IPC-2226A - Microvia: (build-up via) defined as a blind structure with a maximum aspect ratio of 1:1 when measured in accordance with the image below, terminating on or penetrating a Target Land, with a total depth (X) of no more than 0.25 mm [9.84 mil], measured from the structure's Capture Land foil to the Target Land.
µVias (microvias) are used as the interconnects between layers in high density interconnect (HDI) designs, to accommodate the high input/output (I/O) density of advanced component packages and board designs. Sequential build-up (SBU) technology is used to fabricate HDI boards. The HDI layers are usually built up onto a traditionally manufactured double-sided core board or a multilayer PCB. As each HDI layer is built on to each side of the traditional PCB, µVias can be formed using: laser drilling, via formation, via metallization, and via filling. Because the hole is laser drilled, it has a cone shape.
If a connection required a path through multiple layers, the original approach was to stagger a series of µVias using a step-like pattern. Improvements in technology and processes now allow µVias to be stacked directly on top of each other.
Buried µVias are required to be filled, while blind µVias on the external layers do not require filling. Stacked µVias are usually filled with electroplated copper to make electrical interconnections between the multiple HDI layers and provide structural support for the outer level(s) of the µVia.
Supported Types of µVias
The software supports a µVia that traverses from one layer to an adjacent layer.
The other type of µVia that is supported is referred to as a Skip µVia, this type skips the adjacent layer, landing on the next copper layer after that.
The Via Type is detected automatically based on the defined layer span, as shown in the image below.
µVias are automatically stacked when traversing multiple layers.
Click the button to add a new via span definition.
Select the First Layer and the Last Layer that the via is to span in the Properties panel. Note that the order the two layers are chosen defines the drill direction, as indicated by the direction of the conical µVia shape in the image, if the via is a µVia.
To define a µVia, enable the µVia checkbox. This option will be available when the via spans adjacent layers, or adjacent +1 (referred to as a Skip via).
The new via definition is automatically named <Type> <FirstLayer>:<LastLayer> (eg, Thru 1:2). The Type is detected automatically, based on the layers being spanned, and the µVia option.
If the Stack Symmetry option is enabled the Mirror checkbox will be available, enable this to define a symmetrical µVia, drilled from the opposite direction.
The Via Types tab in the LSM is used to define the layers that each Via Type spans. These are not full object definitions, the diameter and hole size of a via placed in the design continues to be controlled by: the default preferences if the via is placed manually; the applicable Routing Style design rule if the via is placed during interactive routing; or the properties defined in the Properties panel if the via is manually edited.
The via object now includes a Name property, this can be used to switch from one Via Type to another. Changing this will change the layers that the via spans, not the via's dimensional properties.
Designing with µVias
Stacked µVias being placed during a layer change from L1 to L3, press 6 to cycle via options.
During Interactive Routing:
As you change routing layers the most suitable Via Type to suit that layer span, is automatically chosen.
The via size properties are set in accordance with the applicable Routing Via Style design rule, define suitable Routing Via Style design rules to ensure that the placed µVias are sized correctly.
If the layer change is more than one layer, stacked vias can be placed if suitable Via Types are defined.
If there are multiple Via Type combinations available to suit the layers being spanned, press the 6 shortcut to cycle through the available combinations.
The proposed Via Type(s) are displayed on the Status bar, for example [µVia 1:2, µVia 2:3], as shown in the image above. This information is also available in the Heads Up display.
A side view of the proposed Via Type(s) is shown in the Properties panel, as shown above.
If there are multiple via solutions for the layer change, they are presented in the order: use µVia(s), use Skip µVia, use Blind via, use Thruhole via.
Working with Stacked µVias:
Stacked µVias can be worked with as if they are a single via, click and drag on the stack to move them all, with the attached routing.
Click once to select the uppermost µVia in the stack. If the mouse is not moved, subsequent single-clicks will select each of the other µVias in the stack, in turn.
Ctrl+Click and drag to move only the selected µVia with its attached routing.
To select all µVias in a stack, click once to select one, then press Tab to extend that selection to include all µVias in that stack.
If there are stacked vias, the displayed numbers are the start and end layers of all vias in the stack. Hover the cursor over the image below to show the µVias in 3D, on the right is a stack of the other three µVias.
The Routing Via Style Design Rule and µVias
The layers that a via spans is defined for that Via Type, in the Layer Stack Manager. The X/Y properties of the via, including its diameter and hole size, are defined manually, or by a Routing Via Style design rule.
To simplify the process of scoping Routing Via Style design rules, the following via-related query keywords are available:
Via Type Query
All via objects, regardless of the Via Type.
All vias that span from the top layer to the bottom layer.
All vias that start on a surface layer and end on an internal layer, that are not a µVia.
All vias that start on an internal layer and end on another internal layer, that are not a µVia.
All vias that have the µVia option enabled, and connect adjacent layers.
All vias that have the µVia option enabled, and span 2 layers.
These query keywords can be used in the rule matching to instruct the software what diameter and hole size to use for each Via Type, as shown below. If required, the rule scopes can be further refined by adding other qualifiers, such as InNetClass.
µVia Output Considerations
The PCB drill table and drill-type output files have been updated to support µVias.
The PCB Drill Table includes µVia drill pairs.
The drill table identifies each hole by size, if the same size is used on multiple drill pair layers it is flagged as mixed.
Drill Fabrication Files
Because µVias use a different hole-creation technique (laser drilled), the hole-detail for µVias is output to a separate drill file for each layer pair being drilled.
NC Drill - a separate file is created for each µVia drill pair.
Gerber X2 - specific setup entries for each µVia plot.
ODB++ - a separate drill fabrication file created for each µVia drill pair.