Tensile strength of fine pitch QFP lead-free soldered joints with diode laser soldering

摘要

Purpose - The purpose of this paper is to investigate the laser soldering of fine pitch quad flat package (QFP) devices using lead-free solders and solder joint reliability during thermal cycling. Design/methodology/approach - QFP devices were selected as the test vehicles and were soldered with four alloy types, Sn37Pb, Sn3.5Ag, Sn3.8Ag0.7Cu and Sn3.8Ag0.7Cu0.03Ce. The experimental samples were QFP-256 devices with lead-free solder paste on the printed circuit boards. The packages were dried for 24 h at 125℃ prior to reflow soldering. Soldering experiments on the QFP devices were carried out with an infrared (IR) reflow soldering oven and a diode laser (DL) soldering system. Reflow soldering was performed at peak temperatures of 210℃ (SnPb), 240℃ (SnAgCu and SnAgCuCe) and 250℃ (SnAg), as determined on the boards. Pull testing was adopted to evaluate the tensile strength of the four solders using an STR-1000 micro-joint strength tester. Findings - The tensile force of the QFP micro-joints increased as laser intensity increased when it was less than an "optimal" value. The maximum tensile force of the QFP micro-joints was gained when the laser intensity had increased to 2,165, 2,127, 2,165 and 2,064 W/cm~2, depending on the alloy used. The thermal fatigue performance of three lead-free solder joints, SnAgCuCe, SnAgCu and SnAg, was determined to be superior to that of the eutectic SnPb alloy. After soldering without thermal cycling tests, the fracture morphology of soldered joints exhibited characteristic toughness fracture with both of the soldering methods. After 700 thermal cycles, the fracture mechanism was also toughness fracture, nevertheless, the dimples became large. The fracture morphology of the soldered joints subjected to 1,500 thermal cycles indicated brittle intergranular fracture on the fracture surface and no intense plastic deformation appeared before fracture with IR soldering. For DL soldering, the pull fracture model of the SnAgCuCe was completely ductile in the soldered joint with 1,500 thermal cycles. Originality/value - The paper usefully investigates the influence of laser intensity on the tensile strength of different soldered joints and the solder joint reliability during thermal cycling.