Solder-Substrate Interactions

When a substrate contacts a molten solder, it may dissolve. At the same time, Sn in the solders interacts with the substrate to form IMC at then-interface. The relative amount of the substrate that may dissolve into the molten solder is determined by its solubility limit at the temperature, and the dissolution rate of the substrate into the molten solder depends on the substrate material and temperature. In general, the temperature dependence follows an Arrhenius behavior. 

Phase diagrams may be used as a guide to identify which intermetallic phase forms at the liquid solder/substrate interface. However, an equilibrium phase diagram merely indicates what phases are thermodynamically stable, given a particular composition and temperature. The kinetics of solder-substrate reactions determine the structure of the solder/substrate inter- 

25 wt%). In Ref 25, a Cu-Pb-Sn isothermal section at 200 °C (392 °F) was presented, and the solubility of Cu in 63Sn-37Pb was approximately 0.05 wt%. The experimentally measured values (Ref 22, 23) are significantly larger than either curve-fitted values (Ref 21) or thermodynamically calculated values. 

The solubility limit of Cu in molten eutectic Sn-Ag was determined experimentally (Ref 8). Solder joints of eutectic Sn-Ag were reflowed on Cu for 5 to 10 h, to achieve the Cu saturation. The Cu concentration in the solder matrix was determined by energy dispersive x-ray (EDX). The measurement was conducted at temperatures between 232 and 300 °C (450 and 572 °F). The test data were curve-fitted to an Arrhenius relationship such as Eq 1, and the results are C o (at.%) = 9870, and Q = 35.3 kJ/mol. 

The solubility limit (C, at.%) of a substrate material in a molten solder may be used to determine the minimum thickness (A/js b) of the substrate or under bump metallization (UBM) for a given solder bump dimension (the ratio of solder volume (V) to contact area (A), or the thickness of the solder joint, / soider)  

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Use of ink-jet technology for printing of fuel cell components and packaging presents most of the same fluid/substrate interaction issues. The printing of Nafion , metal catalyst (e.g., Pt), solder, electrodes, etc. requires deposition of liquid onto a nonporous substrate (similar to printing an overhead transparency). For desired resolution, it is essential to control the spreading of the material. 

The interactions between solders and substrates are described in two classes liquid sol-der/substrate reaction during soldering and solid solder/substrate reaction during subsequent aging. During soldering, two processes occur simultaneously (a) the substrate metal dissolves into the molten metal and (b) the active constituents in the solder combine with the substrate metal to form intermetallic compounds (IMCs) at the substrate/solder interface. 

This chapter reviews Uteratme on the microstructure of solder joints and the interactions of solders with substrates and related solder joint reliability issues. The substrates here are limited to Cu, Ni-coated Cu, electroless nickel/ immersion gold (ENIG), and hot air solder leveled (HASL) Sn-Pb. The solders are mainly Sn, eutectic or near eutectic Sn-Ag, and Sn-Ag-Cu alloys. Specific reliability issues discussed here include black pads of ENIG, gold embrittlement, compatibility of Pb-free solders with Pb-containing surface finish, and Kirkendall voids. 

It is also very important to appreciate the complex interactions of the measures they take and the results they obtain in production with those of all the other subsections involved. For instance, it is not only the direct down-line process of metallization that is affected by a change in the type of plastic. This also has a significant effect on later assembly and connection technology, for example if the new substrate fails to evince the level of temperature resistance required for the soldering process. 

Vianco, P. Grant, R. Solder interactions with multicomponent substrate chemistries, presented at the TMS Annual Meeting, Anaheim, CA, February 4, 1 6. 

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