Research On The Theory & Experiments Of SMT Lead-Free Solder Joint Reliability Under Drop Impact

Surface mount technology (SMT) is a high density and high reliability microelectronics packaging technology. Electronic components are soldered directly on Printed circuit board (PCB) or ceramics substrate. In the SMT field, and the solder joints act as mechanical connect and electronic link between electronic components and PCB, so solder joint reliability has been a great concern. In recent years, electronic devices and systems have been tending to light, thin, small, low power, multi-function and high reliability. These handheld electronic products are more prone to being dropped during their useful service life because of their size and weight. This dropping event can not only cause mechanical failures in the housing of the device but also create electrical failures in the PCB assemblies mounted inside the housing due to transfer of energy through PCB supports. In addition, due to the merits of lead, it is used widely in the SMT field. However, lead is toxic. It not only contaminates our environment, but also endangers our health. Lots of countries have constituted laws to restrict the using of lead. Lead-free in electronic industry has become a tendency. Thus, the research on reliability of SMT lead-free solder joint under drop impact is very significant.In this paper, the reliability of SMT lead-free solder joints of BGA packages, which are currently popular in handheld electronic products, was investigated under drop impact. A series of experimental and theoretical researches were finished, and a new reliability test method which aims at engineering application was developed.First, due to the limitation of JEDEC test board, the design idea of a new test board was demonstrated. A series of modal tests were conducted. The modal tests of PCB (free boundary condition) and PCB assembly (free boundary condition and fixed boundary condition) were carried out. The equivalent elastic modulus and damping parameters of PCB can be gained, and the fixed boundary condition of finite element (FE) model was also determined. These will help to the following work.Second, drop impact tests of three different heights were carried out. Strains and acceleration responses of PCB were measured. Failed BGA packages of different test conditions were dyeing, and cross section analysis of failed solder joints was conducted. It was found in our drop tests that the root cause of lead-free solder joints under drop impact was the combined effect of mechanical shock and PCB bending vibration. From the resistant values of some test points, dye penetration test and cross section analysis, it can be seen that the solder joints at four outermost corners were failed at first. The vulnerable location of lead-free solder joint located at the Ni/intermetallic composite (IMC) interface between package and the solder joint. The failure location, failure interface and failure mode of solder joints were the same as those of failed solder joint of the JEDEC test board, whose drop tests were conducted by Intel. These also verify that the round test board could alternate the JEDEC test board under drop impact. This will provide a theoretical and experimental foundation to improve JEDEC drop test standard.Third, Based on modal tests and finite element simulation, a valid three dimension finite element model was established. The input-G method was adopted and the ABAQUS software was used to carry out the three-dimension FE simulations of lead-free solder joints. The established FE model was very simple and effective, the dynamic responses, such as output acceleration, strains were well correlated between testing and simulating. When the maximum peeling stress was used as a failure indicator to determine initial failure location of solder joint, numerically predicted failure locations of the solder joints accorded with those observed from drop impact tests. Fourth, on one hand, Weibull distribution in statistics was utilized to study the impact life prediction models of lead-free solder joint under the three drop heights. On the other hand, a life prediction model was formulated using Power law to relate the maximum peeling stress and mean impact life.Final, From the point of view of viscoelasticity, dynamic properties of PCB under drop impact were analyzed based on mass-spring-damping, beam and plate theories. It proved that viscoelastic parameters of PCB substrate had a great influence on the solder joint reliability under drop impact. This not only contributes to understanding real dynamic characteristics of PCB under drop impact and perfecting theory research, but also provides a reference for reliability design of PCB and reliability evaluation.