These manufacturing steps are also present in standard PCB production, but in HDI manufacturing, the precision and control difficulty are elevated to a new level.
1. Inner Layer Imaging
Achieving finer lines and spaces, such as 2.5/2.5 mil or smaller, is crucial. This requires high-resolution dry film or liquid photoresist, along with advanced exposure equipment like Laser Direct Imaging (LDI) to minimize alignment errors and light diffraction, ensuring precise circuit patterns.
2. Lamination
Beyond sequential lamination, the choice of materials is also more demanding. Typically, ultra-thin copper foils with low roughness (e.g., RTF, VLP) and high-performance prepregs (PP) and Resin Coated Copper (RCC) are used. This minimizes signal transmission loss and facilitates the creation of fine-line circuitry.
3. Plating
Plated Through Hole (PTH): A thin layer of electroless copper is deposited on the non-conductive walls of laser-drilled microvias to make them conductive, preparing for subsequent electroplating. The metallization of these microvias demands highly active and penetrative chemical solutions.
Electroplating: In addition to via filling, uniform plating across the entire board is critical. This ensures consistent copper thickness in traces and holes, even in areas with varying densities.
4. Surface Finish
HDI boards are often used for advanced packages like BGAs, CSPs, and QFNs, which have small and dense pads. The surface finish (e.g., ENIG, ENEPIG, Im-Sn) must be uniform, flat, and have good solderability. It is also essential to prevent issues like plating bleed-out that could affect soldering.
5. Inspection & Testing
AOI (Automated Optical Inspection): This checks 100% of inner and outer layer circuit patterns for defects and requires a high capability for detecting flaws in fine lines.
AVI (Automatic Visual Inspection): Used to measure the accuracy of drilled hole positions.
Electrical Testing: Due to the high number of net nodes and small spacing, higher-density flying probe testers or dedicated test fixtures are required.
Reliability Testing: Strict reliability tests, such as Thermal Stress Testing (TCT/TST) and Interconnect Stress Test (IST), are mandatory. These tests ensure the connection reliability of microvias under thermal expansion.
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