For the fabricator to successfully implement an HDI strategy, several new competencies must be acquired. These competencies include:
- Material selection
- Small diameter via formation
- Fine line imaging and etching
- Via filling technology
- Improved registration
- Enhanced bonding strength for the sub-laminations
- Metallization technology
These topics will be presented in more detail in this and future columns. But first, a short discussion on materials for HDI is warranted.
Most of the dielectric materials that are used to make printed circuit boards incorporate reinforcement into the resin system. Reinforcement usually takes the form of woven glass fiber. Woven fiberglass is just like any other cloth, made up of individual filaments that are woven together on a loom. By using different diameter filaments, different yarn bundle sizes and different weave patterns, different styles of glass cloth are created. Woven glass fabric adds both dimensional stability and thermal durability to the dielectric, but it does present some problems when used in HDI constructions.
Materials selection is equally as important, especially due to the higher temperatures of lead-free assembly and their subsequent effect on laminate delamination and reliability. Important new capabilities to embrace are:
- Impedance calculations and stackups for high-frequency boards using coplanar waveguides and coplanar stripline models
- Characteristics and scaling/feature compensation for the newer phenolic-epoxy and halogen-free FR-4s
- Ability to add local fiducials to align laser drilling CCD cameras
- System to store laser drilling parameters based on diameter, depth, and material types
- Characteristics of via-plugging to determine if placement of buried vias will create problems
When lasers are used to create vias, the difference in ablation rates between the glass fiber and the surrounding resin can cause poor hole quality. Also, the fiberglass cloth is not uniform due to having areas with no glass, areas with one strand, and the intersections of strands (also known as knuckles), making it difficult to set up drilling parameters for all these varying materials. Usually the drilling is set up for the hardest to drill region which is the knuckle area. Figure 1 shows some examples of poor hole quality due to laser ablation of glass fiber reinforced dielectrics. It seems many fabricators opt to not employ a glass etch in these types of conditions. A properly controlled glass etch will aid in removing the protruding glass fiber bundles and enhance overall plating uniformity.
When migrating HDI manufacturing over to laser-drillable prepregs, improvement in via quality is achievable. These laser-drillable prepregs are made using spread yarns in both the warp and fill directions. Thus the reinforcement is more uniform helping to minimize the areas with no glass fiber, as well as the knuckle area.
To read the full version of this article which appeared in the November 2017 issue of The PCB Magazine, click here.
Trouble in Your Tank: Moving into Microvias—The Interaction of Materials and Processes, Part 1