PCB design and manufacturing is a complex process managing thousands of components and connections across a multi-layer board. It is of critical importance to ensure the production yield is error-free and one way to improve fabrication yields is to perform a design rule check (DRC).
Catching an error in the design phase will prevent a product from being scrapped due to power ground shorts, misaligned vias, and missing pins. DRC for manufacturing is important in ensuring the quality of PCBs fabricated on the production line.
In this article we will cover:
What is design rule check (DRC) in PCB design?
A design rule check is a set of rules used by a designer to ensure the schematic matches all manufacturing considerations and dimensional tolerances set for a board.
Production processes have a margin of error for the variation in the products that are manufactured. Such variations in production yield are adjusted in the design phase itself.
What is DRC and LVS check?
A DRC allows one to verify the schematic and layout with regard to the margin of error you can incorporate in your design. As it allows one to check if a specific board layout correlates with the original circuit diagram or schematic of the design, it is also called layout versus schematic (LVS) check.
What are DRC errors?
Electronic design automation software lets you know when a design margin is violated by stating the parameter that is out of the acceptable range. This is called a DRC error and it helps eliminate rework in the production stage.
DFM stands for design for manufacturing, which is the layout topology in a way that avoids all the problems that could be encountered during the PCB fabrication and assembly processes.
- Drilling checks for drilling holes:
- Drill-to-copper: The drill-to-copper is the land clearance between the edge of a drilled hole to the nearest copper feature. The nearest copper feature can be a copper trace, copper pour, or any other active copper region.
- Annular ring checks: To achieve acceptance for Class 2 and Class 3, follow the tables below published by Altium. The first one gives the annular ring requirements for mechanically drilled blind, buried, and through-holes on ½ oz copper:
|Drill||Pad||Anti-Pad||PCB Thickness||Aspect Ratio|
|0.006″||0.016″||0.026″||up to 0.039″||6.05:1|
|0.008″||0.018″||0.028″||up to 0.062″||7.75:1|
|0.010″||0.020″||0.03″||up to 0.100″||10:01|
|0.012″||0.022″||0.032″||up to 0.120″||10:01|
|0.0135″||0.024″||0.034″||up to 0.135″||10:01|
|Drill||Pad||Anti-Pad||PCB Thickness||Aspect Ratio|
|0.008″||0.023″||0.033″||up to 0.062″||7.75:1|
|0.010″||0.025″||0.035″||up to 0.100″||10:01|
|0.012″||0.027″||0.037″||up to 0.120″||10:01|
|0.0135″||0.028″||0.038″||up to 0.135″||10:01|
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- Signal checks: The first checklist we like to implement after receiving Gerber files is signal checks. This checklist holds key parameters that include conductor width, spacing requirements, hole registration, and many more.
- Solder mask checks: Coming to solder mask clearance, we can typically define it as a general isolation recommendation alongside specific details characterized by the kind of surface element(s) being isolated. Particular specifications apply to pads, which may be solder mask defined or non-solder mask defined, and drill holes, which may be plate through-holes or non-plated through-holes.
- Silkscreen checks: Silkscreen to mask spacing, silkscreen to copper spacing, and silkscreen to hole spacing and route spacing
DFA is the process of designing a device or a PCB by considering the ease of assembly as one of the key criteria. Here are a few DFA guidelines:
- Select readily available components and validate their production. This will prevent production delays.
- Apply component spacing guidelines. The component placement will determine whether the board can be assembled, the soldering techniques and the type of thermal dissipation to be used.
- Use component manufacturer recommendation for footprints. This will prevent pad mismatch while ensuring accurate markings for identification are present.
- Apply board edge guidelines. Board shape and component placement can impact panelization.
DFF stands for design for fabrication. Therefore, as the name suggests, this addresses the issues related to fabrication. DFA stands for design for assembly. In most cases, DFF and DFA together make up DFM.
DRC (resign rule checking) in many cases is used for DFM but is not sufficient. That’s also acceptable to some extent because DRC issues detected in manufacturing can indeed have a direct impact on the manufacturability of a PCB. However, DRC is different from DFA.
DRC and DFM
DRC checks whether a problem exists or not. Like a hard pass/fail detection of a problem in the circuit board. It ensures whether the layout connectivity precisely matches with the schematic diagram’s defined connectivity. DRCs don’t include all the rules required to make a bare PCB or assembled PCB. But that is only one aspect of DRC. Most commonly, DRC includes rules that stand for defining the minimum spacing between components for the entire circuit board or for individual layers. So, if we consider from the spacing aspect, DRC becomes a subset of DFM, but only if the rules checked by DRC reflect a manufacturer’s requirements for spacing. If not, DRC is solely for electrical verification.
DFF and DFA
As in most cases, DFM’s two primary components are DFF (design for fabrication) and DFA (design for assembly). They are more involved in the nuances as compared to DRC. DRC is all about detecting very specific deviations from the intended interconnect. On the other hand, DFM checks PCB topology for potential manufacturing issues. Therefore, we can also say, a DRC defect, suppose a short, will get repeated in every copy of the board, irrespective of the quantity. Whereas it has been witnessed, if the same PCB quantity contains DFM issues, the manifestation of problems may be seen in some circuits only.
For instance, a layout that contains very thin pieces of copper can be correct if we go by the schematic. And it will pass DRC if there is no issue with the spacing. However, the same copper, being so thin, could form a sliver. Therefore, it could potentially detach from the board and form solder bridges with other components during assembly. Physically, this could happen in some PCBs and some PCBs may work as expected. So, this sort of situation can pass DRC, but in real-world manufacturing, it can cause havoc. DFM will detect such issues and save manufacturers and assemblers from scrap and rework.
How DRCs minimize board respins
PCB designs undergoing re-spins will incur a significant cost per re-spin. Electrical DRC capabilities help ensure that designs meet performance requirements and time-to-market cost goals by minimizing and even eliminating potential re-spins. Such checks will alert users of rule violations that could have been overlooked in manual inspection. Such checks include customizable analog, signal integrity (SI), power integrity (PI), electromagnetic interference (EMI), and safety checks. These checks allow designers to identify and correct issues.
Some example of DRC’s in the design include:
- The spacing between the traces, and between the trace and pads, through holes.
- Tight coupling between the power supply and ground wire. Spacing to be widened for the ground lines.
- Design of the key signal lines. Trace length and guard wire length along with the required separation of input and output lines.
- Checking if the analog circuits and digital circuits have separate ground connections.
- Checking if PCB labels might short the circuit.
- Modifying undesirable lines.
- Checking if the resistance welding can meet the production process needs.
- Checking whether the outer rim of the power layer in a multi-layer board is reduced, to avoid short-circuiting when the power layer is exposed..
DRCs for PCB manufacturing is a critical aspect that designers and manufacturers need to know well to design efficient and reliable circuit boards.
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