques to examine the effects of package type, solder paste type and solder reflow technique on the quality of fine pitch surface mount IC package solder joints is described. In particular, the effect of the use of ceramic or plastic packages, copper or Alloy 42 leadframes, silver loaded or non‐silver loaded solder paste and infra‐red, laser or hot‐bar reflow on solder joint metallurgical structure, electrical resistance and mechanical strength is evaluated. In addition to these solder joint parameters, an associated visual inspection was used to find the best process parameters to minimise solder balling, bridging etc. and a correlation between paste contacts at placement and solder bridges after reflow was also conducted. The experiment used an L9 array to find the optimum parameters from three factors, each at three levels. An extension to the basic Taguchi array was included in the form of an outer (noise) factor to include the effect of climatic stress on the solder joints under investigation. Response tables separate out the contribution of each factor level to the mechanical strength and electrical resistance of the assemblies. By comparing the response tables before and after climatic testing it is possible to estimate the effect of each factor level on the long‐term quality of the solder joints. It is shown how Taguchi experimental design techniques can be used to minimise the number of experiments required to predict optimum solder assembly process parameters. The accuracy of the prediction is shown by the results of a confirmation run which yielded mechanical strengths very close to those predicted, both before and after highly accelerated stress testing of the solder assemblies.

Reflow soldering is one of the most significant factors in determining solder joint defect rate. This study aims to introduce an innovative approach for optimizing the multiple performances of the reflow soldering process.

This study aims to minimize the solder joint defect rate of a ball grid array (BGA) package by using the grey‐based Taguchi method. The entropy measurement method was employed together with the grey‐based Taguchi method to compute for the weights of each quality characteristic. The Taguchi L18 orthogonal array was performed, and the optimal parameter settings were determined. Various factors, such as slope, temperature, and reflow profile time, as well as two extreme noise factors, were considered. The thermal stress, peak temperature, reflow time, board‐ and package‐level temperature uniformity were selected as the quality characteristics. These quality characteristics were determined using the numerical method. The numerical method comprises the internal computational flow that models the reflow oven coupled with the structural heating and cooling models of the BGA assembly. The Multi‐physics Code Coupling Interface was used as the coupling software.

The analysis of variance results reveals that the cooling slope was the most influential factor among the multiple quality characteristics, followed by the soaking temperature and the peak temperature. Experimental confirmation test results show that the performance characteristics improved significantly during the reflow soldering process.

The proposed approach greatly reduces solder joint defects and enhances solutions to lead‐free reliability issues in the electronics manufacturing industry.

The findings provide new guidelines to the optimization method which are very useful for the accurate control of the solder joint defect rate within components and printed circuit board (PCB) which is one of the major requirements to achieve high reliability of electronic assemblies.

This paper aims to develop an estimation tool for warpage behavior of slim printed circuit board (PCB) array while soldering with electronic components by using finite element method. One of the essential requirements for handheld devices, such as smart phone, digital camera, and Note-PC, is the slim design to satisfy the customers’ desires. Accordingly, the printed circuit board (PCB) should be also thinner for a slim appearance, which would result in decreasing the PCB’s bending stiffness. This means that PCB deforms severely during the reflow (soldering) process where the peak temperature goes up to 250°C. Therefore, it is important to estimate PCB deformation at a high temperature for thermo-mechanical quality/reliability after reflow process.

A numerical simulation technique was devised and customized to accurately estimate the behavior of a thin printed board assembly (PBA) during reflow by considering all components, including PCB, microelectronic packages and solder interconnects.

By applying appropriate constraints and boundary conditions, it was found that PBA’s warpage can be accurately predicted during the reflow process. The results were also validated by warpage measurement, which showed a fairly good agreement with one and another.

For research limitations, there are many assumptions regarding numerical modeling. That is, the viscoplastic material property of solder ball is ignored, the reflow profile is simplified and the accurate heat capacity is not considered. Furthermore, the residual stress within the PCB, generated at PCB manufacturing process, is not included in this paper.

This paper shows how to calculate PBA warpage during the reflow process as accurately as possible. This methodology helps a PCB designer and surface-mount technology (SMT) process manager to predict a PBA warpage issue and modify PCB design before PCB real fabrication. Practically, this modeling and simulation process can be easily performed by using a graphical user interface (GUI) module, so that the engineer can handle an issue by inputting some numbers and clicking some buttons.

In a common sense manner, a numerical simulation method can decrease time and cost in manufacturing real samples. This PCB warpage method can also decrease product development duration and produce a new product earlier. Furthermore, PCB is a common component in all the electronic devices. So, this PCB warpage method can have various applications.

Because of an economic advantage, the development of a numerical simulation tool for estimating the thin PBA warpage behaviour during reflow process was attempted. The developed tool contains the features of detailed modeling for electronic components and contact boundary conditions of the supporting rails in the reflow oven.

A neural‐network‐based predictive model is proposed to model the second‐side thermal profile reflow process in surface mount assembly with a view to facilitating the oven set‐up procedure and improving production yield.

This study performs a 3⁸⁻⁴ fractional factorial experimental twice to collect the thermal‐profile data from a second‐side board. The first experiment has components on the second side only, while the second experiment also has additional components on the primary side. A back‐propagation neural network (BPN) is then used to model the relationship between control variables and thermal‐profile measures.

Empirical results illustrate the efficiency and effectiveness of the proposed BPN in solving the second‐side thermal‐profile prediction and control problem.

There is no study dedicated to the investigation of the second‐side thermal‐profile variance with and without the presence of primary‐side components. The study suggests that a variant oven‐setting strategy for the second‐side reflow process is important to ensure reflow‐soldering quality.

Increases in component packing density and the consequent decrease in feature size in electronics products continue to place ever more emphasis on process design to manage or predict the outcome of the inherent process/materials interactions. The most significant pressure is for improved first‐off process yields because of high cost and technical difficulty of rework processes and concerns about the life of reworked products. The current dominant process for the production of surface mount assemblies is the reflow soldering of stencil printed solder paste. This paper presents the results of work that begins to describe the sub‐processes of solder paste reflow. It is essential to understand and optimise these complex physical processes when aiming for the six‐sigma level quality demands of electronics manufacture.

This paper aims to study the influence of reflow atmosphere and placement accuracy on the solderability of 01005 capacitor/SAC305 solder joints.

The 01005 capacitors were mounted on OSP-coated pads, and the samples were fabricated in four different atmospheres, i.e. 200 ppm O₂/N₂, 1,000 ppm O₂/N₂, 3,000 ppm O₂/N₂ and air. After the reflow process, visual inspection and X-ray detection were carried out to examine the solder joint shapes and possible defects. Some of the samples fabricated in different conditions were cross-sectioned and the solder joint microstructures were analyzed. On the other hand, besides placing the components on their normal pad positions, a 50 per cent offset of the x-axis (long axis) or y-axis (short axis) was introduced into the chip mounter programs to evaluate the 01005 capacitor’s assembly sensitivity to placement accuracy. The process-induced defects were investigated.

Experimental results indicated that an N₂-based protective atmosphere was necessary for 01005 type assembly, as it could obviously improve the 01005 solder joint quality, compared with the air condition. The protective atmosphere had little effect on the appearance, quality and microstructure of solder joints when the oxygen concentration was below 3,000 ppm. But a very low oxygen concentration could increase the risk of tombstoning defects for the assembly process. The N₂-based protective atmosphere containing 1,000-2000 ppm O₂ was acceptable and appropriate for the assembly of tiny components.

The results of this work provide a set of reflow process parameters and recommendations for 01005 size component assembly in manufacturing.

The objective of this research is to address issues that relate to the assembly of Sn/Ag/Cu bumped flip chips.

Flip chips bumped with Sn/Ag/Cu bumps were assembled onto different lead‐free surface finishes at lead‐free soldering temperatures. Sensitivity to fluxes, reflow profiles, pad finishes and pad designs were all investigated and the potential consequences for assembly yields were calculated numerically.

Soldering defects, such as incomplete wetting and collapse and poor self‐centring were observed in the assemblies. Defect levels were sensitive to contact pad metallurgy and flux type, but not to flux level and reflow profile within the ranges considered. Owing to a particularly robust substrate‐pad design, defects observed in this work were limited to incomplete wetting and collapse, as well as poor self‐centering.

The scope of this work is limited to the lead‐free fluxes available at the time of research. A switch to lead‐free solder alloys in flip chip assemblies raises concerns with respect to the compatibilities of materials and the quality of soldering that is achievable. While this may be less of an issue in the case of larger area array components, such as ball grid arrays and chip scale packages, it is more of a concern for applications that use flip chips due to the smaller size of the solder spheres. Assembly yields tend to become more sensitive to the reduced collapse of the joints. More work is essential to investigate the potential benefits of more active lead‐free fluxes, both no‐clean tacky and liquid fluxes, in reducing or eliminating soldering defects.

The paper offers insights into assembly issues with Sn/Ag/Cu bumped flip chips.

This paper aims to investigate the metallurgical reactions between two commercial AgPt thick films used as a solder land on a low temperature co‐fired ceramic (LTCC) module and solder materials (SnAgCu, SnInAgCu, and SnPbAg) in typical reflow conditions and to clarify the effect of excessive intermetallic compound (IMC) formation on the reliability of LTCC/printed wiring boards (PWB) assemblies.

Metallurgical reactions between liquid solders and AgPt metallizations of LTCC modules were investigated by increasing the number of reflow cycles with different peak temperatures. The microstructures of AgPt metallization/solder interfaces were analyzed using SEM/EDS investigation. In addition, a test LTCC module/PWB assembly with an excess IMC layer within the joints was fabricated and exposed to a temperature cycling test in a −40 to 125°C temperature range. The characteristic lifetime of the test assembly was determined using DC resistance measurements. The failure mechanism of the test assembly was verified using scanning acoustic microscopy and SEM investigation.

The results showed that the higher peak reflow temperature of common lead‐free solders had a significant effect on the consumption of the original AgPt metallization of LTCC modules. The results also suggested that the excess porosity of the metallization accelerated the degradation of the metallization layer. Finally, the impact of these adverse metallurgical effects on the actual failure mechanism in an LTCC/PWB assembly was demonstrated.

This paper proves how essential it is to know the actual LTCC metallization/solder interactions that occur during reflow soldering and to recognize their effect on solder joint reliability in LTCC module/PWB assemblies. Moreover, the adverse effect of using lead‐free solders on the degradation of Ag‐based metallizations and, consequently, on board level reliability is demonstrated. Finally, practical guidelines for selecting materials for second‐level solder interconnections of LTCC module are given.

The benefits of controlling the atmosphere in an infra‐red reflow oven are evident in improved soldering yields and easier post‐soldering cleaning of the assembly. The main benefits arise from reducing the oxygen partial pressure in the atmosphere at the time when the solder is molten. The most common inerting atmosphere used is nitrogen, and to reduce the oxygen concentration to 100 ppm is relatively straightforward, but below this level the equipment and the running costs increase with decreasing oxygen requirement. This paper gives data on the effects of reducing the oxygen level on a number of parameters relevant to high quality manufacture and product reliability. The aim is to identify which aspects of the soldering process are crucially sensitive to the oxygen concentration and which are not, in order to establish a good working compromise between oxygen level and the cost of its attainment.