Acid Traps in PCB Design: What They Are & How to Avoid Them

One of the first design rules I learned as a junior PCB engineer was simple: avoid acute angles in your traces. No angles less than 90 degrees. Use 45-degree routing. The reason given was always the same—acid traps. Fifteen years later, I still follow this guideline, though the manufacturing landscape has changed dramatically. Understanding what acid traps in PCB design actually are, why they matter, and how modern fabrication has evolved helps you make informed decisions about when this rule is critical and when it’s simply good practice.

What Are Acid Traps in PCB Design?

Acid traps are geometric features in your PCB layout where etching chemicals can accumulate and pool during the fabrication process. When traces meet at acute angles (less than 90 degrees), or when small enclosed spaces form between traces, pads, and vias, these areas can trap the corrosive chemicals used to remove unwanted copper from the board.

The trapped etchant continues working on the copper long after the rest of the board has been properly processed. This over-etching can erode through the trace, creating opens, weak connections, or complete circuit failures. The term “acid trap” comes from the acidic etchants historically used in PCB manufacturing, though the problem exists with any chemical etching process.

How Acid Traps Form During PCB Manufacturing

To understand acid traps, you need to understand the PCB etching process. Contrary to what it might look like, copper traces aren’t drawn onto a blank board. They’re revealed by removing unwanted copper from a fully copper-clad substrate.

The Etching Process

The basic PCB fabrication sequence works like this:

1. Start with a fiberglass substrate fully covered in copper
2. Apply etch-resistant material (photoresist or toner) to protect desired traces
3. Expose the board to chemical etchant that dissolves unprotected copper
4. Rinse away etchant and remove the protective layer
5. The remaining copper forms your circuit traces

When etchant gets trapped in a corner or pocket, it can’t be properly rinsed away. It continues dissolving copper underneath the protective layer, tunneling into traces that should have been preserved.

Common Etchants and Their Behavior

Traditional acidic etchants create the highest risk of acid trap problems. Modern photo-activated and alkaline etching processes have significantly reduced—though not eliminated—these concerns.

Common Causes of Acid Traps in PCB Layout

Several design patterns create conditions where etchant can accumulate:

Acute Angle Trace Junctions

The most common cause of acid traps PCB designers encounter is traces meeting at angles less than 90 degrees. The sharp corner creates a pocket where etchant pools and continues etching.

T-Junctions Without Teardrops

When a trace connects to a pad or another trace at a perpendicular junction, the sharp internal corners can trap etchant. Adding teardrops—gradual copper transitions—eliminates these sharp internal angles.

Tight Spacing Between Features

Even without acute angles, spacing that’s too tight between traces, pads, and vias creates narrow channels where etchant can’t flow freely. These enclosed areas become traps. The minimum spacing depends on your fabricator’s capabilities, but 3 mils (0.076mm) is a common lower limit.

Copper Islands and Slivers

Small, isolated copper features or thin copper “slivers” can act as traps. These features may also detach during etching and float to other areas of the board, causing shorts. Modern design rules flag features narrower than 4 mils as potential problems.

Problems Caused by Acid Traps

When acid traps occur, they create several failure modes:

The most insidious problem is the weak connection that passes initial testing but fails in the field. A trace that’s been partially eroded might have enough copper to conduct under ideal conditions but fails under thermal stress or vibration.

Design Techniques to Avoid Acid Traps

Preventing acid traps requires attention during layout. These techniques eliminate or minimize the geometric conditions that trap etchant:

Use 45-Degree Routing

The simplest and most effective prevention is routing with 45-degree angles instead of 90-degree turns. Most PCB design tools default to this routing style.

Implementation: When changing trace direction, use two 45-degree bends instead of one 90-degree turn. This eliminates acute internal angles entirely.

Add Teardrops at Junctions

Teardrops are gradual copper transitions between traces and pads or vias. They eliminate the sharp corners that form at T-junctions.

Implementation: Most EDA tools include automatic teardrop insertion. Enable this feature globally and verify that teardrops are applied to all pad and via connections.

Maintain Adequate Spacing

Don’t design to your fabricator’s absolute minimum capabilities unless space constraints demand it. Leave margin for etchant flow.

Recommended Minimums:

• Trace-to-trace: 5 mils (0.127mm) or greater
• Trace-to-pad: 5 mils minimum
• Via-to-via: 8 mils minimum
• Avoid grouping vias in tight clusters near traces

Eliminate Copper Islands

Review copper pours for small, isolated areas that might trap etchant or detach during fabrication. Most DFM tools flag copper islands below a minimum size threshold.

Run DFM Analysis

Design Rule Checking (DRC) catches many issues, but dedicated Design for Manufacturing (DFM) analysis specifically identifies acid trap risks that basic DRC might miss.

Modern Manufacturing: Are Acid Traps Still a Problem?

The PCB industry has evolved significantly, and modern fabrication processes have greatly reduced acid trap risks:

Photoresist Technology

Modern photoresist provides uniform, high-quality coverage that’s far more resistant to etchant penetration than older methods. UV-hardened resist creates a robust barrier that prevents tunneling even when etchant pools briefly.

Alkaline and Photo-Activated Etchants

Most production PCB facilities have moved away from traditional acidic etchants. Alkaline etching solutions are less aggressive and more controllable. Photo-activated etchants only work when exposed to UV light, providing precise control over where etching occurs.

Improved Process Control

Modern spray etching systems ensure etchant is continuously moving across the board surface, preventing pooling. Automated rinsing removes residual chemicals before they can cause damage.

The Practical Reality

Does this mean you can ignore acid traps entirely? Not quite. While modern manufacturing has dramatically reduced the risk, following good design practices costs nothing and provides margin against process variations. Some fabricators—particularly lower-cost offshore suppliers or DIY processes—may still use older methods where acid traps are more problematic.

The recommendation: continue using 45-degree routing and avoiding acute angles as standard practice. It’s become an industry convention that ensures compatibility across fabricators and provides design margin without any real cost.

Read more about design for series articles:

Acid Traps in PCB Design: What They Are & How to Avoid Them

Best DFM Software Tools for PCB Design in 2026: A Practical Guide from the Workbench

Boundary Scan Testing (JTAG) in PCB Design: A Complete DFT Guide

Boundary Scan Testing (JTAG) in PCB Design: A Practical DFT Guide

Common PCB Manufacturing Defects: How DFM Prevents Costly Errors

Design for Assembly (DFA) in PCB: Best Practices for Efficient Production

Design for Cost (DFC): How to Reduce PCB Production Expenses

Design for Environment (DFE): Sustainable PCB Design Practices

Design for Reliability (DFR): Building Long-Lasting PCBs

Design for Testability (DFT): How to Create Easily Testable PCBs

DFM vs DFA: Key Differences and When to Apply Each in PCB Design

DFMA in PCB Design: Combining Manufacturing & Assembly Optimization

How to Reduce PCB Production Costs with DFX Strategies

Improve PCB Yield Rate: DFM & DFA Techniques That Work

PCB Design for Manufacturing (DFM): Complete Guide to Reduce Costs & Errors

PCB DFM Checklist: 25 Critical Items to Review Before Manufacturing

Test Point Design Guidelines for PCB Testability (DFT)

What is DFX (Design for Excellence)? Complete Framework for PCB Design

DFM Tools and Resources for Acid Trap Prevention

Design Analysis Tools

Industry Standards

• IPC-2221: Generic standard on printed board design
• IPC-2222: Sectional design standard for rigid organic boards
• IPC-7351: Land pattern requirements

Manufacturer Resources

Most PCB fabricators publish design guidelines that include angle and spacing recommendations. Request your fabricator’s design rules before starting layout.

Frequently Asked Questions About Acid Traps in PCB Design

Are 90-degree trace angles really a problem?

For acid traps specifically, 90-degree angles are borderline—they create potential pooling areas but aren’t as problematic as acute angles. The bigger concern with 90-degree angles is signal integrity in high-speed designs, where the extra capacitance at corners can cause reflections. Using 45-degree routing eliminates both concerns with no downside, making it the preferred standard. Modern etching processes handle 90-degree angles reasonably well, but there’s no reason to use them when 45-degree routing is equally easy.

Do I need to worry about acid traps with a reputable PCB manufacturer?

Reputable manufacturers using modern processes have largely eliminated acid trap problems through improved etchants and process control. However, designing to avoid acid traps costs nothing and provides margin against process variations. It also ensures your design works if you later switch to a different fabricator or need to use a lower-cost supplier. Consider it cheap insurance that’s already built into standard design practices.

Can DRC catch all acid trap issues?

Basic Design Rule Checking catches acute angles if you’ve configured angle rules, but it may miss more subtle acid trap conditions like tight enclosed spaces or problematic copper pour geometries. Dedicated DFM analysis tools provide more comprehensive checking. Running both DRC and DFM analysis before release gives you the best coverage. Many fabricators offer free DFM checking as part of their service.

What angle should I use for PCB routing?

Use 45-degree routing as your default for all designs. This creates 135-degree corners when changing direction—smooth transitions that eliminate acid trap risk and provide good signal integrity. For very high-speed designs (multi-gigabit signals), curved arc routing provides even smoother transitions. Avoid angles less than 90 degrees entirely, and use 90-degree angles only when absolutely necessary for space constraints.

How do teardrops help prevent acid traps?

Teardrops eliminate the sharp internal corners that form where traces connect to pads and vias. Without teardrops, these T-junctions create exactly the kind of acute internal angles that trap etchant. With teardrops, the copper transitions gradually, eliminating the pocket where chemicals could accumulate. Most PCB design tools can automatically insert teardrops—enable this feature globally for best results.

Conclusion: Good Practice That Costs Nothing

Acid traps represent a historical concern that modern manufacturing has largely addressed. But the design practices developed to prevent them—45-degree routing, adequate spacing, teardrops, DFM analysis—remain valuable because they cost nothing to implement and provide margin against manufacturing variations. They’ve become industry conventions that every PCB designer should follow.

Whether you’re designing a simple two-layer board or a complex HDI stack-up, using 45-degree routing and avoiding acute angles ensures your design is manufacturable across the widest range of fabricators. It’s one of those best practices that’s easy to follow and has no downside—the definition of a design rule worth keeping.