The PCB assembly drawing is crucial for component assembly.
Once your PCB is out of the fabrication line, you need at least four documents before taking it for assembly.
These are board assembly notes, a schematic diagram, a bill of materials (BOM), and a PCB assembly drawing.
Here’s a close-up look at the basics and elements of the PCB assembly drawing document.
What Is a PCB Assembly Drawing?
A PCB assembly is a graphical representation of your printed circuit board that shows the component reference designators, their values, tolerance, etc. Additionally, it indicates the following.
- Board outline, dimensions, and thickness
- Hole charts
- Layer stack up
- Torque parameters for securing casing screws
- PCB alignment
The document ensures correct and accurate component placement and soldering. And it can help you dismantle and reassemble the board to troubleshoot malfunctions.
A multilayer PCB assembly drawing
When dealing with a single-sided board, manufacturers can place the PCB assembly drawing on the back side because there are no electrical conductors.
But with double-sided boards, you need an additional view to show the front and rear sides. If the board is compact, you can include both drawings in a single-page document.
But if large, you’ll need additional sheets to include extended views that show the mechanical element connections in an easier-to-understand way.
Contents of a PCB Assembly Drawing
Always include these items in your PCB assembly drawing.
Drawing Format
You’ll have to use CAD software to create all your PCB drawings, designs, and layouts. Most CAD systems will force you to use internal drawing formats in the system library. But others generate the drawing format you want automatically.
An engineering drawing of a printed circuit board
Either way, you’ll have to combine this drawing format with the PCB layout created earlier to produce the drawing.
Board Outline
Board outlines include slots & cutouts, and they are high-level PCB design outline representations from the layout database.
The fabrication drawing design document contains the dimensions for the slots and cutouts, but the PCB assembly drawing document only references them.
Identification Information
The drawing should have this information for identifying each unique printed circuit board.
- Drawing ID number
- Assembly drawing revision label
- Board name
- Contract data
- Corporate address
- Date of creation
Part shapes
In addition to showing the reference designators, the PCB assembly drawing should indicate the part shapes in the outline to help with soldering.
A PCB assembly drawing showing the part shapes, polarization, and orientation for easy placement
Mechanical Parts
Some boards require mechanical parts like shields, connectors, and mounting screws. Since they don’t have the typical circuit board footprint, you might have to add them manually or draw them in a separate sheet.
The CAD footprint library might not represent these components accurately. So adding them separately or manually might be a necessary option.
And remember to include these components in the bill of materials because the schematic might not depict them clearly (they are not electrical parts).
A graphic image depicting a bill of materials
For instance, the drawing might not indicate an ejector handle because it is neither electronic nor has a typical board footprint.
If the schematic and BOM don’t include this part, use an expanded view of the assembly drawing to list the mechanical part connections.
Assembly Notes
An assembly note is technically a separate document from the assembly drawing, but it must come with the schematic. This document is a production guideline that lists instructions like:
- Special feature locations
- Industry standards
- Assembly details
- Industry specifications
- Component list (if required by the original manufacturer)
PCB assembly notes have the following functions and benefits.
Designates the Primary Board Side
All circuit boards have two sides: a primary or top side and a secondary or bottom side. This board direction should match the fabrication drawing.
Although it is easy to determine this orientation in the assembly drawing for single-sided boards, it might not be straightforward for double-sided PCBs.
In such a case, the assembly note will highlight the primary side. And you should include an additional view in the PCB assembly drawing to show the layout’s secondary side.
Defines Reference Designators
Usually, the component reference designators get printed on the board. But if there is insufficient space on the surface to include all these assembly markings, manufacturers write them in the assembly notes.
A PCB assembly drawing with reference designators
But generally, PCB assembly drawings are scalable enough to show all the assembly markings. This data should be clear-to-read and be within the component footprint to guide the assembly, field service, and inspection crew.
Specifies Soldering Procedures
The assembly note can combine specifications like J-STD-001 and J-STD-003 to specify the quality of the mechanical connections.
J-STD-001 guides how to inspect through-hole solder joints using X-ray machines. This testing process aims to identify the errors or faults you cannot identify with the naked eye or other means.
On the other hand, J-STD-003 highlights the solderability determinations used to verify the PCB manufacturing processes and storage conditions don’t affect solderability.
So the assembly note will have a statement like “solder as per J-STD-001 and J-STD-003 specifications.”
Lists Component Mounting Instructions
Like the soldering procedures, the assembly notes list the mounting instructions using IPC guidelines. For instance, it can state the PCB component mounting must conform to IPC-CM-770. And its acceptability must be to IPC-A-610.
IPC-CM-770 lists the most effective component mounting methods, while IPC-A-610 defines the acceptable standards for each assembly operation.
A vector image showing a chip on a pick-and-place machine being placed on a PCB
If your board contains manually assembled parts, the drawing/notes should specify the component height restrictions and spacing from other pieces.
Indicates All Specialized Assembly Methods
The notes also specify if the assembly process (soldering) is lead-free. For environmental safety purposes, all PCB soldering processes nowadays utilize lead-free solder.
But specific applications like space and military electronics projects require leaded solder.
A roll of lead wire solder
This specification usually has to do with the IPC-1066 standard. So the document marks the components requiring assembly in-line with this standard.
Manufacturer Label Identification
Lastly, the PCB assembly drawing should have manufacturer identification information like barcodes and assembly tags.
Using a drawing pointer, you can identify the manufacturer label, then reference it in the assembly notes.
Wrap Up
Most people confuse the PCB assembly drawing document with the PCB fab drawing.
But as you can see, the assembly drawing document explains how to fasten every component on the PCB with great detail.
So you can refer to this drawing as the foundation for precise part mounting. And omitting any data can lead to errors and mistakes.
So include the details above to ensure the assembly machine operators program the placement and soldering machine precisely before commencing the process.