Bullseye! Whether you are engaging in a little target practice, playing a video game, or competing at the fair to win the big teddy bear for your sweetheart, there is nothing like hitting your mark. And the universal mark or aim for these activities is the bullseye or center of the target. Hitting anything other than the bullseye can compromise the value of your efforts and can lead to disappointment, agonizing defeat, or in the case of the teddy bear, perhaps even a return to bachelorhood.
During the PCB construction process, holes are drilled in the board that may be used for installation, through-hole technology, or as vias for multilayer PCBs. For through-hole components and vias that connect to traces, the objective of the drilling process is to bore the hole through the bullseye of a copper pad. As these pads are typically circular, the area of the pad that remains after drilling is referred to as the annular ring. For annular ring PCB design, precision is much more than a goal, it is mandated by regulations and essential for successful board build and operation. Let’s take a look at the important issues to consider when designing annular rings and then how to implement them.
Important Considerations for Annular Ring PCB Design
The term annular ring is most likely derived from the word annulus, which defines a ring-shaped region or object. An annulus is most recognizable from its use in mathematics to describe the area between the diameters of two concentric circles. For circuit boards, annular regions are indeed typically ring-shaped; however, their usage as the conductive pad area surrounding a drilled hole can take on other shapes; for example, elliptical, rectangular, or square. Although all annular rings have the same function, irrespective of their shape, they are categorized by IPC-600 as one of the types listed below.
* In the descriptions above for PTH (plated through-hole) or via and NPTH (non-plated through-hole), layer refers only to the single-layer through which the drill hole exists and not the entire PCB structure.
The PTHs or vias listed above may be one of several types, as shown below.
PCB via options
As shown above, all of the via options include annular rings and your selections are primary considerations that affect many PCB layout design decisions, which are discussed below.PCB Annular Ring Design ConcernsCross-sectional and top view of a PCB annular ring (Source)
In the figure above, an example of a through-hole via connected to a BGA component pad is shown, which includes both external and internal supported annular rings. As illustrated, several factors affect annular ring PCB design.
DFM for HDI Printed Circuit Boards
PCB Annular Ring Design Factors
- Type of drill hole
- Location of drill hole
- Layer copper thickness
- Clearance to other elements
- Trace width
- Via wall thickness
Collectively, all of the issues listed above determine the design of the annular ring. Now, let’s see how these factors are integrated to achieve the best annular ring PCB design.
How to Best Design Annular Rings for Your PCB
Annular rings are created during manufacturing; therefore, the regulatory standards that govern the construction of your boards also provide specifications and guidelines for the rings. These include IPC-2221 Generic Standard on Printed Board Design, IPC-A-600 Acceptability of Printed Boards, and IPC-6012 Qualification and Performance Specification for Rigid Printed Boards. For the most part, these standards define the minimal acceptance dimension or lowest tolerance according to the IPC classification that is determined based upon where and how your PCBA will be used. Adhering to these standards, allows for the generation of a set of essential steps, shown below, that if followed will result in a regulatory-compliant, quality-made board.
Essential Steps for Annular Ring PCB Design
❐ Step #1:
Determine the IPC class for your board and understand the standards associated with developing it.
❐ Step #2:
Determine clearance requirements for annular ring.
❐ Step #3:
Determine the annular ring width minimum.
❐ Step #4:
Determine the via wall minimum (if applicable).
❐ Step #5:
Select the size and location of the annular ring.
To optimize your annular ring PCB design, the specifications must fall within the tolerances of your contract manufacturer’s (CM’s) DFM rules and guidelines. Otherwise, breakout, where the drill hole extends outside of the annular ring, solder bridging or solder mask expansion issues may result.
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At Tempo Automation, our smart-factory manufacturing process is quality certified by many standards organizations; including IPC. You can rest assured that your boards will be built quickly, accurately, and to the highest standards.
And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.