A printed circuit board in electronic products is an assembly of components and the bare board, joined together through soldering. This bonding process involves applying solder at designated locations and then solidifying it through high temperatures. However, PCB soldering process is susceptible to various factors such as solder quality, temperature fluctuations, and component placement deviations, all of which can lead to defects, ultimately affecting electrical performance and reliability. In this article, TechSparks will delve into common PCB soldering defects, including their definitions, consequences, causes, and measures.
Solder Bridging
Definition:
Solder bridging refers to a soldering defect caused by solder spanning across two conductors where there should be no electrical connection, which may occur during solder paste printing or after soldering.
Consequences:
Short circuits cause circuit malfunction or even lead to fires.
Altered design intent affecting signal transmission, voltage delivery, or current paths.
Reduced overall product aesthetics and appearance.
Causes:
Excessive solder forming deposits and flowing to connect at high temperatures.
Instability or improper temperature control of the soldering iron during manual soldering.
Higher probability of bridging with high-pin-count and fine-pitch components.
Measures:
Plan component layouts sensibly to avoid close spacing between components.
Use selective soldering machines instead of traditional stencil printing.
Conduct AOI inspection after soldering completion.
Solder Fillet
Definition:
A protrusion formed after solder paste printing, resembling a sharp tip that bridges between adjacent pads.
Consequences:
Poor solder joint appearance.
Excessive fillet length may lead to solder bridging defects.
In high-voltage circuits, increased local electric field strength due to overloading may cause arcing.
Causes:
Excessive detachment speed or slowness between stencil and PCB after printing.
Poor quality stencil with burrs on the hole walls.
Long storage time of solder paste leading to inadequate flowability and wetting.
Measures:
SPI inspection after solder paste printing.
Timely cleaning of the stencil after use.
Check solder paste shelf life before use.
Virtual Soldering
Definition:
After soldering, the solder joint fails to establish an effective connection between the pad and the pin, resulting in a gap between them.
Consequences:
Manifests as circuit open causing non-operation.
Insufficient component soldering reliability may lead to detachment.
Subtle defects are difficult to identify, resulting in equipment becoming completely unusable after long-term use.
Causes:
Oxidation of pads or component pins.
Small stencil apertures or insufficient scraper pressure leading to insufficient solder paste.
Mismatch of thermal expansion coefficients between component pins and solder paste.
Oxidation of solder paste leading to flux loss.
Measures:
Use high-quality solder paste brands and check shelf life before use.
Store components in a moisture-resistant environment and use an oven to bake components with moisture.
Adequate preheating time to fully activate the flux.
Voiding
Definition:
The presence of unfilled gaps or voids in the soldering area, usually at the interface between the solder joint and the pad (or lead).
Consequences:
Reduces effective soldering area, weakens solder joint strength, and lowers reliability.
Squeezing solder may cause short circuits between solder joints.
Causes:
High temperature causes decomposition of organic matter in flux, generating bubbles trapped in the alloy.
Long exposure of solder paste to air results in excessive absorption of moisture.
Stains or oxidation on the surface of pads.
Measures:
Timely cleaning and drying after bare board manufacturing.
Take solder paste out of the refrigerator in advance and use a centrifuge mixer before use.
Strengthen preheating to reduce surface tension between solder and pad.
Tombstoning
Definition:
Also known as “Manhattan effect,” it refers to the phenomenon where one end of a component is firmly soldered while the other end is lifted up after soldering.
Consequences:
May accidentally contact surrounding pads or traces, causing circuit shorts.
Circuit open leading to component failure.
Causes:
Improper pad design such as differing sizes or excessive spacing.
Inconsistent force or template aperture during printing.
Solder or component misalignment.
Uneven heating or excessive temperature during soldering.
Measures:
Ensure parameters align with design requirements.
Inspect quality after initial batch production and adjust equipment promptly if tombstoning is detected.
Attempt to complete printing and component placement in one operation without multiple actions.
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