Table of Contents
Soldering is a critical process in PCB assembly and manufacturing. However, various soldering defects can occur, decreasing reliability and functionality. This comprehensive guide examines the most common soldering defects, their causes, how to prevent them, and repair techniques. With an understanding of potential soldering issues, PCB manufacturers and assemblers can implement targeted strategies to avoid defects and optimize the soldering process.
With ELE, a dependable PCB manufacturer & assembler, you will never have to be concerned about soldering defects. As a one-stop PCB assembler, ELE always pays attention to every link in the production chain, including wave soldering and reflow soldering.
Common Soldering Defects and Failure Modes
This section outlines the most prevalent soldering defects, grouped into categories based on the failure mode.
Open Circuits
First, let’s discuss the open circuit where current cannot flow. For the current to flow through the circuit, an uninterrupted path is needed, the so-called “closed circuit”. If there is any interruption or break in the circuit, it becomes an open circuit, which prevents the current from flowing. In an open circuit, the two terminals are not connected, resulting in a break in circuit continuity. Even though no current is flowing, there may still be a voltage drop between two points in the circuit.
- Solder Opens: An open solder joint occurs when there is no connection between the component lead and the PCB pad. This usually happens when the lead and pad do not bond adequately. Opens lead to an incomplete circuit.
- Lifted Pads: The pad detaches from the PCB surface due to excess heat or vibration during soldering. This results in a loss of electrical connection.
- Solder Skips: Skips happen when solder paste is missed on SMD pads, leaving portions of the joint unsoldered. This can create open circuits.
Engineer’s Log
We once faced a long-standing tombstoning issues on a high component density board using 0402 capacitors. Automated Optical Inspection (AOI) consistently flagged the same component locations. The culprit was not the reflow profile but the PCB design, since one capacitor pad was connected to a very large ground plane, while the other pad was connected to a very small trace.
During reflow, the small trace pad would get hot and reflow faster, pulling the part down by wire-polar forces, while the other side was still a heat sink and not yet reflowed. To solve this problem, we edited the design to add thermal reliefs to the ground plane connection, So sharing the load equally and making the part stable.
Short Circuits
A short circuit is an unexpectedly low resistance path that passes through an unusually high current. A short circuit occurs whenever the resistance of the circuit, or the resistance value of a part of the circuit, drops to almost zero ohms. Short circuits are usually caused by improper wiring or damaged insulation.
- Solder Bridges: Solder forms an unintended connection between adjacent PCB conductors, creating a short. Bridges frequently result from excess solder.
- Solder Balls: Small solder balls may form on the PCB surface during reflow. These can cause shorts if trapped between conductors.
- Solder Splash: A splatter of tiny solder droplets can lead to shorts. Splashes generally arise from solder impurities or contaminants.
Engineer’s Log
A new product using a 0.4mm pitch QFP was failing functional tests due to intermittent shorts. Visual inspection was difficult, but X-ray revealed tiny, hair-like solder bridges between pins. The solder paste volume was correct, but the paste was slumping and spreading before reflow. The problem was twofold: the stencil aperture was cut slightly too large for the pad, and the solder paste itself had poor slump characteristics.
We switched to a laser-cut, electro-polished stencil with a 10% aperture reduction and qualified a new solder paste with higher metal content and better rheology. This provided the crisp print definition needed to eliminate the bridging.
Weak Joints
- Cold Solder Joints: Inadequate heat during soldering leads to incomplete wetting and weak, “cold” joints. These are identifiable by a rough, dull surface.
- Disturbed Joints: Also called “disturbed solder joints.” Movement of components during soldering prevents proper bonding and causes reliability issues.
- Solder Voids: Gaps or holes within the solder joint significantly reduce connection strength. Voids are caused by contamination or poor wetting.
- Insufficient Solder: Too little solder applied to a joint leads to poor electrical and mechanical performance. The joint may appear underfilled.
Engineer’s Log
Gas pockets trapped within the solder joint, visible on X-ray. These are often caused by outgassing from flux volatiles that don’t have time to escape. While small voids are often acceptable per IPC standards, large voids under the thermal pad of a QFN, for example, can severely degrade thermal performance and lead to component overheating.
Other Defects
- Solder Dewetting: The solder does not adequately adhere to the PCB pad or component lead. Dewetting is visible in areas lacking solder coverage.
- Solder Flux Residue: Leftover flux after soldering can lead to corrosion or insulation resistance issues. Flux residues appear as white, brown, or clear residues.
- Solder Cracking: Cracks within solder joints, are often caused by stress or repeated thermal cycling. Cracked joints have reduced strength and conductivity.
- Tombstoning: During reflow, one end of the component lifts from the pad, resembling a tombstone. This misalignment causes opens or shorts.
Causes of Common PCB Soldering Defects
Understanding what causes soldering defects is key to preventing them. Here are some of the most common root causes of the failure modes outlined above:
- Insufficient heat or cooling too quickly during soldering leads to cold joints, voids, poor wetting, and other defects.
- Excessive solder quantity can create bridges, icicles, and tombstoning. Too little solder causes open and dewetting.
- Solder contamination from oils, oxides, grease, or chemicals leads to dewetting, voids, icicles, and other reliability issues.
- Component misalignment or movement during reflow causes tombstoning, distortion, and disturbed joints.
- Design issues like insufficient thermal relief, small pad sizes, or improper pad spacing contribute to tombstoning, bridges, and skips.
- Excessive thermal shock during soldering or operation lifts pads and causes cracking over time.
- Residual soldering flux increases insulation resistance and creates the potential for corrosion.
- Solder paste quality issues like high slump, low metal content, or contamination cause solder balling, spatter, and bridging.
Prevention of PCB Soldering Defects
Soldering defects can be prevented through controls during design, materials selection, process optimization, inspection, and other measures.
Design for Manufacturing (DFM)
- Allow sufficient clearance between conductive elements to prevent bridging.
- Incorporate adequate thermal relief in pad designs to avoid tombstoning.
- Specify component sizes and spacings to match pad layouts.
- Ensure pad dimensions match component lead sizes.
- Eliminate unnecessary pads, vias, and traces. -Verify functionality with prototyping and testing.
Solder Paste Controls
- Select high-quality solder paste suited for the assembly process.
- Implement solder paste inspection to check application quality.
- Store solder paste correctly and discard when expired.
- Use the optimal solder paste print speed, pressure, and separation for the PCB.
- Clean stencils regularly and monitor aperture quality.
Process Controls
- Set reflow oven zones for the ideal thermal profile and atmospherics.
- Control cooling rate to prevent thermally-induced defects.
- Use no-clean flux or clean thoroughly after soldering.
- Inspect boards during manufacturing to catch issues early.
- Monitor process parameters like temperature and conveyor speed.
- Certify operators in proper soldering techniques.
Inspection and Testing
- Visually inspect solder joints for acceptability.
- X-ray inspection can catch hidden defects.
- ICT fixtures check electrical connections.
- Implement automated optical inspection (AOI).
- Functionally test assemblies under temperature cycling.
Repair of Common Soldering Defects
Many soldering defects can be repaired through rework, salvaging the PCB assembly. Here are some guidelines for repairing typical defects:
Solder bridges: Use a soldering iron, solder braid, or sucker to remove excess solder creating the bridge. Prevent overheating nearby joints.
Cold joints: Reheat the faulty joint using a soldering iron until good wetting is achieved, then allow it to cool properly.
Solder balls: Remove with tweezers, vacuum, or tape. Prevent damage to nearby components or joints.
Tombstones: Realign and rework the component while avoiding damage to the PCB or adjacent components.
Lifted pads: Attempt to reattach the pad using adhesive or fiberglass material. Rework components as needed. Severely lifted pads may require board replacement.
Solder voids: Use a soldering iron with flux to reflow the joint and fill any voids. Take care to avoid damaging the PCB or component.
Flux residue: Clean with isopropyl alcohol, either by hand or using an automated cleaning system. Avoid scrubbing.
Opens/skips: Use solder paste or wire with a soldering iron to fill in the missing solder. Match the original joint profile.
For severe defects, full component removal and replacement may be necessary using soldering/desoldering tools. Professional rework specialists can repair some complex defects.
Conclusion
Understanding common soldering failures like cold joints, bridging, tombstoning, and lifted pads is critical for PCB quality management. Implementing design and process controls can prevent many defects from occurring.
When defects do occur, trained operators can repair many issues through selective rework techniques. With vigilance and preparation, the impact of soldering defects on product reliability and manufacturing efficiency can be minimized.
FAQs
A1: Contamination in the solder paste from oxide formation or environmental factors is the primary cause of solder balling during reflow.
A2: Cold joints have a rough, dull appearance compared to a good joint’s smooth, shiny surface. They may also appear grainy or underfilled.
A3: Careful process controls like regulating soldering iron temperature, using the right tip size, and following IPC soldering standards will minimize pad lifting.
A4: The smaller size and mass makes SMD components more susceptible to thermal imbalances during reflow, increasing tombstoning risks.
A5: X-ray inspection and cross-sectioning are effective ways to detect subsurface defects like voids and cracks not visible externally.
CONTACT US
English 
Italian 
Hungarian