Introduction

The silkscreen, also known as the soldermask legend, is the layer of printed text, markings, and graphics on the surface of a printed circuit board (PCB) that identifies components, displays text, and provides reference markers for manufacturing and assembly. This article provides a comprehensive overview of PCB silkscreen including its purposes, printing process, design, applications, and considerations.

Definition of a PCB Silkscreen

The silkscreen refers to the topmost layer on a PCB consisting of text, component outlines, fiducials, logos, and other graphics printed in ink. It takes its name from the fabric mesh historically used in the printing process. Silkscreening allows marking non-conductive areas of the PCB surface.

Modern silkscreens utilize advanced print, photoresist, and laser direct imaging techniques.

Purposes of Silkscreen

Key roles that the silkscreen serves:

• Component Identifiers – Names, designators, values for placement and assembly
• Component Outlines – Displays footprint outlines for BGAs etc.
• Board Information – Title, revision, manufacturer data
• Logo and Text – Company branding information
• Special Markings – Date code, copyrights, approvals
• Solder Mask Dam – Provide solder mask clearance around pads
• Alignment Marks – For secondary operations like assembly
• Board Zones – Identify sections like analog, high-speed digital etc.
• Assembly Instructions – Placement directions, cautions
• Test Points – Identify probe locations

Silkscreen Printing Process

The silkscreen legend goes through a printing and curing process:

1. Liquid Photoimageable Ink – Light sensitive ink applied by curtain coating

2. Mask and UV Exposure – Mask blanks areas, expose to harden printed regions

3. Develop – Remove unexposed ink, leaving printed pattern

4. Cure – Thermally cure the ink

5. Strip Resist – Remove remaining mask to complete legend

Silkscreen inks are epoxy-based to bond with the soldermask layer underneath. Fine pitch designs may requrie reduced ink thickness.

Types of Silkscreen

Several methods print the silkscreen legend:

Liquid Photoimageable

• Most common method
• Light sensitive ink and photomask process
• Wet chemical developing
• Typical thickness of 0.35-0.85 mil

Dry Film

• Laminated photo resin layer
• Exposed and developed like liquid photo
• Slightly wider line/space capability
• Thickness around 0.5-5 mil

Inkjet Printing

• Direct inkjet deposited graphics
• No masking
• Lower durability and resolution
• Layer thickness 0.1-0.3 mil

Laser Direct Imaging

• Direct laser ablation of paint or anodization
• No ink, masks, or developing
• Expensive but high mix capable
• Resolution down to 200 μm

Silkscreen Location

The silkscreen can be printed onto:

• The top (primary) surface of the PCB
• The bottom (secondary) side of the board
• Both the top and bottom of the board

Double-sided printing provides markings on both sides for assembly and test convenience.

Silkscreen Registration

Accurate registration between the copper layer and silkscreen is critical:

• Copper-to-Legend – Aligns silkscreen to pads and traces
• Layer-to-Layer – Aligns front and back silkscreens

Misaligned screens can prevent solder paste deposition or obstruct components.

Typical Silkscreen Design Elements

Common items included on the silkscreen legend are:

• Part reference designators – R1, U3, J7 etc.
• Values – 10K, 22pF, 16MHz etc.
• Component outlines – Displays footprint border
• Text – Titles, board info, warnings
• Lines and boxes – Group related elements
• Logos and graphics
• Special symbols – Component polarization indicators
• Copper pour clearance – Prevent mask shorts

Silkscreen Design Guidelines

IPC-2221A provides silkscreen design guidelines:

• Place text on same side as components
• Ensure text is readable – 1.5mm minimum
• Put reference designators near components
• Avoid placing text on pads or vias
• Include component second line text if helpful
• Pick readable fonts and sizes
• Allow clearance from pads/traces
• Use frame outlines for hidden devices like BGAs

Reviewing with fabrication/assembly partners is recommended.

Silkscreen Design Considerations

Additional considerations when designing the silkscreen:

• PCB Density – Allow space for all required text and symbols
• High Voltage Areas – Keep markings out of danger areas
• Future Production – Include markings for alternate components at same location
• Multi-Function Areas – Allow for multiple reference designators if needed
• Fine Pitch Devices – Use smaller text sizes as needed
• Thermal Relief – Add dams around thermally sensitive parts
• Translation – Design for localization into multiple languages

Silkscreen Solder Mask Dam

A solder mask dam structurally isolates pads from the silkscreen:

Figure 2: Solder mask dam providing clearance between pads and silkscreen legend.

Benefits include:

• Prevents solder bridging from mask to pad
• Eliminates adhesive interactions between pad and legend
• Improves reworkability without legend disturbance
• Allows tighter silkscreen-to-pad spacing

Typical dam is 0.2mm away from pads.

Silkscreen Solder Paste Relief

Silkscreen and solder paste alignment is also critical:

• Legend can block paste deposition if misaligned
• Paste relief openings avoid this
• Creates windows in silkscreen for paste release

But relief openings also reduce available silkscreen space.

Silkscreen Reflow Considerations

Reflowing over the silkscreen requires awareness:

• Cured ink can withstand soldering heat
• But some solvents in paste can attack legend
• Optical brighteners in paste cause discoloration
• Replacing legend may be needed post-reflow

So expect some aesthetic deterioration, or use low-residue paste.

Silkscreen and Lead-Free Solder

Higher lead-free reflow temperatures impact silkscreens:

• Ink curing may need higher thermal stability
• Ink-to-substrate adhesion weakens
• Increased likelihood of delamination or blistering
• Thicker ink resists damage better

Legend as Artwork

Some designs utilize the silkscreen for branding artwork:

• Display company name, logos, slogans
• Nickname or title for the product
• Shows off company or product visual identity
• Makes counterfeiting harder

This allows customization for niche products.

Conductive Ink Safety Markings

Conductive ink printed on the legend provides warning symbols:

Figure 3: Conductive ink ESD symbols printed on PCB silkscreen.

• Allows printing caution symbols on live boards
• ESD, safety grounding, high voltage etc.
• Automated optical inspection can check
• Requires conductive ink printing

Silkscreen Thickness and Durability

Thicker silkscreen prints better withstand environmental stresses:

Table 1: Silkscreen thickness and durability by printing method

Maximum thickness depends on registration tolerances and clearance to conductors.

Silkscreen Damage

Common causes of silkscreen wear during PCB service life:

• Abrasion – Rubbing, scraping against surfaces
• Chemical – Solvents, cleaning agents, solder flux
• Thermal – Overheating during soldering or operation
• UV Exposure – Sunlight, photodegradation
• Humidity – Delamination in damp environments
• Cracking – Thermal expansion mismatch with substrate

Adequate silkscreen thickness reduces damage risks. But expect some degradation over product lifetime.

Silkscreen Repair and Rework

Damaged silkscreens can be repaired:

• Touch Up – Using matching pen or paint
• Printed Overlay – Adhesive legend printed on transparent film
• Full Replacement – Strip and reprint defective areas
• Scraping – Removing damaged silkscreen section

But rework is often easier with liquid photoimageable legend.

Silkscreen vs Laser Engraving

Laser engraving directly marks the substrate as an alternative to silkscreen:

Silkscreen

• Ink-based, masks required
• Fine features, high durability
• Lower cost

Laser Engraving

• Direct ablation marking
• Simpler process, no chemicals
• Crisp text even on BGAs
• Only marks surface

Silkscreen vs Inkjet Printing

Inkjet printers offer moderate print resolution:

Silkscreen

• Higher durability and longevity
• Fine pitch capability
• Lower running cost at volume

Inkjet Printing

• Eliminates screens and chemicals
• Simpler graphics changes
• Only short production runs

Automated Optical Legend Inspection

Automated tools like AOI systems check silkscreen quality:

• Verifies text legibility and print quality
• Checks location and alignment to copper
• Confirms all required markings are present
• Comparators check against golden board
• Can detect defects down to 0.15mm

This automation speeds inspection, improving quality.

Conclusion

The silkscreen legend fulfills a wide variety of invaluable marking, identification, and display purposes on PCB assemblies. As PCBs grow more complex, silkscreens must condense more information into less space while maintaining durability. Fortunately, continued advancements in printing technology empower PCB designers to incorporate high value visual aids without compromising on miniaturization and performance. The humble silkscreen remains a key asset in manufacturing, inspecting, assembling, operating, and servicing virtually all modern electronics hardware.

FAQs

Q: What other names refer to the PCB silkscreen?

A: Common synonyms are solder mask legend, photoimageable legend, screen print, and solder resist legend.

Q: What printing methods allow the smallest silkscreen text?

A: Both liquid photoimageable and dry film printing support text below 1mm for high density designs.

Q: Does silkscreen ink contain lead or other hazardous substances?

A: Most legend inks today are lead-free and RoHS compliant. Some risk exists with older PCB stocks.

Q: Can conductive inks be printed on the silkscreen layer?

A: Yes, using specialty conductive inks, warnings and symbols can be printed within the silkscreen.

Q: Does silkscreen ink withstand soldering heat?

A: Thermally cured legend inks are designed to resist typical soldering, though some degradation can occur over repeated reflow exposure.

How to elegantly arrange PCB silkscreen

PCB silkscreen printing is the use of screen printing technology to make printed circuit boards. The silk screen value is the silk screen layer. When drawing the pcb layer. The layer containing the text legend is used to mark components or add other information. This layer is called the silk screen layer.

The component shape, serial number, and other descriptive text are printed on the component surface or solder surface by screen printing to facilitate the plug-in (including the surface-mounted component patch) of the circuit board production process and the maintenance operation of the product in the future.

The PCB silk screen layer is generally placed on the top layer (Top), but for the electronic products with high failure rate and frequent maintenance, such as the motherboard of TVs, computer monitors, printers, etc., the silk screen layer can be set on the component side and the soldering surface. .

Many PCB engineers who draw PCB think that silkscreen does not affect the performance of the circuit, so it does not pay attention to silkscreen. However, for a professional hardware engineer, you must pay attention to these details.

The following describes the method of elegantly arranging PCB silkscreen.

1.Placement area

In general, the silkscreen printing of resistors, capacitors, tubes, etc., do not use four directions when placing, this will lead to debugging, repair, welding, see silk screen is very tired (the board has to turn several directions) .

Therefore, it is recommended to place it in two directions at most, as shown in the figure below. This way, it will be very convenient to view the silk screen.

As shown in the figure below, if the components are too dense, and you can’t put on the silk screen, you can write a silk screen in the blank place nearby, mark the arrow, and then draw a frame,which is convenient for identification.

2、Do not try to Via’s on the silkscreen

As shown in the figure below, the via is on the silkscreen 8 . After the PCB is made, you will not know whether it is R48 or R49.

3、Do not Arrange the silkscreen on high speed signals(eg, clock lines, etc.).

This recommendation is for high speed signals  on the top or bottom layer, because such signal circuit can be considered as microstrip lines.

The speed of the signal running on the microstrip line (phase velocity) is related to the medium. If the silk screen is pressed to the line, as shown in the figure below, the medium will become uneven, causing the phase velocity to change, and finally the impedance is discontinuous. , affecting signal quality.

Of course, there will be no such problem

with the signal line of the inner layer.

4、The reading direction of silkscreen should be consistent with the direction of use.

As shown in the figure below, the reading direction of the silkscreen is the same as the direction of use of the chip, mainly when welding, reducing the probability of the welding being reversed.

Others, such as electrolytic capacitors, may not follow this recommendation because you can indicate positive and negative polarity.

5、The pin number should be marked on the silkscreen.

As shown in the figure below, the P3 connector is marked with 4 pin numbers for easy debugging/installation. In addition, it is best to mark the pin-intensive places, such as chips, FPC sockets, etc.

At the same time, it also meets the previous recommendation. The reading direction of P3 is the same as the direction of use of the connector.

6、Silkscreen printing of special electroni ccomponents.

For special components such as BGA and QFN, the size of the silkscreen should be exactly the same as the size of the chip (as shown in the figure below). Otherwise, it will be difficult to align and affect the soldering.

7、Silkscreen of the mount hole

Here, the silk screen of the screw is added near the mounting hole, and the length and the total number of screws are marked for easy installation.