As an undergrad, I once had classes cancelled because of an electrical fire in a neighboring building. Even funnier was that I was actually in a junior-level electrical engineering course when we were dismissed. Several students wanted to go check the fire out; their reasoning was that we were electrical engineers, and thus electrical fires fell under our purview. In this class we’d had quite a few labs ending with blue smoke (and even one small fire), so our reasoning wasn’t too far out of left field.
Understanding how PCB shorts can occur is instrumental in designing PCBs that minimize the risk of shorting. At the time, our professor, who wanted to get out of class just as much as we did, informed us that any emergency with a source greater than 9 volts was most definitely not something we would go see. But he did give us additional remedial homework on short circuits so we’d stop blowing up components in his labs. I’ll let you skip the homework, but a reference on PCB shortage causes is always handy.
PCB Contamination and Electrochemical Etching
Contamination can take many forms, all of which are bad news for your PCB. Incompatible chemistry or improper cleaning can leave films and residues on the surface of your board, between the laminates, or between copper layers internal to the PCB. This contamination can cause damage or degradation, but many people don’t realize it can also cause short circuits.
When metallic or conductive salts are present in the layers of PCB, electrochemical etching can cause them to form filaments that are officially called Conductive Anodic Filamentation. These filaments cause shorts through the non-conductive materials of multilayer boards. Copper dendrites are more common on the surface of a PCB; they are also formed by electrochemical etching, and are more likely when the surface of your board is contaminated, especially by acids or ionic solutions. Contamination shorts can also happen with stripped wires going into connectors. My class speculated that the electrical fire was caused by dirty wiring.
Contamination on the surface or internal layers of your PCB can lead to filaments grown by electrochemical etching that will short across pads or layers of the board.
Ensuring Safety with Solder Paste Stencil Alignment
When a PCB is manufactured, the components are placed into position before going into a reflow oven. In this oven, the solder paste melts and forms the electrical connection from the component to the PCB. If the components are placed incorrectly, either shifted or rotated just a little, then the leads may not match up with their corresponding pads. Leads may connect neighboring pads, or be close enough that solder reflow causes a bridge to form between leads.
The stencil used to apply the solder paste might also have aperture sizes that are mismatched to the size of the PCB pad. If they are too big, solder may be spread over multiple pads. Too small, and the solder won’t always reach the lead and form a strong connection. It’s also possible that the pad size might be mismatched to the component leads.
Avoiding Overcrowding and Reducing PCB Shortages
Component sizes are decreasing, resulting in smaller pad pitch sizes and higher component density. With a high component density there is a higher potential for heat-damage, and interference between parts. If you have to re-work the design in the future, then high component density may increase the cost and time that you need to invest.
While the size of your component pads isn’t directly causing electrical shorts on your PCB, smaller pads make shorts more likely. The reason is that smaller pads have less margin than larger pads, so you have less buffer in the event of layout issues. But, if the pads are large compared to the spacing between them, you’re more likely to have solder bridges form over that narrow distance, and Optimum Design reports that solder bridges cause as up to 15% of PCB defects. That means you can’t make pads larger to give yourself some wiggle room for alignment issues, either.
Layout Concerns for Trace Routing Designs
Smaller component pads also make it more difficult to route traces as traces become smaller and closer together. The trace routing can affect the quality of solder joints and depends on:
- Width of the trace connection to the pad
- Number of connections you have to a given pad
- Pad size (larger pads give you more margin to work with)
- Consistency of sizes between different traces
Areas with high trace density are prone to a multitude of problems. You’re more likely to have solder bridges (creating shorts), insufficient solder on the pad (creating openings), and tilted components because the solder isn’t distributed with the uniformity expected by your design.
Although most electrical shorts don’t cause course-cancelling flames, small fires, sparks, and overheating are common.
Managing pad margins, trace widths, and optimizing routing are challenging design tasks for the best of us. Maintaining your operating knowledge of design problems and making sure to take as much care of your PCB layout as possible will help to minimize the risk of pcb failure and shortage.
Choosing software with robust capabilities can help make these challenges a lot more manageable. CircuitStudioCircuitStudio can offline most of this heavy lifting.
Many thanks to Altium for permission to reproduce this post.
