PCB Design Rules Check (DRC) and Its Role in Manufacturing

PCB design and manufacturing is a complex process, requiring management of thousands of components and connections across multiple layers. Ensuring error-free production yield is critical, and one way to improve manufacturing yield is by performing Design Rule Checks (DRC).

Identifying errors at the design stage prevents products from being scrapped due to power ground shorts, misaligned vias, or missing pins. DRC used in manufacturing is essential for ensuring the quality of PCBs produced on the production line.

1. What is Design Rule Check (DRC) in PCB Design?

DRC is a set of rules used by designers to ensure that the schematic matches all the manufacturing constraints and dimension tolerances set for the PCB.

There are always some deviations during the production process, and these variations are adjusted during the design stage itself.

2. What are DRC and LVS Checks?

DRC allows you to verify the schematic and layout based on the tolerance range that can be incorporated into the design. Since it checks if specific PCB layouts match the original schematic design, it is also referred to as Layout vs. Schematic (LVS) checking.

3. What is a DRC Error?

EDA software notifies you when parameters exceed the acceptable range, indicating a violation of the design margin. This is called a DRC error, which helps eliminate rework during the production phase.

4. DFM Rules

DFM stands for Design for Manufacturing, and it refers to layout topology designed to avoid issues that might arise during PCB manufacturing and assembly.

1. Drill-to-Copper Check
   Drill-to-copper refers to the minimum gap between the edge of a drilled hole and the nearest copper feature. The nearest copper feature could be traces, copper pads, or any other active copper areas.
2. Annular Ring Check
   For Class 2 and Class 3 acceptance, the annular ring requirements for blind, buried, and through-hole vias on copper are specified.
3. Signal Check
   The first checklist implemented after receiving Gerber files is the signal check. This list includes key parameters like trace width, spacing requirements, hole alignment, etc.
4. Soldermask Layer Check
   Soldermask clearance is typically defined as a general isolation recommendation, with specific details based on the surface elements isolated.
5. Silkscreen Check
   The silkscreen layer’s clearance from the soldermask, copper, holes, and traces is checked.

5. DFA Rules

DFA stands for Design for Assembly, and it is the process of designing a PCB or device with ease of assembly as one of its key criteria. Below are some DFA guidelines:

• Choose off-the-shelf components and verify their manufacturability to avoid production delays.
• Apply component spacing guidelines. The placement of components determines whether the board can be assembled, soldered, and what type of heat dissipation will be required.
• Use package recommendations from component manufacturers to prevent mismatched pads and ensure accurate identification marks.
• Apply board edge guidelines. The shape of the PCB and the placement of components will affect panelization.

DFF

In many cases, DRC (Design Rule Check) is used in DFM, but it’s not sufficient on its own. This is acceptable to some extent because DRC issues detected during manufacturing will indeed directly impact the PCB’s manufacturability, but DRC is different from DFA.

6. DRC vs. DFM

DRC checks for issues, similar to a pass/fail detection for PCB problems, ensuring the layout’s connectivity exactly matches the connectivity defined in the schematic. DRC does not include all the rules required to make a bare PCB or assembled PCB, but that’s just one aspect of DRC.

Typically, DRC includes rules that define the minimum spacing between components across the entire PCB or individual layers. From a spacing perspective, DRC becomes a subset of DFM, provided that the rules checked by DRC reflect the manufacturer’s spacing requirements. If not, DRC is only used for electrical verification.

7. DFF and DFA

In most cases, the two main components of DFM are DFF (Design for Fabrication) and DFA (Design for Assembly). Compared to DRC, they are more involved in the nuances of the design. DRC focuses on detecting specific deviations from expected interconnections.

On the other hand, DFM checks whether the PCB topology has potential manufacturing issues. We can also say that DRC defects (e.g., short circuits) will occur in every copy of the PCB, regardless of quantity. While DFM issues may only become evident in certain circuits, they might be noticeable in some but not all PCBs.

For example, a layout that contains very thin copper traces may pass DRC if the spacing is correct, as per the schematic. However, those thin traces could become problematic in the manufacturing process, potentially leading to separation from the board or forming solder bridges. This could happen on some PCBs, while others may work as expected. DFM would detect such issues, preventing scrapping or rework during assembly.

8. How DRC Minimizes PCB Rework

Reworking redesigned PCBs incurs substantial costs. DRC functionality helps minimize or even eliminate potential redesigns, ensuring the design meets performance requirements and time-to-market cost objectives. Such checks alert users to rule violations that might be overlooked during manual inspections. These checks include customizable simulations, signal integrity (SI), power integrity (PI), electromagnetic interference (EMI), and safety checks. These checks allow designers to identify and correct issues early.

Examples of DRC to Prevent Rework:

• Spacing between traces, traces and pads, vias.
• Tight coupling between power and ground planes, requiring wider ground plane spacing.
• Design of key signal lines, including trace and protection line lengths and necessary separation between input and output lines.
• Separate ground connections for analog and digital circuits.
• Ensure PCB labeling does not cause short circuits.
• Modify unnecessary traces.
• Ensure that through-hole soldering meets production process requirements.
• Check for shrinkage in multi-layer PCB power layers to avoid short circuits.

DRC for PCB manufacturing is a critical aspect that both designers and manufacturers need to understand in order to design efficient and reliable PCBs.