What Is Legend Print in PCB:Screen Printing Vs PCB Inkjet Printing

When manufacturing printed circuit boards (PCBs), adding legible and durable text and markings is an important part of the process. These markings, known as the “legend”, help identify components, connections and provide other vital information for PCB assembly and troubleshooting.

There are two primary methods for adding legend printing to PCBs during fabrication - screen printing and inkjet printing. Both have their advantages and disadvantages when it comes to factors like resolution, durability, cost and more.

This article will examine legend printing for PCBs in detail, comparing screen printing and inkjet printing processes. It will look at how each method works, pros and cons of each, and what to consider when determining the best option for a particular PCB design.

What is Legend Printing on PCBs?

Legend printing refers to adding text legends, markings, and other printed information to the surface of a PCB. This includes:

• Component designators - The reference labels for each component such as R1, C3, U1, etc.
• Polarities - Plus (+) and minus (-) symbols.
• Warnings - Any caution or warning statements.
• Board information - Names, logos, copyright notices, etc.
• Component values - Resistance, capacitance and other specs.
• Component outlines - The lines showing component shapes and footprints.
• Solder masks - The coatings that define solderable and non-solderable areas.
• Fiducials - The reference marks used for PCB assembly.

This printed legend provides critical information for PCB assembly, testing, troubleshooting and long term maintenance. Good legend quality ensures legibility, adhesion and durability.

The two main industrial processes used for legend printing are screen printing and inkjet printing. Let's look at both in more detail:

Screen Printing Overview

Screen printing, also known as silkscreen, stencil or thick film printing, is a tried and tested process used for legend printing on PCBs. It involves using a fine mesh stencil screen with an image pattern, then pushing ink through the pattern and onto the PCB surface.

The basic process steps are:

1. The artwork is photo-imaged onto a fine stainless steel or polyester screen. This creates a stencil with blank areas that will block ink, and open mesh areas where ink can pass through.
2. The screen is lined up and clamped over the PCB surface.
3. Ink is placed at one edge of the screen, known as flood coating. A rubber squeegee blade is then used to push the ink across the screen, forcing it through the open mesh and onto the PCB surface below.
4. The screen is lifted, leaving the image pattern ink transferred to the PCB. The board is then oven cured.

Multiple overlapping colors can be printed in separate passes to build up complex legend graphics as needed. Screens can also be setup for solder masks.

Overall, screen printing allows fast, high volume PCB legend printing with good opacity and durability. However it does have some limitations.

Inkjet Printing Overview

Inkjet printing refers to non-contact printing using inkjet heads that spray microscopic droplets of ink onto a surface. It is the same basic technology used in desktop inkjet printers.

For PCB production, industrial inkjet printers use precise positioning systems and specialized inks to jet patterns directly onto the boards.

The process involves:

1. Loading PCBs into the printer bed. Optical positioning aligns the board.
2. The print heads jet tiny ink droplets, building up the image. Typical droplet size is 10-40 picoliters.
3. Curing systems such as UV light instantly dry the ink.
4. Further passes can overlay colors to create complex legends.

Inkjet allows on-demand variable printing without screens or tooling. It provides finer resolution than screen printing, but the ink durability is lower. Tradeoffs versus screen printing depend on the PCB application.

Now that we have an overview of both methods, let's compare them in more detail:

Screen Printing vs Inkjet - Resolution and Detail

• Screen printing resolution - Typically 100 μm (0.1 mm) lines and spaces is the finest detail possible. Practical minimum text size is around 1.5mm.
• Inkjet printing resolution - Modern inkjets can print down to around 30-40 μm resolution. This allows finer lines, smaller text, barcodes and other intricate markings.

So inkjet has the advantage for printing very fine or complex legend details. Screen printing is better suited for simpler markings and larger text.

Durability and Layer Opacity

• Screen printing durability - The thick ink deposit cures to a tough, resilient print layer that is highly scratch and solvent resistant. Excellent adhesion to the PCB substrate.
• Inkjet printing durability - Inkjet uses thin ink layers so abrasion resistance is lower. However specialized ink chemistries help improve durability close to screen printing levels. Adhesion can also be very good.
• Screen printing opacity - The thick ink deposit provides high opacity even with the darkest board colors underneath. This ensures good legibility.
• Inkjet printing opacity - Thinner ink layers mean opacity is reduced, especially on dark substrates. May require white ink base layer first to aid contrast.

So screen printing generally has better durability and opacity. But inkjets are improving in these areas with new ink formulations.

Legend Alignment and Positioning

• Screen printing alignment - Screen alignment relies on using registration marks to manually align the screen over the PCB. Typical alignment accuracy is ±0.5mm.
• Inkjet printing alignment - Inkjets digitally register artwork and automatically align to board fiducials for precision positioning. Accuracy can be ±0.1mm or better.

So inkjets have much better capabilities for aligning legends precisely over components and existing board features.

Speed and Volume

• Screen printing speed - Very fast, capable of printing multiple boards per second in production. Excellent for high volume PCB production.
• Inkjet printing speed - Slower than screens due to the scanning print heads building up each image. But throughput is still suitable for moderate to high volume PCB production.

Screen printing has the advantage for the absolute highest volume PCB legend printing needs. But inkjets are still reasonably fast.

Costs

Both methods require significant initial equipment costs and setup:

• Screen printing costs - Screen fabrication and plates add setup costs. But running costs are low. Ideal for printing the same board design at very high volumes.
• Inkjet printing costs - No tooling costs, but printers have high initial costs. Better suited for fast design iterations or lower volume batches where stencil costs are prohibitive.

So screen printing has lower costs at very high volumes. Inkjets are more economical at lower volumes with frequent design changes.

Legend Printing Options Summary

Other Legend Printing Methods

There are some other options for adding legends to PCBs besides traditional screen printing and inkjet:

• Laser printing - Laser light is used to change surface chemistry, either removing copper or ablating an polymer coating to create markings. Allows finer details than screens but lower durability.
• Photolithography - Can pattern legends using photoresist masks and etch processes similar to PCB fabrication. Expensive and complex.
• Thermal printing - Uses thermal print head to transfer foil legends. Typically lower resolution and durability.

However these niche methods have failed to displace traditional screen and inkjet printing as the mainstream PCB legend processes.

Best Applications for Each Method

When to Use Screen Printing

Screen printing excels in these applications:

• High volume production - Fast throughput and lowest running costs suits ultra high volume PCB production.
• Durability critical - Thick print layers resist abrasion. Important for exposed external PCBs.
• Complex graphics - Overlapping colors easily produce complex logos, icons and markings.
• Solder masks - Thick ink gives excellent solder mask edge definition and coverage.
• Low precision requirements - Registration accuracy is sufficient for many PCBs.

So screen printing is ideal where speed, durability, graphics quality and cost are the priorities.

When to Use Inkjet Printing

Inkjet printing is best suited for:

• Lower volume batches - Avoid stencil costs. Fast design iterations.
• Fine features - High resolution prints fine lines, small text, barcodes, etc.
• Precise alignment - Pinpoint registration for dense boards.
• White base layer - Where a white ink pre-coat is needed for contrast on dark boards.
• Late stage customization - Add unique serial numbers, barcodes, texts without changing screens.
• On-demand prints - Avoid stocking pre-printed boards. Print legends at time of order.

In summary, inkjet printing provides precision printing capabilities not easily possible with screens.

Summary of Screen Printing vs Inkjet Printing for PCB Legend

Both screen printing and inkjet printing have their place for legend printing on PCBs:

• Screen printing is the workhorse for high durability and very high volume PCB production. Especially where finer details are not required.
• Inkjet printing provides new levels of precision and flexibility. It expands the capabilities for digitally printable legend markings.

Many PCB manufacturers now utilize both technologies - using inkjets for prototyping and design iterations, then transitioning to traditional screen printing for mass production.

The optimal choice depends on factors like run volume, legend detail, alignment precision, durability needs and production work flows. Working with an experienced PCB fabrication partner experienced in both methods helps determine the best approach.

Frequently Asked Questions

Q: What are some typical legend printing specifications?

A: Common specifications include:

• Text height: Typically 1.5mm minimum for screen printing, 0.7mm capable with inkjet.
• Line/space width: 100μm for screen printing, 30-40μm for inkjet
• Alignment tolerance: ±0.5mm for screens, ±0.1mm for inkjet
• Layer thickness: 10-30μm for screens, 0.5-5μm for inkjet
• Adhesion: 1B-3B tape test for most applications
• Abrasion resistance: Typically >20 rubs with screwdriver test

Q: Does legend printing require special PCB surface finishes?

A: Most standard PCB finishes like ENIG, immersion tin, silver, HASL lead-free are compatible. But finishes like hard gold or OSP can cause adhesion problems with some legend inks. Discuss requirements with your PCB manufacturer.

Q: Can legends be printed on both sides of a PCB?

A: Yes, legends can be screen printed or inkjet printed on either or both sides of a board as required. Registration accuracy becomes more important for double sided applications.

Q: What file formats do I provide for legend print artwork?

A: Common formats are Gerber files, DXF/DWG, PDF, EPS, TIFF, etc. Your PCB supplier will specify what exact files they require. Keep all layers separate.

Q: How are white legends printed on dark soldermasks?

A: A white ink background layer is first printed over the soldermask to provide contrast. Color legends are then printed on top of the white layer. Inkjet offers superior results.

Q: Can legend printing combine both inks and solder masks?

A: Yes, it is common to screen print solder masks and legends in a single pass. Some inks and masks may not be compatible - check with your PCB supplier.