Solidification Of Solder Research Articles

SAC305 Solder Reflow: Identification of Melting and Solidification Using In-Process Resistance Monitoring

The proper control of solder heating and melting leads to successful joining of surface-mount technology devices to the metallization of printed circuit boards. When developing new solder alloys for more miniaturized and lower temperature processes, the improved process control for the heating and melting during solder reflow can be beneficial. To find an optimized temperature profile efficiently, it is helpful to know the exact onset times of the physical mechanisms of soldering including sintering, melting, and solidification. In particular, to exactly know the time, the solder remains in the molten state can help to minimize the temperature exposure of sensitive substrates. To this end, we used standard lead-free SAC305 solder and a real-time resistance monitoring setup to observe the in-process resistance during the soldering of a 0805-type chip resistor to find out exact points in time when the soldering mechanisms occur. Cross-sectioning studies and microscopic observations were performed in parallel to explain three distinct resistance drops observed in the resistance signals. Sintering of solder balls resulted in drop 1, the collapse and melting of all the solder balls resulted in drop 2, and the solidification of solder resulted in drop 3, which actually consisted of two separate drops, one for each solder joint of the chip resistor. The observation of the resistance drop due to solder solidification was achieved because our newly improved setup reduced the signal noise to less than 0.012 $\text{m}\Omega $ (root mean square value). On average, the solder was in a liquid state for 17.6 min ± 0.18 min (seven samples). The solidification drop was 0.2 $\text{m}\Omega $ and happened at approximately 200 °C, indicating not more than 28 K of undercooling. Such an accurate measurement of melting and solidification times may prove useful in the future when optimizing the reflow temperature profiles for advanced soldering applications, e.g., to minimize the melting–solidification interval on heat-sensitive thin flexible substrates for wearables.

Assessment of room-temperature short-term stress relaxation and strain relaxation with recovery in Sn-Bi lead-free solders solidified under rotating magnetic field

In this paper, short-term stress relaxation test (SRT) as well as strain relaxation and recovery test (SRRT) were performed to assessment the minimum creep rate of Sn-Bi solder from the primary creep stage at 25 °C. Both techniques allow to determination the effect of rotating magnetic field (RMF) during solidification of Sn-Bi solder on the viscous stress exponents nv and other parameters, and have been established to be appropriate for assessment the initial microstructure dependent flow properties of the alloy solder. Additionally, the viscous strain rate in irreversible viscous flow is developed during primary creep and a power-law relationship was created from experimental data to predict the creep deformation mechanism. It was found that the Sn-Bi solder solidified under RMF had the higher resistance to stress and strain relaxation than that solidified without RMF. The stress exponent values of 9.4–6.3 for dislocation creep were achieved depending on the initial imposed strain, and indicating the role of RMF on refining β-Sn dendrites that affect the density of mobile dislocations. Significant decrease in undercooling (from 13.4 to 5.6 °C) was attained after applying RMF, which has good prospects for industrial application.

Formation behaviour of reaction layer in Sn-3.0Ag-0.5Cu solder joint with addition of porous Cu interlayer

The morphology and growth of interfacial intermetallic compound (IMC) between Sn-3.0Ag-0.5Cu solder alloy and Cu substrate metal of solder joint is reported. The IMC morphology and IMC thickness layer were observed at three different porosities of porous Cu interlayer. The results revealed that during soldering process, Cu6Sn5 compound with scallop like morphology was formed at the interface of both the solder alloy and Cu substrate and at solder alloy and porous Cu interlayer. By adding porous Cu interlayer at the solder joint, the IMC thickness increased with increasing soldering temperature and the number of pores in porous Cu interlayer. The effect of porosity on increasing the IMC layer was also due to the slower cooling rate during solidification of molten solder.

Possibility of a shape phase transition for solidification of tin at scallop-like surfaces of Cu6Sn5

The possibility of solder-spreading transitions in solidification of solder at a rough rigid intermetallic surface is proven theoretically. Depending on the misorientation of scallops on the interface, one can observe a two-dimensional spreading transition over the scallops or a one-dimensional spreading transition along the triple-junction of two intermetallic compound scallops and liquid solder. The extent of undercooling can be determined not only by different interface energies, but by different angles between neighbouring scallops as well.

The effect of praseodymium (Pr) additions on shear strength and microstructure of SnAgCu joints during the ageing process

PurposeThe purpose of this paper is to investigate the influence of praseodymium (Pr) additions (0, 0.05 and 0.5 wt%) on the mechanical properties and microstructure of SnAgCu solder joint during aging process. Moreover, the authors aim to indicate that the decreased soldification undercooling of Sn3.8Ag0.7Cu solder can suppress the formation of Ag3Sn plate to some extent.Design/methodology/approachThe shear strength evolution of SAC, SAC‐0.05Pr and SAC‐0.5Pr solder joint were studied under 150°C aging process with STR‐1000. The effect of Pr additions on the solidification behavior of SnAgCu solder was also studied by differential scanning calorimetry. To study the microstructure evolution, the cross‐sections of all specimens were observed by means of scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). Meanwhile, the etchant, consisting of 20%HNO3+distilled water was used for deep etching to reveal the interfacial morphology.FindingsThe shear force reduction rate of SAC solder joint during aging was restrained by 0.05%Pr addition but promoted with 0.5% Pr addition. The growth of IMC layer of SnAgCu joint in the aging process was suppressed significantly by different amounts of Pr additions. However, the beneficial effect of Pr addition due to the suppression of IMC layer growth was weakened by the micro‐cracks formed in PrSn3 compounds in SnAgCu‐0.5Pr joint. Pr additions (0.05, 0.5 wt%) decrease the solidification undercooling of SnAgCu solder, which will suppress the formation of Ag3Sn plate to some extent.Research limitations/implicationsFurther studies are necessary for confirmation of the practical application, especially of the manufacturing technology of solder paste containing Pr.Practical implicationsThe appropriate amount of Pr in Sn3.8Ag0.7Cu solder is about 0.05 wt%. It is found that SAC‐0.05Pr solder has an improvement in solder joint reliability in long aging processes. The results suggested the novel solder alloys can meet the requirements of high reliability application.Originality/valueThe paper demonstrates that: Pr additions promote the solidification of SAC solder; shear force reduction rate of SAC solder joint was reduced by 0.05%Pr addition; the IMC layer growth rate of SnAgCu solder joint was suppressed by Pr additions; and micro‐cracks were found in PrSn3 phases after aging.

テープキャリアパッケージの高信頼インナボンディング技術

For higher reliability of bonding on TCP (Tape Carrier Package), research has been done on the bonding of leads coated with Sn and Sn-Pb on Au bumps formed on LSIs. Major defects of bonding are (1) insufficient bonding, (2) vanishing of Au bumps, and (3) solidification of solder in lumps. Defects (2) and (3) occur when the coating reacts with Au bumps excessively, that is, if the coating is thick or the bonding temperature is too high. As for defect (3), it has been revealed that lumps form when excessive solder flows along the lead and solidifies because its melting point rises as Sn is taken in from the coating. This is more likely with Sn-Pb coating than with Sn coating, probably because the melting point of solder formed at the time of bonding falls due to Pb, allowing reaction to take place more easily. As a result, bonding conditions for practical uses have been formed Sn and Sn-Pb coatings.