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14 The PCB Design Magazine • December 2014 • Materials do well in processes requiring RoHS. • Newer materials are available at higher performance and lower costs. These new materials are not suitable for standard or sequential lamination. Disadvantages • Complex via models—many variations and still evolving. • Complex stackup definition. • Effective design methods on large dense designs have not been widely understood and documentation is sparse. • Predictive design guides and cost estimates not yet available. • Although HDI fabrication is pervasive in the Pacific Rim and China, North America has been slow to adopt. Recommendations • HDI is the best alternative to high layer- count and expensive standard laminate or sequentially laminated boards. • The trend is for higher pin-count and finer pin pitch. The tipping point will occur when the >1500 pin BGAs use a .8 mm pitch. The only way to effectively breakout and route multiple instances of these devices on a single board will be with the smaller HDI feature sizes. • HDI currently dominates the fabrication technology for handheld and consumer electronics. For large board designs, it will continue to grow. HDI Stackup Details This section is provided as a reference de- scribing the relevant HDI stackup information based on the IPC standard. In this section, Types I, II and III are described. Type III is the recom- mended configuration for large, dense boards with multiple high pin-count BGAs. IPC-2315 Design Guide for High Density Interconnects and Microvias Published jointly with the Japan Printed Cir- cuits Association, IPC/JPCA-2315 provides an easy-to-follow tutorial on the selection of HDI and microvia design rules and structures. It ad- dresses various considerations when designing an HDI printed wiring board, including design examples and processes, selection of materials, general descriptions, and various microvia tech- nologies. It offers designers and manufacturers one source for reliable design and manufactur- ability information for commonly produced HDI boards. HDI Type I This construction uses both microvias and through vias in a structure consisting of a lami- nated core and a single micro via layer on at least one side. Notes: 1. The number of layers in the laminated core varies and is limited by two factors a. The through-via should have an aspect ratio (total length to hole size) less than 10x to maintain reasonable reliability. b. If the FR-4 dielectrics become too thin, they will delaminate under higher temperatures required for lead-free soldering. Recommendations • In the context of large dense boards with multiple high pin-count BGAs, this stackup will not be significantly better than laminate. The through via pads will need to be large. Using only a single microvia layer will limit the ability to ben- efit from the smaller via and trace feature sizes. HDI Type II This construction uses microvias, buried vias, and may have through vias. HDI LAYER STACkUPS FOR LARGE, DENSE PCBS continues feature Figure 1: IPC Type I construction.