HDI PCB Microvia Types

What is considered a Microvia?

  • Laser Drilled Blind Via from the outer layer to an inner layer
  •  150 µm (0.006”) diameter laser drill (range 100 µm (0.004”) – 200 µm (0.008”)
  • Laser drill directly into BGA or SMT footprint (Via-in-Pad)
  • Eliminate through-hole vias
  • Increases routing density & enhance electrical characteristics
  • Maintain 0.5:1 aspect ratio or advanced 0.8:1
  • Provides fan-out solutions
  • Microvias terminate to the underlying pad without penetrating the copper

Microvia Types

  • Standard – Single Lamination cycle (no Buried Via)
  • Standard – with buried via
  • Staggered or stair-step
  • Off-set Microvias
  • Stacked Microvias are required for 0.4 mm pitch
  • Deep Microvias

Standard Microvias create routing density (eliminate through vias)

  • Reduce layer count
  • Enhance electrical characteristics
  • Standard Microvias limited to layers 1 – 2 & 1 – 3

Stacked Microvias allows increased routing on multiple layers

  • Provide Solutions for next generation applications such as  1 mm – 0.8 mm – 0.65 mm – 0.5 mm – 0.4 mm – 0.3 mm & 0.25 mm
    • SMV provides solid copper plate eliminating potential solder voiding

Deep Microvias

  •  provides additional dielectric material & small geometry features, improved Impedance performance & provides RF Microvia solutions

Deep Stacked Microvias

  • Provides additional dielectric while maintaining small geometries on multiple layers
HDI PCB Microvia types

Single Microvia  

HDI PCB Microvia types

Stacked Microvias

Electroplated Copper via filling Mechanism

  • Bottom-up filling behaviour is attributed to the action of organic additives (must be controlled to prescribed limits)
  • Suppressor rapidly forms current inhibiting film on Cu surface. Film has little geometric dependence due to high suppressor solution concentration
  • Accelerated bottom-up fill behaviour is due to a local accumulation of brightener species at the feature base
  • As surface area is reduced during deposition, the concentration of brightener species increases, resulting in a non- equilibrium surface concentration. This local concentration of brightener accelerates the plating rate relative to the surface.
  • Brightener
HDI PCB Microvia
Planar Microvia

Planar Microvia

Stacked Microvia

Stacked Microvia

Copper Via filling Advantages:

  • Design flexibility (SMV post or buried SMV)          single or stacked
  • Provides a solid copper plate
  • Improves Current Carrying Capability & Thermal Management
  • Provides a Planar surface for BGA (Via-in-Pad)
  • Increases routing density for Fine Pitch BGAs (0.65 mm, 0.5 mm, 0.4 mm, 0.3 mm& 0.25 mm)
  • Allows Design Fan-out on multiple layers using 0.010” (250 µm),  0.008” (200 µm), and 0.007” (175 µm) pad diameters

Design Guidelines:

Pad diameters are driven by device pitch, drill diameter driven by dielectric thickness

  • First consideration for via hole diameter is aspect ratio and reliability
  • Regardless of hole via hole diameter IPC Class III requires a larger minimum pad diameter than IPC Class II
  • When designing to class II with minimum pad diameters it is important to maintain a minimum of 8 mil from the primary drill hole edge to any copper features.
  • Signal routing channels come in integer values (i.e. 1,2,3…. n)
    • Signal track width should be selected based on the electrical requirements
    • Track width and pad diameter should be maximized to take advantage of via pitch with respect to the maximum integer value of signal routes (in other words, don’t shrink pads and or track widths if it does not create another signal route!)

Standard

  •  0.012” (300 µm) Pad diameter & 0.006” (150 µm) laser drill
  • 0.5:1 aspect ratio
  • 0.010” (250 µm) Pad diameter & 0.005” (125 µm) laser drill

Advanced

  • 0.0086” (218 µm) Pad diameter & 0.005” (125 µm) laser drill

Flip Chip Solution

  • 0.007” (175 µm) Pad diameter & 0.004” (100 µm) laser drill

Cerra Systems Printed Circuit Board Fabrication capabilities support high-frequency PCB, High-temperature Boards, Thick PCB, ultra-thin PCB, heavy copper PCB, Metal Core PCB, HDI boards with Blind Vias, Buried Vias, Micro Vias, Embedded passives, bonded heat sink, Impedance Control, Depth control drilling, Back drilling, Edge Plated PCB, Bump Pads, cavity with ledge, Via on pad and stacked micro-Via technology.

CerraSystems – http://cerrasystems.com

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