The function of a solder bridge.
How a solder bridge forms.
Precautions and tips for preventing solder bridges.
Directly above the soldering iron is a solder bridge. Notice the presence of solder between the two pads.
What Is a Solder Bridge? Here Is What You Need to Know
If you are wondering what a solder bridge is–look no further, we have the answer! A solder bridge is a connection of two or more pads by an excessive application of solder that creates a “bridge” between them. The solder bridge is on top of the PCB solder mask, resulting in the pads becoming inadvertently electrically and thermally connected.
While other soldering issues can often be visibly identified (for example, tombstoning), microscopic solder bridges might be harder to catch. Undetected solder bridges may result in a short circuit, a trace or component burning up, or general circuit malfunctioning, depending on the two connected nodes and their design. That being said, some designers may use deliberate solder bridges to make or break temporary circuit options with closely spaced pads.
How a Solder Bridge Forms
In general, it’s the job of the solder mask layer to keep solder from excessively being applied. Solder-resist coating is added to the PCB during manufacturing to protect areas of the PCB that should not have solder. For example, the space between two pins of a TSOP package-type IC may have a solder mask applied so that when the solder is applied, it’ll only be applied to the pads.
As pads are separated by a non-conducting solder mask gap in the manufacturing and assembly process, the leads are soldered to the pads and a sufficient amount of solder forms a filet on the pad. However, occasionally, an excess of solder will bridge the gap of the copper, connecting the two pads.
When solder reflows, it has a lot of surface tension, causing the solder to ball up and hold onto things. Anything that disrupts this tendency can lead to bridging, including:
- Bad solder paste stencils
- Insufficient placement registration
- Messy assembly lines
- Clumsy rework technicians
Other elements that may lead to solder bridges include too much solder on SMT pads, a bad seal between the stencil and the board during the printing process, or solder pads that are too big relative to the gaps between them. Furthermore, imprecisely placed components or too great of a pad size-to-component lead ratio may result in excess solder applied and a bridge forming.
The underside of a PCB with soldered through-hole components. Notice the small gap between the two pads nearest to the soldering iron—no solder bridge!
Preventing Solder Bridges
There is no method to guarantee a circuit board without solder bridges, however, the precautions below can help reduce the risk of their formation.
- Use correct lead lengths for through-hole parts. Through-hole components with too long of a lead can result in solder bridges. The correct length depends on PCB thickness, size of components, and type of soldering application (selective, wave, etc.). Consider contacting your assembly house for any predefined specifications.
- Utilize the correct hole size and pad diameter for through-hole parts. As we’ve discussed, surface pads that are too large or even annular-ring-sized decrease the gap between conductors. Utilizing datasheets to ensure correct through-plated holes and pads is essential.
- Ensure proper solder mask application. Solder inherently doesn’t want to adhere to the solder mask, and should therefore be applied everywhere except around the leads of components. Not having a solder mask, especially between pins and small gaps, will create solder-bridge risks.
- Utilize fiducials on PCBs. Fiducials are precise marks used by automated machines to align placements for every part of your board. When done correctly, machines will be able to more easily align and accurately place components. The IPC recommends two marks in opposite corners and a third in another. Inaccurately placing these will result in a higher risk of misplaced parts.
- Work with reliable PCB assembly houses. Most of the causes of solder bridges occur in the assembly process, so working with a manufacturer who knows what they’re doing is essential. Having an open line of communication for questions and understanding the assembly process will allow for better designs.
- Verify your solder mask relief. This relief is essentially the negative of the solder mask and is usually an outline surrounding SMD, through-hole pads, test points, vias, and any other conducting materials. Many PCB design applications will let you manually inspect this area.
- Use solder mask-defined pads for dense areas. There may be times when there isn’t space for a full solder mask dam (both a barrier and mask relief) between pads, such as when using BGA or LGA components. In this case, use a solder mask-defined pad that has a solder mask relief the same size as the copper pad it protects. If you do use this, it is recommended to leave a note to the manufacturer to not apply standard mask clearances.
- Verify the solder reflow profile, as liquid solder tends to flow towards areas of higher temperature. Analyzing and modifying the solder profile can help reduce undesired solder accumulation in specific parts.
- Other small tips include performing a design rule check to identify where solder mask application can be minimized, maintaining a minimum of 2 mils of a solder resist dam between pads, cleaning the stencil after use, and ensuring you do not mix old and new flux when preparing solder paste.
Now that you know what a solder bridge is, consider utilizing an advanced PCB editor such as Cadence’s OrCAD PCB designer to perform design rule checks and verify the appropriate safe distance between pads, reducing solder bridge formation.
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