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I'm looking at a trace that goes from Pad 2 of a component to pin 22 of the QFN. What's the point of the thick trace at the beginning if it ends up being as small the pin where it is run?

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    \$\begingroup\$ The wide trace minimizes the total end-to-end resistance. If the trace needs to be narrow att some point, it's a good idea to keep the narrow part as short as possible, too. \$\endgroup\$ – Dave Tweed Jul 22 '15 at 22:05
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The pin size of the QFN is too small to allow such a wide connection without shorting a neighboring pin. However, the total resistance is reduced by having the trace as wide as possible for as much of the trace as possible. A typical resistance for a PCB trace would be in the range of 1 mOhm/square. That means that if you run a current through a 1 cm square of trace, it will be 1 mOhm of resistance. A 3 cm square is the same resistance irrespective of the fact that it is three times longer because it is also three times wider. Think of it as three resistors with three times the resistance each, but in parallel so the total resistance is 1/(1/3+1/3+1/3) = 1.

The wide trace takes fewer squares to fill and therefore has less resistance than a narrow trace.

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You may have heard of the term Necked track I have seen this on 12V automotive stuff The Idea is that under fault conditions when the circuit hasn't been correctly fused the Deliberatly thin PCB trace will go open preventing FIRE I think that a necked track is relevant anywhere where the prospective fault currents are high I don't know how they behave at high voltages but I have tested them on large 12V marine batteries and they work I run a fat 100 thou trace then a short say 3mm length of 10 thou then bark to the 100 thou trace Most importantly you can make sure that the wiring doesn't burn because burning plastic emits bad gases that could be really bad in the confines of a car

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    \$\begingroup\$ I think you're taking about spark gap, here is a video explaining this concept : eevblog.com/2014/10/31/eevblog-678-what-is-a-pcb-spark-gap \$\endgroup\$ – Yahya Tawil Jul 23 '15 at 6:22
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    \$\begingroup\$ @yahyatawil I think this answer is about using a deliberately narrowed down, short length of track as a fuse - although that's not what the OP was asking about. Dave's comment on the question gets my vote. \$\endgroup\$ – brhans Jul 23 '15 at 11:52
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    \$\begingroup\$ I just found that SPARK gap have the same usage of what Autistic talking about (making a fuse from PCB traces) and I found useful to share David link. \$\endgroup\$ – Yahya Tawil Jul 24 '15 at 14:35

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