In the modern electronics manufacturing industry, PCB design is a critical phase. A well-executed and precise PCB design not only optimizes circuit performance but also reduces production and maintenance costs. To help you design PCBs that are suitable for assembly, this article provides key tips on effective PCB design.

What is PCB Design for Assembly (DFA)?

Before we dive deeper, let’s first define PCB assembly and PCB assembly design.

• PCB assembly is the process of integrating the circuit board with other electronic components (such as connectors, enclosures, heat sinks, etc.) to incorporate it into the final product.
• PCB assembly design involves considering the assembly process early in the design phase to ensure the product is optimized for efficient assembly.

One common issue, which is often faced by newcomers but less frequently by experienced PCB designers, is the failure to fully consider assembly during the initial circuit board design phase. In these cases, designers tend to focus on the PCB itself without fully understanding how the board will be used within the product or application. Overlooking the PCB assembly process can lead to a series of problems.

Although the individual circuit board design may seem acceptable at first glance, certain design decisions can complicate future assembly. For example, placing components too close to each other can cause issues during the assembly process and affect the product’s performance.

Component availability issues: If certain components are out of stock, it could delay the entire manufacturing process.

PCB Assembly Design Tips

1. Pay Attention to Component Spacing

One of the most common problems faced by less experienced PCB layout engineers is improper spacing between components. Placing components too closely together can lead to a variety of issues, requiring redesigns and remanufacturing, which results in time and cost losses.

Experienced PCB layout engineers use several techniques to prevent spacing problems during the assembly process. For instance, when designing the component footprint, they create a place_boundary (in Allegro), which is larger than the physical component outlines. This ensures adequate spacing between components during placement, minimizing potential assembly issues.

PCB layout engineers must ensure that place_boundary shapes for components do not overlap.

Different component types have different spacing requirements. For example, the minimum spacing between discrete components like capacitors and resistors should be at least 10 mils, with 30 mils being the ideal spacing. Below are additional spacing rules:

The values above may vary depending on design constraints and the capabilities of assembly suppliers.

2. Select Components Early in the Design Phase

Experienced engineers choose components early in the design phase to minimize conflicts between the design and the actual assembly process.

By considering component sizes from the beginning, the PCB assembly process can proceed smoothly. Smaller components typically occupy less space, and PCB layout engineers can collaborate with circuit design engineers to explore options for reducing component sizes, thus freeing up space on the circuit board.

3. Separate Leaded and Lead-Free Components During Assembly

It is essential to never mix lead-free components with those that require traditional lead-based soldering. If any component requires lead-free assembly and there are no suitable alternatives for traditional solder, the entire PCB must be processed with lead-free assembly, ensuring all components comply with lead-free requirements. This step is crucial for maintaining solder quality and meeting environmental regulations.

4. Even Placement of Large Components

During the layout phase, aim to distribute large components evenly to achieve uniform heat distribution during the solder reflow process. Additionally, ensure that the assembly contractor customizes a heat profile specifically for each reflow oven used in the assembly process.

5. Choose Appropriate Footprint Sizes

During the layout phase, carefully review the components used in the design. If there is enough space on the PCB and the design does not require smaller components, opting for larger components can be more advantageous for assembly.

For instance, it is often better to use 0402 resistors instead of 0201 resistors. If the design can accommodate 0805 resistors, there’s no need to choose 1206 resistors, which can unnecessarily increase the size of the PCB.

6. Avoid Mixed Manufacturing Processes

Whenever possible, avoid mixing manufacturing processes. The advantages of using a single through-hole component do not outweigh the additional time and costs involved. It’s more efficient to use multiple through-hole components or none at all. If through-hole technology is used, placing all through-hole components on one side of the circuit board will streamline the manufacturing process.

7. Choose Components with Suitable Lead Times

The availability of components is directly linked to the final product’s delivery schedule. Engineers should verify the availability of all components (inventory, lead times, and potential obsolescence) before starting the design. If any components have long lead times, it is wise to place advance orders.

8. Keep the BOM Up to Date During the Design Process

The Bill of Materials (BOM) is a critical document for both design and assembly. If there are issues with the BOM, the assembly facility may need to halt the project until the issues are resolved. To avoid this, review and update the BOM whenever changes are made to the design. When new components are added while the layout is in progress, ensure the BOM is updated with the correct part numbers, descriptions, and component values. Engineers may change components due to long lead times, size constraints, or availability, but they must not forget to update the BOM accordingly. Failure to do so can lead to assembly issues and delays.

Each BOM should be formatted as follows to ensure all components to be assembled on the circuit board are correctly identified. A properly formatted BOM should contain the following columns:

• Item Number
• Reference Designators (separated by commas)
• Quantity of Components
• Manufacturer Name
• Full Manufacturer Part Number
• Full Part Description
• Component Footprint Name

Click to download the Wellcircuits BOM Template

Additional useful information might include:

• Quantity per PCB
• Distributor Name
• Distributor Part Number

9. Carefully Verify the Component Footprint

Component footprints are another vital aspect of layout design. PCB layout engineers must ensure that the footprints are accurately created according to the recommended pad patterns in the datasheets. Properly identifying the correct components and their corresponding pad patterns using the part number key in the datasheet is essential. Misinterpreting datasheet guidelines can lead to incorrect footprints, requiring a complete redesign and remanufacturing of the circuit board.

The diagram below shows a part number key and examples of various footprint types:

For more information on creating footprints in Allegro, please refer to our guide.

10. Ensure All Instructional Markings (Designators, Pin1, Polarity, etc.) Are Present

During assembly, the absence of critical markings—such as Pin1 indicators, component polarity, or orientation—on the silkscreen layer can cause significant issues. Ensure that all essential markings, including designators, Pin1 markings, and polarity indicators, are included in the component