July 20, 2020
Layers are not always aligned and drills wander. Unless you are requesting class 3 fabrication, which is expensive, your via can break 90° out of the annular ring, eliminating up to 1/4 of the circumference of the annular ring.
Image of class breakout from IPC Standards document.
If the drill breaks out in the direction of your trace, you can lose electrical connectivity. So how do you deal with that eventuality in your next design?
Image shows a variety of random breakout directions.
Teardrops / Keyholes
One way to deal with break out is with teardrops or keyhole via pad design. The names teardrop and keyhole come from their resemblance of the pad to a teardrop shape and an old-fashioned ward lock.
Teardrops and keyholes increase the amount of copper in the direction of the trace. That way, if the drill is offset in the direction of the trace, there is still plenty of copper to conduct your current and your signal.
Images of via without teardrop (left) and via with teardrop (right).
An example layout with teardrops is shown below.
If your EDA does not allow teardrops, consider using keyholes — a small length of copper trace that has a trace-width that is a bit wider than the via-diameter.
An example Keyhole via connection provides additional copper in case the drill is off-target.
What happens if you need to make a connection to another layer in your design? Many designers will place a via in-line in the trace and then continue their route on the other layer. But that technique can provide three opportunities for break-out that will break connectivity on your board. That decreases your yield per panel, which in turn, increases your overall cost. PCB Designers almost never see this type of failure because fabricators throw these boards into the trash after they fail visual inspection or flying probe testing.
If the drill breaks out to the left, right, or top of the annular ring, electrical connectivity can be compromised.
One way to deal with the problem is to change the size of the pad on your in-line vias. Using teardrops along with an oblong pad will increase the amount of copper around the drill hole, so break-out is no longer a problem. Another way you can deal with this issue is to offset the via from the trace. Combined with an oblong pad or teardrops/keyholes, this design technique allows the drill to wander significantly in any direction without breaking connectivity. Yet another way to deal with the problem is by using multiple vias that are offset from the trace. The offset is needed to decrease the chance of breakout, since two in-line vias would increase, rather than decrease, the likelihood of a breakout.
Image shows standard via routing (1), oblong-pad (2), offset via routing with ellipse pad (3), and redundant via routing (3).
But why stop there? If you have the space for it, add even more copper around the via hole with a copper pour.
If you have ever reworked an electronics project, there’s a chance that you have caused the copper traces to peel up from the laminate. Copper is not permanently attached to substrates, it is held on by intermolecular adhesive forces between the copper and the epoxy resin.
Peel strength is measured in units of [pli], which is equivalent to [Newtons]/[Linear Inch] or [Newtons]/[25.4 mm]
Copper adhesion is on the order of 100-101 pli. That the same range as household tapes.
That’s not a great deal of holding force to keep a USB connector secured to the board. So to augment the adhesive bond between the copper and the epoxy, you can add small mechanical vias to the copper pads.