What is blind and buried via in PCB?
Blind and buried vias technology emerges in the requirement of miniaturization and high integration of electronic products, in order to improve circuit density of PCB boards.
Blind Vias: blind vias is connected from inner layer to outer layer,it does not penetrate the entire board.
Buried Vias: the buried vias is connected between inner layers, which can not be seen from the surface of PCB.
The process method of blind and buried vias
1.Depth drilling method
Use mechanical to drill the depth of blind via achieve the connection between inner layer and out layer. The weakness of this method that productivity is very low,only one piece can be machined at one time,the processing is difficult and low reliability.
Through repetitious pressing, made out the buried vias, the blind vias and the through hole successively.The weakness of this method is not easy control the expansion after repeatedly pressed,the processing is also long.This method can’t make cross blind and buried vias.
Blind via(L2-L4,L4-L8) Blind vias(L1-L3)
The method of laser drilling increases layer by layer for progressive lamination.Disadvantages: high level equipment requirements, high plating ability and high cost.
We dedicated a FA team to calculate pressure coefficient, reduce the pressure times lead to quality problems.
X-ray shooting machine and OPE punching machine ensure the accurate registration.
High precision CNC drilling machine to ensure drilling accuracy.
The buried blind vias be made by plugging resin process, reduce the blasting hole without copper plate and risk
CCD automatic parallel light exposure machine for small trace and high accurate registration.
Blind and buried vias fabrication Capability
Layer: Max. 20 Layer
Min via size: 0.1mm for laser drilling, 0.15mm for traditional drilling
Aspect ratio: < 12:1 for traditional drilling, >0.75:1 for laser drilling
Design suggestion of blind and buried vias
1.In production difficulty and cost,the HDI laser buried blind vias PCB is higher than the multiple lamination blind vias PCB, so try to avoid the design of cross buried blind vias.
HDL laser drilling Traditional blind vias
2.The connection of metallized hole and circuit.
Design ring width = minimum finish ring width + hole tolerance + etch tolerance.
Pad diameter = drill diameter +2 x minimum finish ring width + hole tolerance + etch tolerance.
Minimum finish ring width: 0.025mm.
Hole tolerance: +/-0.075mm.
Etch tolerance: +/-0.025mm.
When spacing is allowed, often use teardrop pad to ensures the safety connection between pads and wires.
3.Connection of metallized hole and copper area
A. Direct connection
B. Thermal pad connection (for reducing heat dissipation, making heat is concentrated on the solder)
Slit width: >=0.125mm.
Ring width is same as above requirement.
Inside diameter= drill diameter + 2x ring width.
4.Isolation of hole and line
The distance between hole to line and pad >=0.25mm.
The isolated pad diameter >= drill diameter+ 0.6mm.
Pay attention to the distance between the isolated pads when placing inner isolated pads.
Examples of common design errors：
5.Design points of blank area
Do not leave large blank area without copper on inner layer, it’s easy to be warped and copper foil wrinkle when laminate press as internal stress of board is uneven.
The outer layer should be as even as possible, do not leave a large blank area without copper, it can be filled with nonfunctional square pad. Otherwise plating,PTH, copper thickness will be uneven.
Fabrication suggestion of blind and buried vias
The best is a symmetrical structure to prevent the expansion caused by inconsistent PCB in serious warpage.
Use a core thickness as much as possible.
The inner layer try to use same type copper thickness, and same copper thickness at both sides of each core.
Try to use the standard specification material of core and prepreg.
The better buried vias diameter is 0.3mm~0.5mm,too large or too small is not conducive to plugging resin.
The minimum ring is 0.15mm for buried and blind vias,the minimum ring is 0.10mm for laser blind vias.