In recent years advanced interconnection technology such as flip-chip technology and ball grid array (BGA) is enabling microelectronic packaging to move toward finer pitches, higher speed, greater packaging densities and lighter weight. In flip-chip technology and BGA, the electronic components are interconnected electrically and mechanically to the substrate by solder bumps and solder balls, respectively. Thereby the interconnection by the solder joint is essentially important in the electronic packaging. Generally the interconnection by the solder joint is completed by the formation of intermetallic compound (IMC) at the interface between the solder and the substrate. The presence of IMCs formed during the liquid soldering influences the solder joint reliability. The presence of interfacial IMCs usually improves the wettability of liquid solder by reducing the interfacial energy between the liquid solder and the substrate and indicates that a good metallurgical joint has formed. However, a solder joint may become embrittled if excessive amounts of IMCs are formed at solder joint during a soldering operation or in use. IMCs that have grown at the molten solder-base metal interface could lead to the loss of strength at this interface under certain loading conditions. The influence of the interfacial IMCs upon the solder joint reliability increases as the size of solder bumps becomes smaller for the advanced packaging applications such as BGA, CSP (Chip Scale Package) and WLP (Wafer Level Package).
Generally, there are two largely different types of grain morphology of interfacial IMCs at the solder joint. One is scallop-shaped and rough (rounded) type and the other is polyhedral-shaped and faceted type. The typical example of scallop-shaped and rough (rounded) type IMC is $Cu_6Sn_5$ which usually forms on soldering of Sn-rich alloys on Cu substrate and the typical example of polyhedral-shaped and faceted type is $Ni_3Sn_4$ which forms on soldering of Sn-rich alloys on Ni substrate. By the reaction of molten solder alloy with compositions of 96.5Sn-3.5Ag with either the thick Cu or the thick Ni substrate at 250℃, the rounded $Cu_6Sn_5$ grains formed over Cu and the faceted $Ni_3Sn_4$ grains precipitated over Ni. As the soldering time changed from 1 min to 60 min, normal grain growth occurred for rounded $Cu_6Sn_$5 grains while abnormal grain growth (AGG) mode was observed for faceted $Ni_3Sn_4$ grains. The measured grain size distributions also confirmed the difference between normal grain growth and abnormal grain growth. Also the transition in morphology of $Ni_3Sn_4$ grains that formed at the interface between liquid Sn-3.5Ag solder and Ni substrate has been observed at 250 ~ 650℃. The morphological transition of $Ni_3Sn_4$ is due to the decrease of entropy of formation of the $Ni_3Sn_4$ phase and has been explained well by the change of Jackson’s parameter with temperature. According to the variation of solder joint strength with temperature, it decreased rapidly between 350℃ and 450℃, where the thickness of the $Ni_3Sn_4$ layer was around 6.5 ㎛. However, the solder joint strength decreased slowly with increase of soldering time without a significant drop although the thickness of IMC was larger than 6.5 ㎛. The notable drop of solder joint strength and the fracture mode transition with an increase of the soldering temperature appears to come from excessive lateral growth of IMC grains between 350℃ and 450℃.
The $Cu_6Sn_5$ phase formed in the reaction of molten Sn-Ag-Cu solder alloy with a composition of up to 1wt.%Cu. The shape of $Cu_6Sn_5$ grains was changed from facet type to round type with increasing soldering time and increase of Cu content. The transition of the Cu6Sn5 grains shape was explained by Cu content in the solder. The lower Cu content causes the dissolution of interfacial IMC, therefore the IMC grains have facet face. With the increasing soldering time, the dissolution of interfacial IMC is stopped and the shape of $Cu_6Sn_5$ grains become round because the molten solder are saturated by substrate element. The thickness of interfacial $Cu_6Sn_5$ layer was thicker with Cu content at the early stage of soldering, however the growth rate decreased more quickly with increasing soldering time. This phenomena was explained by lateral growth of IMC grain and reduction of grain boundary. The thickness of interfacial $Cu_6Sn_5$ layer and lateral size of $Cu_6Sn_5$ grains between molten Sn-3.0Ag-0.5Cu solder alloy and Cu substrate increased with soldering temperature. However the shape of Cu6Sn5 grains became more faceted because the solubility of Cu in molten solder increased with soldering temperature.
A numerical method has been developed for calculating the thickness of interfacial IMC layers formed between Cu substrates and solders during the soldering process. Comparison between experimental results and calculated results showed that the numerical method predicted well the IMC layer thickness during soldering by reflow profile. Calculation of the IMC thickness also showed that dissolution has a significant effect on the growth behavior of an interfacial IMC. Also a convection of molten solder at an early stage of soldering was found during soldering by calculation.
