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Electronics Forum / CAD / March 2005How important are chamfered routes?
I was once told by a professional layout person that leaving a 90 degree inside corner on intersections of traces and/or pads was not good. Seems the acid is harder to wash out of the little corners and can result in over etching. I have seen tons of boards that don't bother with this and I have found few who have even heard of this. Is this a real issue? Is it only an issue on boards with very fine traces, like 6 or 5 mil? I have not seen much in layout packages that would make this easy to do. In fact, it can be very hard to do without direct support from the package. >I was once told by a professional layout person that leaving a 90 >degree inside corner on intersections of traces and/or pads was not [quoted text clipped - 7 lines] >do. In fact, it can be very hard to do without direct support from the >package. This is called an 'acid trap' for inside 90 degree corners and more acute intersections. Many CAD packages will DRC for it and flag them as problem points. However, fab processes have improved such that I don't think anyone really cares anymore. If you've got a really low-tech fabricator, maybe its still a problem. BTW I think the term usually used is 'mitering'. Mitering is not necessary for signal integrity, unless you are well into multiple gigaHertz. For lower frequency signal integrity, extensive mitering might reduce trace length, and that's usually always a good idea. BTW, what is the difference between a champfer, and a bevel? I've looked and never found good distinguishing definitions. Gary Crowell CID Micron Technology > However, fab processes have improved such that I don't think anyone > really cares anymore. If you've got a really low-tech fabricator, [quoted text clipped - 10 lines] > BTW, what is the difference between a champfer, and a bevel? I've > looked and never found good distinguishing definitions. Just to be clear, I am not just referring to using 45 degree corners in mid trace turns. More specifically the hard part is joining to a pad with all angles of 45 degrees. chamfer (chm'fr) pronunciation tr.v., -fered, -fering, -fers. 1. To cut off the edge or corner of; bevel. Seems like the non-jargon definitions are pretty much the same. miter (m'tr) pronunciation v.tr. 1. To bestow a miter upon. 2. a. To make (two pieces or surfaces) join with a miter joint. b. To bevel the edges of for joining with a miter joint. Even miter is about the same. For now I am not going to worry about it since the software does not support it. I tried beveling the inside corners using just traces on a very small board I did a while ago and it was a real PITA. It was not so hard to do the first time, but moving anything was a lot more work. >> However, fab processes have improved such that I don't think anyone >> really cares anymore. If you've got a really low-tech fabricator, [quoted text clipped - 36 lines] >very small board I did a while ago and it was a real PITA. It was not >so hard to do the first time, but moving anything was a lot more work. Ah, at trace-pad intersections, what you are referring to is usually called 'teardropping', that is, using added traces at the trace-pad joint to 'feather' the trace into the pad. Done for three reasons that I can think of. 1. to eliminate the acid trap. (and, as I mentioned, not usually important anymore.) 2. to relieve the stress concentration and cracking potential at the joint. 3. to reduce the potential for cutting the trace by a wandering drill. You're right, this would be extremely tedious to do by hand, though many CAD/CAM systems will do it automatically. You could even ask the board fab house to do it for you, and it should be no charge in most cases. Two notes: In some pin layouts, interstitial connector pin patterns, in particular, teardropping can cause DRC errors due to reduced clearance. Teardropping with arcs can reduce this problem, but I've not seen a CAD system that does this. An alternate to teardropping is a 'snowman'; just a smaller round pad, off-center from the pad, that overlaps the trace-pad joint. Probably easier to do if you have to do it by hand. Gary Crowell CID Micron Technology Guys, guys, guys, Entering a pad at 90 degrees is not considered an acid trap, never was. Not even a 90 degree trace corner, it is any acute angle entry below 90 degrees. For the 90 degree unmitered corners, it does effect to some degree impedance controlled traces simply because of the change in trace width area at the 90 degree corner, effectively changing the impedance of your trace at each 90 degree corner. A 45 degree miter has a much smaller change in trace width area at the corner and therefore a much smaller change in impedance at that corner. There is an old wife's tale in the industry about electrons flying off the trace when it runs into the 90 degree corner,or that it radiates wildly (not in normal circuits below several Ghz). Sort of like a pile of wrecked cars at the bottom of a cliff where there is a 90 degree corner at the top of a cliff. Possibly the previous designer offering advice was referring to this or other variations of this myth over the years. As Gary said, teardrops are actually for two mechanical reasons. They decrease the probability that a drill mislocated towards the trace entering the pad/via will result in a minimum annular ring violation severely reducing the traces connectivity to the via/pad. It also reduces the chance of mechanical cracking of the trace at the point it enters the pad/via due to thermal shock, solder joint stresses and board loading in high stress applications. I disagree that teardrops ever had anything to do with acid traps because angles of 90 degrees were never considered an acid trap, acute angles less than 90 degrees was an acid trap. As for Gary's comments on the state of technological affairs, I also agree that it is not so much of an issue with most fabricators these days. However, it has not gone completely the way of the Dodo bird. Cleaning techniques are much improved these days and acid traps are nearly a thing of the past with most fabricators. At least until you get down to acute angles less than 45 - 30 degrees.  SignatureSincerely, Brad Velander > Ah, at trace-pad intersections, what you are referring to is usually > called 'teardropping', that is, using added traces at the trace-pad [quoted text clipped - 23 lines] > Gary Crowell CID > Micron Technology >Guys, guys, guys, > Entering a pad at 90 degrees is not considered an acid trap, >never was. Not even a 90 degree trace corner, it is any acute >angle entry below 90 degrees. Oh sure, we agree; I mentioned teardropping, meaning for the case of an acute pad entry. Of course 90 degrees wouldn't be an acid trap or the DFM check would leave marks all over the place. Come to think of it, I wonder what it does consider an acid trap, is 89 degrees? I've never thought to check (CAM350). A pinhole check is a good idea too. Gary Crowell CID Micron Technology |
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