How to Design Through-Hole Pads with Copper Exposure on One Side and Solder Mask Coverage on the Other

In conventional PCB design, through-hole components are generally designed with solderable copper pads exposed on both sides of the circuit board. However, some applications require a through-hole pad to be soldered from only one side while the corresponding pad on the opposite side remains covered by solder mask.

This design approach is commonly used when a component must be soldered from a specific side of the board, while soldering or electrical contact on the opposite side needs to be prevented.

Understanding how to correctly define the solder mask openings in the PCB design files is important for ensuring that the finished board matches the intended assembly process.

For projects that require professional fabrication, you can also review our PCB Manufacturing Services to understand available manufacturing capabilities and production options.

Conventional Through-Hole PCB Pad Design

In a standard through-hole component design, components such as leaded resistors and capacitors may be soldered from either side of the PCB.

The corresponding through-hole PCB pads are therefore normally designed with solder mask openings on both the top and bottom sides. This exposes the copper surface around the plated hole and allows solder to form a reliable connection.

In this configuration:

• The top-side pad is exposed and available for soldering.
• The bottom-side pad is also exposed and available for soldering.
• The plated through-hole provides electrical continuity between the two sides.
• The solder mask is opened around the pad on both surfaces.

This is the conventional approach for through-hole components when soldering from either side is acceptable.

When Should One Side of the Pad Be Covered?

Some PCB designs have a different requirement: the component needs to be soldered from only one side, while the pad on the opposite side should remain covered with solder mask.

For example, the bottom side may be designated as the soldering side, while the corresponding top-side pad needs to be covered.

The purpose of this arrangement is to control the soldering area and prevent unintended solder deposition or contact on the opposite side.

The design can therefore be summarized as follows:

• Soldering side: Solder mask opening with exposed copper.
• Opposite side: Solder mask remains over the copper pad.
• Plated hole: Maintains electrical connection through the PCB.
• Component: Installed according to the specified assembly direction.

Correctly defining these areas in the design files is essential because the solder mask layers determine where the finished PCB will have exposed copper.

For more information about PCB fabrication capabilities, including multilayer, HDI, rigid-flex, impedance-controlled, and other advanced board technologies, see our PCB Capabilities.

How to Design One-Sided Exposed Copper Pads

When creating this type of PCB pad design, the key requirement is to control the solder mask opening independently on the two sides of the board.

The pad itself remains electrically connected through the plated hole, but the solder mask definition determines whether the copper surface is accessible for soldering.

The basic design concept is:

1. Define the Copper Pad

Create the required through-hole pad according to the component footprint and hole dimensions.

The copper pad should provide sufficient annular ring around the drilled hole to meet the PCB manufacturer’s fabrication requirements.

2. Open the Solder Mask on the Soldering Side

On the side where the component will be soldered, the solder mask opening should expose the copper pad.

This allows solder to wet the pad and establish the intended mechanical and electrical connection.

3. Keep the Opposite Side Covered

On the opposite side, the solder mask should remain closed over the pad.

This prevents the copper surface from being directly exposed and helps avoid unwanted soldering or contact during assembly.

4. Verify the PCB Layer Configuration

Before generating the manufacturing files, carefully check the top solder mask and bottom solder mask layers.

A typical configuration is:

This layer relationship should be clearly reflected in the final Gerber data.

For complex boards, professional PCB Design Services can help verify layer definitions, footprints, DFM requirements, and manufacturing data before production.

Designing the Pad in Altium Designer

When using Altium Designer, the footprint and solder mask settings should be configured so that the required side has a solder mask opening while the opposite side remains covered.

The general workflow is:

Step 1: Open the PCB Footprint

Open the required component footprint and locate the through-hole pad.

Step 2: Configure the Pad Properties

Set the pad as a through-hole pad and define the appropriate hole size, pad diameter, and plating requirements.

Step 3: Check Solder Mask Expansion

Review the solder mask settings associated with the pad.

The solder mask opening should exist only on the side where soldering is required.

Step 4: Inspect Both Sides of the Board

Use the PCB layer display to verify that:

• The required side has an opening in the solder mask.
• The opposite side remains covered.
• The copper pad exists on both sides as required by the plated through-hole structure.
• No unintended solder mask openings are generated elsewhere.

Step 5: Generate and Review Manufacturing Files

After completing the layout, generate the Gerber files and inspect the solder mask layers independently.

A visual Gerber review can help identify incorrect openings before the files are sent for PCB manufacturing.

Designing the Same Structure in PADS

The same design principle can be implemented in PADS by controlling the pad stack and solder mask definitions.

The basic process is similar:

Step 1: Open the Component Footprint

Locate the required through-hole component and open its pad or decal properties.

Step 2: Configure the Through-Hole Pad

Set the hole and copper pad dimensions according to the component and manufacturing requirements.

Step 3: Define the Solder Mask

Configure the solder mask opening on the side where soldering will take place.

The opposite side should be configured without a solder mask opening so that the copper remains covered.

Step 4: Verify the Pad Stack

Check the top and bottom pad definitions carefully.

The final pad stack should reflect the intended manufacturing structure rather than simply using the default through-hole footprint settings.

Step 5: Review the Generated Gerber Files

After exporting the production files, inspect the top and bottom solder mask layers to confirm that only the intended side is exposed.

Important Considerations Before PCB Manufacturing

Although this design is relatively simple, several details should be checked before ordering the board.

Confirm the Assembly Direction

Clearly determine which side of the PCB will be used for soldering.

If the assembly process requires soldering from the bottom side, the bottom solder mask should be opened while the top side remains covered.

Check the Solder Mask Layers

Do not rely only on the PCB editor’s 3D appearance.

Always verify the actual solder mask layers in the manufacturing output because incorrect layer settings can result in an unintended exposed pad.

Verify Pad and Hole Dimensions

The hole diameter, copper pad diameter, annular ring, and solder mask opening should all satisfy the selected manufacturer’s capabilities.

Consider Soldering Requirements

The soldering method can influence the final pad design. Hand soldering, wave soldering, selective soldering, and other assembly processes may have different requirements.

For projects requiring complete fabrication and assembly, our PCB Assembly Services support SMT, through-hole, mixed-technology, and other PCB assembly requirements.

Review DFM Requirements

A design that looks correct electrically may still create manufacturing problems if the pad geometry, solder mask clearance, hole size, or copper-to-edge spacing falls outside production capabilities.

A DFM review before fabrication can help identify these issues early.

Why Correct Solder Mask Design Matters

The solder mask is more than a protective coating. Its opening pattern directly affects which copper surfaces are available for soldering.

Incorrect solder mask settings can lead to:

• Unwanted soldering on the opposite side.
• Insufficient solderable copper area.
• Accidental electrical contact.
• Solder bridges or solder accumulation.
• Assembly difficulties.
• Differences between the intended PCB design and the finished board.

Therefore, solder mask configuration should be treated as an important part of the PCB manufacturing data rather than as a cosmetic layer.

Final Design Checklist

Before submitting your PCB files for production, verify the following:

• The through-hole pad dimensions match the component requirements.
• The plated hole size is correct.
• The soldering side has the required solder mask opening.
• The opposite side remains covered by solder mask.
• The top and bottom solder mask layers have been checked independently.
• The Gerber files correctly reflect the intended pad configuration.
• The design complies with the manufacturer’s manufacturing capabilities.
• The assembly method has been considered.
• DFM requirements have been reviewed.

For additional production support, you can contact our PCB manufacturing team with your Gerber files, BOM, drawings, and technical requirements for project evaluation.

Conclusion

Designing a through-hole pad with copper exposed on one side and solder mask coverage on the other is primarily a matter of correctly controlling the solder mask layers in the PCB footprint and manufacturing files.

The copper pad and plated hole can maintain the required electrical connection, while selective solder mask openings determine which side is available for soldering.

Whether you use Altium Designer, PADS, or another EDA platform, always verify the complete pad stack and inspect the generated Gerber files before production. Proper PCB design, accurate solder mask definition, and compatibility with the manufacturer’s process are essential for achieving reliable assembly results.

If your project requires both fabrication and assembly, GOPCBA provides integrated PCB manufacturing, component sourcing, PCB assembly, testing, and related electronic manufacturing services. You can also review our PCBA Capabilities to understand available assembly technologies and production capabilities.