Brian, Sorry for the late response, but here goes.
The problem with calling out 4 oz. Cu, or even 2 oz. for that matter, is that the board house will probably "pattern plate" the Cu, and it may not be uniform. There is additionally the problem of etching small traces in the same layer, due to the thickness. Solder, or solder plate (with or without soldermask so that you can put more on), as brought out in an earlier post, is a very very poor design choice, simply because solder will only carry about 16 percent of the current that copper will. You should never rely on solder to carry any current whatsoever. This is why solder is always excluded in any current carrying capacity calculations, whether they be the old MIL STD 275 calculations or charts, or any of the numerous newer ones (the IPC charts are the same as the old 275 charts). Bus bars do offer a solution, as does plain old wire. I would opt for the wider traces, as brought out in one earlier post, and I would distribute the copper on both sides of the board, and stitch it together with a very liberal sprinkling of vias, which should allay your fears about the connections that you bring up below. Remember to account for the size of the vias in your trace width calculations (subtract out the hole size(s) from the width of the trace)). Is it possible that the caps are large enough to have screw type terminals? If so, you could possibly use a wire with a terminal, in addition to the copper traces. Someone also mentioned a 10 - 15 degree C rise in conductor temperature in an earlier post. That is one problem with most "current capacity calculators" today, is that they start at 10 degrees C, and the fact is that you really do not want to design in a 10 degree C rise in temperature to your product, not to mention 15 degrees C or anything higher. In reality, unless you have a lot of airflow and lots of extra cooling capacity, you really should be designing for something mush less than 10 degrees C for the normal operation of the product. If you get large surges in current that last for any considerable length of time, and you get them often, then maybe it is time to increase your nominal / normal current rating a bit. JaMi ----- Original Message ----- From: "Brian Guralnick" <[EMAIL PROTECTED]> To: "Protel EDA Forum" <[EMAIL PROTECTED]> Sent: Friday, August 08, 2003 10:26 AM Subject: Re: [PEDA] PCB Copper thickness VS mounted rails. > > One other possibility is to make the board double sided, with 3 Oz foil > > on each side. This will be easier for them to etch/plate, and > > paralleling the > > high current traces on two layers gives the same resistance. I guess this > > won't work if this is a thermal board to be bonded to a heat sink. > > > > Jon > > I don't like making power supply PCBs with power traces on both sides. > Especially with large snap-in caps. It is too difficult to ensure that the > top of the PCB under the cap has a good solder to the huge fat traces just > like the bottom. I've experienced such designs where power supply sections > get a fine odd crackling type noise, which may be mistaken for a bad caps, > but it really was fine cracks in the solder on the top layer just under the > cap. With a 1 layer board, such a problem is easily caught. > > > _____________ > Brian Guralnick > [EMAIL PROTECTED] > > > ----- Original Message ----- > From: "Jon Elson" <[EMAIL PROTECTED]> > To: "Protel EDA Forum" <[EMAIL PROTECTED]> > Sent: Monday, August 04, 2003 1:59 PM > Subject: Re: [PEDA] PCB Copper thickness VS mounted rails. > > > > > > > > Brian Guralnick wrote: > > > > >I'm designing a power supply with will have a large ripple current. This > > >power supply will be on it's own PCB and it's 1 layer. Am I better off > > >mounting high current rails, or, increasing the PCB copper thickness from > > >1oz to something like 4-6oz? > > > > > >The power supply will be 90 vdc, continuous dc current of 4 amps, with > > >current surges & ripple current above 15 amps. > > > > > > > >Check with your PCB vendor on how much the extra thickness of copper > > > > > will cost you. Then, compare with the rails, including the cost of having > > the assemblers deal with it. If the 4 or 6 Oz foil will carry the > > current with > > acceptable electrical characteristics, it sounds like the best solution, > > unless > > the extra cost is prohibitive. My guess is the extra foil thickness will > be > > cheaper than all the extra handling to assemble the whole thing. > > > > One other possibility is to make the board double sided, with 3 Oz foil > > on each side. This will be easier for them to etch/plate, and > > paralleling the > > high current traces on two layers gives the same resistance. I guess this > > won't work if this is a thermal board to be bonded to a heat sink. > > > > Jon > > > > > > > > > > > > > > > > * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Forum Guidelines Rules: * http://www.techservinc.com/protelusers/forumrules.html * * Browse or Search previous postings: * http://www.mail-archive.com/[EMAIL PROTECTED] * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
