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PCB007-July2019

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52 PCB007 MAGAZINE I JULY 2019 Copper deposit in the vias with electroless copper or alternatives, such as carbon-based direct plate processes to the vias, depends on process control, equipment design, and chemi- cal parameters. When these are not in control, defects arise. Introduction In "Moving Into Microvias, Part 3," I dis- cussed the importance of desmear, particularly as it relates to higher-performance materials. In Part 4, I will present metallization for HDI blind via processing. There are a few points to consider first. With respect to HDI and blind vias, the chemistry must find a way into these cup-shaped vias. The chemistry must be replenished as it is de- pleted to maximize the diffusion of the active ingredients to the vias. Of course, these same principles apply for through-hole vias as well. Designing plating cells to ensure plating solu- tion movement is critical to providing void-free deposits for HDI designs. I will delve into these cell design methods and other techniques in a future column. For now, let's discuss metalli- zation and potential issues. HDI Via Defects The big concern for HDI and vias is not get- ting a sufficient thickness of copper in the blind vias. This includes voids at the capture pad, thin or tapered plating as the via wall meets the capture pad, poor throwing power, adhesion failures, etc. Figure 1 depicts an ideal state for plating of blind vias. The plating is uniform in thickness at the capture and along the via sidewall. However, not everything is this easy. As blind via structures become deeper (3–5 mils) and diameters shrink (from 4 mils to 3 mils or less), fluid dynamics play an increasingly criti- cal role. Allowing for sufficient plating solution exchange in these cup-shaped vias is neces- sary to ensure that key addition agents—such as brighteners and leveling agents as well as copper ions—are replenished, as these are de- pleted during the plating operation. The same holds true for conventional electroless copper and direct metalization processes. Secondly, starting with a quality-formed blind via plays a significant role in plating continuity and uniformity. Figure 2 depicts a poorly drilled via. Glass fibers and more of a cup-shaped via negatively impacts the plating quality. With poorly formed vias, there is an extreme difficulty with moving plating solutions into the via. A poorly formed via will bring about mass Moving Into Microvias, Part 4 Trouble in Your Tank by Michael Carano, RBP CHEMICAL TECHNOLOGY Figure 1: Ideally plated blind via.

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