Electronic – eagle cad: enlarging the pads

Simply put, toe and heel are the areas of the solder joint that extend from the front and rear of the lead/pad. Think of a solder pad on your PCB as a piece of paper, and think of your hand as one of the leads coming off an IC package. If you place your hand in the middle of the piece of paper, the "toe" and the "heel" are the distance between the tip of your fingers, and the palm of your hand, to the edges of the paper. That's the extra solder in the joint that extends farther than the actual lead/pad on the IC itself.

Here, they are recommending a specific amount of toe because it will provide mechanical reinforcement for the joint. Since there is no room for a suitable heel, given that this is a castellated package that will sit pretty much flush against the main PCB, the only way to add more solder to the joint is to expand outwards, and thus they recommend to give it some extra toe.

To comply with their recommendations, you'll have to turn off the default stop layer and cream layer for the pads, and draw them by hand. This is as easy as going to the properties for the pads to turn both of those features off, and using the rectangle tool, selected to the tStop and tCream layer, to draw the features.

Based of the picture, it sounds like they recommended a tStop that matches the recommended pad exactly, and a tCream layer that matches the pad exactly except for the edge opposite the package, which would extend out 4 mils further.

The most prevalent reason for avoiding 90 degree bends is simply FUD - fear, uncertainty and doubt. There's a stack of websites miles high that recommend or show using 45 degree angles for traces but don't necessarily say 90 degree angles are bad. To this end, it seems people have taken up the idea that 90 degree angles ARE bad... for reasons they don't actually know.

Here are some potential issues, but why they don't matter for most of what people do here:

1.) You could potentially have signal integrity problems. Reading this PDF, among other sources, indicate there are very small SI problems that could occur at very high frequencies - in the GHZ+ range. For most of us, we're not hitting those rise times so it's not a big issue. Also alluded to is that stacking these 90 degree angles very close to each other (like if you meandered a trace with only 90 degree angles) could be bad. For most of us, again, we're not meandering traces that much.

2.) There are potential issues during etching because of the creation of "acid traps" in the 90 degree angles. For modern manufacturing processes, this has been described as far less of an issue than people believe. For imprecise home etching, it may be a big concern.