Pad design, soldering technology and reliability of package on package — Study on the deformation of printed board

In the last half a century, the complexity of electronic systems has undergone a sustainable rapid development stage following the Moore's law: the density of integrated circuits (ICs) was doubled every two years. As a result, electronic devices become more miniaturized in space and contain more power in unit volume. Electronic package has to deal with highly increased I/Os with fine soldering pitch. A symbol of the fast development of the package technology is using staked technique to develop three-dimensional (3D) package from 2D and 2.5D. The representative products are the package-on-package (PoP) technology in which packages are staked and through-silicon-via (TSV) technology in which chips are staked. The PoP has been widely used in handheld products such as cell phones and PDA. The harsh environment in service and high integration in composition have brought big challengers to the design and process of the package. PoP technology has attracted great attentions. The staked composition has largely reduced the length of connection lines between packages. It is also coupled with modules design concept in which different functions can be contained in different packages and thus results in a convenience for efficient production. In view of the advantages, PoP is favored by cell phones and other handheld products. A great deal of work has been done on the general issues. However, given the short life period, detailed studies on specific aspects and individual products have not been covered in full. These include such aspects as the effects of pad design, stencil design and soldering profile on the reliability of PoP systems. The issue for how to increase the pass rate of soldering satisfactory under extreme deformations has also not been seen by the authors in report. PoP has raised challengers to package design. As a staked system, interactions are intensified between packages: the deformation due to the difference of the coefficient of thermal expansions (CTE) will be accumulated and the heat cannot be dispersed through the surfaces between packages. Furthermore, the staked packages become difficult to bend following the PCB as performed by single thin package. All the above factors can deteriorate mechanical damage. So we will work on investigation of the manufacturability and reliability of a PoP system through an integration of modeling, testing and design modification. This includes (i) finite element analysis (FEA) of the PoP system to identify the critical regions and deformation; (ii) experimental test of the PoP system to validate the FEA results; (iii) evaluation of the impact of thermal deformation on the quality and reliability of solder joints to form guidance for detail designs of PoP systems. Agreed results have been achieved from our modeling and testing. The thermal damage to the PoP system has been realized. The effect of manufacturing details of pad, stencil and soldering on the reliability of PoP systems has been analyzed. A guide for design procedure has been offered. Through this integrated investigation, we have obtained a good understanding to the mechanical performance of PoP systems and found the way for how to design relevant details for enhanced reliability. This has practical reference to PoP design and testing. In this paper, we work on the parts of integration modeling: research on printed board's deformation. Finite element method is used to predict the deformation of printed board in different copper case, testing the PCB s deformation by use moire, and correcting finite element model, after we master PCB deformation, we could design PCB such as layout, routing, and wiring to pave the way for the production of POP. Subsequent work with experimental continue to be in a future paper published in succession.