Microstructures and degradation of heat sink very-thin quad flat package no-leads Pb-free Sn-Ag-Cu solder joints after thermal aging

Heat sink very-thin quad flat package no-leads (HVQFN) packages soldered with Sn-3.8Ag-0.7Cu on metallized laminate substrates have been put to thermal aging. Temperatures from 140°C to 200°C for times up to 30 weeks were applied. The solder joint microstructure develops intermetallic compound layers and voids within the solder. Due to this, the mechanical reliability of the HVQFN inner lead solder joints is degraded. The intermetallic layers are of the type (Cu, Y)₆Sn₅, with Y=Ni, Au or Ni+Au, as well as Cu₃Sn, and follow a power law with aging time: X=C·tⁿ, where n=0.4 to 1.9 depending on temperature. The voids within the solder are attributed to Sn depletion of the solder in favor of the growth of (Cu,Ni)₆Sn₅. They are more pronounced the less the solder volume is in proportion to the intermetallic diffusion area. The amount of voids is quantified as a percentage of the residual solder. The time to reach the failure criterion of 50%, i.e., t_50%, is related to the absolute temperature according to an Arrhenius equation with an activation energy E_a=0.95 eV. This equation is used for determination of the maximum allowable temperature at a certain required operating lifetime.