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Compiled by Cookson Electronics
Bridging is defined as solder misconnecting two or more adjacent pads that come into contact to form a conductive path. Bridges can be caused by problems at various points in the assembly process. This article examines root causes and solutions at stencil manufacture, screen print, component placement, reflow, PCB manufacture, and solder paste.
PCB ManufactureWhen PCBs lead to bridging, it is because SMD pads created a coplanarity issue resulting in poor gasketing during printer setup. To eliminate the problem, remove solder mask between adjacent pads, especially for fine-pitch components.
StencilsAt the design stage, control the aperture size. For fine-pitch components, it is highly recommended to have the opening slightly smaller than landing pad size to improve stencil to PCB gasketing.
A dirty stencil with paste underneath will contaminate the bare board on the next print, attributing a potential bridge. To mitigate the possibility of this error, verify zero print gap set up, use the minimum necessary print pressure, increase wipe frequency, and/or change the specified cleaning chemicals.
Stencil tension also affects defect formation. If the tension is loose, consistent printing is impossible, and printed patterns will be ill-defined.
Paste and Screen Print ProblemsScreen printing can be a source of solder bridges due to poor gasketing, misalignment, bad cleaning, dented squeegee blades, and incorrect print speed or support. Gasketing and alignment are corrected by optimizing the print tool. Smearing and bridges caused by poor cleaning can be resolved by ensuring stencils are dry before the next print and optimizing the wet, vacuum, dry sequence. Poor print definition, with dog ears, is commonly found on fine-pitch apertures. To reduce the risk of bridges from dog ears, check the board support, adjust the separation speed, and make sure solder paste chemistry is taken into account for each set up.
When solder paste causes print defects, it can be the result of dry paste phenomenon, which leads to irregular print shape and inconsistent print volume. Verify that solder paste is not expired, operating temperatures are within supplier's recommendations, and printer temperature is around 25°C at 50% relative humidity (RH). Do not mix using new and old paste. Out-of-spec operating temperatures can also cause good paste to ooze out of pads, which may cause connections with adjacent pads. If temperatures are normal and this persists, verify with another batch of paste to confirm that the problem is batch-related. Perform cold and hot slump tests using IPC-TM-650 Method 2.4.35.
Component PlacementPlacement inaccuracy will narrow the gap between pads, increasing the chance of bridging. Verify component placement pressure for each product. Use X-ray inspection to verify correct BGA placement; microscopy for QFNs.
Excessive component placement pressure will squeeze paste out of pads. Verify the actual component height against data entered in the machine if this occurs. Component placement height should be ±1/3 of paste height.
Reflow ProfileExtended soak will input more heat to the paste and result in paste hot slump phenomenon. If possible, adopt a straight ramp to spike profile, without a soak zone.