ASSEMBLY AND DESIGN CHALLENGES FOR NEW GENERATION 0.4/0.4MM PITCH PACKAGE ON PACKAGE (POP) AND 0.3MM PITCH CHIP SCALE PACKAGE (CSP)

摘要

Miniaturization and the integration of a growing number of functions in portable electronic devices require an extremely high packaging density for the electronic components. One way to increase the packaging density is to reduce the size of the packages stack them with as Package on Package (PoP) and at the same time increase the density of the I/Os. This means that the size and pitch of solder balls or pads in the electronic component packages will continue to shrink and that component stacking with so called Package on Package (PoP) will be heavily used. Another important factor is that not only is the area density increasing and the drive to make portable electronics thinner also drivers thinner components and thereby warpage becomes one of the key challenges. The use of fine pitch CSP and PoP component's, equal and below 0.4mm pitch, poses a number of challenges for PCB Design, SMT Assembly process and Reliability. First, a feasible assembly process must be achieved. The assembly process ranges all the way from screen-printing, dipping in flux or paste, reflow soldering in air or nitrogen and in many cases underfill of CSP's and PoP's. Many factors influence the quality of the assembly process and with the reduced pitch the process capabilities for both assembly and PCB fabrication will be tested to its limit and beyond. The basic processes to control are screen-printing, pick & place, reflow soldering with or without the aid of Nitrogen and underfill. Second, the right materials (such as PCB material, PCB surface finish, solder paste, dipping flux or paste and underfill) and PCB design need to be selected to ensure a high yielding, cost effective and reliable interconnect. Of course the mechanics of the products makes a big difference as well but it is very product dependant and many of today's products leave little room for designing the mechanics in the most reliable way due to total cost and overall looks of the product. This paper will discuss different design & layout alternatives and assembly & material selection alternatives for 0.3-0.4mm pitch CSP's and 0.4/0.4mm pitch PoP devices and the challenges with those. Different stencil types and dipping materials together with air vs. nitrogen refiow is a big part of this study.