| With the gradual promotion and application of high-power devices,the density of the package is getting higher and higher,the size of the solder joint is getting smaller and smaller,the number of solder joints is increasing,and the power of the internal chip of the component is getting larger and larger.The heat flux density inside the package is also getting higher and higher,in order to make use of its performance preferably,it must be cooled rapidly.The internal solder joints of the components will be subjected to high frequency and sharp temperature changes during service,which will inevitably cause the thermal expansion coefficient mismatch of various materials inside the components,and the joints of the joints will undergo cyclic changes of stress and strain,which in turn causes thermal fatigue damage of the solder joints,leading to the failure of the entire packaged device.At present,there is no relevant experimental or scientific research equipment at home and abroad,and there is no corresponding evaluation method.It is urgent to develop rapid thermal fatigue experimental equipment and technology to study the thermal reliability and thermal fatigue failure behavior of high-power solder joints.The influence,competitiveness and right to speak,guaranteeing the safety of the electronic information industry and saving energy and reducing emissions are of extremely important practical significance.The rapid thermal fatigue test device was designed and developed.SAC305 solder ball and solder bump were used to conduct rapid thermal fatigue tests,and the solder ball and solder bump under extreme conditions were studied.The internal crack formation,solder ball and solder bump interface organization evolution,the main conclusions are as follows:(1)Designed an electric soldering iron-heated rapid thermal fatigue experimental device,a mechanical reciprocating rapid thermal fatigue experimental device,and an electromagnetic induction heated rapid thermal fatigue experimental device.After comparing the heating source,cooling source,temperature control method,temperature sensor and equipment cost,it is found that the electromagnetic induction heating type rapid thermal fatigue test equipment has high efficiency,more intelligent,high commercialization possibility,and can meet the rapid thermal fatigue test conditions.So,it is its ideal choice.The electromagnetic induction heating type rapid thermal fatigue test equipment was trialproduced.The temperature range of the test platform is-15~190?,the temperature rise and fall time is 12 s,and the cycle time is 24 s.The rate of temperature rise and fall reaches 17.08?/s,which can fully meet the design and practical requirements.(2)The germination period of the surface crack of the 0.1g solder ball is 400 cycles,the germination period of the surface crack of the 0.2g~0.3g solder ball is 650 cycles,and an "X" type of grid cracks appears on the top of the surface of the solder ball,the surface crack growth rate of 0.4g solder ball is significantly lower than that of 0.1g solder ball.As the rapid thermal fatigue cycle increases,the oxide film continues to thicken and expand,the stress of the oxide film growth is very prominent or cannot be released through creep,the inner surface will appear a crack of the oxide film.The greater the curvature of the metal oxide film,the greater the Mises stress and equivalent plastic strain,and the more likely the surface of the solder ball to crack.The appearance of cracks will quickly release the stress in the film and lead to changes in the oxidation rate.At the same time,the cracks act as a fast channel for the diffusion of oxygen elements also will directly promote the oxidation behavior.The thermal stress is released by increasing the width of the crack,and no new stress concentration point is generated on the oxide film,so no new crack will be generated after the crack distribution is stable.(3)During the initiation period of the surface crack,there was no crack inside the 0.2g solder ball.The directional dendrites increased significantly at 1500 cycles,and several short inter-granular cracks appeared.At 2500 cycles,dendrite coarsening is obvious,and there are many cracks on the edge of the solder ball and near the edge.At 3500 cycles,two large cracks appeared in the lower left corner of the solder ball,and the tissues around the cracks were clearly differentiated.At 4500 cycles,a large main crack along the temperature gradient appeared in the lower left corner of the solder ball,and there were many small dendrites around the crack.At 5500 cycles,a long crack penetrated the entire solder ball,and many cracks also appeared at the bottom of the solder ball.The dendrites around the crack grew obviously along the temperature gradient.The internal crack growth of 0.4g solder ball is slower than that of 0.2g solder ball.The crack growth is distributed in a “zigzag” shape at 4500 cycles.Under the action of heat flow,the recrystallization and grain directional growth preferentially occur at the bottom of the solder ball.With the increase of the thermal fatigue cycle,the growth of dendrite increases,resulting in excessive local stress,coupled with various defects inside the solder ball.Eventually,cracks inside the solder ball.(4)With the increase of thermal fatigue cycle,the surface of conventional thermal fatigue solder bump becomes rough and dark,and there are almost no cracks on the solder bump surface.Cracks are gradually formed on the surface of rapid thermal fatigue solder bump,and the surface cracks are concentrated on the top of the solder bump.As the cooling rate increases,the temperature gradient of the sample surface and core increases,the thermal stress increases,the incubation period of crack initiation decreases,and the growth rate increases after the crack is formed.The larger the solder ball,the greater the amount of deformation,the greater the tensile stress on the solder bump,and the easier the solder bump to crack.With the increase of the conventional and rapid thermal fatigue cycles,cracks appear at the solder bump interface,and the crack propagation direction is basically symmetrically distributed in the center,and the crack propagation is very close to the pad interface.The interface crack of rapid thermal fatigue is much larger than the interface crack of conventional thermal fatigue in both the early and late stages.Due to the different expansion coefficients,the residual stress in the solder bump rises.At the same time,the volume shrinkage of the solder ball reflow will also cause the residual stress in the solder bump to rise,and the interface is more likely to crack under rapid temperature changes.(5)Whether it is conventional thermal fatigue or rapid thermal fatigue,the IMC thickness at the solder bump interface almost varies between 2 and 3?m.In the early stage of rapid thermal fatigue,the IMC of the three different sizes of solder bump interface changes greatly.Then the growth rate of the interface IMC slows down significantly.Finally,approaches a steady state that is equivalent to the thickness of the initial cycle,while the conventional thermal fatigue solder bump interface IMC,The thickness is about 1?m higher than the initial period.At 5500 cycles,the shear strength of conventional thermal fatigue solder bump(0.2g,0.3g,0.4g)was 24.80 MPa,24.69 MPa and 20.97 MPa,which were reduced by 35.68%,36.87% and 43.49%,respectively.The thermal fatigue solder bump(0.2g,0.3g,0.4g)shear strength are 23.53 MPa,19.08 MPa and 18.52 MPa,which are 38.98%,51.21% and 50.09% lower than the thermal fatigue respectively.The trend of rapid thermal fatigue shear strength decline is greater.As the diameter of the solder bump increases,the width and depth of the dimples at the fracture become smaller and shallower.The fracture mode is mainly ductile fracture.With the increase of the thermal fatigue cycle,the internal grains of the solder structure increase,dendrites and cracks increase,and the fracture mode changes from ductile fracture to brittle fracture. |
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