Research On Reliability Of High Power Light Emitting Diode And Wafer Level Packaging Product

Reliability issues are the key factors that hinder large-scale application of electronicproducts. This thesis focuses on the reliability problems of high power light emitting diodes(LED) and wafer level packaging (WLP) products. Due to its superior characteristics, such ashigh luminous efficiency, long lifetime and low power consumption, high power LED willbecome the most potential substitute for traditional lamps. Meanwhile， because of theirhigh productivity and low cost, the WLP products also have attracted more and moreattention. However, since the external environment is complex and volatile, many reliabilityproblems emerge. Therefore， it is significant to study the reliability of LEDs and WLPproducts. Based on finite element method, the detailed contents of this thesis are as follows:(1) Reliability of LED’s gold wire with triangle-shaped loop profile andtrapezoid-shaped loop profile under thermal shock loadings is studied respectively,and it is found that the gold wire with trapezoid-shaped loop profile has higherreliability. And copper ring is applied around the silicone lens to suppress the largedeformation of lens during thermal shock test, thus the stress concentration isreduced at the second bond of gold wire. Taking both the optical and mechanicalperformance of the LED model into account, a0.6mm copper ring in height is thebest choice.(2) Failure analysis techniques of high power LEDs is introduced, and then one of thefailure samples undergoing thermal shock cycles is analyzed and it concludes thatbecause of delamination, the entire tensile stress stemming from shrink andexpansion of silicone is imposed on the gold wire, and then plastic deformationhappens on the wire, resulting in ductile fracture finally.(3) The thermo-mechanical reliability of fan-in and fan-out WLP products is studied and the fatigue life of solder ball is considered as the evaluation indicator. And it isfound that the polymer film with4GPa Young’s modulus and40ppm CTE(coefficient of thermal expansion) is helpful to enhance the reliability of solder ball.One fan-in WLP product is evaluated, and it reveals that thin chip and PCB, smallchip and small solder ball diameter have positive effects on the solder ball’sreliability. What’s more, the solder balls which are furthest away from the chipcenter have the lowest reliability. So coating glue to protect these balls is proposed,and it is found that high Young’s modulus and low CTE glue is good for solder ball’sreliability. One fan-out WLP product’s thermo-mechanical reliability is analyzed,and it is found that the outmost ball right below the chip has the largest chance tofail to work. The accordingly partial array solder ball arrangement and peripheryarray solder ball arrangement are proposed. Furthermore, the solder ball’s reliabilitywith different layouts is studied. Taking the solder ball’s reliability and I/O countsinto account, partial array solder ball arrangement is recommended.(4) Studied the board level drop reliability of fan-out WLP product, a new simulationmethod is applied, using the displacement to be the loading, and it is found that thesolder balls fringed under the chip have the lowest reliability and during drop impact,the peeling stresses in Z direction have the dominant effect on the solder ball’sreliability. Thus, coating glue protecting those balls is proposed and the glue’s effecton the thermo-mechanical reliability of solder ball is also evaluated.