Circuit Shorts: PCB Fasteners
Some connectors are notorious for tending to snap from the board and lifting pads and tracks from the PCB substrate with them. Here's a trick. But is it useful?
Some connectors are notorious for tending to snap from the board and lifting pads and tracks from the PCB substrate with them. Normally, they’re edge connectors and, almost always, they're surface-mount micro-USB connectors. The problem is that when we attach and detach a cable to and from these connectors the cable becomes a lever and the forces become quite powerful that can easily weaken or break the solder joints.
An experienced engineer taught me a trick: place vias in the SMD pads to create z-axis anchors through the PCB; you lose some real-estate on the other side, but in return, you get what may be a stronger mechanical bracing of the connector.
To be clear, we’re talking about vias in mechanical or power pads. Using vias unnecessarily in high-speed signal paths will create stubs that would affect signal integrity, so best avoided there. What I’ve described is also known as via-in-pad, and that comes with a set of potential issues we must consider, as they can:
These aren't necessarily a problem if you’ll be hand-soldering, or be the only user of the device, but it might be if your board will be machine assembled, or go to volume production. In that case, it’s best to stick to what the IPC recommends if you’re fortunate enough to have access to their specs. Or, ask your PCB house for their advice, which is always good advice.
All that said, if you have the board space on the bottom side, then just go with connectors that have through-hole mechanical pins. If your PCB is going into a case, then you’ll need to create a very robust mechanical bracing anyway, and that would make the additional vias not all that meaningful.
So, it’s a nice trick, but it is not all that useful in practice. Is it more useful elsewhere?
An experienced engineer taught me a trick: place vias in the SMD pads to create z-axis anchors through the PCB; you lose some real-estate on the other side, but in return, you get what may be a stronger mechanical bracing of the connector.
To be clear, we’re talking about vias in mechanical or power pads. Using vias unnecessarily in high-speed signal paths will create stubs that would affect signal integrity, so best avoided there. What I’ve described is also known as via-in-pad, and that comes with a set of potential issues we must consider, as they can:
- wick solder paste away from the pad down the hole, weakening the solder connection
- distribute temperature in unexpected ways that could result in a weakened joint
- create an uneven pad surface that could create weak spots
These aren't necessarily a problem if you’ll be hand-soldering, or be the only user of the device, but it might be if your board will be machine assembled, or go to volume production. In that case, it’s best to stick to what the IPC recommends if you’re fortunate enough to have access to their specs. Or, ask your PCB house for their advice, which is always good advice.
All that said, if you have the board space on the bottom side, then just go with connectors that have through-hole mechanical pins. If your PCB is going into a case, then you’ll need to create a very robust mechanical bracing anyway, and that would make the additional vias not all that meaningful.
So, it’s a nice trick, but it is not all that useful in practice. Is it more useful elsewhere?
Additional Resources, Info on PCB Fasteners, and More
Interested in circuit design, PCBs, fasteners, and related topics?- S. Drimer, "The Art of Routing: Via Fasteners," BoldPort.com, 2015.
- Electrical Engineering Stack Exchange, "Using a Via to Strengthen Surface Mount Connector On or Near Pad," 2020.
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