Package-on-package (PoP) technology is coming on strong and continues to overshadow the ever popular system-on-chip (SoC) and other similar packaging types. The rationale behind PoP is simple.  More OEMs are miniaturizing their products, but are intent on adding more functionality. Hence, the only way to go to meet these challenges is to make the printed circuit board (PCB) smaller and stack packages on top of each other.

PoP-populated PCBs are expected to continue gaining popularity in three stages over time as shown in the chart. Today, most designs are at the first stage with two-level ball grid array (BGA) PoThe logic or microprocessor (µP) chip is the bottom BGA packaged chip while the BGA packaged memory chip is on top.  

Designing PoP-populated boards is certainly challenging. But when BGA packaging moves from 0.5-mm pitch to ultra-fine 0.4-mm and 0.3-mm pitch, you’re adding in a major dimension to the design. That means major design flaws can be introduced when 0.5-mm pitch design guidelines are inadvertently used for 0.4/0.3-mm pitch BGA/PoP-based PCB designs.  

When designing with BGAs and pitch sizes of 0.5 mm and earlier ones, there are certain rules of thumb associated with pad sizes and the solder mask opening. One is to keep pad size to about 85 percent of a BGA’s ball size. Another is to use a non-solder mask defined (NSMD) pad. In this case, the solder mask is larger than the BGA pad. Typically, the solder mask is opened to a diameter of about the ball size of the BGA. The recessed solder mask provides stress relief to the NSMD pad during reflow. In short, it creates a protective barrier and lets solder go around it.  

Read the complete article on ECN. 

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Original Hyperlink: http://www.ecnmag.com/articles/2013/10/pop-based-pcbs-move-center-stage..

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