<p>I'm trying to print a large piece with polycarbonate but it keeps warping, I'm using a Taz 5 printer and setting 290 C in the extruder and 145 C in the heating bed. </p>

<p>Other setting I have are:</p>

<ol>
<li>printing speed: 20 mm/s</li>
<li>layer height: 2.5mm</li>
<li>infil: 20%</li>
<li>brim: 15mm</li>
</ol>

<p>Can anyone tell me any tips or suggestions to avoid warping?</p>

I would take a look
here
and
here
.
Literally the first two results of Google.
To summarize what they say,
You need good bed adhesion to keep the first layer from warping near the edges like you will get normally with ABS or PC(Polycarbonate). Some use hairspray or gluestick. I manage myself with buildtak surface. I would stay away from blue painters tape. Your looking for something very sticky. This of course makes it a nightmare to get the part off the bed but well worth the trouble if it keeps your print on the bed and flat.
Make sure the part receives adequate heat. Like someone wrote in the comments, a enclosure works best to keep the temperature of the entire print warm instead of just the few layers closest to the bed.
More perimeter layers works very well by providing more structural strength to try and combat the warping. I've stumbled upon this solution when I had trouble with ABS.
Lastly, first layer is always the most important part of any print. You want to make sure it is as level as possible and a good tip is to raise your bed just so slightly than usual so the first layer is jammed hard against the bed. This provides better surface area. Just be careful not to clog your printhead cause if you constrict the flow out of the nozzle too much, you'll might end up with plastic trying to flow upwards or maybe grind the filament with the extruder gear.