Solder joint thermal properties

Alloy selection depends on several factors, including electrical properties, alloy melting range, wetting characteristics, resistance to oxidation, mechanical and thermomechanical properties, formation of intermetaUics, and ionic migration characteristics (26). These properties determine whether a particular solder joint can meet the mechanical, thermal, chemical, and electrical demands placed on it. 

The circuits schematized below show two possible utilizations of the Seebeck effect, one (left) in closed circuit (and therefore with a potential difference equal to zero) producing a current from thermal energy and the other (right) in open circuit, therefore in the absence of current, called thermocouple and used for measuring temperature differences. These circuits are both made up of two soldered joints of two materials having distinct thermoelectric properties. Case study J8 is devoted to the description and modeling of the thermocouple, also called thermoelectric junction, which is recalled here for comparison. 

The accepted method of nondestructive testing used to control the underfill process is SAM. The thin layer allows this technique to detect voids in the underfill material, which when located near the solder interconnections can be responsible for a significant loss of thermal mechanical fatigue reliability. X-ray techniques can be used to monitor the density of the underfill material, specifically, the distribution of filler material within the layer under the die. Density variations can indicate a larger distribution of underfill mechanical and physical properties, which may affect long-term reliability performance of the solder joints. Quantitative image analysis can be coupled into SAM and x-ray analysis data to provide valuable process control tools for the factory floor. 

Interconnect failures due to thermal fatigue of solder joints can be reduced by closely matching the x-y plane thermal expansion properties of the substrate to at-risk components. Large leadless ceramic components that are used because of their hermeticity pose a particular risk. Possible approaches include altering the laminate reinforcement material, adding constraining metal cores or planes, and switching to a ceramic substrate. The first two approaches are discussed here. A more extensive discussion of these options can be found in Ref. 33. 

Recent advances in the development of TMF predictive models have included the incorporation of solder microstructure as a state variable in the constitutive equation [92,93]. A contour map (Fig. 30) shows the Pb-rich phase coarsening predicted in a leadless chip solder joint subjected to six thermal cycles having temperature limits of —50° and 80°C, ramp rates of 6°C/ sec, and dwell times of 10 min at both limits. The microstructure-based, viscoplastic constitutive equation for Pb-Sn solder improves the accuracy of solder fatigue strain predictions throughout the interconnection geometry. It provides a real-time adjustment of the local solder mechanical properties resulting from local changes in the solder microstructure (i.e., the Pb-rich phase). 

The only SM components with obvious fatigue failures after more than 6700 cycles of 0-100°C, or 5000 cycles of —55 to + 125°C were LCCCs and 1206 chip resistors. No leaded SM devices exhibited failures. There were no unexpectedly early or catastrophic chip carrier or passive component failures. Those failures that occurred followed the same component order as observed for eutectic Sn-Pb. The ranking of alloys relative to eutectic Sn-Pb varied with thermal cycle and component type. Each solder alloy is able to withstand different amounts, types, and rates of loading, which are dependent upon the different coefficients of thermal expansion (CTE) and mechanical properties of the board, components, and alloys, and upon solder joint configurations. 

Sn-Ag Cu alloys wet and form good-quality solder joints with copper. It is a very promising solder system for replacing the conventional eutectic Sn-Pb solder in avionics and automotive applications where solder joints are subjected to harsh thermal cycle conditions and mechanical vibrations, and are expected to sustain operational temperatures up to 150°C [39-42]. The thermomechanical properties of these Sn-Ag alloys are reported to be better than conventional eutectic Sn Pb solders, as shown in Table 6 and depicted in Fig. 18 [31]. Various studies conducted on Sn-Ag-Cu solder alloys are listed in Table 7 [31,39 52]. 

Fabrication of a thermocouple [34] requires some skill and familiarity, especially when using small-diameter wires. The measuring junction should be a joint of good thermal and electrical contacts produced without destroying the thermoelectric properties of the wires at the junction. For applications below 500°C, silver solder with borax flux is sufficient for most base metal types, whereas junctions formed by welding are recommended for use above... 

The topics discussed in the chapter cover the fundamental aspects such as the microstructure and mechanical/physical properties of Pb-free solder alloys, their processing issues (wetting behaviors, interfacial reactions, and joint defects), and their reliability issues (thermal fatigue, creep resistance, tin pest, and others). The existing and potential applications of each solder system for microelectronics have been extensively evaluated. In addition, the recent patent literature for Pb-free solders has been reviewed to discuss the trends in solder alloy development activities. 

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