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PCB-Nov2017

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28 The PCB Magazine • November 2017 Introduction For the fabricator to successfully implement an HDI strategy, several new competencies must be acquired. These competencies include: • Material selection • Small diameter via formation • Fine line imaging and etching • Via filling technology • Improved registration • Enhanced bonding strength for the sub-laminations • Metallization technology These topics will be presented in more detail in this and future columns. But first, a short dis- cussion on materials for HDI is warranted. Materials Most of the dielectric materials that are used to make printed circuit boards incorporate rein- forcement into the resin system. Reinforcement usually takes the form of woven glass fiber. Wo- ven fiberglass is just like any other cloth, made up of individual filaments that are woven to- gether on a loom. By using different diameter filaments, different yarn bundle sizes and dif- ferent weave patterns, different styles of glass cloth are created. Woven glass fabric adds both dimensional stability and thermal durability to the dielectric, but it does present some prob- lems when used in HDI constructions. Materials selection is equally as important, especially due to the higher temperatures of lead-free assembly and their subsequent effect on laminate delamination and reliability. Im- portant new capabilities to embrace are [2] : • Impedance calculations and stackups for high-frequency boards using coplanar waveguides and coplanar stripline models • Characteristics and scaling/feature compensation for the newer phenolic- epoxy and halogen-free FR-4s • Ability to add local fiducials to align laser drilling CCD cameras • System to store laser drilling parameters based on diameter, depth, and material types • Characteristics of via-plugging to determine if placement of buried vias will create problems FEATURE COLUMN: TROUBLE IN YOUR TANK by Michael Carano RBP CHEMICAL TECHNOLOGY

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