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78 PCB007 MAGAZINE I FEBRUARY 2023 Tooling and Materials Selection HDI boards are characterized by smaller fea- tures and tighter registrations. To this end, fea- ture compensation and scaling are much more important than on regular boards. HDI boards are also used for high-frequency applications, and thus, solving these demands creates new challenges for an engineering/tooling depart- ment. Table 1 shows three major challenges in engineering high-frequency boards. Materials selection is equally important, especially because of lead-free assembly tem- peratures and their effect on laminate delami- nation and reliability. Important new capabili- ties to embrace are: 1. Impedance calculations and stackups for high-frequency boards using "co-planar waveguides" and co-planar stripline models. 2. Characteristics and scaling/feature compensation for the newer phenolic- epoxy and halogen-free FR-4s. 3. Ability to add local fiducials to align laser drilling CCD cameras. 4. System to store laser drilling parameters based on diameter, depth, and materials types. › Characteristics of via-plugging to determine if placement of buried vias will create problems 1 . From a materials standpoint, low Dk and low loss laminate materials are the laminates of choice. Signal integrity and impedance match- ing are required. Minimizing signal loss at high frequencies is paramount to IC substrate tech- nology that supports advanced packaging. is brings one to gain a better understanding of via formation and desmear/metallization. Small Hole/Microvia Formation Although HDI is normally associated with laser drilling, small blind vias can also be formed by mechanical drilling and chemical etching. What's important is a process that guarantees each board will receive the correct microvia drilling parameters. Ideally, the shape of the formed via will show a wider opening at the top of the via before gradually tapering down (Figure 1). Figure 1: Ideal microvia formation shape with plated copper.