What is a Solder Bridge?

A solder bridge is an unintended connection of solder between two or more pads or pins on a printed circuit board (PCB) or electronic component. Solder bridges are a common defect that can occur during manual soldering, wave soldering, or reflow soldering processes. They create short circuits which can prevent the circuit from functioning properly or even damage components.

Solder bridges form when excess molten solder flows and connects adjacent pads/pins that are not meant to be connected. Several factors can contribute to solder bridge formation:

• Too much solder applied
• Pads/pins spaced too closely together
• Improper soldering temperature
• Incorrect soldering technique
• Excessive flux
• Component misalignment

Solder bridges can be difficult to spot visually, especially on circuit boards with high component density and small pin pitches. Magnification and good lighting are often needed to identify bridging. Solder bridges can also be detected using continuity testing, x-ray inspection, and optical/visual inspection systems.

Risks and Impact of Solder Bridges

Solder bridges create unwanted short circuits between pins or pads. This can cause a variety of circuit problems depending on what gets erroneously connected:

In some cases, a solder bridge may have no impact if the two pins are meant to be connected anyway, like two ground pins. However, most unintended solder bridges will negatively affect the circuit in some way.

Potential consequences of solder bridges include:

• Device won’t power on
• Erratic/glitchy behavior
• Overheating and component damage
• Excessive power consumption
• Reduced reliability
• Safety hazards (shock, fire)

Even if a solder bridge doesn’t cause an immediate hard failure, it can still degrade performance and lead to intermittent problems or premature failure over time. Latent solder bridge defects that pass initial testing may manifest as early field failures.

Common Locations for Solder Bridges

While a solder bridge can theoretically form between any two adjacent pads given the right conditions, certain types of components and packages are more prone to solder bridging.

Surface Mount Components

Surface mount technology (SMT) components are soldered to the surface of the PCB. SMT parts are more susceptible to solder bridging compared to through-hole parts due to their smaller size, finer pin pitches, and lack of drilled holes to contain the solder.

Common SMT packages prone to solder bridges:

• Quad Flat Pack (QFP)
• Small Outline Package (SOP)
• Ball Grid Array (BGA)
• Land Grid Array (LGA)

These SMT packages have high pin density and tight pin spacing, making it easier for solder to bridge adjacent pins. Without proper process control, solder bridges are a frequent problem when assembling SMT PCBs.

Pin Headers

Another common spot for solder bridges is on pin headers and connectors. Pin headers typically have rows of male pins with 0.1″ pitch. When Soldering Wires or connectors to these closely-spaced header pins, it’s easy to accidentally create solder bridges between adjacent pins.

IC Chips

Integrated circuit (IC) chips in DIP, QFP, SOP and other packages have rows of pins that are close together. Solder bridges between IC pins are a risk, especially when hand soldering. Even if a solder bridge between two IC pins doesn’t cause a hard short, it may still interfere with the chip’s operation.

Preventing Solder Bridges

Preventing solder bridges is easier than fixing them. Following good design practices and proper soldering techniques goes a long way in minimizing solder bridging.

PCB Design Considerations

• Maximize pad/pin spacing: Increase the space between SMT pads and through-hole pins as much as possible to reduce bridging risk. Avoid squeezing parts too close together.

• Optimize pad size and shape: Pads should be the right size for the component pin – not too big or small. Certain pad shapes like tear-drop can help with bridging.

• Solder mask between pads: Include solder mask dams between pads to prevent solder bridges. Solder mask acts as a barrier to contain solder flow.

• Thermal relief connections: For large ground/power planes, use thermal relief connections instead of solid connections. Thermal reliefs have a smaller cross section which reduces solder wicking.

Soldering Process Control

• Use appropriate amount of solder: Apply the right amount of solder for the joint size. Excess solder increases chance of bridging.

• Maintain proper iron temperature: Soldering iron temp should be hot enough to quickly melt solder but not so hot that it causes bridging or component damage. Typical temp range is 600-750°F.

• Select compatible soldering tools: Make sure the soldering iron tip is the right size and shape for the pads. Tweezers and magnification also help with SMT soldering.

• Control flux amount: Use enough flux to promote good soldering but avoid applying too much. Excessive flux can contribute to solder bridging.

• Confirm part alignment: Double check component positioning before soldering. Misaligned parts are more likely to have bridges.

• Automated soldering processes: For volume production, wave soldering or reflow ovens with controlled profiles can yield more consistent results than hand soldering.

Detecting and Locating Solder Bridges

Despite best efforts to prevent them, solder bridges can still happen from time to time. Quickly detecting and locating solder bridges is important for timely rework.

Common methods to find solder bridges include:

Visual Inspection

Careful visual examination of the solder joints, ideally under magnification, can reveal some solder bridges. Look for excessive solder, joints that are touching, or irregular solder shapes/fillets.

However, visual inspection is limited in its effectiveness. Very small bridges or ones hidden under components may be missed. Solder bridges don’t always look like obvious blobs – a thin strand of solder can be hard to see.

Continuity Testing

Checking continuity (electrical connection) between pins/pads that shouldn’t be connected is an effective way to find solder bridges. A multimeter set to continuity or resistance mode is used to probe between suspect pins.

Zero or very low resistance indicates a solder bridge short circuit. Pinpointing the exact location can be tricky though, especially if multiple pins are involved in the bridge. Selective continuity testing between all adjacent pins can help isolate the bridged ones.

Automated Optical/Visual Inspection

For production PCB assembly, automated optical inspection (AOI) or visual inspection systems are commonly used for solder bridge detection. These machines have high resolution cameras and advanced computer vision algorithms that can spot defects like bridges.

AOI is fast, repeatable, and doesn’t require physical contact with the board. The inspection program can be tailored to check for certain solder bridge conditions. However, AOI has some limitations – it can only inspect visible areas and may have trouble with reflective surfaces or tall components that obscure the view.

X-Ray Inspection

For hidden solder joints like those in BGA packages, x-ray inspection is used to check for solder bridges. X-rays can image the solder joints under the package to show any bridging between balls.

X-ray inspection requires special equipment which can be costly. It’s typically used selectively to check suspected problem areas rather than inspecting the whole board. Like AOI, x-ray inspection is non-contact and good for production testing.

Fixing Solder Bridges

Once a solder bridge has been identified, it needs to be carefully removed to eliminate the short circuit. The right tools and techniques are important to avoid damaging the PCB or components.

Solder Wick

Solder wick, also known as desoldering braid, is a copper braid used to remove excess solder. To remove a solder bridge:

1. Place the solder wick over the bridge
2. Press the hot soldering iron tip onto the wick
3. The solder will melt and wick up into the braid
4. Remove the iron and let the solder solidify
5. Lift away the wick and excess solder
6. Repeat if necessary to remove any remaining bridge

Solder wick works well for small bridges in accessible areas. It can be tricky to remove a bridge without disturbing adjacent solder joints though. Using flux can help the solder wick up better.

Solder Sucker

A solder sucker is a vacuum tool used to suck up molten solder. To clear a solder bridge with a solder sucker:

1. Heat up the bridged joint with the iron
2. Once the solder melts, quickly place the solder sucker nozzle close to the joint
3. Activate the vacuum pump to suck up the molten solder
4. Remove the iron and let the joint cool
5. Repeat if needed to fully clear the bridge

Solder suckers can remove bigger solder blobs but require more manual dexterity to use effectively compared to wick. There’s also a risk of lifting pads if too much force is applied.

Hot Air Rework

For larger bridges or ones in hard to reach areas, a hot air rework station can be used. Hot air rework uses a focused stream of heated air to melt and disperse the solder bridge.

1. Direct the hot air nozzle at the solder bridge
2. Melt the bridged solder
3. Use tweezers to gently separate the bridged leads
4. Remove the heat and let cool

Hot air rework gives more control than an iron for tricky bridges. However, the heat can potentially damage nearby parts so it must be applied carefully. Removing all flux residue is also important.

After removing the solder bridge, always visually inspect the joint to confirm complete bridge removal. Re-test continuity to verify the short is gone.

FAQ

How much solder should I apply to avoid bridges?

Use only as much solder as needed to make a good joint – typically a small bead slightly larger than the lead diameter. A joint that looks like a volcano or ball likely has too much solder. Applying the right amount of solder takes practice.

Can you prevent solder bridges by using less flux?

No, using less flux will not necessarily prevent solder bridges. In fact, using too little flux can actually make bridges more likely by causing poor solder flow. Flux helps the solder flow properly. The key is applying the right amount of flux and cleaning any residue.

What’s a safe pin-to-pin resistance value that indicates no bridging?

For general continuity testing, a resistance value above 1k ohm between pins typically means no solder bridge. However, the exact value depends on the circuit – some pins may have lower resistance by design. It’s best to compare measurements to a known good board if possible.

Can solder bridges form over time or only during soldering?

Most solder bridges happen during the initial soldering process. However, certain conditions like physical shock, vibration, or overheating could potentially cause a bridge to form later if there is excess solder present and pads are close together. Proper initial soldering and underfill epoxy minimize this risk.

How can you tell if a solder bridge damaged a component?

Solder bridges that short power to ground can overheat and burn out components. Visible signs of damage include discoloration, charring, and cracks. Electrically test the component to check for proper function. An unintended solder bridge may have damaged the part even if it looks OK.