A perplexing challenge for RoHS compliance is adapting large, complex and thick, high-layer count multilayers to lead-free assembly. These are typically dense, complex assemblies with large BGAs and significant heat spreading features integrated into the design. Fine-pitch packages (QFPs and BGAs) and increasing pin count of packages further complicates the conversion to RoHS. To provide RoHS robustness to an assembled multilayer that has a very high heat-sinking characteristic requires that the total layers be reduced, as well as its overall thickness. But how can this be accomplished? The answer is to increase signal routings per layer by 2X to 4X, and thus reduce overall signal layers and their referenced plane layers. The other new design feature is to change 'topology' so that a majority of vias are now 'blind vias', thus freeing up innerlayer space for this to be accomplished. HDI is the interconnect technology that has been developed to respond to these needs. Microvias are the principal feature of HDI, along with thinner dielectrics and smaller traces and spaces. The important feature is the "Design For Lower Layers Using HDI". This paper covers the major design solutions from HDI that allows designers to implement fewer layers in a multilayer: 1. Reduction in layer count for thickness control and RoHS compliance (Lead-Free Assembly) 2. How to integrate high-I/O and fine-pitch devices without adding layers 3. How to achieve higher component density and component I/Os without adding layers The resulting new multilayers are not only thinner and easier to design but are less expensive and suitable for lead-free assembly. The resulting new multilayers are not only thinner, cheaper, and easier to design but are less costly and suitable for lead-free assembly.
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