Ing. Iva Králová

The wetting balance method is used for the precise classification of solderability of chosen substrates by solder alloys. This work deals with a weakness of the wetting balance method during the wettability measurement of connectors with the wetting issue. The wetting issues at examined pins connector appeared during the serial manufacturing production, and therefore, the connector pins were analysed using the wetting balance method. The wetting balance method showed a good wetting of the connector pins. The wetted pins were examined by scanning electron microscopy (SEM) to find the reason for the wetting issue. This analysis showed a non-wetted area at pins edges. Following investigation of pins microsections using confocal/optical microscopy showed the reason for the wetting issue, when the surface finish was much thinner or was missing on the edges of the pin. This was the reason for the wetting issue of the connector pins in serial manufacturing, even though the wetting balance test showed good wettability results because most parts of the pin surface had good wetting.

Due to the restrictions on lead solder alloy, there is an ongoing search for alternative lead-free alloys with the best possible solder properties similar to Pb-based solder alloys. Therefore, phosphorus and gallium are, for example, added to solder alloys, which could lead to an improvement in their solder properties (wettability, mechanical, lower melting point). These additives are usually used for the improvement of SAC alloys. This work focused on lead-free low-temperature solders based on tin and bismuth. The effect of added 1 wt.% gallium and traces of phosphorus on wettability and spreading was studied. These properties were observed on FR4 boards with three different surface materials: copper, copper with hot air solder leveling surface finish (HASL) and copper with electroless nickel-immersion gold surface finish (ENIG). Examined alloys were Bi58Sn42, Bi58Sn42P, Bi59Sn40Ga1 and Bi59Sn40Ga1P. The results showed that although the addition of Ga and P exhibited no significant improvement or even decrease in the wetting and spreading ability of the solder alloy on copper and ENIG surfaces, the wetting behavior of the doped alloys was better on HASL surface compared to the eutectic solder alloy.

The goal of this work was to evaluate changes in mechanical properties and microstructure of the lead-free bismuth-tin alloys when adding gallium and phosphorus. These elements were added originally on account of melting temperature decrease (Ga) and improvement of wetting (P). Six different solders were examined: the eutectic one Bi58Sn42 as a reference, then Bi59Sn40Ga1 and Bi57Sn40Ga3. In addition, the investigation was also performed on all the mentioned alloys with an added trace amount of phosphorus. Mechanical properties were measured by the shear test of a solder ball on a copper substrate with Organic Solderability Preservatives (OSP) surface finish. Furthermore, the small balls of solder alloys were reflowed on boards with three different surface finishes: copper-plated, Hot Air Solder Levelling (HASL), and Electroless Nickel Immersion Gold (ENIG). These coupons were left to aging in the climatic chamber for 500 and 1 000 hours at a temperature of 100 °C. The metallographic cross-sections were made, and the microstructure of the intermetallic layer (IML) was analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis. The shear test showed a significant decrease in the shear force by adding gallium. It was also found that the addition of phosphorus has only a minor (but still statistically significant) impact on the shear force. The addition of gallium affected the IML thickness and caused a considerable decrease compared to alloys without Ga. The reason was the composition of the IML. IML of Bi-Sn solder joints consisted of Cu-Sn, whereas the gallium-containing alloys formed IML consisted of Cu-Ga.

The aim of this work is to evaluate a wettability improvement and microstructure changes by addition of gallium and trace elements of phosphorus to novel low-temperature lead-free Bi-Sn solder alloys. Four different alloys Bi59Sn40Ga1, Bi57Sn40Ga3, Bi60Sn40 and the eutectic alloy Bi58Sn42 were chosen. Furthermore, all these solders were investigated with an added small amount of phosphorus as well. For the wettability comparison, the wetting balance test in combination with three different fluxes was used. Moreover, these alloys were soldered to a copper plated test board and aged in a climatic chamber at the temperature of 80 °C for 24 days. Subsequently, metallographic cross-sections were made and analyzed by scanning electron microscope (SEM) and energy dispersive X-ray analysis (EDX). The results of the wettability analysis showed the dominance of the chosen flux while soldering. However, it is still possible to draw the conclusion that phosphorus as an additive in Bi-Sn-Ga alloys supports the wetting, which is a crucial property of the solders. On the other hand, by the addition of gallium to the Bi60Sn40, the wetting force decreased. Regarding the microstructure, two different intermetallic compounds were identified. Namely, Cu6Sn5 at the interface between Cu board and alloys Bi60Sn40P and the eutectic one Bi58Sn42. The second detected IMC was CuGa2 between Cu and solder alloys with one and three weight percent of added gallium.

The objective of this work is to examine the mechanical and thermal properties of novel lead-free bismuth-tin solder alloys with low melting temperature, each with a different percentage of added gallium and with phosphorus as a trace element. The motivation was to determine solder alloy with decreased melting point and maintain mechanical properties of eutectic bismuth-tin solder alloy at the same time. BiSn40, BiSn40P, BiSn40Ga1, BiSn40Ga1P, BiSn40Ga3, and BiSn40Ga3P were among the analyzed solder alloys. In addition, the same properties of BiSn42 were analyzed for comparison as a reference. The thermal properties were examined by differential thermal analysis, where the melting and solidification points were determined. The Vickers micro-hardness test was conducted for the observation of mechanical attributes. Moreover, the microstructure of the alloys was observed in a scanning electron microscope. The result of this study showed that the temperature of the melting point significantly decreased in the alloy BiSn40Ga3. Also, this alloy seems to have as sufficient mechanical properties as BiSn42. Therefore, it could possibly be used as a suitable substitution for the eutectic solder alloy.