What is an open on a PCB?

Types of PCB Opens

There are several types of opens that can occur on a PCB:

Trace Open

A trace open is a break in the copper trace that connects components on the PCB. This can be caused by:

• Manufacturing defects like overetching or underplating
• Physical damage to the trace
• Thermal stress during soldering
• Electromigration over time

Via Open

A via open occurs when the plated hole used to connect traces on different layers of the PCB fails to provide electrical continuity. Causes include:

• Insufficient plating in the via barrel
• Voids or cracks in the plating
• Mechanical stress from thermal expansion

Pad Open

A pad open happens when there is no electrical connection between a component pad and its corresponding trace. This can result from:

• Poor solder wetting during component attachment
• Trace lifting off the pad due to adhesion issues
• Mechanical stress on the solder joint

Inner Layer Open

An inner layer open is a break in a trace located on one of the internal layers of a multi-layer PCB. These are often caused by manufacturing defects and can be difficult to detect and repair.

Impact of PCB Opens

The impact of a PCB open depends on the function of the affected connection in the circuit. Some potential consequences include:

• Total loss of device functionality if the open is in a critical path
• Reduced performance or intermittent issues
• Overheating of components due to current crowding through alternate paths
• Electromagnetic interference (EMI) from unintended antenna structures

In safety-critical applications like medical, automotive, or aerospace electronics, a PCB open can have catastrophic consequences. Thorough testing and inspection are essential to ensure reliability.

Preventing PCB Opens in Design

Careful PCB Design practices can help minimize the risk of opens:

Trace Width and Spacing

Use appropriate trace widths for the expected current load to prevent overheating and electromigration. Maintain adequate spacing between traces to avoid shorts.

Via Design

Size vias appropriately for the hole diameter and plating thickness to ensure reliable interconnects between layers. Avoid placing vias in high-stress areas like under large components.

Pad Design

Design component pads with sufficient size and soldermask relief to promote good solder wetting. Use teardrops or fillets to reinforce the trace-to-pad connection.

Material Selection

Choose PCB Materials with good mechanical and thermal properties to withstand the stresses of manufacturing and operation. Consider high-temp or low-CTE materials for demanding environments.

Detecting PCB Opens in Manufacturing

Various inspection and testing methods are used to detect PCB opens during manufacturing:

Visual Inspection

Manual visual inspection can spot obvious opens like broken traces or damaged pads. Automated optical inspection (AOI) uses cameras and image analysis to detect visible defects.

Electrical Testing

Flying Probe testing checks electrical continuity by probing individual points on the PCB. In-circuit testing (ICT) verifies the connectivity and functionality of the assembled board.

X-ray Inspection

X-ray inspection can reveal opens in vias, BGA solder joints, and inner layers that are not visible from the surface.

Boundary Scan

Boundary scan testing uses built-in test circuitry (JTAG) to test connectivity and isolate faults on the PCB.

Repairing PCB Opens

The approach to repairing a PCB open depends on the type and location of the defect:

Trace Repair

Small trace opens can sometimes be bridged with conductive ink or epoxy. For longer opens, a wire jumper may be soldered in place.

Via Repair

Opens in via barrels can be repaired by filling the via with conductive epoxy or by drilling out the via and replacing it with a solid wire rivet.

Pad Repair

Lifted or damaged pads can be replaced by carefully cutting away the old pad and gluing down a replacement pad. The new pad is then soldered to the trace.

Board-Level Rework

For opens in BGA packages or other hard-to-access areas, specialized rework equipment may be needed to remove and replace the affected component.

In some cases, a PCB open may be unrepairable and require scrapping the board. The cost and feasibility of repair should be weighed against the cost of replacement.

FAQ

What is the difference between an open and a short on a PCB?

An open is a lack of connection between two points that should be electrically connected, while a short is an unintended connection between two points that should not be connected.

Can PCB opens be prevented entirely?

While careful design and manufacturing practices can minimize the risk of opens, it is not possible to completely eliminate the possibility of defects. Testing and inspection are important for catching opens before they cause problems in the final product.

How much does it cost to repair a PCB open?

The cost of repairing a PCB open varies widely depending on the type of defect, the location on the board, and the rework equipment and skill required. In some cases, it may be more economical to scrap the board and start over.

Can PCB opens develop over time in a finished product?

Yes, opens can develop over the lifetime of a PCB due to factors like mechanical stress, thermal cycling, or electromigration. Proper design and material selection can help mitigate these risks.

What is the most common cause of PCB opens?

Manufacturing defects are the most common cause of PCB opens, particularly in the etching and plating processes. However, design issues and physical damage during assembly or use can also lead to opens.

Conclusion

PCB opens are a common and potentially serious issue in Electronic Manufacturing. By understanding the types, causes, and impacts of opens, PCB designers and manufacturers can take steps to prevent and detect these defects. Careful design, material selection, and testing are essential for producing reliable PCBs that will perform as intended over their intended lifetime.

While it is impossible to totally eliminate the risk of opens, following industry best practices and staying up to date with the latest technologies and techniques can help minimize their occurrence and impact. When opens do occur, having a well-defined repair strategy can help salvage affected boards and reduce waste.

As PCB technology continues to advance, with ever-finer trace widths and denser component packaging, the challenge of preventing and detecting opens will only become more critical. Investing in robust design, manufacturing, and testing processes will be essential for companies that want to stay competitive in the fast-moving world of electronics.

Here is a table summarizing the types of PCB opens:

Type of Open	Location	Common Causes
Trace Open	Copper trace on surface	Overetching, physical damage, thermal stress
Via Open	Plated hole between layers	Poor plating, voids, mechanical stress
Pad Open	Component pad on surface	Poor solder wetting, trace lifting
Inner Layer Open	Trace on internal layer	Manufacturing defects