Copper, thermomechanical properties

PCB performance In addition to the preceding rehability challenges posed by lead-free assembly processes, it also has to be guaranteed that all other performance characteristics of the printed circuit board stay the same. This includes dielectrical properties such as dielectric constant Dc (which influences impedance), dissipation factor Df, and thermomechanical properties such as copper peel strength, glass transition temperature, or coefficient of thermal expansion (CTE). These properties should not be affected by the assembly processes applying higher temperatures. 

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]. 

The majority of base materials for circuit boards are combinations of a copper foil with a laminate, where the laminate itself consists of a carrier material and a resin. Thus properties of the base material such as mechanical strength, dimensional stability, and processi-bility are determined primarily by the carrier material. On the other hand, the resin materials are responsible for the thermomechanical and electrical properties as well as for its resistance against chemicals and moisture. Frequently used carrier materials are based on glass and carbon fibers, papers, and polyamide, whereas the majority of the laminating resins are thermosets such as epoxies, phenolics, cyanates, bismaleimide triazine (BT) resins, maleimides, and various combinations of these [13]. 

The properties of aluminum alloys (mechanical, physical, and chemical) depend on alloy composition and microstructure as determined by casting conditions and thermomechanical processing. While certain metals alloy with Al rather readily [9], comparatively few have sufficient solubility to serve as major alloying elements. Of the commonly used alloying elements, magnesium, zinc, copper, and silicon have significant solubility, while a number of additional elements (with less that 1% total solubility) are also used to confer important improvements to alloy properties. Such elements include manganese, chromium, zirconium, titanium, and scandium [2,10]. 

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