Why are there teardrops on PCB pads?
There are primarily two reasons to use teardrops:
- It avoids a pocket (where the trace meets the pad) that could collect acid from the PCB etching process which would later do bad things.
- It reduces mechanical & thermal stress resulting in less hairline cracks in the trace.
That being said, in professionally made PCB's teardrops are rarely needed. It's almost more of an aesthetic thing than a solution to a real problem. I've done many boards with and without teardrops and I have yet to notice a difference. In my opinion, they are more trouble than they are worth.
The teardrop is to help the drilling process. It prevents drill breakout where the trace joins the via or through-hole. Sometimes it's not necessary to do it because the manufacturer can do it for you.
Where I work, our repair department recommends we do it because it increases the strengthening of connections between pads and tracks.
If you look at the picture the Ben Jackson posted in answer to a question about solder mask expansion, you'll see that the drilled holes can be significantly off-center from the pad centers.
In extreme cases, you could actually end up with holes that leaves no or very little annulus to make the connection to outgoing traces. The tear drop ensures that there's enough pad material left over to have a solid connection to the pad.