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Design Limitations of Blind and Buried Vias

Three thermal press cycles are the maximum that is specified by Underwriters Laboratories. The previous example required two cycles: (1) layer lamination from 1/2 and 3/4 and (2) layer lamination from 1/2 and 3/4 to 5/6.

The minimum thickness of the core is 0.075 mm (0.003 inches).

Note: BBV layers require half ounce copper. In the through-hole plating process, each BBV layers receives 18 µm (0.0007 inches) of electrolytic copper. Hence, the complete copper thickness is 35µm (0.0014 inches).

For blind or buried via substrates, the minimum drill size is 0.2 mm (0.008 inches) with a maximum of 7:1 aspect ratio.

Note: Later lamination cycles will plug all BBV holes with epoxy.

After every lamination cycle, the ability to register drilled holes to inner layers is affected.

Annular Ring Minimum: Drilled prior to the first press cycle – 0.1 mm on each side
Drilled subsequent to the first press cycle – 0.1 mm (0.004 inches) on each side
Drilled subsequent to the second press cycle – 0.15 mm (0.006 inches) on each side
Drilled subsequent to the third press cycle – 0.23 mm (0.009 inches) on each side

All previous recommendation for multi-layer design apply. See section 2.9.

Information that must be on the board drawings:
Plated through-holes and BBV holes must be listed separately on the drawing’s hole chart.
 
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