“You’re going to need to add teardrops to that before you send it off to the fab house” said my coworker before going home last Friday.
“A teardrop? WTF is a teardrop?!” I thought to myself.
So I had to do some research, and here’s what I found. If you’re new at this like I am, hopefully it will be helpful to you too.
A teardrop is extra copper, straight or rounded, at the junction of a pad or via and a trace, or when a trace transitions between thick and thin. For example, if part of your trace width is changing from 10 mils to 4 mils, then the teardrop is added at the transition point to reduce any stress or hairline cracks.
There are two types of teardrops you could add to your design – Filleting (or straight) and snowman. Filleting is a more commonly used technique. With the snowman method you would add a smaller pad at the junction of your existing pad and trace.
So, what are teardrops used for?
Reduce Breakout and Improve Manufacturing Yield
While manufacturers hope to develop successful and reliable products with high production yield, it’s never going to be perfect. Sometimes, the drill bit wanders or the layers in the board shift during the lamination process. And if that happens, you could end up with a hole created outside the pad or via, breaking the connection. This is known as breakout.
Adding teardrops during the fabrication process is a method used to guard against drill breakout because it increases the amount of copper for the connection. If the breakout is at the back of the pad or via, away from the trace, it doesn't matter because the connection is still good. But if the drill wanders towards the trace, the teardrop is thick enough so that the connection doesn’t get broken.
So how thick should a teardrop be? Well, each manufacturer has their own Design for Manufacturability (DFM) guidelines, but here is a standard formula that follows IPC requirements to get you started.
Teardrops help with drill tolerance, strengthen the reliability of the connection and helps improve yield.
Prevent Acid Traps and Improve Reliability
Alkaline and the other chemicals used to remove the photo resist and unwanted copper are harsh. They’ll eat away at the copper traces if not washed away during the etching and plating process and compromise the connection. Narrow spacing and sharp angles in routes and traces are notoriously hard to wash and are known as acid traps.
Teardrops have no sharp acute angles and a great for preventing acid traps in your designs.
Adding Teardrops to Your Design
Each manufacturer has their own DFM checklist so be sure to work closely with them to get your design ready. Sometimes they’ll add teardrops for you, but if you want to take every precaution to make sure your design will be manufacturable, it’s good to add teardrops yourself to some critical or risky routes. Keep in mind that trace widths of 20 mils or greater don't require teardrops to be added. So, if you’re going to add teardrops to your design, it’s really easy, just:
1. Find ‘Route’ on the menu and select the ‘Teardrop/Tapered Trace’ command.
2. Select ‘Add Teardrop’ and add it to the junction where the trace meets the via or pad.
3. After you’ve added teardrops to your design, right click and select done.
And you can even use the dynamic feature to move pads, vias, and traces around without the teardrop getting disconnected.
About the AuthorMore Content by Deepti Bankapur