Characteristics of Sn8Zn3Bi solder joints and crack resistance with various PCB and lead coatings

Reliability of QFP (quad flat package) solder joints after thermal shock was investigated for PCB's and connecting leads plated with several different alloy coatings before soldering. Sn-8 wt%Zn-3 wt%Bi (hereafter, Sn-8Zn-3Bi) was selected as a solder, and FR-4 PCB's finished with Cu/Sn, Cu/OSP and Cu/Ni/Au were used as substrates. The leads of the QFP were Cu plated with Sn-10 wt%Pb, or Sn, or Sn-3 wt%Bi. The QFP chips were mounted on the substrates using a Sn-8Zn-3Bi solder paste and reflowed in air atmosphere. The pull strength and microstructure for the soldered leads of QFP were evaluated before and after thermal shock testing. The leads plated with Sn or Sn-3Bi showed approximately 40-50% higher pull strength than the reference value of a Sn-37%Pb solder joint for all PCB-finishes. However, in the case of leads coated with Sn-10Pb, the pull strength of the leads soldered to a Sn-finished PCB was 21% lower than the reference value. In microstructure analysis of the joints with Sn-10Pb-plated leads, cracks were found along the bonded interface for Sn-finished PCB. The cracks were believed to start from the low melting temperature phase, 49.38 wt% Pb-32.58 wt%Sn-18.03 wt%Bi, found around the crack, and then propagated through Cu-Zn intermetallic compound. Meanwhile, even when using Sn-10Pb-plated leads, the PCB's finished with Cu/Ni/Au coating had about 30% higher strength than the reference value, and cracks were hardly found on the soldered joint. Thus, even with Sn-10Pb-plated leads the Cu/Ni/Au-finished PCB's were evaluated to be as reliable as the reference joint.