Temperature Resistance Strain Characteristics Of Lead-free Solder Joints Under Thermal Load And Stress-strain Simulation,

Under running condition, the solder joints endure stress and strain cycles because of the cyclic variation of inner power dissipation and environment temperature plus the thermal expansion mismatch among different material, which result in cracks in high stress and strain spot in solder joints and destroying of the whole component. Thus, the research of solder materials under thermal load of mechanical properties and thermal cycling reliability is very important.This paper introduces a means of micro-resistivity measurements to study Sn3.5Ag reliability assessment under thermal load of solder joint thermal power measurement methods, and process finite element simulation. Lead-free solder joints under thermal load is analysised synthetically for failure mechanisms. A theoretical foundation is lay by the establishment of solder joint damage relations with a simple model of resistance.The experiments of a single lead-free solder under high temperature creep and thermal cycle fatigue were studied by means of solder joint creep-resistance testing system. In the investigative process of heat reliability of lead free solder joint, hysteresis loops were discovered between resistance strain and temperature of lead free solder joint under the condition of circular load at 40℃-125℃. The characteristic of hysteresis loops were analyzed and discussed by the way of mechanical anylyse. The result shows that resistance strain is result from plasticity. The result show that the maximum plastic resistance strain is 0.00158 at one cycle in stabilization period and resistance strain is latter than temperature 18.31s.The result show that the change under high temperature of loops was faster than that under low temperature and the disparity of variation range is 0.0036749. The result shows that the loops change from unsteady to steady and finally to un-closed, and its slope were to become lower.The stress-strain characteristics is simulated by the use of finite element software ANSYS of a single modal and multi-solder modal under thermal cycle load. The result show that the maximum stress of solder joints appear the contact surface between the solder joint and the contact , while the minimum stress appear in the middle spot. This result show that the connection between solder joint and substrate solder is the high stress areas, which is more fragile ; Shear stress and shear strain of solder joint follow the temperature cyclic loading , present the cyclical change and increase with the increase of the number of recycling. The phenomenon show that the damage in the solder joints accumulate with the increase of the number of recycling. The damage is divided to reversible and irreversible parts, which the latter is the reason of the increasing of internal stress in the solder joint.In a cycle,the high-stress produce in the low-temperature phase of thermal cycling and the low-stress bring in the high-temperature phase.The stress relaxation appear at high-temperature and low-temperature of heat preservation. The stress-strain curves parallel with each other, which reflects the Sn3.5Ag lead-free solders have the steady-state character of creep.