Understanding the core differences between through-hole, blind hole, and buried hole is the core prerequisite for solving the problem of low yield of blind buried hole plates. The three are completely different in terms of structural definition, process path, and applicable scenarios, directly determining the manufacturing complexity and yield performance of printed circuit boards.
Through hole: Drilling directly from the top layer to the bottom layer of the PCB, completely penetrating the entire thickness of the board, and the existence of this hole can be seen in all layers. It is the most basic form of through-hole in PCB.
Blind hole: drilled into the top or bottom surface of the PCB, only connecting the surface layer with a specified inner layer, without penetrating the entire board, and the hole cannot be seen from the other side of the board.
Buried hole: completely hidden inside the PCB, only connecting the wiring between two or more inner layers, not extending to any surface of the board, and the presence of holes is completely invisible from the board surface.
| Only 1 drilling and 1 electroplating, with a minimalist process | |||
| Only processed within the independent core board, the alignment difficulty is lower than blind holes, but the overall tolerance will be increased after multi-stage compression |
Through hole: One hole occupies the wiring space of all layers. When the pin spacing in BGA area is less than 0.8mm, it is difficult to complete fan out, and high-density design will seriously waste the inner layer area.
Blind hole: It only occupies space on the surface and corresponding inner layer, and can be directly drilled on BGA pads (via hole process), supporting high-density chips with 0.5mm or even 0.4mm pitch, greatly freeing up surface wiring space.
Buried hole: It does not occupy any surface space at all, and the inner layer has the highest℃of freedom in wiring. It can continue wiring above and below the buried hole, and is the core structure for achieving ultimate density in high-order HDI boards.
The vast majority of blind buried orifice plates with a yield rate of less than 85% are caused by the disconnect between design and manufacturing due to differences in the process characteristics of the three types of holes:
Excessive aspect ratio: Blindly using non-standard micropores below 0.1mm during design, with a ratio of dielectric thickness to pore size exceeding the industry's safety threshold of 2:1, insufficient copper layer thickness at the bottom of the hole during electroplating, and high risk of circuit breakage after thermal shock.
Layering and alignment defects: After multiple presses, there is no dynamic compensation for the expansion and contraction of the board, the hole displacement exceeds ± 0.03mm, and the annular solder pad is less than 50 μ m required by IPC standards, resulting in batch open circuit scrap.
Lack of process control: Small factories omit the plasma residue removal process, resulting in residual resin residue in the holes after laser drilling. The adhesion of copper plating is insufficient, and after long-term use, the copper layer falls off and opens up, directly increasing the defect rate by 25%.
Inconsistent electroplating process: Ordinary DC electroplating cannot penetrate deep into the bottom of blind holes, and the copper thickness at the bottom of the hole is less than 10 μ m. After long-term power on and heating, the hole resistance increases and the circuit is broken. Manufacturers without pulse electroplating equipment find it difficult to maintain stable yield.
Standardized blind hole parameters: TOP-L2 and BOT-L3 blind holes are uniformly selected with a laser aperture of 0.15mm, a fixed medium thickness of 0.12mm, an aspect ratio controlled within 1.2:1, and non-standard micro holes below 0.1mm are rejected.
Optimize the stacking structure: use the universal TOP-GND-PWR-BOTTOM stacking, lay grid copper in the blank area of the inner layer to balance the copper density, and use low flow adhesive semi cured sheets in the buried hole area to avoid resin overflow and blockage of the holes during lamination.
Delete redundant blind holes: Replace blind holes with array through holes for power and ground networks, and only retain blind holes for high-speed differential signals, which can reduce the number of blind holes by 30%, while reducing processing costs and process risks.
Pre DFM verification: Complete the calculation of the depth to diameter ratio of blind buried holes in the entire board before casting, intercept excessive hole positions in advance, and avoid discovering batch defects only after the entire board is put into production.