Micro-via‘s are minute holes in circuit boards with a diameter of 152 microns (6 mils) or less that are placed within or underneath component pads. These structures allow the interconnection of outer and inner layers, which results in an increased routing area or available space for the placement of denser components. A via-in-pad is a plated-through hole that is used as an interlayer connection from a component land, but in which there is no intention to insert a component lead or other reinforcing material. However, the ability to either place vias on or off the pads gives designers greater flexibility to selectively create routing room in denser parts of the substrate.
The tendency in electronic devices is toward smaller and lighter devices with an increase in functionality. The size of products, such as cellular phones and camcorders, is a fraction of the size they were before. The increases in the number of I/Os require PCBs to allow signal distribution from these packages to the rest of the board. Different solutions have been proposed, such as fabrication of fine line circuitries and a reduction in the spaces between the lines. However, micro-via technology is one solution to the challenges imposed by the miniaturization of components utilized in current electronic assemblies. Micro-vias are the structures that allow the interconnection between different layers of the circuit board and thereby enhance the routing area for the connection of denser components.
Micro via photos:
Micro-vias are defined by IPC-2315 and IPC-6012A standards, as blind and buried vias that are equal to or less than 152 microns (6 mils) in diameter and have a target pad equal to or less than 356 microns (14 mils). The target pad is defined as the land on which a micro-via ends and makes a connection. The micro-vias are placed on the substrate pads, namely via-in-pad, and are used to interconnect the different outer and inner layers of a PCB. The use of micro-via technology, coupled with the reduction of geometries of a conventional multilayered PCB, is termed as a High Density Interconnection (HDI) structure. Typically, these structures use a traditional FR-4 core with build-up dielectric layers.
Some of the reported advantages of the use of micro-vias are listed below:
- They require smaller pad sizes, which save space in the PCB;
- Increase the wiring density in conventional FR-4 by a factor of 4;
- Reduce layer counts (33% of conventional boards);
- More chips can be placed on the PCB or a smaller PCB can be fabricated;
- In some cases, micro-vias improve the electrical performance due to the smaller via lengths and diameters and shorter pathways in comparison to the through hole; they reduce the cross talk and switching noises;
- Support surface mount components and DCA devices;
- Provide for better reliability in comparison to through hole vias due to lowest aspect ratio.
- Can be manufactured in rigid, flex or rigid/flex substrates.
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