Why do We need PCB fiducials and How to use it?
What are PCB fiducials?
Fiducials, also known as registration marks, are small metallic patterns on a printed circuit board (PCB) that serve as reference points for various fabrication and assembly processes.
They facilitate precise alignment and accurate positioning of various production tools like photo plotters, drilling machines, pick and place equipment etc. relative to the bare PCB avoiding cumulative positional errors. This becomes vital to meet the tight tolerances demanded by high density boards with fine features.
Fiducials come in various shapes and sizes. Some common varieties include:
Copper patterns are preferred since they offer superior contrast under automated optical inspection systems compared to other markings.
Below image shows some sample fiducial placements on a PCB:
Key Benefits
Thus, fiducials form an integral navigational framework facilitating efficient fabrication, component placement and testing benefits.
Where to place fiducials on PCBs?
Since fiducials guide alignments, their locations require careful consideration for maximum effectiveness. Below are some placement guidelines:
1. Around board perimeter
Placing fiducials close to board edges and corners allows easily sighting them under production camera systems besides confirming dimensional stability.
2. Near component groups
Local fiducials adjacent to component clusters ensure accurate positioning w.r.t nearby parts. This minimizes cumulative alignment errors when assembling large boards.
3. Across non-functional areas
Fiducials consume real estate, hence focus on vacant board sections without encroaching upon critical traces or pads.
4. On both sides for double sided boards
For reliable double sided SMT assemblies, align top and bottom fiducials coaxially along the Z-axis enabling accurate mounting despite flipovers.
5. 45° rotation across sides
Angular offset between layers improves true positional correction instead of replicated errors in certain axis.
6. Near testpoints
Assists in accurately probing them during in-circuit testing (ICT).
7. Symmetrical distribution Balanced peripheral and infield placements offer best global alignments.
Adhering to these principles allow optimizing their locations suitable for a given board layout and assembly needs.
How many fiducials are needed?
The optimum number of fiducials required depends on PCB size and complexity aspects.
Some guidelines regarding fiducial count:
More fiducials offer higher correction accuracy by mitigating random errors. But practically limit beyond 20 for most applications from area standpoint.
For irregular shaped boards, place additional fiducials along non-linear sections which are hard to physically access and align during fabrication/assembly.
What are the design considerations?
To serve their intended purpose, fiducials need to adhere to certain design aspects:
1. Size
Typical industry standards recommend a minimum size of 0.8mm up to 2mm diameter for most fiducials considering camera recognition needs and fabrication capabilities. For MICROvias boards, use smaller fiducials (~0.3 mm).
2. Clearance
Provide sufficient clearance of at least 0.3mm from conductive features. This prevents shorting or metal smearing issues when drilling starts erroneously from fiducials.
3. Copper weight
Define same copper thickness as adjacent pads/traces, usually 1 oz. This offers similar metal visibility aiding recognition and prevents over/under-etching.
4. Solder mask opening
Remove solder mask completely from fiducial area during solder masking process for maximum contrast and reflectivity.
5. Non-plated
Keep fiducials non-plated since plating uniformity cannot be tightly controlled on thin entities prone to localized plating thickness variations.
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6. Fine line widths
For optical systems, thin lines and spacing offer better recognition than solid filled patterns. Hence define outlines instead of filled squares/circles.
Sample fiducial implementation
The fiducial construction example below demonstrates a practical implementation following the above design guidelines on a PCB:
Here it uses a 4-spoke crosshair pattern in 1 oz. copper with adequate clearance gaps from nearby conductor features. The thin skeletal structure aids recognition while non-plated construction avoids process variations.
How do fiducials facilitate PCB fabrication?
Now that we have understood fiducial basics, it helps to see how they assist various fabrication steps:
Photoplotting
During photo-lithography process which transfers conductor patterns onto bare boards, optical plotters use fiducials for precise alignment of phototools containing required circuit layouts. This avoids imaging distortions between projected master drawings and panelized boards.
Mechanical drilling
Drilling machines orient the stacked PCB panel correctly by sighting perimeter fiducials minimizing placement inaccuracy and drill wander issues especially for microvias.
Laser drilling
Similar to mechanical drilling, laser drillers employ fiducials to precisely position boards avoiding incorrect z-axis ablations. This accuracy is vital given laser holes show negligible wander due to absence of mechanical bits.
Automated optical inspection
Post-fabrication process quality checks like AOI systems use fiducials as reference to validate conductor widths/spacings, alignment integrity across layers etc. against source data for matchingboard dimensions or detecting defects.
Panelization
Fiducials assist in accurately placing and fixing daughter PCB panels on larger fabrication panels. This minimizes misalignments when separating individual boards after population.
Thus, integrated use of fiducials at each production step accumulates substantial quality and yield benefits.
Role of fiducials in PCB assembly
Besides fabrication, fiducials provide similar alignment assistance when stuffing boards with components either through:
1. SMT Pick-n-Place
The surface mount machines use in-built camera systems to recognize board and component fiducial markers for precise table positioning ensuring accurate fine-pitch placements.
2. Manual assembly
Even hand-assembly utilizes fiducials markings around key component groups or board sections serving as visual cues by operators when inserting parts avoiding mixups.
3. Post-assembly inspection
X-Ray systems employ fiducials again as positional references when checking internal defects like opens/shorts with board level components.
4. Rework/Repair
Helps identify and restore defective components during corrective works relative to board fiducial markers minimizing adjacent damage.
Alternative markings
Fiducials remain the most preferred and reliable alignment system across fabrication to field deployment stages assurring optimal yield and performance. However certain applications employ supplementary alignment marks as well for additional benefits:
1. Tooling holes:
Large mechanical holes help tooling pins grip boards stronger compared to fiducials during rigorous fabrication handling. However they consume significant board real estate.
2. Non-metal marks:
Laser etched circles, inkjet printed patterns etc. on solder mask avoid any shorting concerns with metal fiducials but offer relatively lower contrast.
3. Component outlines:
Metallic shapes resembling nearby component pads/footprints aid visual alignment for manual assembly instead of generic fiducial patterns.
But such markings only complement rather than replace dedicated fiducials which still remain indispensable for automated optical processes like drilling, AOI etc. critical for production quality and repeatability.
FQA
Q1. Where is the best location to place fiducials on PCBs?
The ideal fiducial locations are towards board periphery, inside non-functional areas, near component clusters, at symmetrical infield spots allowing global alignment corrections for fabrication through final assembly.
Q2. What are the typical fiducial sizes used?
Around 0.8mm to 2mm range with minimum clearances of 0.3mm from adjacent copper features are preferred sizes granting optimal visibility under camera systems while avoiding short risks.
Q3. How do fiducials help in improving PCB assembly process?
Both surface mount machines as well manual assembly operators rely on fiducial visual markers across PCBs and components for accurate alignments meeting precision placement requirements critical for high density boards.
Q4. Can component outlines serve as alternate fiducials?
While component body patterns help manual assembly to some extent, for critical automated optical stages like photoplotting, drilling etc. dedicated metallic fiducials only can provide the required level of process control essential for build quality.
PLM (Production Line Manager), VICE HDO (Head Operations) Production, Process and Maintenence Manager, SMT Specialist, Quality Manager.
1 年Fiducials are one of the most important marks in PCBs and Metal Masks (Stencils), this provide and asure the rigth and exactly impression of solder paste and components mounting. Thanks for share.