Evolution of Pb-Free Solders

This chapter discusses the evolution of lead free (Pb-free) solder, from tin-silver-copper (SAC) system with silver content of 3.0, 3.8, and 4.0 to low SAC system such as SAC0307 and SAC105 and the emerge of high reliability Pb-free solder. The discussion covers the reason and the driving force of industries implementing this change. The solder composition has evolved further recently to fulfill high reliability requirement of certain sectors such as automotive, aerospace, and military which are preparing to go green in soldering technology. This kind of high reliability solder involves additional microalloying of tin (Sn)-based solder in making it to be more robust. In this chapter, the author will introduce the techniques used by solder makers and researchers in enhancing the Pb-free solder strength in the recent evolution. Recently, attention has been drawn to low temperature joining technology again such as silver sintered joint and liquid-phase diffusion bonding material used in high power density and high junction temperature-integrated circuits. Pb-free joining material is required to replace the high Pb solder, which is still commonly used in such high-power devices

Determination of the creep properties of Pb-free solders for harsh environments using meso-scale testing

Solder joints in electronic packages are prone to failure due to the evolution of thermal expansion mismatch strains during thermal cycling. The comparatively wide operating temperature range and long lifetimes of aerospace electronics require high reliability solder joints. Since 2006, high reliability industries (aerospace and military amongst others) that are exempt from lead-free RoHS regulation on account of concerns over the reliability of Pb-free solders have found it increasingly difficult and expensive to continue using traditional Sn-Pb-based solders. Hence there is a pressing need to find a suitable alternative that can match the manufacturing and reliability performance of Sn-Pb. There remains a dearth of data for the constitutive behaviour of Pb-free solders under harsh environment scenarios. Unfortunately, conventional test approaches, particularly in the case of creep behaviour which is critical to solder lifetimes, are expensive and time-consuming. High temperature nanoindentation has been recently developed as a quick method for the determination of creep properties of solder alloys. This paper compares and contrasts nanoindentation creep results for bulk Pb-Sn and lead-free solders. However, there are limits to nanoindentation creep, in particular the load-dependence of the technique. A new meso-scale test approach that lies between nanoindentation and bulk creep testing has been developed. Real ball grid arrays using Pb-free solders have been creep tested in the temperature and stress ranges of operating solder joints. High temperature creep constitutive data has been obtained. The technique offers promising time and materials savings in obtaining important mechanical property data for subsequent use in life-prediction models

ISOTHERMAL MECHANICAL AND THERMO-MECHANICAL DURABILITY CHARACTERIZATION OF SELECTED PB-FREE SOLDERS

Due to the hazards of Pb in the environment and its effect on humans and marketing competition from Japanese electronics manufacturers, the conversion to Pb-free solders in the electronics industry appears imminent. As major mechanical, thermal, and electrical interconnects between the component and the PWB, solder joints are crucial for the reliability of the most electronic packages. There is an urgent need for constitutive properties, mechanical durability and thermo-mechanical durability of Pb-free solders. A partitioned constitutive model consisting of elastic, plastic, primary creep and secondary creep models is obtained for the Sn3.9Ag0.6Cu solder and the baseline Sn37Pb solder from comprehensive monotonic and creep tests conducted on Thermo-Mechanical-Microscale (TMM) setup. The comparison between two solders shows that Sn3.9Ag0.6Cu has much better creep resistance than Sn37Pb at the low and medium stresses. The isothermal mechanical durability of three NEMI recommended Pb-free solders, Sn3.9Ag0.6Cu, Sn3.5Ag, Sn0.7Cu, is tested on the TMM setup under low creep and high creep test conditions. The damage propagation rate is also analyzed from the test data. The generic Energy-Partitioning (E-P) durability model is obtained for three Pb-free solders by using the incremental analytic model developed for TMM tests. The scatter of the test results from the prediction by these E-P durability model constants is small. The thermo-mechanical durability of the Pb-free Sn3.8Ag0.7Cu solder is investigated by a systematic approach combining comprehensive thermal cycling tests and finite element modeling. The effects of mixed solder systems, device types, and underfill are addressed in the tests. Thermal cycling results show that Sn3.8Ag0.7Cu marginally outperforms SnPb for four different components under the studied test condition. The extensive detailed three-dimensional viscoplastic FE stress and damage analysis is conducted for five different thermal cycling tests of both Sn3.8Ag0.7Cu and Sn37Pb solders. Power law thermo-mechanical durability models of both Sn3.8Ag0.7Cu and Sn3Pb are obtained from thermal cycling test data and stress and damage analysis. The energy-partitioning durability models of two solders are also obtained. It is found that the slopes of the plastic and creep curves in the E-P damage model of Pb-free solders for thermal cycling are steeper than those for mechanical cycling and those of Sn37Pb solders

Применение газопламенной пайки при изготовлении узлов охладителей пива

Выполнен анализ возможных дефектов при получении паяных трубчатых соединений узлов охладителей пива. Приведены примеры паяных трубчатых соединений из однородных материалов (медь-медь) и разнородных (латунь-нержавеющая сталь, медь-нержавеющая сталь).Fusibility curves and structural peculiarities of alloys of the Sn-Zn-Ni systems in a range of Sn-rich compositions were studied using differential thermal analysis. The resulting data can be employed for development of commercial Pb-free solders for soldering parts used in radio electronics

Controlling tin nucleation and grain orientations in Pb-free solders

Lead-free solder joints made with Sn-Ag-Cu or Sn-Ag solders usually contain only a single βSn grain or three twinned βSn grains which are oriented differently in every joint. Due to the anisotropy of βSn every joint therefore has unique thermomechanical properties and, in an array of numerous joints, it is likely that some will be poorly oriented and could cause early failure of a component. The problems of few grains with variable orientation can be attributed to the nucleation of βSn. This thesis explores catalyzing βSn nucleation using heterogeneous nucleation and develops methods to control βSn microstructures and orientations in Sn-3Ag-0.5Cu/Cu solder joints. The developed droplet nucleation technique in this study provides a new ‘motorway’ for heterogeneous nucleation study, and the introduced orientation control method paves the way to solve chronic problems, such as electromigration, thermomechanical fatigue, and shear fatigue in the electronic packaging industry. It is found that Co additions and Co substrates are effective at catalyzing βSn nucleation. This is demonstrated to be due to heterogeneous nucleation on αCoSn3 crystals. By using a ‘droplet nucleation technique’, in which Sn droplets are solidified directly on an intermetallic compound (IMC) particle, it is proved that αCoSn3 forms a reproducible orientation relationship (OR) with βSn that has a good lattice match. Strong grain refinement occurred in 60g Co-microalloyed Sn-3.0Ag-0.5Cu samples but only weak grain refinement occurred in 550μm solder balls and joints. When soldering Sn-3.0Ag-0.5Cu on Co substrates, βSn is always observed to grow from the interfacial αCoSn3 layer with an orientation inherited from the αCoSn3 layer texture. However, it is shown that nucleation on the αCoSn3 layer does not give useful βSn orientation control. The droplet nucleation technique was then applied to gain a deeper understanding of βSn nucleation on a range of IMC phases. A family of transition metal stannides, PtSn4, PdSn4, and βIrSn4 that have similar crystal structures to αCoSn3, were identified as potent nucleants for βSn. The common solder IMCs, Cu6Sn5, Ag3Sn, Ni3Sn4, were also investigated. It was found that reproducible ORs formed on all IMCs studied and the nucleation mechanisms were explored by combining nucleation undercooling measurements with measured ORs. The nucleation potency of all studied intermetallics is: αCoSn3>βIrSn4 >PdSn4>PtSn4 >Ni3Sn4> Ag3Sn, Cu6Sn5. The droplet nucleation technique also generated new insights into solidification twinning in solder joints. It was found that cyclic twins formed in droplets when the undercooling was sufficiently high and the liquid contained Ag, Cu and/or Ni. Complex interrelated cyclic twins were found in droplets on Cu6Sn5, Ag3Sn, and Ni3Sn4 where up to five rings of cyclic twins formed each related by a common . The twinning mechanisms in these cases were explored and discussed. The thesis then applies the new understanding developed in the previous chapters to develop a technique to reliably control the orientation of βSn in solder joints. Ball grid array (BGA) joints were fabricated reproducibly by introducing an extra step into the manufacturing process: bonding a nucleant IMC ‘seed crystal’ onto each Cu pad so as to control the nucleation location, nucleation undercooling and crystallographic orientation of βSn at the moment of nucleation. Each joint made by this technique had a uniform single-grain microstructure with the c-axis of βSn parallel with the substrate plane. This orientation is reported in the literature to give the best resistance to electromigration and shear fatigue.Open Acces

In situ imaging of microstructure formation in electronic interconnections

The development of microstructure during melting, reactive wetting and solidification of solder pastes on Cu-plated printed circuit boards has been studied by synchrotron radiography. Using Sn-3.0Ag-0.5Cu/Cu and Sn-0.7Cu/Cu as examples, we show that the interfacial Cu6Sn5 layer is present within 0.05 s of wetting, and explore the kinetics of flux void formation at the interface between the liquid and the Cu6Sn5 layer. Quantification of the nucleation locations and anisotropic growth kinetics of primary Cu6Sn5 crystals reveals a competition between the nucleation of Cu6Sn5 in the liquid versus growth of Cu6Sn5 from the existing Cu6Sn5 layer. Direct imaging confirms that the β-Sn nucleates at/near the Cu6Sn5 layer in Sn-3.0Ag-0.5Cu/Cu joints

Developing the knowledge-based human resources that support the implementation of the National Dual Training System (NDTS): evaluation of TVET teacher's competency at MARA Training Institutions

Development in the world of technical and vocational education and training (TVET) on an ongoing basis is a challenge to the profession of the TVET-teachers to maintain their performance. The ability of teachers to identify the competencies required by their profession is very critical to enable them to make improvements in teaching and learning. For a broader perspective the competency needs of the labour market have to be matched by those developed within the vocational learning processes. Consequently, this study has focused on developing and validating the new empirical based TVET-teacher competency profile and evaluating teacher’s competency. This study combines both quantitative and qualitative research methodology that was designed to answer all the research questions. The new empirical based competency profile development and TVET-teacher evaluation was based upon an instructional design model. In addition, a modified Delphi technique has also been adopted throughout the process. Initially, 98 elements of competencies were listed by expert panel and rated by TVET institutions as important. Then, analysis using manual and statistical procedure found that 112 elements of competencies have emerged from seventeen (17) clusters of competencies. Prior to that, using the preliminary TVET-teacher competency profile, the level of TVETteacher competencies was found to be Proficient and the finding of 112 elements of competencies with 17 clusters was finally used to develop the new empirical based competency profile for MARA TVET-teacher. Mean score analysis of teacher competencies found that there were gaps in teacher competencies between MARA institutions (IKM) and other TVET institutions, where MARA-teacher was significantly better than other TVET teacher. ANOVA and t-test analysis showed that there were significant differences between teacher competencies among all TVET institutions in Malaysia. On the other hand, the study showed that teacher’s age, grade and year of experience are not significant predictors for TVET-teacher competency. In the context of mastering the competency, the study also found that three competencies are classified as most difficult or challenging, twelve competencies are classified as should be improved, and eight competencies are classified as needed to be trained. Lastly, to make NDTS implementation a reality for MARA the new empirical based competency profile and the framework for career development and training pathway were established. This Framework would serve as a significant tool to develop the knowledge based human resources needed. This will ensure that TVET-teachers at MARA are trained to be knowledgeable, competent, and professional and become a pedagogical leader on an ongoing basis towards a world class TVET-education system

Developing a NASA Lead-Free Policy for Electronics - Lessons Learned

The National Aeronautics and Space Administration (NASA) is not required by United States or international law to use lead-free (Pb-free) electronic systems but international pressure in the world market is making it increasingly important that NASA have a Pb-free policy. In fact, given the international nature of the electronics market, all organizations need a Pb-free policy. This paper describes the factors which must be taken into account in formulating the policy, the tools to aid in structuring the policy and the unanticipated and difficult challenges encountered. NASA is participating in a number of forums and teams trying to develop effective approaches to controlling Pb-free adoption in high reliability systems. The activities and status of the work being done by these teams will be described. NASA also continues to gather information on metal whiskers, particularly tin based, and some recent examples will be shared. The current lack of a policy is resulting in "surprises" and the need to disposition undesirable conditions on a case-by-case basis. This is inefficient, costly and can result in sub-optimum outcomes

Lead-Free Electronics: Impact for Space Electronics

Pb is used as a constituent in solder alloys used to connect and attach electronic parts to printed wiring boards (PWBs). Similar Pbbearing alloys are electroplated or hot dipped onto the terminations of electronic parts to protect the terminations and make them solderable. Changing to Pb-free solders and termination finishes has introduced significant technical challenges into the supply chain. Tin/lead (Sn/Pb) alloys have been the solders of choice for electronics for more than 50 years. Pb-free solder alloys are available but there is not a plug-in replacement for 60/40 or 63/37 (Sn/Pb) alloys, which have been the industry workhorses

Development of thermodynamic and kinetic databases in micro-soldering alloy systems and their applications

AbstractRecent progress in the development of thermodynamic and kinetic databases of micro-soldering alloys, which were constructed within the framework of the Thermo-Calc and DICTRA software, was presented. Especially, a thermodynamic tool, ADAMIS (alloy database for micro-solders) was developed by combining the thermodynamic databases of micro-solders with Pandat, a multi-component phase diagram calculation software program. ADAMIS contains 11 elements, namely, Ag, Al, Au, Bi, Cu, In, Ni, Sb, Sn, Zn and Pb, and can handle all combinations of these elements in the whole composition range. The obtained thermodynamic and kinetic databases can not only provide much valuable thermodynamic information such as phase equilibria and phase fraction, but also shows the kinetics and the evolution of microstructures when they are combined with some appropriate software programs and models, such as the phase field method and ADSTEFAN software. From the viewpoints of computational thermodynamics and kinetics, some technical examples were given to demonstrate the great utility of these databases for the applications in the development of micro-soldering materials. These databases are expected to be powerful tools for the development of micro-solders and Cu substrate materials, as well as for promoting the understanding of interfacial phenomena and microstructure evolution between solders and substrates in electronic packaging technology