Mechanical shock robustness of different WLCSP types

Abstract

WLCSP, real wafer size package, usage has grown heavily during last years and volume is expected still to grow a lot. Main drivers for WLCSP usage are size and thickness reduction in portable devices and in some applications also price reduction by simplifying packaging structure. The main factor affecting these is reduction of component IO pitch. Growing usage volume has also increased the amount of WLCSP manufacturers, giving large variation of different package structures to the markets. As a consequence field of reliability is large, since different structures are behaving very differently especially under mechanical shock, which is considered to be most critical behavior of WLCSP packages in portable devices. However thermo-mechanical reliability should not be compromised either. Today there are many studies showing comparison of reliability test results between some factors, like dimensions. However from WLCSP end-user point of view whole system is more complex; there are several factors effecting to the overall mechanical and thermo-mechanical robustness of WLCSP packages, especially metallurgical factors play very remarkable role, including different underbump material and solder ball combinations. From WLCSP user point of view it is important to understand the variety of structures in the field and the complexity of factors effecting reliability in the area. Aim of the study is to increase knowledge of complexity of this area and factors effecting reliability of various WLCSP structures. As an introduction, data from last years WLCSP volumes in Nokia portable devices will be shown, together with an estimate of further usage volume. Overview of most common WLCSP structures will be presented, to give an overview of variety of different structures in the field. Reliability test results of mechanical shock will be presented and different package types and bump pitches will be compared. Also "weak links" between different package types will be compared, in terms of failure analysis results. As a conclusion there will be discussion of pros and cons of different package types, there is also not only one correct type to be used but several possibly good ones, defined also by electrical design.