Defect Detection Of Surface Mount Devices Based On Eddy Current Pulsed Thermography

In recent years,surface mount devices(SMDs)have been widely applied in every walk of life,such as military,aerospace,medical treatment,etc.The smaller element size of components and higher density of packaging make heat dissipation worse.Poor heat dissipation causes local overheating which triggers solder joint failure,and then single solder ball defect can result in malfunction of the whole circuit system.It's difficult to detect solder joint defects owing to their tiny sizes.Traditional inspection methods may have complex operations,too high cost,lower detection efficiency or low accuracy.To solve above problems,this paper applied eddy current pulsed thermography(ECPT)detection to defects inspection of tiny solder ball.Surface temperature distribution was recorded by infrared thermal imager for recognition of solder joint defects.The main contents of this thesis are as follows:The process of solder ball detection based on ECPT is analyzed to determine interaction between three physical fields(electrical,magnetic,and thermal).The heating mode and excitation parameters are set according to solder joint characteristics.At the same time,the influence factors are discussed and corresponding improvement measures are put forward.Finally,the heat conduction model of solder joints is solved analytically.Pseudo soldering defects of three positions are defined newly based on the mechanism of ECPT.The initial conditions,boundary conditions and load conditions are ascertained and then modeling analysis is carried out using finite element software.Pseudo soldering defects are introduced in the model,and defect effects on eddy current distribution and temperature distribution are studied.In the meantime,thermal resistance is redefined based on heat analogy method.The variation of thermal resistance is extracted to describe influence of pseudo soldering in heat conduction through solder joint,further characterizing thermal performance of solder joints.The effects of pseudo soldering with different sizes are discussed,and thus acreage of pseudo soldering is proposed to express different degrees of pseudo soldering.The ECPT experimental system is established to carried out comparison experiments about pseudo soldering.Pseudo soldering in different pins,pseudo soldering with different degrees and pseudo soldering in different types of packaging components are all taken into account.In terms of the detection results about pseudo soldering at different pins,defect location can be realized.It also can be assumed component inside connection has less effects on temperature distribution which can be ignored.Then the principal component analysis and Fourier transform are utilized for thermal image processing.Solder joint defects are recognized on the base of the reconstitution thermal images.Different degrees of pseudo soldering,described by different pseudo soldering acreages,are produced to analyze defects quantitatively.The detection capability of three types of packages(SOIC?QFP and TSOP)are compared in accordance with thermal images and temperature-time curves.It can thus be seen that detection capability of QFP component is the best one,but pseudo soldering of TSOP component can not be discovered.As a result,the detection results become better with increase of pin acreage.This article shows that the ECPT method can be used to detect effectively solder joint defects of components with gull-wing leads.Meanwhile it also make sure that the pseudo soldering acreage can be applied to represent different levels of pseudo soldering,realizing quantitative analysis of pseudo soldering.This can lay the foundation for solder joint reliability.