Research On The Reliability Of Soldered Joints And Optimum Simulation For QFP Gull Wing Lead Sizes

The reliability of the SMT soldered joints is a crucial problem for the application of electronical product used in many fields. To meet the needs of the special requestments of QFP devices for the certain product, the FEM method was used to study the effects of lead geometry sizes, stress-strain distribution of the soldered joints and other factors on the reliability of QFP soldered joints, and the thermal cycling experiment was used to validate the FEM results.With the help of 2-D FEM model of J-lead, the simulated results indicate that the distortion of the whole J-lead device was obvious; the stress and strain of the J-lead soldered heel was bigger than the other place of the soldered joints and the stress distributed a wide area, where was the weakest place of the whole soldered joints, so it was easy to lose efficacy. The stress and strain in the J-lead soldered toe was less than that in the heel of the soldered joint, the stress and strain in the center place of J-lead soldered joints was smallest and the simulated results were concordant with the experimental results.The reliability was mainly researched for the gull wing lead of QFP devices. The 2-D analysis results indicate that the inboard of the soldered joints was the weakest position, where the stress was hardly concentrated, so it was easily to be destroyed. When the lead thickness is 0.15mm, the lead height is 0.65 mm, the touch length between ceramic and lead is 0.15mm and the touch length between solder pad and lead is 0.5mm, the 2-D structure will be the most stable. The 3-D analysis results indicate that when the lead pitches are the same, the maximum equivalent stress of the soldered joints increases with the increasing of lead widths. When the lead widths are the same, the maximum equivalent stress of the soldered joints doesn't decrease straightly with the increasing of lead pitches, and but a minimum value exists in the curves of the maximum equivalent stress values. The simulated results also indicate that lead width of 0.15mm and lead pitch of 0.2mm maybe the limit value of lead pitches of QFP.On the bases of the numerical simulation, the tensile tests and the accelerated aging tests of the soldered joints were made in this paper. The results indicate that when the lead pitch of QFP devices was fixed, with the increasing of the lead width, the tensile strength of the soldered joint was decreasing. When the lead width was 0.22mm, the tensile strength of the QFP-100 devices was the highest, the tensile strength of QFP-48 was lower and the tensile strength of QFP-32 is lowest, which were concordant with the experiment results. The accelerated aging tests show that the real life of 186 times was close to the theory result of 213 times and with the increasing of the thermal cycling times, the tensile strength of the soldered joints decreased gradually, and the fracture mechanism changed from tough fracture to brittle fracture.The research results provided data support for some type military products and possess favorable guidance meaning for predicting the working life of soldered joints in electronic industry.