Prediction On Thermal Fatigue Life & Study On The Reliability Of QFP Soldered Joint

Fine pitch device has been used widely with the continuous development of surface mount technonology, especially with requirement of high density microelectronic packaging devices. Although the packaging technology and form are updated continuously, the QFP is used more universal in the actual production. The reliability of soldered joints is more important, as in the SMT, the main function of soldered joints is to realize the electrical and mechanical connection among the chip, external circuit and components. Therefore, the reliability of soldered joints is one of the hot research issues in the world all the time.Effects of lead number, solders and soldering methods on the tensile strength of the joints of QFP device are investigated in this paper. The results indicate that the tensile strength of the joint is improved with the increasing of lead number, and the tensile strength of lead-free soldered joints is greater than that of Pb-containing soldered joints obviouly. Moreover, the tensile strength of joints soldered by laser reflow is increased by 25% than that by infrared reflow. Finite element method is used to simulate the residual stress in soldered joints of QFP devices with the different number of leads and solders respectively. The results indicate that the stress concentration areas in soldered joints locate at the heel, toe of the soldered joint, as well as at the area between the lead and the soldered joints; the largest value of the stress is in the heel of the soldered joints, which would be the weakest area of the joints. The stress of soldered joints for QFP100 is smaller than that for QFP48, and the Sn3.0Ag0.5Cu is smaller than Sn63Pb37 by comparing the lead number and solders, so the reliability of soldered joints are improved through selecting of lead-free soldered joints and lead number for QFP devices. The research results provide technology support for high density and lead-free of soldered joints.Practical thermal cycling test indicate that the tensile force of soldered joints will decrease with the increase of thermal cycling times for both Sn63Pb37 and Sn3.0Ag0.5Cu, and the tensile force of soldered joints for Sn3.0Ag0.5Cu is higher than that of Sn63Pb37 in every cycle, but the decrease rate of them are basically identical. After 1500 thermal cycling times, the crack extended completely leading to the failure of solder joints.The results indicate that the IMC layer of Sn63Pb37/Cu interface is thinner than that of Sn3.0Ag0.5Cu/Cu obviously, but the growth rate of Sn3.0Ag0.5Cu is smaller than that of Sn63Pb37, therefore, Sn3.0Ag0.5Cu solder has stable microstructure under high temperature. In addition, a new layer of Cu3Sn which is very smaller than Cu6Sn5 layer appears in the IMC after thermal cycling. Comparison with the infrared reflow method and laser reflow method, the tensile force of them decreases with the increase of thermal cycling times, moreover, the tensile force of soldered joints for laser reflow is higher than that for infrared reflow in every cycle. The IMC layer of interface for laser reflow is thinner than that for infrared reflow. Moreover, the IMC grains of soldered joints for laser reflow are more fine and uniform than that for infrared reflow.Effects of lead number, solders and soldering methods on the reliability of soldered joints are studied in this paper, it is indicated that the tensile force of soldered joints will decrease with the increase of thermal cycling times, but the tensile force of soldered joints for QFP100 is higher than that for QFP48, the tensile force of soldered joints for Sn3.0Ag0.5Cu is higher than that for Sn63Pb37, and the tensile force of soldered joints for laser reflow is higher than that for infrared reflow. The simulated results coincide well with those of the practical experiment, and the research results provide technology support for high density and lead-free of soldered joints.