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PCB-Oct2017

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42 The PCB Magazine • October 2017 Why use Microvias in PCBs? From a physical and electrical standpoint, microvias offer several distinct advantages over their mechanically created counterparts. Sys- tems with higher circuit densities and better electrical performance can be created using the smallest and most advanced components avail- able. As a result, smaller, lighter, and more ro- bust products can be built [1] . The major benefits of using microvias are: • Lower costs through board size reduction (easily up to 40%) and layer elimination (up to 33%) • Product size reduction: lower substrate weight, thickness, and volume • Increased wiring density: closer compo- nent spacing with more connections per component, which permits higher density at a lower cost. At higher densities, HDI costs less per connection • Improved reliability: The thin nature and 1:1 aspect ratio of microvias deliv- er increased reliability over larger drilled through-holes. • Improved electrical performance (signal integrity): HDI has one-tenth the parasitic inductance, and capacitance of through-holes, fewer stubs, less reflec- tions, less ground bounce, and better noise margins. • Lower RFI/EMI: Since ground planes are closer to or on the surface and distributed capacitance is available, RFI/EMI is signifi- cantly reduced. • Improved thermal efficiency: The thin di- electrics and higher Tg of HDI improve thermal performance. • Greater design efficiency: Microvias allow ease of part placement on both sides of an assembly as well as improved component escape routing (via-in-pad). • Faster time-to-market: 100% completion by HDI auto-routers because of placement and easier innerlayer free space routing. Lower PCB Costs HDI boards can usually provide higher den- sity (increasing past eight layers) at a lower cost. When complex blind and buried vias are pro- duced by sequential lamination, a simpler HDI board will usually cost less. Product Size Reduction Routability area gains increase the potential for a PCB layer count reduction in the design, and for a physically smaller form factor gained through increased circuit density. This allows for better component placement, improved layout options, and better electrical performance. This will be illustrated in the case study provided later. Increased Circuit Density Because microvias can be incorporated with- in the pad structure, there is a major reduction in fanout. Further, microvias provide not only additional gains in routable areas, but also im- proved cost-effectiveness during manufacture. Greater via density per given area provides a greater number of routing tracks per the same area. The benefits of this are twofold—designers can place components in much closer proxim- ity to each other while achieving a correspond- ing increase in trace routable area. Improved Reliability The thin nature and 1:1 aspect ratio of mi- crovias deliver increased reliability over larger drilled through-holes. HDI boards have long been used in European military, transportation, and spacecraft. The data is available from IPC's ITRI on reliability testing of HDI multilayers. Improved Electrical Performance/ Signal Integrity Due to the physical structure of microvias, there is a reduction in switching noise. This is attributable to the decreased inductance and ca- pacitance of the via as its physical size becomes smaller and shorter. A microvia will have nearly one-tenth the electrical parasitics of a through- hole. Another advantage of using microvia tech- nology for creating interconnects is a reduction in circuit noise, signal reflections and crosstalk between traces. Miniaturization offers the op- portunity for smaller current loops on critical nets. The corresponding increase in routability area also allows designers to place traces further apart to reduce crosstalk. More on this later in the article. HDI'S BENEFICIAL INFLUENCE ON HIGH-FREQUENCY SIGNAL INTEGRITY

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