The solder mask clearance in PCB refers to the space intentionally left between the edge of a copper pad and the edge of the solder mask layer. This clearance is crucial for ensuring proper soldering and preventing issues like solder bridging.
Understanding Solder Mask Clearance
Solder mask clearance, sometimes referred to as mask swell, is a designed gap. It prevents the solder mask from overlapping onto the copper pads. This prevents solder from being wicked away onto the mask during reflow soldering. This gap is needed for several reasons, including:
- Preventing Solder Mask on Pads: The solder mask should not cover the pads where components are to be soldered. This ensures that solder makes direct contact with the copper.
- Accommodating Manufacturing Tolerances: There are inevitable tolerances in the PCB manufacturing process. The solder mask application might shift slightly during fabrication. The clearance accounts for this variation, reducing the likelihood of the mask encroaching on pads.
- Facilitating Soldering: Adequate clearance provides space for the solder to adhere correctly to the pads and to form a proper solder joint.
Standard Solder Mask Clearance
According to the provided reference, standard solder mask clearance is approximately 2 mil (0.002 inches). This means the solder mask is typically printed with a 2 mil gap away from the copper pads. There is also a tolerance of +/- 2 mil, or 0.05 mm, on the placement of the mask in relation to the copper.
Key Points
- Default Value: The default solder mask aperture size is adjusted to provide a ~2mil clearance from the copper pad edges.
- Tolerance: There is a manufacturing tolerance of +/- 2 mil (0.05mm) on the mask’s alignment relative to the copper pads.
- Purpose: The gap prevents the solder mask from interfering with the soldering process by ensuring the solder adheres correctly to the copper pads.
Importance of Proper Clearance
Maintaining the proper solder mask clearance is vital for a number of reasons:
- Reduced Solder Bridging: Sufficient clearance reduces the chance of solder flowing from one pad to another, which can cause electrical shorts.
- Improved Soldering Quality: Proper clearance facilitates the formation of strong, reliable solder joints.
- Enhanced Manufacturing Yield: It decreases the number of defects during PCB assembly and increases the overall manufacturing yield.
Example
Here’s an example of a pad and its corresponding solder mask aperture:
| Feature | Size |
|---|---|
| Copper Pad Edge | Reference |
| Solder Mask Edge | +2 mil |
| Tolerance | +/-2 mil |
In this example, the solder mask edge will be placed 2 mils away from the edge of the copper pad. This small gap allows solder to adhere properly to the pad without being hindered by the mask. The tolerance means that during manufacturing, the mask may shift slightly relative to the pad by a maximum of 2 mils.
