If the component and circuit track density of a multilayer PCB design is so high that even small vias of 150 or 200 µm affect the layout, the use of hidden and partial vias (blind and buried vias) can be useful to gain surface area. Blind and buried vias are commonly used in micro-ball grid arrays, flip chips and large BGAs.
Drilling of micro vias, with a diameter according to ≤ 150 µm, on HDI boards is usually done by laser and alternatively with drills. If the target is a 100 µm blind via, the outer layer including copper should have a thickness of about 100 µm. If a conical drill is used for the blind vias, the thickness of the outer layer should be about 150-200 µm. Therefore, new PCB base materials for multiple layers are being developed. In contrast to single plated through hole (PTH) vias, the use of buried vias requires an additional metallization step for the layers of the center core. A design that uses only blind vias on top and bottom side in addition to PTH, and no buried vias, requires a drill tool with depth control.
For example, in a six-layer board, a core layer on both sides can be used. In case of buried vias, the first stack is pressed together after structuring the inner layers 3 and 4. Following the drilling of the buried vias, the metallization of the through-holes (THP, Through-hole Plating) and the structuring of layers 2 and 5 follow. In the second pressing phase, the complete multilayer, previously structured stack with two cores is produced with another prepreg layer and the copper foil on both outer sides. After drilling of the PTH and blind vias, chemical metallization of the vias and top-bottom structuring is carried out.
Designers have various reasons for using blind vias on four-layer boards. For instance, it is possible to create free connections of the mass surfaces on the upper side with inner mass layers without affecting the outer layers. Furthermore, in the case of laser-drilled blind vias with 100 µm, solder pads can be reduced. However, an additional drilling tool is required for blind vias.
Do you have questions regarding this matter?
Your contact person: Lars Führmann
Company: LPKF Laser & Electronics AG
