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20 PCB007 MAGAZINE I SEPTEMBER 2018 Market Trends and Drivers Handset Design As is usually the case, the drive for improve- ments in PCBs usually comes from the mar- ket, and with HDI boards this has really meant changes in mobile handset design. Looking at Figure 1, we can see an evolu- tion of the mobile phone, and several interest- ing observations appear: • Handset thickness has steadily reduced and is currently 7-8 mm • Handset area has increased, with 90-100 cm 2 now being typical • Battery size has generally increased in proportion with overall handset size • Primary PCB area has decreased from approx. 25 cm 2 to 15-20 cm 2 • PCB layer count has remained pretty constant at approx. 10 layer • PCB complexity has increased, L/S reduced from 60 mm to approx. 30 mm So, in order to allow for modern handset de- sign, while leaving sufficient space for all the other components to fit into the handset case, HDI PCBs had to become thinner, smaller and much more complex! Die Packaging Whilst some of the changes in PCB thickness and area demands can be attributed to handset physical design, which has certainly contribut- ed towards increased PCB complexity, much of the additional complexity can be traced to die packaging needs. And, as many components are now packaged in some form of area array, we need to consider solder ball count, diame- ter and pitch. In simple terms, die are more complex than ever, and packaged or not, this generally means they have more I/Os than they had in the past. In order to push I/O count, and still minimize package footprint, one general trend is to try and decrease both solder ball diam- eter and pitch. At the same time, an increase in I/O count also means more traces are re- quired to route those I/Os, and this can usu- ally be achieved through finer lines, assuming the area and layer count restrictions outlined above are to be followed. Figure 1 shows the most recent handset de- signs have a minimum L/S requirement of 30 mm, dropping from 50 mm in earlier genera- tions. This cannot be achieved with fully sub- tractive PCB processes as issues during etch become unacceptable, so there is a need to adopt one of the variations of the SAP process- Figure 1: Evolution of the smartphone. (Sources: iFixit, TechInsights, Teardown.com, IHS, Prismark Partners)

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