High-Density Interconnect (HDI) PCBs are the future generation of printed circuit board technology, which allows high-performance, space-saving electronics of modern smartphones, medical devices, and aerospace systems. But these same characteristics that enable HDI PCB design to be so powerful, microvias, fine-pitch traces, and multilayer stackups, also present severe manufacturability problems that can push production schedules off schedule and push the budget.
The knowledge of preventing typical HDI layout errors is not merely a good practice, but it is a prerequisite to effective product development. This professional manual discusses the most serious mistakes that designers commit and the action plan that can be used to make sure that your HDI designs move efficiently between the conceptualization stage and the PCB fabrication phase.
The Stakes: HDI and its manufacturability
HDI PCBs incorporate new technology like laser-drilled microvias, blind and buried vias, as well as trace widths down to 3 mils (0.075mm). The characteristics require delicate production methods such as sequential lamination, controlled-depth drilling, and advanced plating methods that only allow a small degree of error.
The result of designing decisions compared to manufacturing capacities is dire: the production is held up, the rate of defects increases, the yield is lost, and the costs may rise 30-50 times more than initially estimated. The remedy is in Design for Manufacturability (DFM) principles specified for HDI structures.
Mistakes in Critical HDI layout and their prevention
1. Aspect Ratio Violations on a Microvia Scale
The Error: You can design microvias with an aspect ratio larger than what can be done by manufacturers, which, in most cases, is larger than 0.75:1 for blind vias and 1:1 buried vias.
Why It Matters: With high aspect ratio microvias, plating is a challenge. Copper deposition is not uniform across the via depth, causing voids, dimples, and connections to be incomplete. These defects can be used to concentrate strain during thermal cycling, leading to corner cracks and separation of target pads; one form of failure these devices may experience only when they are deployed to the field.
The Solution:
- Keep blind through aspect ratios of not more than 0.75:1.
- Staggered mixtures of microvia instead of deep stacked mixtures.
- Only two levels of stacked microvias are recommended. Stagger further the mechanical reliability of interconnects.
- Best filled and capped vias on stacked designs, which need to be planarized.
Studies have shown that two-level stacked microvias resist about 20 cycles of thermal operation compared to a four-level design, and therefore, a conservative design choice is important to reliability-sensitive applications.
2. Insufficient Annular Ring Specifications
The Error: Annular rings below 0.05mm (2 mils) were designed, which did not leave enough copper around via pads.
Why It Matters: The fabrication of layers may cause some changes in registration tolerances, thermal expansion during lamination, as well as the variation of drilling accuracy of positions relative to pads. Poor annular rings cause breakout – the hole that has been drilled does not fully or completely contact the pad, thus forming open circuits or high-resistance bonds.
The Solution:
- Keep minimum annular rings on microvias at 0.05mm (2 mils).
- Take into consideration manufacturer registration tolerances (usually less than 25micrometers stacked microvias)
- Increase annular ring specifications in drilling accuracy, which may fluctuate at the outer layers.
- Checks of annular mode have a stackup of worst-case tolerance.
3. Stackup Design: Minimal or no manufacturer consultation
The Error: Making layer stackup configurations without verifying the availability of the material used by the manufacturer and their capability to perform the process.
What It Matters: HDI Stacks entail certain material choices such as core thicknesses, prepreg type, and component weight of copper, amongst others, that fit the inventory and lamination equipment of these manufacturers. Ungrateful stackup specification may cause material changes with implications on impedance control, or may have to be custom ordered at the cost of weeks to lead times.
The Solution:
- Engage your PCB design company or manufacturer when planning firstly in planning.
- Use IPC-2226 standards of HDI stackup configurations (Type I to Type VI)
- Check that the dielectric thicknesses have the right values of controlled impedance.
- Sequential lamination capabilities are consistent with confirming requirements through spanning.
- Ask the manufacturer for stackup templates to be used as design bases.
4. No Proper Fill Specifications Via-in-Pad
The Problem: Putting the vias in SMD pads but failing to stipulate the use of the correct via filling and planarization.
Issues: The gap between vias in component pads results in solder wicking when the component is reflowed. The solder will not make correct connections but will move inside the via barrel and create weak connections, tombstone on small components, as well as BGA failures. The problem with this assembly defect is especially critical in fine-pitch BGAs, in which escape routing requires via-in-pad routing.
The Solution:
- Indication: Choose conductive or non-conductive via fill according to the needs of the electricity.
- Demand through capping and planarization of the company’s soldering surfaces.
- Check that fill requirements are fulfilled fully IPC-4761 Type VII (filled and capped) requirements are fulfilled.
- Provide notes on the acceptance criteria of fabrics, notes on fill quality.
5. Leaving Trace Width and Spacing Minimums unobserved
The Error: Trace design: The trace width and spacing are designed to be smaller than what a manufacturer is able to produce, for example, 3 mils (0.075mm) in smaller HDI processes.
Why It Matters: Fine traces need high-resolution X-ray images and control of etching. At the point where designs fall below process minimums, yield declines rapidly because shorts, opens, and variation of impedance are high. The loss of one point of yield is directly proportional to higher costs per board.
The Solution:
- RW Check with your manufacturer before routing on minimum trace/space requirements.
- Design to 3.5-4 mil minimums in 3 mil capability specification (giving process margin)
- Calculate and use controlled impedance calculation to get the actual required trace widths.
- Always avoid necking between pads that are below certain minimums.
6. Inadequate Solder Mask Specifications
The Judgment error: Leaving clearance of solder mask, dam, and registration tolerances of the fine-pitch component not specified.
Why It Matters: With modern HDI assemblies, where solder mask accuracy is the construction of 0.4mm and 0.5mm pitch BGAs, solder mask accuracy is directly proportional to assembly yield. Lack of clearances between masks and copper leads to solder bridging, lack of dam widths between the pads leads to mask migration during application.
The Solution:
- Indicate the solder mask openings required by the various pads.
- Keep the minimum dams (between openings) between solder masks to 3 mils.
- Check solder mask registration tolerance for fine pitch.
- Take into account solder mask defined pads (SMD) and non-solder mask defined pad (NSMD) depending on the needs of components.
7. Lacking or missing Fabrication Documentation
The Flaw: Giving partial fabrication notes that do not state important HDI-specific specifications.
Why It Matters: HDI manufacturing includes many process choices, which might include drill sequences, lamination cycles, plating specs, and so on, which manufacturers need to know based on design documentation. Lack of information results in assumptions that might not match design intent, which can result in functional failure or reliability problems.
The Solution:
Provide exhaustive notes of fabrication stating:
- Full stack-up of materials, thicknesses, and weights of copper.
- Through types, size, and spanning requirements of each via class.
- Controlled impedance: Keep values and tolerances.
- Finish specifications Surface finish (ENIG or ENEPIG suggested with fine pitch)
- Through the fill requirements and acceptance requirements.
- Requirement in the form of IPC classes (Class 2 or Class 3).
Installation of DFM Checks in the Design Process
HDI PCBs should be developed with effective DFM to ensure that it is verified not only in the end but also during the process. Checks at these points of concern need to be put into place:
- Library Review: Secure component footprints to manufacturer capabilities of pad sizes, spacing, and via-in-pad.
- Place: It allows sufficient routing paths and thermal control.
- Routing: Checks are made during routing on DFM violations before the layout is made.
- Final Check: Run full DFM checks such as Gerber file checks, Drill file accuracy checks, and BOM to design cross checks.
The tools of modern EDA offer automated DFM checking facilities, which check the designs against the manufacturer-specific sets of rules. Use these tools during the design process instead of the use of post-layout verification.
The Bottom Line: Teamwork Is the Answer
The best approach to prevent HDI manufacturability errors is to establish uninterrupted cooperation with your manufacturing partner early. Skilled providers of PCB fabrication are able to provide DFM review services that help uncover possible problems before these problems cost more money.
Provide your design specifications, performance criteria, and quantity along the way in advance. Knowing the abilities and limitations of the manufacturer allows design choices that can strike a balance between performance needs and producibility to produce HDI PCBs that are within specifications, on schedule, and at the target cost.
These are some of the pitfalls to be avoided, and these stringent practices of DFM will ensure that the development of the HDI PCB is not a high-risk affair but a predictable engineering process. This investment in manufacturability design pays off over the product lifecycle: more rapid time-to-market, reduced cost of production, and increased field reliability.
