Solder bridges, tombstoning, cold joints – understand causes and prevent them.
Why Do Soldering Defects Occur?
Despite automated manufacturing, soldering defects can occur. The causes usually lie in one or more of these areas:
- Solder paste: Wrong amount, wrong viscosity, or aged material
- Reflow profile: Temperature too high/low or incorrect ramp time
- Layout: Asymmetric pads, missing thermal reliefs
- Components: Moisture-sensitive components not stored correctly
Solder Bridges
Symptom: Unwanted connection between two adjacent pads or pins.
Causes:
- Too much solder paste (stencil opening too large)
- Insufficient spacing between pads
- Incorrect reflow temperature
Solution: Optimize stencil design, verify pad clearances (min. 0.2mm for fine-pitch), implement SPI inspection after solder paste application.
Tombstoning (Tombstone Effect)
Symptom: A small passive component (0402, 0603) stands vertically on one pad – like a tombstone.
Causes:
- Uneven wetting forces during soldering
- Asymmetric trace routing to the pads
- Different copper area under the pads
Solution: Symmetric layout, equal trace widths to both pads, thermal reliefs on ground planes.
Cold Joints
Symptom: Matte, grainy surface on the solder joint instead of shiny. Poor or no electrical connection.
Causes:
- Reflow temperature too low
- Too short a time above liquidus temperature
- Heat dissipation through large copper areas (missing thermal reliefs)
- Oxidized pads or component leads
Solution: Optimize reflow profile, provide thermal reliefs in the layout, store solder paste and components correctly.
Void Formation
Symptom: Cavities (voids) in the solder joint, especially under BGA and QFN components.
Causes:
- Outgassing from the solder paste or board material
- Incorrect reflow profile (too rapid heating)
- Moisture absorption by the circuit board
Solution: Vacuum reflow soldering, optimize preheat profile, store boards dry, use nitrogen atmosphere in the oven.
Quality Assurance: AOI and X-Ray
Modern EMS providers use multiple inspection methods:
- SPI (Solder Paste Inspection): Checks the solder paste application BEFORE components are placed
- 3D-AOI (Automated Optical Inspection): Camera system checks solder joints, component position, and polarity
- X-ray inspection: For hidden solder joints (BGA, QFN) – sees what the eye cannot
- ICT (In-Circuit Test): Electrical testing of all connections
At SMTEC AG, 3D-AOI inspection is standard for every assembly – for maximum quality and reliability.
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