Wafer-Level Chip-Scale Packaging (WLCSP) has become increasingly popular in portable electronics. One of its main characteristics is its reduced scale and that the solder balls are attached directly to the device. One of the main challenges in WLCSP is how to overcome the effects of thermal mismatch between the silicon die and the printed circuit board that arise from these characteristics. Use of new solder alloys is one of the ways to mitigate thermal fatigue stresses resulting from coefficients of thermal expansion mismatch. Tensile tests and high temperature creep tests were used for initial screening of the alloys and understanding the potential impact of each addition on the reliability of the solder in the final application. Here improvements in thermal, mechanical and metallurgical properties of the new alloy Maxrel Plus are discussed and compared to SAC405. Based on drop shock test, single ball shear test (high temperature storage, PCT and MSL1), thermal cycling test and intermetallics measurement results, we conclude that Maxrel Plus is specially recommended for use in WLCSP.

Several new applications requiring solder materials that would perform for extended periods under harsh operating conditions have recently emerged. Clearly there is a need for a ROHS compliant solder with thermal and mechanical reliability better than Sn-Ag3-Cu0.5/ Sn-Ag4-Cu0.5 but with a similar melting range so that it can be a drop in replacement for these solders. In the work shown here, Alpha focused on improving the mechanical properties of the bulk solder as well a controlled growth of interfacial IMCs and alloy microstructure. Major composition additions do impact the melting behavior and the bulk mechanical properties. Minor alloy additions can also alter the diffusion kinetics and have significant impact on the long term reliability. Tensile tests and high temperature creep tests were used for initial screening of the alloys and understanding the potential impact of each addition on the reliability of the solder in final application. In this paper, a detailed study of the effect of small composition changes (major additions) and of micro additions is presented. Improvements in thermal, mechanical and metallurgical properties of the new alloys are discussed and compared to Sn-Ag3-Cu0.5. We show that the newly developed Pb-free solder alloy Maxrel Plus performs better than Sn-Ag3-Cu0.5 in high strain rate tests such as drop shock and vibration tests as well as in thermal fatigue tests.

We present here the findings of Alpha’s Alloy Development Program on the next generation of low temperature alloys that can be used in reflow soldering temperatures from 170 to 200oC. Approaches in alloy development, test methodologies and results are discussed. The alloy properties targeted for improvements include: Alloy strength, alloy ductility, microstructure stability, improvements in thermal cycling, high temperature creep and drop shock. We show how the use of micro-additives in eutectic Sn-Bi alloys improves these properties. Further, comprehensive reliability studies were undertaken for new low temperature alloys. Thermal Cycling was undertaken from -40°C to +125°C with a 10 minute dwell time. Compared to standard Sn-Bi systems, improvements in thermal fatigue resistance are discussed. The basic properties of the new alloys shown here were fully characterized and their use in SMT applications evaluated, especially in drop shock and temperature cycling tests. In summary, we present here a new generation of low temperature Pb-free alloys, which are capable of delivering high reliability performance at low soldering temperatures. Various alloy compositions were evaluated and alloy B showed superior performance versus the benchmark and other low temperature alloys. Overall, the new low temperature alloys show significant improvements in metallurgical properties, soldering properties for SMT assembly, and thermal and mechanical reliability.

In this paper, we present details of a very systematic study undertaken for the development of lead-free and RoHS compliant alloy for high operating temperature applications. Currently, Innolot is one of the most suitable alloy for such ultra-high reliability requirements, such as, Automotive and high CTE mis-match applications. The aim of this study was to develop a new lead-free alloy with improved thermal and electrical properties when compared to standard Innolot. This paper summarizes basic alloy properties, including mechanical, thermal and electrical properties of a set of these new lead-free alloys. Overall, the new alloys present significant enhancements in metallurgical properties and soldering properties for SMT assembly.

Low temperature alloys are used to achieve peak reflow temperatures from 170 to 200°C. Sn-Bi alloy stands as logic choices due to its low melting point, higher strength and low cost. However, use of Sn42-Bi58 alloy as soldering material is limited by a series of drawbacks such as low ductility, and poor thermal and mechanical reliability. Here we show how the use of micro-additives in eutectic Sn-Bi alloys improves thermal fatigue and mechanical shock properties. The basic properties of the new alloys shown here were fully characterized and their use in SMT applications evaluated, especially in drop shock and temperature cycling tests. Among the new alloys proposed, alloy B demonstrates superior mechanical properties, thermal cycling, drop shock and creep resistance, against benchmarks. Use of this new alloy in applications such as portable devices, PV ribbons and high efficiency LEDs will be highly beneficial due to its superior performance.

Low temperature lead-free alloys continue to gain popularity for use in electronics assembly. In most cases, Sn-Bi systems are preferred as compared, for example, to Indium containing alloys which tend to be more costly. Typically, Sn42Bi58 and Sn42Bi57.6Ag0.4 are the most commonly used alloys in PCB assembly and other electronic applications. In this paper which is the first in a series, we present details of a very systematic study to further improve the properties of Sn42Bi58 based alloys. Some of the alloy properties targeted for improvements included: alloy strength, alloy ductility, microstructure stability, improvements in thermal cycling and high temperature creep. These improvements were accomplished while maintaining the desirable attributes such as alloy spread and melting temperature close to the Sn-Bi eutectic. Significant improvements in metallurgical properties, soldering properties for SMT assembly and thermal mechanical reliability have been achieved. In this first report of the series we present basic alloy properties including mechanical, thermal and electrical properties. Introduction The low temperature, lead-free alloy systems used in electronic assemblies, typically have a soldering temperature between 170°C to 200°C. Sn-Bi and Sn-In are the most common, low temperature alloys used in electronic assemblies. In most cases, Sn-Bi systems are preferred as compared, for example, to Indium containing alloys which tend to be more costly. Typically Sn42Bi58 and Sn42Bi57.6Ag0.4 are the most commonly used alloys in PCB assembly and other electronic applications. The Sn42Bi58 system fulfills the basic requisites of a low-cost, lead-free, eutectic alloy, with a melting point of 138°C. However, the Sn42Bi58 alloy has a series of limitations such as low ductility, poor electrical and thermal conductivity, and poor reliability, when compared to other lead-free alloys such as Sn-Ag4.0-Cu0.5. An alternative to the Sn42Bi58 system is a Sn42Bi57.6Ag0.4 alloy. Solder Pastes with this alloy have shown improvement over pastes using Sn42Bi58, including proven mechanical and electrical reliability. In this paper, we present details of a very systematic study to further improve the properties of existing Sn42Bi58 and Sn42Bi57.6Ag0.4 based alloys. These alloys were developed through elemental additions to improve mechanical strength, fatigue life and drop shock resistance. For example, reducing the amount of Bi tends to increase the ultimate tensile strength (>10wt.% Bi) and elongation (>30wt.% Bi) of Sn-Bi alloys . Further, toughness of an alloy can be defined as the energy absorbed during impact. Solder alloy impact resistance can be improved by reducing the level of Bi in the alloy. This property was measured through the Charpy Impact test and is covered in the Methodology Section. Another approach to improve the ductility of the Sn42Bi58 is by the addition of alloying elements. For example, Ag has limited solubility in the Sn-Bi eutectic alloy, but its addition in small amounts results in higher elongation [2] and improved thermal fatigue properties . We have developed various new alloys mainly through alloying additions. The compositions were developed to improve mechanical strength, fatigue life and drop shock. The effect of these elemental additions on alloy strength, ductility, thermal conductivity, copper dissolution, bulk and interfacial microstructure stability, and creep properties are shown for three Alloys – Alloy A, Alloy B and Alloy C. Methodology A comprehensive evaluation of the new low temp alloys was undertaken using different test methodologies. These included: Thermal Analysis The liquidus and solidus temperature of the alloys was measured using a differential scanning calorimeter (DSC), as per ASTM E794 standard. Multiple samples of same alloy were measured to reduce any variation. Coefficient of Thermal Expansion (CTE) was measured using a thermal mechanical analyzer from TA Instruments (model TMA 2940), according to the RT-500C standard. The thermal diffusivity of the alloys was measured using a Nanoflash instrument, in which the front side of the sample was heated by a light pulse and the resulting temperature signal versus time on the rear surface was measured using an infrared detector. This temperature distribution depends on the thermal diffusivity (α) of the material, which was then used to calculate the thermal conductivity (K). So, K=ρ α cp, where ρ is the density and cp the specific heat. Copper Dissolution The copper dissolution is assessed by measuring the time taken for a wire to break under load when immersed in solder. For this test, a 0.05 mm diameter copper wire was fluxed and dipped in molten solder, which was kept at 190°C. The copper dissolves in the solder and after a given time it has insufficient strength to support the weight attached to it. This procedure was repeated multiple times for each alloy and the rate of copper dissolution was calculated for the time it took the copper to dissolve. Wetting Balance A typical wetting balance curve shows the variation of the wetting force of the solder on a test vehicle surface with respect to time (Figure 1). Basically, six distinct events can be identified during the wetting test: (A) The initial stage just before immersion in the solder, (B) As the test vehicle is immersed in the solder, there is an opposite (negative) force applied by the solder on the surface, (C) The wetting force increases and returns to zero upon wetting (t0), (D) The wetting force increases and reaches its maximum value (Fmax) as the test vehicle withdraws from the molten solder, (E) The wetting force increases again at the minimum contact of the test vehicle with the solder, (F) The wetting force decreases as the test vehicle is completely removed from the solder. So, a lower zero wetting time and higher wetting force are desirable properties for an alloy. The wetting balance test of the alloys was carried out on Cu substrates, in accordance to the JIS Z 3198-4 standard. The tests were performed using a Rhesca solder checker to report the zero wetting time (t0) and maximum force (Fmax). The 30 x 10 x 0.30 mm Cu coupons were cleaned and completely immersed in RF 800 flux and allowed to dry for 10s. The coupons were then immersed to a depth of 4-5mm in the molten alloy at a speed of 20mm/s and held for 10s. Tests were performed at 190°C. -4 -3 -2 -1 0 1 2 3 4 5

As handheld devices become increasingly smaller and complex, there is a shift in reliability requirements of solder pastes. Considering that thermal management and drop resistance of such devices become more challenging, improved thermal fatigue and mechanical shock properties grow into must have requirements. Additionally, multi-step assembly process and a surge in use of temperature sensitive components bring additional challenges that necessitate the use of low temperature alloys. Here we present the findings of our Alloy Development Program on the next generation of low temperature alloys that can be used in reflow soldering temperatures from 170 to 200°C. By using micro-additives we have created low temperature alloys with superior mechanical properties, higher drop shock resistance and improved fatigue life.

Customer expectations for light emitting diode (LED) based luminaries (Solid State Lighting) are very high due to the relatively high cost of such luminaries. For commercial and outdoor residential applications, a B50, L70 of 35,000 hours and a 3 year warranty is needed to meet EnergyStar Category A requirements. For such high reliability and lifetime requirements, it is critical to have excellent assembly interconnect reliability (i.e. Package to Insulated Metal Substrate attach). This study presents the results of initial work related to understanding the reliability of Solid State Lighting assembly interconnects in a LED Package-Insulated Metal Substrate system.

Sn–Ag–Cu (SAC) solder alloys are the best Pb free alternative for electronic industry. Since their introduction, efforts are made to improve their efficacies by tuning the processing and composition to achieve lower melting point and better wettability. Nanostructured alloys with large boundary content are known to depress the melting points of metals and alloys. In this article we explore this possibility by processing prealloyed SAC alloys close to SAC305 composition (Sn-3wt%Ag-0.5wt%Cu) by mechanical milling which results in the formation of nanostructured alloys. Pulverisette ball mill (P7) and Vibratory ball mills are used to carry out the milling of the powders at room temperature and at lower temperatures (−104 °C), respectively. We report a relatively smaller depression of melting point ranging up to 5 °C with respect to original alloys. The minimum grain sizes achieved and the depression of melting point are similar for both room temperature and low-temperature processed samples. An attempt has been made to rationalize the observations in terms of the basic processes occurring during the milling.

The drop shock reliability of solder joints has become a major issue for the electronic industry partly because of the ever increasing popularity of portable electronics and partly due the transition to lead free solders. Most of the commonly recommended lead-free are high Sn alloys which have relatively higher strength and modulus. This plays a critical role in the reliability of Pb-free solder joints. Further, even though metallugically it is the Sn in the solder alloys that principally participates in the solder joint formation, details of the IMC layers formed with SnPb and Pb-free alloys are different. The markedly different process conditions for SnPb and Pb-free alloys also bear on solder joint quality. Brittle failure of solder joints in drop shock occurs at or in the interfacial IMC layer(s). This is due to the inherent brittle nature of the IMC, defects within or at IMC interfaces or transfer of stress to the interfaces as a result of the low ductility of the bulk solder. In developing improved performance alloys, Cookson Electronics has addressed both issues -improved ductility and modification and control of the intermetallic layer. A broad range of base alloy compositions together with selected micro-alloying additions to SnAgCu alloys have been evaluated with the objective of controlling bulk alloy mechanical properties and the diffusion processes operating in the formation and growth of the intermetallic interfacial layer(s). In the present article a detailed study of a range of micro-alloy additives is presented. The alloy additives generally act as diffusion modifiers slowing interdiffusion between substrates and solder thereby reducing IMC thickness or the propensity for void formation. Alternatively additions can be made that act as diffusion compensatorscopy. It should be noted that the level of the micro-additions does not measurably modify the bulk mechanical properties of the base alloys. Our results show that dramatic improvements in the solder joint reliability, as demonstrated by high-speed ball pull and drop shock tests, can be achieved.