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46 PCB007 MAGAZINE I SEPTEMBER 2018 ditionally a PCB or even HDI board has fea- ture sizes greater than 30/30 mm; an IC sub- strate, on the other hand, has L/S that is often >15/15 mm. Nevertheless, SLP has reached L/S that is smaller than 30/30 mm, and this has defined the name for SLP as it is a PCB that has the feature sizes close to that of an IC substrate. Traditionally, PCBs are fabricated using a sub- tractive process that requires etching the pre- existing copper-clad laminate (>5 mm thick- ness). This often results in over-etching that cannot be compensated for afterwards. As a result, the minimum L/S using this technolo- gy is limited. Different from the subtractive process, mSAP uses a negative pattern design to "grow" cop- per by electroplating on top of the pre-exist- ing thin copper clad (<5 mm thickness, which serves only as the seed). Then the negative pattern is etched. mSAP is usually used for IC substrate fabrication and allows a further min- iaturization of feature size for L/S. Until now, within the few smartphone models that have already adapted SLP, the L/S remains around 30/30 mm. However, the adaptation of mSAP for board is to further decrease L/S to increase fixed area. As can be seen in Figure 1, starting from iPhone 5s, Apple has gradually decreased the PCB area of the iPhones compared to the total smartphone area. Until the latest iPhone X, they have decreased 3% of the area while adding more functions and increasing the bat- tery capacity. In the meantime, iPhone X not only adapted SLP but also stacked two SLPs like a sandwich to further increase integration in a fixed area. This surface reduction was enabled by the increase of density and reduction of intercon- nection. The semiconductor industry's trends are af- fecting the semiconductor package and pack- age-to-board interconnect level. PCB tradition- ally serves as the interconnection of the chips and the final product, but nowadays it is also an integration solution. SLP is one of the an- swers today to the scaling demand while also responding to the functional roadmap. SLP is so-named because this "board" has blurred the definition between a PCB and an IC substrate. Although usually fabricated by different technologies, the main difference be- tween a PCB and an IC substrate is the fea- ture sizes, especially line and space (L/S). Tra- Figure 1: PCB analysis of Apple flagship smartphones.

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