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30 The PCB Design Magazine • November 2017 When utilizing HDI technology, two basic HDI structures exist: 1. Build-up or sequential build-up (SBU) structures 2. Any-layer structures Figure 1 shows the breakdown of sequential build-up technologies for HDI. The three basic elements are dielectric format, via formation, and metallization method. A key aspect of HDI technology is the use of microvias. For reference, the IPC HDI De- sign Committee has identified microvias as any hole equal to or less than 150 microns. Multiple types of HDI stack-ups associated with blind and buried microvias can be used to meet the density and cost requirements for today's prod- ucts. Design teams should develop stack-ups in conjunction with the board fabricator to mini- mize cost and meet signal integrity require- ments. There may also be additional require- ments related to plating and specific materials. As a rule, the vendor will adjust all the stack-up variables as needed during their process to meet the end-product requirements. Common stack- up approaches when utilizing blind and buried microvias include: 1. Sequential lamination & drill 2. Type III staggered vias 3. Type III stacked vias 4. Type VI any-layer vias Each stack-up option utilizes the same trace spacing and layer thicknesses, but the Type III and Type VI layer buildups allow for greater flexibility (Figure 2). Introduction to Power Integrity (PI) The goal in designing a power distribution network (PDN) for a PCB is simple: minimize the impedance between power and ground for the products frequency range. Successful execu- tion of this goal however is not always easy. With multiple voltage rails and a limited num- ber of planes available to carry those voltages, in addition to ever-shrinking real estate avail- able for capacitors, the task can become quite complex. The PDN consists of a combination of the DC-to-DC converter or voltage regulator mod- THE IMPACT OF HDI ON PCB POWER DISTRIBUTION Figure 1: Elements of the HDI sequential build-up process.