Why Microvia Technology?
HDI (high density interconnection) circuit boards and Microvia Technology are inextricably linked.
Microvias are minute holes drilled by a laser to generate the electrical connection between the layers in a multilayer circuit board.
This technology offers many advantages:
- Option of component layout with the smallest BGA pitch
- Miniaturisation with ‘Via in Pad’ technology
- Cost-effective generation of high wiring density
- Future-proof technology – components are becoming smaller all the time
- High reliability
- Reduction in inductance and capacitance effects
Dimensions / key parameters / dielectric thicknesses
The most important parameter for microvias is the aspect ratio, the relationship between the hole diameter and the hole depth:
Hole diameter (1)
Aspect ratio (AR) = -----------------------------------------
Hole depth / length (2)
(1) Hole diameter
(2) Hole depth
(3) End of hole diameter
(4) Cu thickness (acc. to IPC 6012
min. 10 µm)
(5) Dielectric layer thickness
The maximum aspect ratio for microvias
is 1 : 0.8
2 standard prepreg types are used as the dielectric material (5):
Prepreg type 1080 60 – 65 µm dielectric layer thickness(5) end of hole diameter(3) = 100 µm
Impedance-controlled conductors with Microvias
Electromagnetic waves propagate at a finite speed. This propagation speed depends on the material used (ε dielectric constant) and can be determined approximately with the formula cSignal ≈ cLight / √εr .
For FR4: cSignal ≈ 150.000 km/s.
Due to the finite propagation speed, the following rule of thumb formula applies:
The spatial dependence of voltage and currents becomes significant for a conductor length > λ / 10.
Impendence matching should be implemented to avoid transmission faults.
Production-compatible conductor tracks can be realised for impedance-matched system through the use of microvias:
The following 4 models present a selection of the different microvia configurations.
As the calculated impedances are dependent on the Cu layer thickness and this in turn is determined by the multilayer construction and the production process, it is necessary to calculate individually for each project.
Edge-Coupled Coated Microstrip
Coated + Embedded Microstrip
Edge-Coupled Coated + Embedded Microstrip