Manufacturing High-Density Interconnect (HDI) PCBs involves more critical and complex processes than a standard PCB. The core challenge is achieving smaller vias, finer lines and spaces, more layers, and more precise interconnections.
These are the specialized or highly demanding steps that ensure the high-density characteristics of an HDI board.
1. Laser Drilling
Importance: This is the foundation of HDI technology. Traditional mechanical drilling can't reliably create micro-vias or buried vias smaller than 0.15mm. Laser drilling (typically using UV or CO₂ lasers) can precisely ablate micro-vias with diameters from 50μm to 100μm.
Challenges:
Alignment Accuracy: Vias must be precisely aligned with the inner layer pads, with minimal deviation.
Via Shape Control: The process needs to form a good "cup shape" to ensure proper filling during subsequent plating.
Material Compatibility: The process must account for the different laser absorption rates of various materials (like copper foil, resin, and glass fiber) to prevent incomplete drilling or over-ablation.
2. Plating Filling (Via Filling)
Importance: For designs with any-layer interconnects or stacked vias, the micro-vias must be completely filled with copper, not just plated on the walls. This is essential for drilling new vias on top of the filled ones to ensure reliable connections, and it also improves thermal dissipation and electrical performance.
Challenges:
Process Complexity: This requires specialized plating solutions, additives, and a pulsed power supply to achieve a perfect, void-free fill through a long deposition process.
High Cost: This is a time- and material-intensive step, making it a significant part of the overall cost of an HDI board.
Surface Flatness: The filled via openings must be completely flat, with no depressions or bumps that could affect subsequent circuit fabrication.
3. Sequential Lamination
Importance: HDI boards are typically built using a buildup process, where lamination occurs in multiple steps. For example, a core board is created first. A resin dielectric layer with copper foil (like ABF or PP) is then laminated onto both sides. New vias are then laser-drilled, plated, and circuit patterns are created on this new layer. This process is repeated multiple times.
Challenges:
Alignment Accuracy: The cumulative expansion and contraction from multiple lamination cycles must be precisely controlled to ensure accurate alignment between all layers.
Process Control: The temperature, pressure, and vacuum for each lamination cycle must be precisely controlled to prevent delamination between layers and to avoid issues like insufficient resin fill or circuit distortion from excessive resin flow.
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