Solid solder-substrate reactions

Activation energies for the Cu6Sn5 is lower than that in the CuaSn phase (Ref 75, 76). It follows that the proportion of Cu Sns in the total intermetallic thickness increases as the temperature decreases. 

The thickness, X, of the intermetallic layer can be represented by an equation of the usual form  

Miniature Charpy tests were conducted on four different solder balls (eutectic Sn-Pb, near eutectic Sn-Ag-Cu, eutectic Sn-Zn, and Sn-Zn-Bi) bonded to Cu or electroless Ni(P)-immersion Au, and subsequently aged at 150 °C (302 °F) for up to 1000 h (Ref 79). A large number of voids were observed in the 8-phase after 500 h 

At least four separate studies have reported void formation in the 8-Cu3Sn phase and degradation of the joint strength after long-term solid state aging. An SEM photo of a cross section of a Cu/eutectic Sn-Pb interface after baking for 20 days at 125 °C (257 °F) is shown in Fig. 7. 

The interface microstructure and shear strength of 96.5Sn-3.5Ag and 62Sn-36Pb-2Ag on Cu substrate after aging at 150 °C (302 °F) 

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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. 

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