In PCB design, removing via annular pads on non-functional connection layers (i.e., no annular design) to enhance routing and signal integrity is a precision technique that requires careful consideration. The table below summarizes its core advantages and disadvantages, helping you quickly establish an understanding.
|
Dimension |
Advantages of No Annular Design (Benefits) |
Challenges and Risks of No Annular Design (Costs) |
|
Routing Space |
Releases more space for high-density routing, especially in tight inner layers |
Requires more precise manufacturing processes, placing certain demands on PCB manufacturers |
|
Signal Integrity |
Improves insertion loss for high-speed signals, reducing impedance discontinuities and reflections caused by excess pads |
Poor design may introduce new impedance discontinuities |
|
Thermal Management and Reliability |
Reduces excess copper layers in the Z-axis, helping to improve reliability under thermal stress and avoid “resin starvation” and “telegraph effect” |
In some high aspect ratio vias, removing pads may actually reduce mechanical reliability |
|
Manufacturing Costs and Processes |
Reduces drill wear |
Typically requires additionalresin plug + electroplating fill processes to achieve reliable pad surfaces, which significantly increases costs |
Mastering Key Processes and Design Trade-offs
To achieve a reliable no annular design, you need to focus on the following core processes and design trade-offs:
Core Process: Resin Plug and Electroplating FillThis is the most mainstream and reliable process for achieving via-in-pad no annular design. The process involves: after laser drilling micro-holes, filling the vias with specialized epoxy resin, grinding flat, and finally electroplating the surface to form a completely flat pad. This fundamentally prevents solder from being sucked into the holes during reflow soldering, leading to cold solder joints, and provides a perfect soldering surface for subsequent surface mount.
Better Solution: HDI Blind and Buried Hole TechnologyFor designs pursuing higher reliability and performance, adoptingblind holes andburied holes in HDI processes is the fundamental solution. Blind holes only connect the surface layer and specific inner layers, completely eliminating the risk of solder loss and gas entrapment from a physical structure perspective. Although this increases costs, for high-end products, it can prevent greater losses due to failures and repairs later on.
Important Design Trade-off: When to Use No Annular DesignDecision-making requires balancing multiple factors. For example, inhigh aspect ratio (large ratio of board thickness to hole diameter) vias, retaining non-functional pads may help improve mechanical reliability; while inlow aspect ratio vias, removing them is more beneficial for reliability. Additionally, consider the board material’sZ-axis thermal expansion coefficient,current carrying capacity, and potential impacts onhigh-speed signal integrity.
Avoiding Common Design Risks
When applying no annular design, be sure to avoid the following “minefields”:
Do not use untreated through holes in soldering areas: If using unfilled no annular through holes on pads for BGA or small package components, capillary action during reflow soldering can draw away a large amount of solder paste, leading tosolder loss rates exceeding 50%, causing tombstoning and cold solder joints, resulting in significant scrap.
Use with caution for cost-sensitive projects: For ordinary consumer electronics, if the BGA pin pitch is 0.65mm or greater, traditional “dog bone” fan-out designs are more cost-effective, potentially savingover 50% of PCB costs compared to using via-in-pad solutions.
Core Decision Principles
In summary, whether to adopt a no annular design, you can follow these principles:
Not enough space, BGA too small? Resin plug is the key. When pushed into routing constraints by 0.4/0.5mm pitch BGAs, a reliable resin plug no annular design is your best choice.
Cost tight, process uncertain? Traditional fan-out is the most reliable. Facing low-cost and process-uncertain projects, traditional fan-out methods carry lower risks.
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