Introduction

A printed circuit board (PCB) is the foundation of an electronic device, providing mechanical support and electrical connections between components. The PCB fabrication process involves many steps, including applying a solder mask layer on top of the copper traces. The solder mask serves several important functions:

• It prevents solder bridges from forming between adjacent traces during soldering
• It provides electrical insulation between traces
• It protects the copper from oxidation and corrosion
• It improves aesthetic appearance by coloring the substrate

Two key terms related to the solder mask are “solder mask bridge” and “solder mask opening”. Understanding these defects is critical for PCB designers and manufacturers to ensure proper function and reliability of the boards.

Solder Mask Bridge

A solder mask bridge occurs when a thin section of solder mask inadvertently connects two adjacent conductors on a PCB that are meant to be electrically isolated. This bridge can form during the solder mask application process if proper care is not taken.

Solder mask is typically applied by screen printing or photoimaging. In screen printing, a thick liquid solder mask material is squeezed through a fine mesh screen onto the PCB surface. If the mesh holes become clogged or the mask material flows excessively, it can create a bridge between traces. In photoimaging, a liquid photosensitive solder mask is applied and then selectively cured using UV light through a mask. Overexposure can potentially connect adjacent traces.

Solder Mask Opening

A solder mask opening, also called a solder mask defined pad, refers to an opening in the solder mask that exposes the underlying copper pad. This allows solder to wet to the pad during component assembly. The size and shape of the opening precisely define the solderable area.

Solder mask openings are created by selective exposure and development of the photosensitive solder mask material. The openings must properly align with the copper features below. Any misalignment, undersize opening or pinhole can result in poor solder connection.

Causes of Solder Mask Bridge and Opening Defects

Many factors during PCB fabrication and assembly can lead to solder mask bridges or inadequate openings. Understanding the root causes is key to preventing these defects.

Solder Mask Bridges

• Insufficient or excessive solder mask thickness
• Clogged screen mesh during screen printing
• Misaligned or damaged solder mask stencil
• Overflow of solder mask material between traces
• Insufficient developing of photoimageable solder mask
• Overexposure of UV light during photoimaging

Solder Mask Openings

• Misalignment between solder mask layer and copper layers
• Undersized openings in solder mask master pattern
• Insufficient developing of photoimageable solder mask
• Plugged holes in screen mesh during screen printing
• Solder mask pinholes due to contamination or damage

Effects of Solder Mask Bridge and Opening Defects

Solder mask bridges and openings can lead to several issues if not addressed. It is critical for PCB designers and assemblers to understand the potential effects.

Effects of Solder Mask Bridge

• Electrical short between traces that should be isolated
• Inability to solder components to bridged pads
• Short circuit and impaired functioning of assembled PCB
• Extreme cases can cause overheating, fire or device damage

Effects of Insufficient Solder Mask Opening

• Poor wetting and weak solder joint between pad and component lead
• Intermittent or open connection due to partial solder coverage
• Tombstoning of components during reflow soldering
• Stress on components leads due to misaligned connection

Effects of Excessive Solder Mask Opening

• Reduced insulation between adjacent solder joints
• Increased risk of solder bridges forming between pads
• Possible corrosion or dendrite growth between closely spaced traces

How to Prevent Solder Mask Defects in PCB Fabrication

Proper control during the solder mask application process is key to avoiding bridge or opening defects. Here are some tips for PCB fabricators:

Preventing Solder Mask Bridges

• Maintain optimum solder mask thickness, typically around 25-75 μm
• Use high-quality solder mask materials and processes
• Clean screen mesh thoroughly and regularly during screen printing
• Inspect stencil for damage between runs and replace if needed
• Use solder masks designed for fine pitch between traces
• Control exposure time and lamp height for photoimaging
• Validate developing process to clear all unexposed mask
• Adjust mask viscosity if overflow occurs between traces

Ensuring Adequate Solder Mask Openings

• Use automated optical inspection to catch any alignment issues
• Design openings with sufficient overlap onto pads, around 150 μm
• Validate accuracy of solder mask master patterns
• Use emulsions and mesh size appropriate for pad/opening pitch
• Inspect inside openings for plugging after screen printing
• Control UV exposure intensity and time for clean openings
• Verify fully developed openings by optical measurement

Solder Mask Inspection Methods

To catch solder mask defects early, PCB fabricators should utilize the following inspection methods during manufacturing:

Optical Inspection

Optical inspection with an automated vision system or microscope can quickly identify bridging or misaligned/missing openings. Backlighting and color filters aid in highlighting defects. Image analysis software can automatically compare to a reference.

Electrical Testing

In-circuit testing or flying probe testing can reveal electrical shorts or opens caused by solder mask issues. Test points between adjacent traces will show a short if bridged. Continuity tests indicate poor contact in openings.

Solder Paste Inspection

After solder paste application, optical inspection of the print pattern can infer potential solder mask issues. Misalignment, skips or bridging of the paste correspond to underlying mask defects.

Automated X-ray Inspection

X-ray imaging reveals hidden solder mask defects under components after soldering. Bridge or void analysis assists in identifying problem areas.

Cross-Sectioning

Physically cutting a sample and examining under a microscope is a definitive way to measure solder mask thickness and opening dimensions. It also validates alignment between layers.

How to Overcome Solder Mask Issues During PCB Assembly

If a PCB containing solder mask defects reaches assembly, there are ways to mitigate or overcome the issues:

Remedying Solder Mask Bridges

• Carefully scrape or slit the bridge using a knife or razor blade
• Use solder wick or vacuum desoldering tool to remove solder and break the bridge
• For severe cases, isolate the bridged traces by cutting them apart

Improving Solder Coverage in Openings

• Increase solder paste deposition in the opening
• Adjust pad geometries to allow sufficient filleting
• Switch to solder paste alloy with better gap-filling ability
• Reshape component lead to match pad for surface-mount devices

Mitigating Excessive Mask Openings

• Apply high-temperature non-conductive coating to exposed areas
• Cover nearby pads during rework soldering to prevent bridging
• Conformal coat PCB when assembled to secure loose contamination

While not ideal, these rework steps can salvage PCBs with solder mask issues and avoid scrapping expensive boards due to minor defects.

Solder Mask Bridge and Opening Analysis

To fully characterize solder mask defects and determine root causes, additional analysis steps are employed:

Optical Microscopy

Detailed examination under a high-magnification microscope reveals bridge or opening dimensions. Image capture and on-screen measurements determine spacing, overlap, and alignment.

Scanning Electron Microscopy (SEM)

SEM provides higher magnification views of solder mask features and edges. Elemental analysis can identify contaminants causing pinholes or blocking openings.

Fourier-Transform Infrared Spectroscopy (FTIR)

FTIR generates a chemical fingerprint of the solder mask material. Comparison to a reference helps determine if chemical composition played a role.

Ion Chromatography

This technique detects and measures ionic contaminants that may have contributed to solder mask defects by interfering with adhesion or curing.

Bond Strength Testing

Mechanically testing the adhesion of the solder mask to copper and substrate via peel or pull tests helps quantify bonding issues.

Improving Solder Mask Process Control

Based on analysis and root cause investigation, PCB manufacturers can implement solutions to achieve better solder mask process control and prevent defects:

Statistical Process Control

Track critical parameters like mask thickness, bonding strength, alignment accuracy, and pinhole counts using control charts. Monitor for excursions and trends.

Operator Training

Educate machine operators on proper setup, handling, and inspection to reduce human errors. Include defect photos in training materials.

Screen and Stencil Maintenance

Schedule regular cleaning and replacements of screens and stencils. Ensure proper tensioning, contact, and alignments.

Exposure Optimization

Dial in the optimal exposure energy and duration through design of experiments. Account for lamp aging and intensity variations.

Solder Mask Material Handling

Control storage, thawing, and ambient conditions for solder mask chemicals. Use in-line viscosity and tack measurements.

Surface Preparation and Cleaning

Standardize surface treatments and cleanliness testing of innerlayer boards prior to solder mask coating. Reduce contamination risks.

The Future of Solder Mask Technology

While existing solder mask processes are well-established in PCB fabrication, innovations continue to improve the technology:

Laser-Direct Imaging (LDI)

LDI solder mask eliminates screens and photo-tools by using a laser to directly ablate the image pattern layer-by-layer. This enhances accuracy and reduces defects.

Multi-Functional Solder Resists

Solder masks are moving beyond mechanical and insulating properties, offering built-in functions like heat dissipation, EMI shielding, corrosion inhibitors, and more.

Flexible and Stretchable Masks

New acrylic, silicone and polyimide solder mask materials enable reliable PCBs on flexible and bendable substrates for wearable devices.

3D Printing

Additive processes like stereolithography, aerosol jetting, and inkjet are being adapted to construct solder mask layers from CAD models with micron precision.

Laser Cut Openings

In an additive process, the entire substrate can be coated with solder mask first, then a laser cuts out openings precisely aligned to conductor features underneath.

Conclusion

As a critical layer in PCB fabrication, proper application of solder mask is essential to manufacturing quality boards. Solder mask bridges can lead to shorts or impaired soldering, while inadequate openings result in poor connections. Cause analysis paired with process improvements enables the prevention of solder mask defects. Advancements in materials and additive processes promise continued enhancement in solder mask performance and capabilities. With diligent process control and inspection, PCB fabricators can avoid costly solder mask issues during assembly and produce robust, reliable circuits.

FQA

Here are some common questions and answers about solder mask bridges and openings:

Q1: What is the typical maximum allowable size for a solder mask bridge?

A1: The industry standard IPC-A-610E states solder mask bridges up to 0.1 mm (3.9 mils) are generally acceptable, however many manufacturers specify tighter criteria of 0.05 mm (2 mils) max bridge width.

Q2: What are common causes of solder mask pinholes?

A2: Contamination from dust, fibers or oil is the most common cause of pinhole defects in solder mask. Insufficient cleaning, poor mask adhesion, and damage during handling can all contribute to pinholes.

Q3: How much misalignment between solder mask openings and pads is acceptable?

A3: As a guideline, the IPC standard allows up to 25% reduction of overlap around a pad before it is deemed a defect. Around 75 μm of overlap is recommended for a robust solder joint.

Q4: What are some key solder mask properties that aid application and performance?

A4: Important properties include adhesion strength, viscosity, thermal stability, chemical resistance, flexibility, and high dielectric strength. These impact the coatability, resolution, and functionality of the solder mask.

Q5: What are some alternatives to traditional epoxy-based solder mask materials?

A5: Some options include UV-curable acrylic, silicone-polymer composite, polyimide, and flexible ink-based masks for specialized applications requiring heat/chemical resistance or stretchability.