Defect 1: Solder Bridges

What are solder bridges?

Solder bridges are unwanted connections of solder between two or more conductors or component leads that should not be connected. They create short circuits that can prevent the PCBA from functioning properly.

Causes of solder bridges

Some common causes of solder bridges include:

• Excessive solder paste
• Incorrect stencil design or placement
• Component misalignment
• Tombstoning (vertical chips)
• Insufficient solder mask between pads

Impact of solder bridges

Solder bridges can have the following negative impacts:

• Short circuits
• Incorrect PCB operation
• Overheating and damage
• Rework and scrap
• Reliability issues in the field

Solutions for solder bridges

To prevent or fix solder bridges, try these solutions:

• Optimize solder paste volume
• Verify stencil design and setup
• Use SPI to inspect solder paste deposits
• Ensure precise component placement
• Increase solder mask width between pads
• Use visual inspection and ICT to detect bridges
• Remove bridges manually or with a solder sucker

Defect 2: Cold Solder Joints

What are cold solder joints?

Cold solder joints are weak, unreliable connections caused by improper wetting of the solder to the pad and component surfaces. They often look dull, porous, or cracked compared to good shiny, smooth joints.

Causes of cold solder joints

Cold solder joints can be caused by:

• Insufficient heat during soldering
• Contaminated/oxidized surfaces
• Incorrect solder paste composition
• Inadequate flux activity
• Vibration during cooling

Impact of cold solder joints

The potential impacts of cold solder joints include:

• High electrical resistance
• Intermittent connections
• Cracks and mechanical weakness
• Premature joint failure
• Inconsistent PCB performance

Solutions for cold solder joints

To avoid or correct cold solder joints, consider these options:

• Optimize reflow profile parameters
• Verify solder paste quality and handling
• Ensure proper surface cleanliness
• Increase flux activity if needed
• Minimize movement until joints solidify
• Use x-ray inspection for hidden joints
• Rework or replace affected joints

Defect 3: Component Shift/Rotation

What is component shift/rotation?

Component shifting or rotation occurs when parts move out of their intended positions during placement or reflow. The result is misaligned components that may have poor solder joints or interfere with other parts.

Causes of component shift/rotation

Factors that can contribute to component shift/rotation are:

• Uneven or excessive solder paste
• Incorrect pick and place settings
• Lack of adhesive or part anchoring
• Bumping or vibration of PCBs
• Rapid heating or outgassing

Impact of component shift/rotation

Shifted or rotated components can lead to:

• Solder bridges or opens
• Mechanical interference
• Visual defects
• Nonfunctional circuits
• Difficulty in rework

Solutions for component shift/rotation

Steps to prevent component shift/rotation include:

• Control solder paste volume and placement
• Optimize pick and place parameters
• Use adhesive dots for large/heavy parts
• Reduce PCB handling and movement
• Adjust reflow profile if needed
• Utilize machine vision for inspection
• Rework or replace problem components

Defect 4: Tombstoning

What is tombstoning?

Tombstoning, also known as drawbridging or Manhattan effect, happens when small chip components stand up on one end during reflow, looking like tombstones. This leads to one pad having no solder connection.

Causes of tombstoning

The primary causes of tombstoning are:

• Unequal solder wetting forces
• Imbalanced solder paste volumes
• Misaligned component placement
• Fast temperature ramp rates
• Long, narrow component geometries

Impact of tombstoning

Tombstoned parts result in:

• Open circuits on one side
• Unreliable or nonfunctional PCBAs
• Visual defects
• Rework challenges
• Scrap and yield loss

Solutions for tombstoning

To combat tombstoning, try these techniques:

• Equalize pad sizes and solder volume
• Center components on pads
• Optimize pick and place process
• Slow down temperature ramp rates
• Use components with better aspect ratios
• Visually inspect after reflow
• Replace tombstoned parts and hand solder

Defect 5: Insufficient Solder

What is insufficient solder?

Insufficient solder refers to solder joints where there is not enough solder volume to form a strong, reliable connection between the pad and component lead. The joint may look concave or have exposed base metal.

Causes of insufficient solder

Common causes of insufficient solder include:

• Low solder paste deposits
• Stencil apertures too small
• Solder paste not releasing from stencil
• Poor wetting during reflow
• Solder wicking away from joint

Impact of insufficient solder

The negative effects of insufficient solder are:

• Weak, unreliable solder joints
• Open circuits or high resistance
• Mechanical stress and fractures
• Intermittent performance issues
• Premature joint failure in use

Solutions for insufficient solder

To ensure adequate solder volume, consider these options:

• Increase stencil aperture sizes
• Optimize solder paste print process
• Verify paste release from apertures
• Improve pad solderability and wetting
• Reduce solder wicking with dam or mask
• Use SPI and AOI to measure solder volume
• Add solder and reflow affected joints

Defect 6: Excess Solder

What is excess solder?

Excess solder is when there is too much solder paste deposited and reflowed on the pads and component connections. The joints may look bulbous, rounded, or even form solder balls or bridging.

Causes of excess solder

Excess solder defects can be caused by:

• Oversized stencil apertures
• Excessive solder paste printed
• Solder paste slumping
• Component floating during reflow
• Solder balls or splash

Impact of excess solder

Problems arising from excess solder include:

• Solder bridges and short circuits
• Difficulty inspecting joint quality
• Increased risk of thermal damage
• Solder balls causing shorts elsewhere
• Need for touch-up and rework

Solutions for excess solder

To avoid or fix excess solder issues, try these methods:

• Reduce stencil aperture dimensions
• Control solder paste volume and release
• Optimize print parameters to avoid slump
• Adjust reflow profile if needed
• Use selective solder masks dams
• Employ AOI and x-ray to find defects
• Remove excess with wick or sucker

Defect 7: Lifted Pads/Traces

What are lifted pads/traces?

Lifted or peeling pads and traces occur when the copper foil delaminates and separates from the PCB base material. This may happen during rework or normal assembly processes and results in broken connections.

Causes of lifted pads/traces

Some key factors that contribute to lifted pads and traces are:

• Poor laminate bond strength
• Mechanical stress during handling
• Excess rework heat
• Pads not supported by vias
• Traces too thin or narrow

Impact of lifted pads/traces

Negative consequences of lifted pads and traces include:

• Open circuits and loss of continuity
• Weakened or failed solder joints
• Scrapped PCB substrates
• Limitations on rework
• Field failures and returns

Solutions for lifted pads/traces

To minimize lifted pads and traces, consider these approaches:

• Source quality PCB laminates
• Avoid excess handling forces
• Use appropriate rework temperatures
• Anchor pads with vias when possible
• Specify suitable copper weights and widths
• Inspect for visual signs of lifting
• Remove affected pads/traces and jumper

By being aware of these 7 common PCBA manufacturing defects, their causes, impacts, and solutions, assemblers can take proactive steps to improve their processes and output quality. Implementing the right controls, parameters, and inspection methods helps reduce the occurrence of these defects and catch them early when they do happen. The result is smoother production, lower costs, and higher customer satisfaction.

PCBA Defects FAQ

Q1: What are some of the most common PCBA defects?

A1: Some of the most frequently occurring PCBA defects include solder bridges, cold solder joints, component shift/rotation, tombstoning, insufficient solder, excess solder, and lifted pads/traces. Each of these issues can cause a variety of problems that impact the quality, reliability, and functionality of the final assembled boards.

Q2: How can solder bridges be prevented?

A2: Solder bridges can be prevented by optimizing the solder paste printing process, using proper stencil designs, ensuring accurate component placement, and providing adequate solder mask spacing between pads. Inspection methods such as SPI, AOI, and ICT can help catch bridging defects early in the process.

Q3: What causes tombstoning of components?

A3: Tombstoning, where small chip components stand up vertically on the board, is primarily caused by uneven wetting forces on the component pads. This can be due to unequal pad sizes, misaligned placement, or imbalanced solder paste volumes. The component geometry and reflow profile can also influence tombstoning risk.

Q4: How does insufficient solder affect joint reliability?

A4: Insufficient solder in a joint can lead to weaker, less reliable connections between the pad and component. The reduced solder volume may not fully wet the surfaces, resulting in a low-strength bond that is prone to cracking or separation under mechanical or thermal stress. Joints with insufficient solder are more likely to fail in the field.

Q5: What are some methods to address excess solder defects?

A5: Excess solder defects can be corrected and prevented by reducing solder paste volumes, optimizing stencil aperture sizes, controlling print parameters, and using selective solder masks or dams as needed. Inspection techniques like AOI and X-ray can help identify problem areas. Rework to remove excess solder can be done with braid, wick, or a solder sucker.