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To give an overview of the issues encountered, and changes that need to be made in the various types of soldering process when converting them from conventional to lead‐free assembly.

This paper has been written to provide a review of the lead‐free reflow, wave and hand soldering processes. Problem areas highlighted and methods for adjusting and optimising each type of soldering process for compatibility with lead‐free solders are described.

The move to lead‐free soldering in electronics assembly can lead to a number of issues that affect process performance, yields and reliability. Problems that are sometimes encountered with conventional lead‐bearing solders can exacerbated when moving to lead‐free. Many of the issues are associated with the higher melting points of the recommended lead‐free solders. Fortunately, these issues are now well known and, with care and attention to process optimisation, they can largely be avoided.

The value of the paper lies in its ability to provide information on the types of problems and issues encountered when moving to lead‐free solders and the advice it gives on how to avoid them. It also describes how to convert the various lead‐free soldering processes used in PCB assembly using a range of measures that can minimise defects, avoid common problems and optimise yields. Sources of additional assistance are also identified.

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.

Over the last few years, the emergence of new European draft legislation has focussed electronics industry attention on the likely ultimate proscription of lead in electronics assembly. Much work has already been undertaken to identify the possible alternatives to conventional tin‐lead solders and to evaluate their performance benefits and limitations in comparison with the traditional materials. Although, some companies are already offering products manufactured using lead‐free products, there is still a widespread lack of activity in many areas. With this none‐too‐distant deadline rapidly approaching, Envirowise has sponsored this paper as part of its coordinated activities to assist the UK electronics industry and to promote environmental efficiency and best practice. This paper details the current situation with respect to the drivers towards the adoption of lead‐free assembly before giving an overview of the current situation. This paper concludes with details of sources of further information.

The purpose of this paper is to investigate the laser soldering of fine pitch quad flat package (QFP) devices using lead‐free solders and solder joint reliability during thermal cycling.

QFP devices were selected as the test vehicles and were soldered with four alloy types, Sn37Pb, Sn3.5Ag, Sn3.8Ag0.7Cu and Sn3.8Ag0.7Cu0.03Ce. The experimental samples were QFP‐256 devices with lead‐free solder paste on the printed circuit boards. The packages were dried for 24 h at 125°C prior to reflow soldering. Soldering experiments on the QFP devices were carried out with an infrared (IR) reflow soldering oven and a diode laser (DL) soldering system. Reflow soldering was performed at peak temperatures of 210°C (SnPb), 240°C (SnAgCu and SnAgCuCe) and 250°C (SnAg), as determined on the boards. Pull testing was adopted to evaluate the tensile strength of the four solders using an STR–1000 micro‐joint strength tester.

The tensile force of the QFP micro‐joints increased as laser intensity increased when it was less than an “optimal” value. The maximum tensile force of the QFP micro‐joints was gained when the laser intensity had increased to 2,165, 2,127, 2,165 and 2,064 W/cm², depending on the alloy used. The thermal fatigue performance of three lead‐free solder joints, SnAgCuCe, SnAgCu and SnAg, was determined to be superior to that of the eutectic SnPb alloy. After soldering without thermal cycling tests, the fracture morphology of soldered joints exhibited characteristic toughness fracture with both of the soldering methods. After 700 thermal cycles, the fracture mechanism was also toughness fracture, nevertheless, the dimples became large. The fracture morphology of the soldered joints subjected to 1,500 thermal cycles indicated brittle intergranular fracture on the fracture surface and no intense plastic deformation appeared before fracture with IR soldering. For DL soldering, the pull fracture model of the SnAgCuCe was completely ductile in the soldered joint with 1,500 thermal cycles.

The paper usefully investigates the influence of laser intensity on the tensile strength of different soldered joints and the solder joint reliability during thermal cycling.

The electronics industry will implement lead‐free soldering in the near future. Lead‐free implementation steps are divided into lead‐free process and lead‐free product. The eutectic Sn/Ag/Cu alloy seems to have become the most widely used alloy in the implementation of lead‐free processes. In this study, the requirements for component placement are discussed from the lead‐free process point of view. Experiments concerning the self‐alignment capability and tack strength of both tin‐lead and lead‐free solder pastes are presented. According to the results, a bigger variation in self‐alignment capabilities can be expected when using a lead‐free paste. The paste properties affecting the self‐alignment mechanism and tack strength are also discussed.

The dissolution rates of iron and alloyed steels in molten lead‐free solders were investigated in order to clarify the effect of erosion of iron plating on soldering iron tips. The dissolution rates of iron‐based alloys in lead‐free solders were found to be about three times greater than in conventional Sn‐Pb eutectics, indicating that the iron plating of a soldering iron tip is subjected to heavier damage when used with lead‐free rather than eutectic Sn‐Pb. Several steel alloys showed dissolution rates similar to that of pure iron, suggesting that compositional changes in the iron plating may have little influence on the erosion depth. Decreases in the reaction temperature and time, and a small addition of iron into the solder was found to be effective in suppressing both dissolution of iron wire and erosion of iron plating.

Though lead‐free replacements for SnPb eutectic alloys for reflow, wave, and hand soldering have been developed, relatively little has been reported on practical experience of lead‐free wave soldering processes. In wave soldering, the interaction between the PCB, flux, solder alloy and processing equipment makes it desirable to develop the consumables and the wave soldering machine concurrently. A crossfunctional project team was formed and a lead‐free wave soldering process developed and validated through nine months of industrial use in production of broad‐band communications technology products.

Results for reflow soldering are presented from a three‐year EC funded project “IDEALS” to develop lead‐free soldering solutions. On the basis of fundamental data from the literature, a shortlist of candidate lead‐free solders was selected, and results from tests on physical and soldering characteristics, and wetting balance testing, led to the choice of SnAg3.8Cu0.7, melting at 217°C. Implications for solder paste medium development are discussed. Differences in alloy density, melting point, and surface tension relative to conventional solders were found to give higher levels of internal voids, reduced spread on copper, and rougher, duller joints. Reflow process window studies showed that sound reliable joints could be obtained with a peak temperature as low as 225°C. Reliability was tested on soldered test boards using thermal shock cycling, power cycling, and vibration. Overall the SnAg3.8Cu0.7 gave results approximately equivalent to conventional solders, and different board finishes had no significant effect. The effects of Sb and Bi were also evaluated. No justification was found for minor additions of Sb, but 2‐5 per cent Bi was found to allow a reduction of the peak reflow temperature, though at the cost of reduced reliability if any Pb was present.

The High Density Packaging Users Group conducted a substantial study of the solder joint reliability of high‐density packages using lead‐free solder. The design, material, and assembly process aspects of the project are addressed in this paper. The components studied include many surface mount technology package types, various lead, and printed circuit board finishes and paste‐in‐hole assembly.

The purpose of this paper is to present challenges met during package-on-package (PoP) technology implementation in real surface-mount technology assembly processes.

The properties and behavior of different combinations of soldering materials, PoP components and soldering profiles were investigated, both in the laboratory and during production trials. The purpose of such an approach was identification of existing problems and challenges in lead-free PoP systems assembly as well as checking which soldering material designed to PoP is more suitable for this technology.

Technological trials are needed to select adequate soldering materials for PoP systems assembly, as laboratory tests of materials alone were not sufficient. The challenges of PoP technology were associated with the equipment utilized, the soldering materials, operational parameters and the soldering profile used for assembly. The localization of defects in PoP systems is very difficult and, in many cases, destructive methods have to be used on solder joints for the assessment and confirmation of failures.

This paper shows main materials and soldering challenges in lead-free PoP technology. In particular, the problem related with selection of soldering materials and soldering profiles for PoP was presented. Moreover, the issues that have to be taken into consideration during the planning of a PoP system assembly procedure are presented.