High Current Traces

OK, final post on this.  After a bunch of tweaks, I've gotten everything to route with this last problem.

I have a QFN24 package that has 6 output pins which together provide up to 5 amps.  Each pin has a width of 0.25 mm.  The 6 pins are overlaid with a polygon which is connected to one of my high current traces.  My high current trace is 0.5 mm.  The router creates traces which are the width of the wide netclass trace, 0.5 mm, and even though there is a polygon connecting the pins, the router runs traces between all the pins.

Here's the problem.  At the end points of the polygon connecting the 6 pins, the 0.5 mm traces extend past the 0.25 mm pads, violating the spacing to the next pins.  I can't change the width of the pins or the width of the traces.  Do I rip just those traces then route manually?

Thanks,

Dave

Hi Dave,

For easier visualisation, a screenshot of this problematic PCB part would be useful... ;-)

Anyway, I can read your fear about high currents and traces width. That's ok, but current is not the only parameter to be considered during traces width selection. Actually, it is the temperature which you should have in mind. When you have good air flow or larger enclosure, you can decrease traces width.

To avoid guessing, you can use one of online calculators which return min trace width based on your temperatures (environment and max allowed trace temperature) and few laminate properties. Hopefully, you will find out that 0.25mm is enough.

Best wishes,  Ivan.

Ivan Zilic wrote:

 

Hi Dave,

 

For easier visualisation, a screenshot of this problematic PCB part would be useful... ;-)

 

Anyway, I can read your fear about high currents and traces width. That's ok, but current is not the only parameter to be considered during traces width selection. Actually, it is the temperature which you should have in mind. When you have good air flow or larger enclosure, you can decrease traces width.

To avoid guessing, you can use one of online calculators which return min trace width based on your temperatures (environment and max allowed trace temperature) and few laminate properties. Hopefully, you will find out that 0.25mm is enough.

 

 

Best wishes,  Ivan.

Hi Ivan,

Yes, I've considered trace temperature and I think I'm being conservative regarding trace width.  I'd rather err on the safe side and I think I'm pretty close to routing with 0.50 mm traces on the high current lines, including the low current traces on the same net as the high current traces.

But I still have this one clearance issue when the 0.25 mm pad is routed with a 0.50 mm trace.  A picture of this is below.  Two pins on the top right (pins 16 & 15) and four pins coming down the right side from the top (pins 14 through 11) are on the same net.  You can see the bulge on pin 16 which is too close to pin 17, and a similar bulge between pins 11 and 10.  These bulges are the 0.50 mm traces added by the router.  I can rip those traces, but even if I route manually they are the same 0.50 mm width and I have the same problem.  There is also a polygon with the same net sitting on top of these pads and extending to the right.  (Some caps sit on the other end of the polygon.)

I tried eliminating 5 of the pins in the symbol, then putting a polygon in the package to cover the 1 remaining pin and where the other 5 pins would be.  But that gives me an overlap error.

I'll keep trying things but if anyone has a good idea, let me know.

Thanks,

Dave

Registered, but still cannot post a day later.

d.

Registered, but still cannot post a day later.

d.