I am putting together a high current motor controller with Eagle and have run into a problem with power planes. While I can build a power plane with a named polygon, Eagle does not completely merge corresponding pads into the plane. Instead, it isolates the pad and then connects to it using wire wide of the polygon. The result is small little disconnected regions between the plane and pad. Interestly, if you place a named via on a named polygon, Eagle correctly recognizes they are the same and does not do this.
Attached is an image showing the problem (red dots show problem areas):
The upper pad is the most troublesome. The pad is in the blue plane, but you can see that Eagle has isolated it from the bulk of the plane. Only a single 60 mil vertical trace from below connects it (you have to look close). In a high current application like this, it is effectively reducing the size of my trace which is bad.
Anybody got an ideas how to tell Eagle not to do this? Right now my only option is to post-process the Gerber to fix these, but I would prefer to make Eagle generate the proper data. Thanks.
Those limited connections between pads & planes are called “thermals”, and you’ll be glad they’re there when you try to solder those pads - otherwise, your iron would try to heat up the entire plane. They’re short enough that they shouldn’t impact your current carrying capacity.
I don’t know Eagle, but I’d assume it has some way of adjusting the parameters of thermals. Even if you can’t turn them off entirely, you could probably have it use more and/or wider connections.
Vraz:
The upper pad is the most troublesome. The pad is in the blue plane, but you can see that Eagle has isolated it from the bulk of the plane. Only a single 60 mil vertical trace from below connects it (you have to look close). In a high current application like this, it is effectively reducing the size of my trace which is bad.
It is eliminating some of your thermals due to geometry of the region. I find that the wider your thermals are (width of polygon), the more often it eliminates these. Anyway, you can put a real wire there between your pad and the area of the polygon that you think it should connect to. As long as that wire has the same name as the polygon, it will do the right thing.
For instance, For the area with the top red dot, I would place a wire from the pad to the filled area of the polygon diagonally to the upper right of it. You could try doing the same with the left side of that pad, but I don’t think there’s enough copper to attach to there, which is why Eagle skipped it in the first place. An even better approach, if it is possible, is to just move (or bend) that diagonal trace (to the upper left of the pad) further up or to the left or both, and that will automatically give you two more thermals. You’ll never get the fourth unless you can move BL-R3.
Thanks to all for the quick and informative replies!
For this particular application, turning off thermals for the polgygon (thanks Jason) seems like the way to go. Because of the design, there are adjacent open areas to act as thermals for soldering. This particular board contains just the high current switching components (mosfets, resistors and capacitors) so I can tolerate more challenging soldering in exchange for better current handling.
I did also experiment with adding explicit wires between the pad and the polygon (Lou’s suggestion) which I almost went with. It worked well, though once I placed the wire I was unable to delete it (cannot delete due to inability to perform back annotation). If you add an explicit wire on the board, how do you later delete it? Hmm… I wonder if ripup would have done it-- I didn’t think to try that.
For this particular application, turning off thermals for the polgygon (thanks Jason) seems like the way to go. Because of the design, there are adjacent open areas to act as thermals for soldering. This particular board contains just the high current switching components (mosfets, resistors and capacitors) so I can tolerate more challenging soldering in exchange for better current handling.
So you realize… the issue is that the plane will wick heat away from the joint connected to it faster than you can apply heat. To overcome this requires a lot more heat (i.e., higher-watt iron, higher temp, and fat tip), which runs the risk of delaminating the PCB or overheating your part.
Something that helps a lot is to pre-heat the board to just below the solder melting point (e.g., heat gun) - this is usually in a range tolerable long-term by most parts, and lets you add just a little more heat to the joint to melt the solder.
Vraz:
I did also experiment with adding explicit wires between the pad and the polygon (Lou’s suggestion) which I almost went with. It worked well, though once I placed the wire I was unable to delete it (cannot delete due to inability to perform back annotation). If you add an explicit wire on the board, how do you later delete it? Hmm… I wonder if ripup would have done it-- I didn’t think to try that.
Yes; ripup is the way to remove any wire or via which corresponds to a net in the schematic.
So you realize… the issue is that the plane will wick heat away from the joint connected to it faster than you can apply heat. To overcome this requires a lot more heat (i.e., higher-watt iron, higher temp, and fat tip), which runs the risk of delaminating the PCB or overheating your part.
Understood. The planes will act as a heatsink and make it difficult to localize the heat during soldering. Should be an interesting learning experience. Assuming my analysis is correct, only two caps and two of the three MOSFET legs are impacted by this change. Everything else has isolation due to the design of the board.
Soldering the MOSFETs is certainly of concern since they are the bulk of the component cost. They are rated at 300C for 10 seconds for soldering purposes. I did google some other high current controllers (OSMC and another) and they also skipped thermals around the MOSFETs so it may be a common tradeoff in this application.
If you can’t live with thermals but still want to solder quickly, I’d advise heating up the board before placing and soldering the components. So - use something like a skillet or toaster oven or any other method conventionally used for smt soldering - and once it gets hot (but not past the solder’s melting point) remove the boards, place the components, and solder them. Naturally you won’t be able to handle the boards bare handed. This way the planes will already be plenty hot and won’t wick away much heat.
