PCB Design Considerations for Assembly

Board Size and Panelization

PCB size impacts many aspects of the design and assembly process. Smaller boards are generally cheaper and easier to assemble. However, very small boards can present challenges with limited space for components and traces. Very large boards are more expensive and may encounter issues fitting into standard assembly equipment.

Typical PCB sizes range from 0.5″ x 0.5″ up to around 16″ x 20″. For high volume production, multiple PCBs are often grouped into a panel that goes through assembly. Panelization allows assembling multiple boards at once to save time and cost. The panel is then depanelized into individual boards. Fiducial marks and breakaway tabs are used to facilitate this process.

Component Selection and Placement

The types of components and where they are placed on the board have a big impact on assembly:

• Use standard, readily available components whenever possible. Custom or obsolete parts increase cost and lead time.
• Avoid components that are very small, large, or have fine pitch pins which are more difficult to place and solder.
• Provide sufficient clearance between components for the assembly equipment to access each part. Avoid placing parts too close to board edges.
• Orient components in the same direction where feasible. This allows faster placement in one pass.
• Group similar components together to optimize placement programming.
• Place sensitive components away from board edges and provide shielding if needed for protection.

Component	Placement Guidelines
Chip resistors, capacitors	Min 0.040″ between, 0.020″ from board edge
SOICs, QFPs	Min 0.020″ between, 0.040″ from board edge
BGAs	Min 0.080″ between, 0.125″ from board edge
Connectors	Min 0.100″ from board edge, aligned to board edge

SMT vs Through-Hole

Surface mount technology (SMT) components are mounted directly onto the surface of the PCB, while through-hole parts have leads that are inserted into holes and soldered on the opposite side.

SMT is the most common assembly technology today. SMT components are smaller, cheaper, and can be placed very quickly by machine. Whenever feasible, use SMT parts over through-hole.

However, through-hole is still used for parts like large connectors or components subject to mechanical stress. If using through-hole parts, pay attention to lead lengths and hole sizes. Avoid using both SMT and through-hole parts on the same side of the board.

Solder Mask and Silkscreen

Solder mask is the coating applied over bare copper on the PCB, while silkscreen is used to print component outlines and labels.

For solder mask:
– Ensure sufficient spacing between exposed copper pads and traces
– Avoid solder mask in areas that will contact connector pins or require soldering
– Specify mask color (green is standard, other colors like red, blue, black are possible)

For silkscreen:
– Provide component outlines to aid assembly and rework
– Label key components, connectors, test points, and polarity indicators
– Ensure labels are legible at the fabricated size (typically minimum 15 mil text height)
– Avoid silkscreen over pads or holes

Vias and Mounting Holes

Vias are used to route signals between layers of the PCB. For assembly:

• Avoid vias under BGA pads which can cause soldering defects
• Plug or tent vias under components to prevent solder wicking
• Cap larger vias to prevent vacuum nozzles from picking up the board

Mounting holes are used to attach the PCB to an enclosure or chassis. Typical mounting hole size is 0.125″ diameter. Provide sufficient clearance around holes for screws and standoffs. Avoid routing traces between mounting holes.

PCB Layout Guidelines for Assembly

Trace Routing and Spacing

The width and spacing of copper traces on the PCB impact assembly:

• Provide sufficient trace width for current carrying capacity and manufactureability
• Ensure minimum trace spacing for voltage clearance and to prevent solder bridging
• Avoid acute angles in traces which can cause acid traps in manufacturing
• Provide ground planes or copper pours to improve current return paths and EMI

Typical minimum trace width and spacing is 0.006″ for inner layers and 0.008″ for outer layers.

Trace	Minimum Width/Spacing
Signal	0.006″ internal, 0.008″ external
Power/Ground	0.020″ internal, 0.025″ external
High voltage (>50V)	0.025″ spacing

Copper Balancing and Panelization

The amount of copper on each layer of the PCB should be balanced to prevent warpage during manufacturing. Warpage can cause assembly issues like poor soldering or inability to pick up components. Use copper pours or teardrops to balance copper coverage across layers.

For panelization:
– Route boards at least 0.060″ apart to allow space for depanelization
– Provide breakaway tabs or mouse bites to facilitate depanelizing individual boards
– Include fiducial marks for assembly equipment to register board position

Thermal Management

High power components like FETs, voltage regulators, and processors generate significant heat which must be dissipated to avoid damage. For thermal management:

• Provide wide traces or copper pours connected to thermal pads to spread and sink heat
• Use large SMT pads or vias to conduct heat to inner ground plane layers
• Consider thermal interface material between the PCB and heat generating components
• Specify high temperature solder mask and silkscreen if board will operate at elevated temperature

Design for Test

Including design for test (DFT) features in your PCB makes it easier to verify assembly and diagnose issues:

• Provide test pads or vias connected to key signals that can be probed
• Include clear silkscreen labels for test points and signal names
• Locate test points to be easily accessible for probing
• Consider built-in self test firmware or boundaryscan/JTAG interfaces for more complex boards

Documentation and Manufacturing Preparation

Bill of Materials and Centroid File

Creating a complete, accurate bill of materials (BOM) is critical for successful PCB assembly. The BOM lists all the components to be placed on the board along with quantities, manufacturer part numbers, and reference designators.

Avoid issues by:
– Carefully checking BOM against schematic for missing or incorrect parts
– Ensuring quantities and part numbers exactly match the components to be used
– Specifying alternates in case primary parts are unavailable

In addition to the BOM, a centroid (XY) file specifies the coordinate location of each component on the board. This is used to program the pick-and-place assembly equipment. Centroid data must perfectly match the physical board and components. Errors in rotation, mirroring, or offset can cause misplaced parts.

PCB Layer Stackup

The layer stackup defines the arrangement and thicknesses of the copper and insulating layers in a multi-layer PCB. Specifying an appropriate stackup is important for:

• Controlling impedance of critical high speed traces
• Providing shielding or crosstalk isolation between layers
• Ensuring sufficient copper weight for current carrying capacity
• Meeting total board thickness targets

A typical 4 layer stackup might be:

Layer	Cu Weight	Thickness
Top	1 oz	1.4 mil
Gnd plane	0.5 oz	0.7 mil
Power plane	0.5 oz	0.7 mil
Bottom	1 oz	1.4 mil
Total		62 mil

Mechanical Considerations

Besides the PCB itself, mechanical considerations are important for assembly:

• Provide 3D models of custom mechanical components like enclosures, heat sinks, or displays
• Specify torque and assembly sequence requirements for connectors, screws, or press fit parts
• Call out any labels, stampings, or serialization to be applied
• Ensure PCB mounting and mating features align properly with mechanical components

Design Review and Signoff

Before sending design files for production, perform a thorough design review:

• Double check schematic, BOM, and layout against product requirements
• Verify components are available and alternatives specified if needed
• Confirm all design rules and guidelines have been followed
• Review manufacturing drawings and documentation for completeness
• Obtain final signoff on design package from all stakeholders

Taking the time up front to properly review and approve design files prevents expensive and time consuming revisions later.

Frequently Asked Questions

What are the most common PCB design issues that impact assembly?

Some of the most frequent problems that can disrupt assembly include:

• Incorrect or missing BOM information
• Components not properly spaced or oriented for placement equipment
• Silkscreen labels not legible or missing polarity indicators
• Traces and pads not sized appropriately for current and voltage
• Inadequate thermal relief or heat sinking for power components

How small of a pitch can SMT handle?

Standard SMT equipment can handle component pitches down to about 0.020″. For finer pitches down to 0.012″, more advanced equipment and processes are required. Most PCB designs can be accomplished without ultra-fine pitch components. If very fine pitch parts are required, consult with your manufacturer on their specific capabilities.

What’s the best way to panelize PCBs for assembly?

The two main options for panelizing are tab routing and V-scoring. With tab routing, a 0.060″ gap is routed around each board perimeter, leaving small tabs intact to hold the boards together. The tabs are then broken to separate individual boards after assembly. V-scoring uses a special V-shaped router bit to cut part way through the panel so boards can be easily snapped apart.

Tab routing is the most common and economical approach. It works for most standard FR-4 PCBs. V-scoring provides a cleaner edge and is better for thicker PCBs over 0.093″, but is more expensive. If tight tolerances are required on board edges after depanelization, specify these up front so the right method can be used.

Do I need to include fiducial marks on my PCB?

Fiducial marks are reference points on the PCB used by pick-and-place equipment to align the board for component placement. Using fiducials (typically three round copper pads in an “L” pattern) improves placement accuracy and speed, especially for smaller components.

For most PCBs, fiducials are recommended as cheap insurance for consistent assembly results. Smaller boards under 2″ x 2″ can often be assembled accurately without fiducials using just tooling holes for alignment. Larger, more complex boards benefit the most from fiducial marks. Confer with your assembly partner on their preferred fiducial mark sizes and locations.

How do I verify my design is ready for assembly?

Doing a design for manufacturability (DFM) check is the best way to ensure your PCB is optimized for assembly. Many CAD tools include automated DFM checks that will flag issues like insufficient pad sizes, spacing violations, or drill problems.

Carefully reviewing the design against the guidelines covered in this article will also help identify any oversights. Having an experienced PCB designer double check your work provides an added level of confidence.

When in doubt, contact your PCB manufacturing and assembly partner early in the design process. They can advise you on any special requirements and do a final manufacturability review before production. Taking the time to get your design right up front avoids surprises and delays down the line.