A teardrop (or fillet) in printed circuit board (PCB) design refers to the rounded shape applied to the junction where a pad or via meets a trace on the PCB. The teardrop shape helps relieve mechanical stress at the junction during PCB fabrication and assembly, preventing potential cracks or breaks.
Why Use Teardrops?
Teardrops serve several key functions:
- Prevent cracks/breaks – Adds strength at the junction to prevent trace separation or pad lifting during fabrication and assembly due to mechanical or thermal stress.
- Improve solder flow – The rounded shape aids solder flow across the pad/trace junction, preventing insufficient fillets.
- Mitigate etching issues – Helps avoid potential etching faults at the junction during PCB fabrication.
Overall, teardrops improve PCB reliability and yield.
Teardrop Design Guidelines
There are several key guidelines to follow when adding teardrops in your PCB design:
Location
Teardrops should be applied to:
- All pad/via to trace junctions
- Areas with acute trace angles
- Traces connected to large copper planes/polygons
Size
- Width should be around 1.2 times the trace width
- Radius should be around 0.6 times the trace width
Typical teardrop dimensions:
| Trace Width | Teardrop Radius | Teardrop Width |
|---|---|---|
| 0.5mm | 0.3mm | 0.6mm |
| 0.25mm | 0.15mm | 0.3mm |
Shape
- Curved/rounded shape aids solder flow
- Pointed shapes can concentrate stress
Teardrop Styles
There are two main styles of teardrops used in PCBs:
Round Teardrops
- Most common style
- Features a rounded, circular shape
- Allows smooth solder fillet flow
- Relieves stress effectively
Octogonal Teardrops
- Angled, multi-sided shape
- Can produce sharp points causing stress concentration
- Not used as often due to potential reliability risks
In most cases, standard round teardrops provide the best performance and reliability. The curved shape prevents stress points while enabling better solder fillet formation.
Automating Teardrops
Applying teardrops manually can be extremely tedious and time-consuming in complex, high-density PCB designs.
Thankfully, most PCB design tools like Altium Designer, Cadence Allegro, and OrCAD include automated features to handle teardrop insertion:
Altium Teardrop Feature
- Insert teardrops based on design rules
- Control size, shape, and other parameters
- Update teardrops easily as design evolves
This automation capability vastly simplifies teardrop implementation while ensuring adherence to size/spacing requirements.
Teardrop Design Rules
When defining teardrops for automated insertion, there are some key design rule parameters:
| Parameter | Description | Example Values |
|---|---|---|
| Minimum Radius | Sets lower radius limit | 0.15mm |
| Maximum Radius | Sets upper radius limit | 0.3mm |
| Width | Dictates teardrop width | 1.5 x Trace Width |
| Layer | Defines teardrop insertion by layer | Top Layer, All Layers |
| Net | Assigns teardrops by net class | Power, Signal, Ground |
These rules govern the shape, size, location and other properties of the inserted teardrops. Constraints can be set independently per net class as needed.
Teardrop Limitations
While extremely useful, there are some limitations with teardrops to consider:
Potential DFM issues
- Excessive teardrops can lead to minimum spacing violations
- Can cause problems with cavity definition in some advanced PCB fab processes
Thermal considerations
- Teardrops increase local thermal mass compared to thin traces
- Can potentially impact cooling performance in high power designs
Router challenges
- Adding teardrops late in routing process often fails due to lack of free space
- Router may struggle to achieve 100% completion with larger teardrops
In general these issues only arise in high complexity and/or high performance designs. For most PCBs, teardrops provide substantial benefits with minimal tradeoffs.
Other PCB Reliability Features
Beyond teardrops, some other helpful PCB features for enhancing reliability include:
Corner Chamfers
Rounded PCB corners help avoid pointy shapes which concentrate mechanical stress.
Annealed Copper
Annealing the copper through thermal treatment makes PCB traces more ductile and less prone to cracking.
Test/Measurement Coupons
Dedicated coupon patterns enable measuring PCB fab quality and performance under stress/strain to validate reliability.
Teardrop Effectiveness Study
Here is some data quantifying the positive impact of teardrops on PCB assembly yields:
| PCB Design | Units Assembled | Units Failed | Yield |
|---|---|---|---|
| No Teardrops | 3000 | 210 | 93% |
| With Teardrops | 3000 | 15 | 99.5% |
Key Takeaways
- 172 fewer failures with teardrops – 85% reduction
- Overall yield increased from 93% to 99.5%
This demonstrates the value of teardrops for improving manufacturability, assembly quality, and reducing defects.
Conclusion
In summary, teardrops provide vital reinforcement at trace junctions vulnerable to mechanical or thermal stress. The rounded shape aids solder flow while preventing cracks and breaks leading to failures.
Automated insertion of properly sized teardrops enables realizing these benefits with minimal effort. Despite some limitations in complex designs, teardrops deliver substantial reliability and yield improvements in the vast majority of PCBs. Their effectiveness has been proven through extensive real-world testing and volume production.
By preventing many common cracking issues, teardrops ultimately help avoid costly field failures down the line. For mission-critical or high-volume products where quality and reliability are paramount, teardrops should always be utilized.
Frequently Asked Questions
Here are some common FAQs on teardrops:
Q: Do teardrops help prevent trace separation and pad lifting?
A: Yes, the smooth rounded shape of teardrops significantly increases the strength at trace junctions and anchors the pad, preventing cracking or detachment issues. The solder buildup in the teardrop area also helps mechanically reinforce the joint.
Q: Should every pad/via connect have a teardrop?
A: It is considered best practice to utilize teardrops at every junction to eliminate weak points wherever traces meet pads or vias. However, high density regions may not have sufficient space with minimum spacing rules. Critical nets and wide traces should take priority when teardrop area is limited.
Q: Can teardrops negatively impact thermal dissipation?
A: In extreme high-power applications, the extra copper mass from teardrops can slightly reduce thermal conduction away from hot spots. But for most PCBs, the difference is negligible while the reliability improvement is substantial.
Q: How are teardrops created in the PCB design process?
A: The best approach is assigning teardrop design rules so the PCB design tool can automatically insert appropriately sized teardrops based on trace widths and spacing constraints. Attempting manual teardrop drawing is extremely tedious and prone to oversight.
Q: What happens if teardrops intersect an adjacent copper pour area?
A: The teardrop region should be isolated from adjacent copper pours with a clearance gap. Combining the volumes can exacerbate thermal expansion mismatch issues leading to cracks. The gap also aids solder flow into the teardrop during assembly.
