HDI Microvia Technology | |
A microvia is a small plated through hole that electrically connects HDI conductive layers in a PCB. The most distinguishing feature of HDI circuit boards is the extensive use of microvias in the design. Standard PCB vias range from 8 to 12 mils in diameter. In contrast, microvia diameters typically fall between 3 to 8 mils. The reduced size of microvias is a critical technology that facilitates the miniaturization and component density achievable with HDI. By shrinking the vias, finer routing features can be implemented in the layers above and below. Microvias also improve electrical performance by reducing parasitic capacitance and insertion losses at high frequencies compared to traditional vias. Key Benefits of Microvias Enable trace and space of 4 mils or below Allow higher wiring densities Reduce parasitic inductance and capacitance Improve signal integrity at high speeds Permit greater routing flexibility Decrease layer requirements and cost Microvia Challenges While microvias offer significant advantages, their ultra-small dimensions also impose fabrication and reliability challenges including: Precise Drilling Accuracy – Laser drilling must be tightly controlled to hit targets as small as 3 mils and avoid creating defects. Thin Dielectric Concern – Thin dielectrics between layers increases the risk of microvia barrel cracking during layers lamination. Reliable Metallization – Achieving reliable plating of ultra-small via holes requires optimized chemical processing. Registration Control – Tighter registration tolerances are necessary to ensure landing pads line up across layers. Aspect Ratio Limits – Maintaining low microvia aspect ratios reduces reliability risks from stresses. Microvia Design Rules Key microvia design guidelines include: Minimum microvia diameter is typically 3 to 5 mils Maximum aspect ratio (height/diameter) of 1:1 to 1:3 Capture pads should allow for +/- 0.5 to 1 mil registration tolerance Avoid placing microvias in high current paths Limit the daisy chaining of microvias (use stacked vias instead) Minimize the number of layer transitions through microvias where possible Following these rules will help maximize microvia manufacturability and performance. | |
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