Assembly Drawing Essentials

assembly drawing essentials: what this playbook covers (and who it’s for)

This playbook is designed for hardware engineers, procurement leads, and project managers who need to transition a PCB design from a digital CAD file to a physical, reliable product. While Gerber files define the copper and BOMs define the parts, the assembly drawing is the governing document that tells the manufacturer how to build the board correctly. It bridges the gap between electrical intent and mechanical reality.

In this guide, we focus specifically on assembly drawing essentials—the critical notes, views, and specifications that prevent costly manufacturing errors. You will learn how to define requirements that eliminate ambiguity regarding component orientation, mechanical assembly, and special processing like conformal coating. We move beyond basic theory into actionable lists that you can copy directly into your documentation packages.

We also address the commercial side of assembly documentation. You will find a risk assessment framework to identify where poor drawings lead to yield loss, a validation plan to verify your contract manufacturer (CM) is following instructions, and a supplier checklist to audit their capabilities. Whether you are working with APTPCB (APTPCB PCB Factory) or another partner, this guide ensures your data package is robust enough for scale.

When assembly drawing essentials is the right approach (and when it isn’t)

Understanding when to invest time in detailed assembly drawings is the first step in efficient project management; while every production run benefits from clarity, the depth of documentation required varies by stage.

This approach is right when:

You are moving to NPI or Mass Production: Reliance on "tribal knowledge" or email threads is impossible at scale. The drawing must stand alone as the source of truth.
You have polarized or ambiguous components: LEDs, diodes, and ICs with subtle markings require explicit orientation diagrams to prevent reversed placement.
You have mechanical requirements: If your PCBA requires heat sinks, daughterboards, or specific screw torques, these cannot be conveyed in a standard BOM or centroid file.
You require special processes: Instructions for masking, conformal coating, underfill, or "do not wash" components must be visually defined.
You are subject to compliance standards: Industries like automotive or medical require a paper trail showing that assembly instructions were formally documented and followed.

This approach might be overkill when:

You are building a "looks-like" prototype: If the board is non-functional and only for mechanical fit checks, a full assembly drawing may be unnecessary.
You are physically present at the line: If you are hand-soldering the first five units yourself or standing next to the technician, verbal instruction can temporarily replace formal documentation (though this is risky).
The design is purely digital with no special parts: A simple breakout board with non-polarized passives might survive with just a silkscreen and a BOM, provided the silkscreen is descriptive enough.

Specs & requirements (before quoting)

To ensure your manufacturer can quote accurately and build correctly, your assembly drawing must contain specific, quantifiable data points rather than vague requests.

Governing Standards: Explicitly state the IPC class required (e.g., "Workmanship per IPC-A-610 Class 2"). This sets the baseline for solder joint quality and cleanliness.
Component Orientation: Provide a clear diagram for Pin 1 location on all ICs and polarity markings for capacitors and diodes. Do not rely solely on the silkscreen, as it can be clipped or covered.
Critical Dimensions: Define maximum component height restrictions, especially if the PCBA fits into a tight enclosure. Specify tolerance ranges (e.g., "Max height 5.0mm +0/-0.2mm").
Mechanical Assembly Specs: List torque values for all screws and nuts (e.g., "Torque M3 screws to 0.6 Nm ±10%"). Specify if thread-locker is required.
Masking Requirements: Clearly outline areas that must be masked before wave soldering or conformal coating. Use hatched zones in the drawing to indicate "Keep Out" areas.
Labeling and Serialization: Define exactly where labels should be placed, the font size, and the content (e.g., "Serial Number: YYWW-XXXX"). Specify if the label must be high-temperature resistant.
Depanelization Method: Specify if the boards should remain in the panel or be broken out. If they are to be broken out, specify the method (V-score, mouse bites, or router) to prevent stress on ceramic capacitors.
Cleaning Instructions: State whether the board requires aqueous cleaning or must remain "No-Clean." Identify components that are sensitive to washing (e.g., unsealed buzzers or switches).
Revision Control: The drawing must have a title block with the current revision number, date, and author. This must match the BOM revision to avoid version conflicts.
Special Soldering Requirements: Note any through-hole parts that require selective soldering or hand soldering due to thermal sensitivity.
Adhesives and Underfill: If BGA underfill or staking adhesives are required for vibration resistance, specify the material type and the curing process.
Test Points: Identify critical test points that must remain accessible for ICT or functional testing, ensuring they are not covered by labels or mechanical parts.

Hidden risks (root causes & prevention)

Even with a drawing in hand, specific gaps in assembly drawing essentials can lead to catastrophic failures during volume production; identifying these risks early allows you to mitigate them before the line starts moving.

Risk: Ambiguous Polarity Markings
- Why it happens: Designers use custom footprints where the "dot" or "line" is unclear, or the silkscreen is covered by the component body.
- How to detect: Review the Gerber assembly layer against the physical part datasheet.
- Prevention: Add a magnified view in the assembly drawing explicitly showing the component body relative to the pad layout.
Risk: BOM and Drawing Conflicts
- Why it happens: The BOM is updated with a new part number, but the drawing still references the old MPN or footprint.
- How to detect: Perform a line-by-line comparison of the BOM against the drawing notes during the DFM phase.
- Prevention: Add a note stating "In case of conflict between BOM and Drawing, the BOM takes precedence" (or vice versa, depending on your process).
Risk: Missing Centroid Data Correlation
- Why it happens: The centroid file basics (XY coordinates) do not match the rotation defined in the drawing (e.g., 0 degrees in CAD vs. 90 degrees on the reel).
- How to detect: The pick-and-place machine places parts rotated 90 or 180 degrees wrong during the first run.
- Prevention: Require a "First Article" photo verification before the full run is populated.
Risk: Unspecified Glue/Adhesive Locations
- Why it happens: The drawing says "glue capacitor," but doesn't say where, leading to glue on the pads or vents.
- How to detect: Visual inspection reveals inconsistent glue application.
- Prevention: Draw specific "glue zones" and "no-glue zones" on the assembly layer.
Risk: Through-Hole Lead Length Interference
- Why it happens: Leads are not trimmed short enough on the bottom side, causing shorts against the chassis.
- How to detect: Fit check with the enclosure fails.
- Prevention: Specify "Trim leads to max 1.5mm" in the assembly notes.
Risk: Conformal Coating on Connectors
- Why it happens: Masking zones are not defined, so the spray covers contact pins.
- How to detect: Connectivity failure during functional test.
- Prevention: Use a dedicated layer in the drawing to show exact masking boundaries.
Risk: Heat Sink Shorting
- Why it happens: Mechanical assembly drawings fail to specify an insulating washer or thermal pad.
- How to detect: Short circuit upon power-up.
- Prevention: Include an exploded view of the mechanical stack-up in the drawing.
Risk: Incorrect Depanelization Stress
- Why it happens: The drawing doesn't specify "Router Cut Only," so the operator uses a guillotine, cracking MLCCs.
- How to detect: Field failures due to cracked capacitors.
- Prevention: Explicitly ban high-stress separation methods for panels with sensitive components near the edge.
Risk: Illegible Silkscreen References
- Why it happens: High-density designs remove reference designators to save space.
- How to detect: Repair technicians cannot find "R45" to debug a board.
- Prevention: Include a searchable PDF assembly map where designators are clearly legible, separate from the physical silkscreen.
Risk: Moisture Sensitivity Mismanagement
- Why it happens: The drawing doesn't specify baking requirements for MSL parts if the package is open.
- How to detect: Popcorning (cracking) of ICs during reflow.
- Prevention: Add a standard note: "Handle MSL components per IPC/JEDEC J-STD-033."

Validation plan (what to test, when, and what “pass” means)

To ensure your assembly drawing essentials are being respected, you must implement a validation plan that checks the physical output against the documentation at specific milestones.

Objective: Verify Component Polarity
- Method: 100% Visual Inspection (AOI) and manual check of polarized caps/ICs on the First Article.
- Acceptance Criteria: All polarity notches/stripes match the assembly drawing diagram exactly.
Objective: Confirm Mechanical Fit
- Method: Physical installation of the PCBA into the final enclosure (or a 3D printed mock-up).
- Acceptance Criteria: No interference with housing; mounting holes align; connectors mate without stress.
Objective: Validate Soldering Quality
- Method: X-Ray inspection for BGAs/QFNs and visual inspection for through-holes.
- Acceptance Criteria: Meets IPC-A-610 Class 2 (or 3) criteria; acceptable voiding percentage (e.g., <25%).
Objective: Check Cleanliness
- Method: Ionic contamination testing (ROSE test).
- Acceptance Criteria: Contamination levels below the specified limit (e.g., <1.56 µg/cm² NaCl equivalent).
Objective: Verify BOM Consistency
- Method: BOM cleanup tutorial steps applied to the "As-Built" BOM provided by the supplier.
- Acceptance Criteria: The manufacturer's purchase list matches your approved vendor list (AVL) exactly.
Objective: Validate Conformal Coating
- Method: UV light inspection (if coating has UV tracer).
- Acceptance Criteria: Coating covers required areas; keep-out zones (connectors) are 100% clean.
Objective: Confirm Labeling
- Method: Barcode scanner test.
- Acceptance Criteria: Scanner reads the correct string; label is straight and in the defined location.
Objective: Test Mechanical Fasteners
- Method: Torque check on a sample of screws.
- Acceptance Criteria: Screws do not loosen; torque values are within the specified tolerance.
Objective: Verify Depanelization Quality
- Method: Edge inspection under magnification.
- Acceptance Criteria: Edges are smooth; no delamination or stress marks on nearby components.
Objective: Functional Verification
- Method: Automated Functional Test (FCT) or bench test.
- Acceptance Criteria: Board passes all electrical parameters defined in the test spec.
Objective: Documentation Traceability
- Method: Audit the traveler/router document.
- Acceptance Criteria: Every process step (paste, place, reflow, AOI) is signed off and dated.
Objective: Packaging Validation
- Method: Drop test of the packaged PCBA.
- Acceptance Criteria: No damage to the board or ESD bag after simulated shipping drops.

Supplier checklist (RFQ + audit questions)

Use this checklist to vet potential partners or to ensure your current supplier, such as APTPCB, has all the necessary information to execute your assembly drawing requirements.

RFQ Inputs (What you send)

Assembly Drawing (PDF): Visual representation with notes, dimensions, and views.
Pick and Place File: The machine-readable coordinate file (often linked to centroid file basics).
BOM (Excel): Cleaned list with MPNs, quantities, and reference designators.
Gerber Files: Including paste layers, silkscreen, and copper layers.
3D STEP Model: For mechanical clearance verification.
Test Specification: If functional testing is required.
Approved Vendor List (AVL): Acceptable alternates for critical components.
Special Process Instructions: Conformal coating, potting, or underfill specs.
Packaging Requirements: ESD bags, bubble wrap, or custom trays.
Volume & Timeline: EAU (Estimated Annual Usage) and target delivery date.

Capability Proof (What they confirm)

IPC Class Adherence: Can they build to Class 2 or Class 3 as requested?
Minimum Pitch: Can they handle the finest pitch component on your board (e.g., 0.4mm BGA)?
X-Ray Capability: Do they have in-house X-Ray for BGA inspection?
AOI Availability: Is Automated Optical Inspection used for all batches?
Conformal Coating: Is this done manually or with an automated spray machine?
Selective Soldering: Do they have selective soldering machines for mixed-technology boards?
Cleanliness Testing: Can they perform ionic contamination testing in-house?
Programming: Do they support IC programming (pre-programming or in-circuit)?

Quality System & Traceability

Certifications: ISO 9001, ISO 13485 (medical), or IATF 16949 (automotive) as needed.
First Article Inspection (FAI): Do they provide a formal FAI report before mass production?
Component Sourcing: Do they buy only from authorized distributors to prevent counterfeits?
ESD Control: Is the facility fully ESD compliant (floors, smocks, grounding)?
Moisture Control: Do they have dry boxes and baking ovens for MSL parts?
Record Retention: How long do they keep production records and quality data?
Non-Conformance Handling: What is the process if a board fails inspection?

Change Control & Delivery

ECN Process: How do they handle Engineering Change Notices during a build?
Deviation Procedure: If they need to substitute a part, how is approval obtained?
Firmware Version Control: How do they ensure the correct firmware version is flashed?
Overage Policy: Do they build extra boards to cover yield loss?
Shipping Documentation: Do they include a Certificate of Compliance (CoC) with shipment?
RMA Process: What is the warranty and return procedure for defects?

Decision guidance (trade-offs you can actually choose)

Drafting assembly drawings involves balancing detail with flexibility. Here are the trade-offs you will face.

Strict AVL vs. Open Sourcing:
- If you prioritize reliability and consistency, specify a strict Approved Vendor List (AVL) in your drawing/BOM.
- If you prioritize cost and lead time, allow "generic equivalents" for passives (resistors/caps), letting the CM choose based on stock.
Detailed Notes vs. Industry Standards:
- If you prioritize custom requirements, write explicit notes for every process (e.g., "Solder fillet height > 50%").
- Otherwise, simply reference "Per IPC-A-610 Class 2" to save documentation time and rely on established industry norms.
Panelization in Drawing vs. Manufacturer Discretion:
- If you prioritize assembly efficiency for your specific line, define the panel array in the drawing.
- Otherwise, let the manufacturer optimize the panel layout for their specific machines and material utilization.
100% X-Ray vs. Sample Testing:
- If you prioritize zero defects on BGAs, mandate 100% X-Ray inspection (adds cost/time).
- Otherwise, accept sample-based X-Ray (e.g., first 5 and last 5 of a run) for standard consumer products.
Hard Copy vs. Digital Only:
- If you prioritize auditability, require a signed hard copy of the drawing to travel with the batch.
- Otherwise, rely on the digital PDF/Gerber package for speed and paperless operations.
Washing vs. No-Clean:
- If you prioritize aesthetic and coating adhesion, mandate aqueous cleaning.
- Otherwise, use No-Clean flux to reduce process steps and cost, accepting some visible flux residue.

FAQ

What is the difference between an assembly drawing and a fabrication drawing? A fabrication drawing instructs the factory how to make the bare PCB (drill hits, stackup, copper weight). An assembly drawing instructs the factory how to populate the board with components (orientation, soldering, mechanical assembly).

Can I just send the Gerber files and a BOM? Technically yes, but it is risky. Without an assembly drawing, the manufacturer has to guess the orientation of ambiguous parts and may miss special requirements like "do not wash" or specific label placement.

How do I handle BOM changes after the drawing is released? Best practice is to update the drawing revision to match the BOM. However, you can add a note to the drawing saying "Component values and MPNs per BOM; Drawing is for reference only" to avoid constant drawing updates for minor value changes.

What file format should the assembly drawing be? A searchable PDF is the industry standard. It allows operators to zoom in and search for reference designators. DXF files are also helpful for automated equipment setup but should accompany a PDF.

Do I need to include the centroid file in the drawing? No, the centroid file basics dictate that it is a separate text/CSV file used to program the pick-and-place machines. The drawing is a visual reference for humans to verify the machine's work.

How do I indicate "Do Not Populate" (DNP) parts? Mark them clearly in the BOM. On the assembly drawing, you can cross them out or omit them, but the BOM is usually the primary driver for DNP status. A note "DNP components per BOM" is helpful.

Should I specify solder paste stencil design in the drawing? Usually, you should leave stencil design to the manufacturer as they know their process best. However, if you have a specific requirement (e.g., "reduce aperture by 10% for this pad"), include it as a note.

Why is a "First Article Inspection" (FAI) important? FAI is the physical validation of your assembly drawing. It proves that the manufacturer correctly interpreted your notes, orientation marks, and BOM before they build the remaining thousands of units.

First Article Inspection (FAI) – Learn why validating the first unit against your assembly drawing is the most critical step in quality control.
Components & BOM Services – Understand how to structure your BOM to align perfectly with your assembly drawing notes.
DFM Guidelines – Discover how Design for Manufacturing reviews can catch drawing errors before they reach the production floor.
Box Build Assembly – See how assembly drawings expand to cover mechanical integration, cabling, and final enclosure assembly.
Conformal Coating Services – Explore how to specify coating zones and masking requirements in your documentation.

Request a quote

Ready to move your design into production? At APTPCB, we review your assembly drawing essentials as part of our standard engineering check to ensure every detail is clear before we build.

Request a Quote Now

For the fastest response, please include:

Gerber files (RS-274X format)
Bill of Materials (BOM) with MPNs
Assembly Drawing (PDF) with orientation and special notes
Centroid/Pick-and-Place file
Quantity and lead time requirements

Conclusion

Mastering assembly drawing essentials is about more than just satisfying a documentation requirement; it is about controlling the outcome of your manufacturing investment. By clearly defining requirements, anticipating risks like polarity errors, and validating the results through structured inspections, you transform a digital design into a reliable physical product. A comprehensive drawing package eliminates guesswork, empowers your supplier to deliver quality, and provides the solid foundation needed to scale from prototype to mass production with confidence.