IPC-SM-840 Solder Mask Requirements: How to Specify Class T and Class H

If you’ve ever specified solder mask on a PCB fabrication drawing, you’ve likely referenced IPC-SM-840. This standard governs everything from material qualification to performance testing for permanent solder mask and flexible cover materials. Getting the specification right matters—the wrong class designation or missing requirements can lead to boards that fail in the field or get rejected at incoming inspection.

In this guide, I’ll walk through everything you need to know about IPC-SM-840, from understanding the difference between Class T and Class H to properly calling out solder mask requirements on your drawings. Whether you’re a design engineer specifying boards or a quality engineer evaluating incoming material, this information will help you work confidently with this essential PCB standard.

What is IPC-SM-840?

IPC-SM-840, officially titled “Qualification and Performance Specification of Permanent Solder Mask and Flexible Cover Materials,” establishes requirements for evaluating liquid and dry film solder mask materials. The standard determines whether a solder mask is acceptable for use on printed circuit boards based on standardized testing.

The current revision is IPC-SM-840E, published in December 2010, which added requirements for flexible cover materials (coverlays) used on flex circuits. The standard runs approximately 19 pages and provides comprehensive test requirements, pass/fail criteria, and classification guidance.

Purpose of IPC-SM-840

The standard serves three primary purposes:

1. Material Evaluation – Establishes standardized tests for solder mask vendors to qualify their products
2. Production Qualification – Enables board fabricators to qualify their solder mask application processes
3. End-Use Classification – Provides class designations (T and H) based on reliability requirements

IPC-SM-840 ensures that solder mask materials can withstand manufacturing processes (soldering, cleaning, handling) and provide long-term protection in the end-use environment.

Why Solder Mask Matters for PCB Reliability

Before diving into the specification details, let’s understand what solder mask actually does and why proper qualification matters.

Functions of Solder Mask

Function	Benefit
Prevents solder bridging	Confines solder to intended pad areas during assembly
Electrical insulation	Provides dielectric barrier between adjacent conductors
Environmental protection	Shields copper traces from oxidation, contamination, and moisture
Mechanical protection	Guards against handling damage, scratches, and debris
Electromigration prevention	Retards dendritic growth and other conductive formations

A solder mask that passes IPC-SM-840 qualification has demonstrated it can perform these functions reliably under standardized test conditions.

IPC-SM-840 Class T vs Class H: Understanding the Difference

The most critical decision when specifying solder mask is selecting the appropriate class. IPC-SM-840 defines two classes based on end-use requirements:

Class T – Telecommunications

Class T applies to:

• Computers and data processing equipment
• Telecommunication equipment
• Sophisticated business machines and instruments
• Certain non-critical military applications
• High-performance commercial and industrial products

Solder mask qualified to Class T is suitable for applications requiring extended performance life but where interrupted service is not life-threatening. Think commercial electronics where reliability matters but failure doesn’t create safety hazards.

Class H – High Reliability/Military

Class H applies to:

• Equipment where continued performance is critical
• Systems where downtime cannot be tolerated
• Life support equipment
• Critical military and aerospace applications

Solder mask qualified to Class H must meet more stringent test requirements and tighter pass/fail criteria. This class is mandatory when uninterrupted service is essential.

Class T vs Class H Comparison

Requirement	Class T	Class H
Intended Use	Commercial, telecommunications	Military, aerospace, life support
Test Severity	Standard	More stringent
Flammability	UL 94 V-1 minimum, oxygen index ≥28%	Cannot raise UL 94 V-number of base laminate
Typical Applications	Consumer electronics, computers, industrial	Avionics, medical devices, defense
IPC-6012 Correlation	Class 1 and Class 2 products	Class 3 products

Historical Note on Class Designations

Older IPC documents reference Class 1, Class 2, and Class 3 solder masks. For practical purposes:

• There is no Class 1 solder mask (consumer grade doesn’t require SM-840 qualification)
• Class 2 is equivalent to Class T
• Class 3 is equivalent to Class H

If you encounter legacy drawings with the old class designations, use this translation when specifying to current standards.

IPC-SM-840 Test Requirements Breakdown

IPC-SM-840 qualification involves a comprehensive series of tests covering physical properties, chemical resistance, soldering compatibility, electrical performance, and environmental durability.

Physical Property Tests

Visual Inspection (Section 3.3.1) The cured solder mask must be uniform in appearance, free from defects that would affect functionality.

Non-Nutrient (Section 3.2.6) The solder mask material shall not contribute to biological growth. This prevents fungal contamination in humid environments.

Pencil Hardness (Section 3.5.1) Minimum hardness of “F” on the pencil hardness scale. Many commercial masks achieve 4H to 6H hardness, well exceeding the minimum requirement.

Adhesion (Section 3.5.2) Tests adhesion to various substrates including:

• Copper surfaces
• Nickel surfaces
• FR-4 laminate
• Double-layered solder mask (mask over mask)

The solder mask must maintain adhesion after exposure to thermal stress and soldering processes.

Machinability (Section 3.5.3) The cured mask must withstand machining operations (drilling, routing) without cracking or tearing.

Chemical Resistance Tests

Resistance to Solvents and Cleaning Agents (Section 3.6.1.1) The solder mask must resist attack from common solvents and cleaning chemicals used in PCB assembly. IPC-SM-840 specifies exposure to multiple solvents including:

Solvent Category	Examples
Alcohols	Isopropyl alcohol (IPA)
Chlorinated solvents	Various cleaning agents
Flux removers	Saponified and semi-aqueous cleaners
Process chemicals	Photoresist strippers, etchants

The mask must show no evidence of softening, wrinkling, blistering, or loss of adhesion after solvent exposure.

Hydrolytic Stability/Aging (Section 3.6.2) Tests long-term stability under heat and humidity:

• 28 days exposure to 95-99°C at 90-98% RH
• No visible change (cracking, blistering, discoloration, loss of adhesion)

This accelerated aging test simulates years of field exposure in demanding environments.

Flammability Requirements

UL 94 Flammability (Section 3.6.3)

Class	Requirement
Class H	Solder mask cannot raise the UL 94 flammability “V” number of the base laminate
Class T	V-number cannot be raised by more than one rating; minimum rating V-1; oxygen index ≥28%

Flammability testing ensures the solder mask doesn’t compromise the fire resistance of the PCB assembly.

Soldering Requirements

Solderability (Section 3.7.1) The solder mask coating shall not adversely affect solderability of exposed pad areas when tested per J-STD-003. This ensures the mask doesn’t contaminate or interfere with solder wetting.

Resistance to Solder (Section 3.7.2) Tests resistance to molten solder contact:

• No solder sticking to mask surface
• No degradation of mask properties after solder exposure

This is critical for wave soldering and reflow processes where the mask contacts molten solder.

Electrical Requirements

Dielectric Strength (Section 3.8.1) Minimum dielectric withstanding voltage requirements:

Condition	Requirement
Standard	500 VDC per 25 µm (0.001 in) of thickness
Thin mask (<25 µm)	Absolute minimum 500 VDC breakdown voltage

This ensures the solder mask provides adequate electrical insulation between adjacent conductors.

Moisture and Insulation Resistance Tests electrical performance after humidity exposure to ensure the mask maintains insulating properties in moist environments.

IPC-SM-840 and Flexible Cover Materials

Revision E of IPC-SM-840 added requirements for flexible cover materials (coverlays) used on flexible printed circuits. These materials serve a similar protective function but must also withstand repeated flexing without cracking or delaminating.

Cover Material Classifications

Class	Application
Class TF	Telecommunication flex applications
Class HF	High reliability flex applications

Cover materials undergo additional flex testing to verify they maintain adhesion and protective properties through repeated bending cycles.

How to Specify Solder Mask per IPC-SM-840

Properly specifying solder mask on your fabrication drawing prevents misunderstandings and ensures you get boards that meet your requirements.

Basic Specification Format

A complete solder mask callout should include:

SOLDER MASK OVER BARE COPPER PER IPC-SM-840 CLASS [T/H], COLOR [GREEN/OTHER], [MATTE/SATIN/GLOSS] FINISH.NO MASK ALLOWED IN HOLES, ON FIDUCIALS, OR ON SOLDERABLE SURFACES.

Key Elements to Specify

Element	Options	Notes
Standard Reference	IPC-SM-840E	Always specify current revision unless legacy required
Class	T or H	Based on end-use reliability requirements
Color	Green, red, blue, black, white, yellow	Green is most common; affects inspection visibility
Finish	Matte, satin, semi-gloss, gloss	Matte reduces solder balling during reflow
Coverage	SMOBC (Solder Mask Over Bare Copper)	Most common; mask applied before surface finish
Exclusions	Holes, pads, fiducials, test points	Define keep-out areas clearly

IPC-6012 Integration

If your drawing calls out IPC-6012 (Qualification and Performance Specification for Rigid Printed Boards), solder mask is automatically covered under Section 3.7:

“When solder mask is required on printed boards, it shall meet the qualification/conformance requirements of IPC-SM-840. If a solder mask performance class is not specified for Class 1 or 2, IPC-SM-840 Class T shall be used. For Class 3, IPC-SM-840 Class H shall be used.”

This means:

• IPC-6012 Class 1 or 2 boards → Default to IPC-SM-840 Class T
• IPC-6012 Class 3 boards → Require IPC-SM-840 Class H

You should still explicitly specify the solder mask class on your drawing for clarity.

Read more IPC Standards:

Solder Mask Types and IPC-SM-840 Applicability

IPC-SM-840 covers both liquid and dry film solder mask materials. Understanding the different types helps you select appropriate materials for your application.

Liquid Photoimageable Solder Mask (LPI/LPISM)

LPI is the most common solder mask type used today. It offers:

• High resolution for fine-pitch components
• Excellent adhesion to uneven board surfaces
• Cost-effectiveness for high-volume production
• Compatibility with lead-free soldering temperatures

LPI is applied by screen printing, curtain coating, or spray coating, then exposed through a photomask and developed to create the pattern.

Dry Film Solder Mask (DFSM)

Dry film provides:

• Consistent, uniform thickness
• Excellent for controlled impedance applications
• Preferred for some RF/microwave applications

DFSM is vacuum-laminated onto the board, then exposed and developed similar to LPI.

Application Method Comparison

Characteristic	LPI	Dry Film
Resolution	Excellent (<50 µm features)	Good
Thickness uniformity	Variable (depends on topography)	Excellent
Cost	Lower	Higher
Volume suitability	High volume	All volumes
Topography conformance	Excellent	Good (best on flat surfaces)
Common applications	General PCB	RF, high-frequency, controlled impedance

Both types must meet IPC-SM-840 requirements when qualified.

Solder Mask Thickness Requirements

IPC-SM-840 doesn’t specify a fixed solder mask thickness like “20 µm” or “25 µm.” Instead, it requires:

“The printed board fabricator shall confirm that the minimum thickness of the solder mask on the printed board is sufficient to meet the requirements of 3.8.1 [Dielectric Strength].”

Practical Thickness Guidelines

Location	Typical Thickness	Notes
Over traces	12-25 µm (0.5-1.0 mil)	Minimum for adequate protection
Between traces	25-50 µm (1.0-2.0 mil)	Fuller coverage in valleys
Trace edges	5-8 µm minimum	Critical for dielectric strength
Overall range	12-75 µm	Varies by application method

The key requirement is meeting the 500 VDC per 25 µm dielectric strength specification. Thinner coatings must still achieve 500 VDC absolute minimum breakdown voltage.

Related IPC Standards for Solder Mask

IPC-SM-840 works alongside several related standards:

Standard	Title	Relationship
IPC-HDBK-840	Solder Mask Handbook	Companion handbook with practical guidance
IPC-6012	Qualification and Performance of Rigid PCBs	Invokes SM-840 for solder mask requirements
IPC-6013	Qualification and Performance of Flexible PCBs	References SM-840 for coverlay requirements
IPC-A-600	Acceptability of Printed Boards	Visual inspection criteria for solder mask defects
IPC-TM-650	Test Methods Manual	Referenced test procedures
J-STD-003	Solderability Tests	Solderability testing methodology

IPC-HDBK-840: The Solder Mask Handbook

For detailed practical guidance on solder mask selection, application, and troubleshooting, IPC-HDBK-840 provides comprehensive information including:

• Solder mask material properties
• Application process considerations
• Interactions with surface finishes
• Troubleshooting methodology
• Design guidelines

Qualification vs Conformance Testing

IPC-SM-840 distinguishes between two types of testing that serve different purposes:

Material Qualification Testing

Material qualification is performed by solder mask vendors to prove their products meet IPC-SM-840 requirements. This involves:

• Complete testing per Table 1 of the specification
• Testing on standardized IPC-B-25A test boards
• Documentation of all test results
• Identification of qualified class (T, H, or both)

Once a solder mask material is qualified, the vendor can claim IPC-SM-840 compliance for that specific formulation. Any formulation changes beyond specified thresholds require requalification.

Production Process Qualification

Board fabricators must also qualify their solder mask application processes. This ensures that the qualified material performs correctly when applied using the fabricator’s specific equipment and procedures. Production qualification considers:

• Application method (screen print, spray, curtain coat)
• Cure parameters (time, temperature, UV exposure)
• Pre-treatment and cleaning processes
• Post-application handling

Ongoing Quality Conformance

After initial qualification, IPC-SM-840 requires ongoing conformance testing to verify continued compliance. This includes periodic testing of production lots to ensure batch-to-batch consistency.

Common Solder Mask Defects and IPC-SM-840

Understanding what IPC-SM-840 qualified materials should prevent helps identify process issues:

Defect	Cause	SM-840 Related Test
Poor adhesion	Surface contamination, inadequate cure	Adhesion testing (3.5.2)
Solder bridging	Mask too thin, poor registration	Resistance to solder (3.7.2)
Mask lifting	Solvent attack, thermal stress	Solvent resistance (3.6.1)
Discoloration	Excessive heat, UV exposure	Hydrolytic stability (3.6.2)
Cracking	Brittle cure, thermal shock	Machinability (3.5.3), adhesion
Solder balling	Surface energy issues, wrong finish	Solderability (3.7.1)

A properly qualified IPC-SM-840 material applied with correct process parameters should resist these defects.

Resources for IPC-SM-840

Where to Purchase IPC-SM-840

Source	URL	Notes
IPC Store	shop.ipc.org	Official source, PDF or print
ANSI Webstore	webstore.ansi.org	PDF format with DRM
Document Center	document-center.com	Various formats

Related Documents Available from IPC

Document	Description
IPC-SM-840E	Current solder mask specification
IPC-HDBK-840	Solder mask handbook (companion document)
IPC-SM-840E TOC	Table of contents (preview)
IPC-TM-650	Test methods manual (free download)

Industry Resources

• Taiyo America – Major LPI solder mask manufacturer with technical resources
• Peters (Lackwerke Peters) – European solder mask supplier with datasheets
• Sun Chemical – Solder mask material supplier
• NCAB Group – PCB fabricator with solder mask design guidelines

Frequently Asked Questions About IPC-SM-840

What is the difference between IPC-SM-840 Class T and Class H?

Class T (Telecommunications) is for commercial and industrial applications where extended performance life is required but interrupted service is not life-threatening. Class H (High Reliability/Military) is for critical applications where continued performance is essential, including military, aerospace, and life support equipment. Class H has more stringent test requirements and tighter flammability specifications than Class T.

Does IPC-SM-840 specify solder mask thickness?

No. IPC-SM-840 does not mandate a specific solder mask thickness. Instead, it requires that the applied thickness be sufficient to meet the dielectric strength requirement of 500 VDC per 25 µm (0.001 inch). For coatings thinner than 25 µm, the mask must still achieve an absolute minimum 500 VDC breakdown voltage. Typical thicknesses range from 12-75 µm depending on application method and board topography.

How does IPC-SM-840 relate to IPC-6012?

IPC-6012 (Qualification and Performance of Rigid Printed Boards) references IPC-SM-840 for solder mask requirements in Section 3.7. When IPC-6012 is specified, solder mask automatically must meet IPC-SM-840. IPC-6012 Class 1 and 2 products default to IPC-SM-840 Class T; Class 3 products require IPC-SM-840 Class H. Explicitly specifying the solder mask class on your drawing is still recommended for clarity.

What solder mask colors are covered by IPC-SM-840?

IPC-SM-840 does not restrict solder mask color—any color can be qualified to the standard. Green is most common because it provides good contrast for visual inspection. Other available colors include red, blue, black, white, and yellow. Note that different color formulations may have slightly different processing parameters, and white or colored masks are typically less translucent than green, which can affect visual inspection of underlying features.

Is IPC-SM-840 applicable to flexible circuit coverlays?

Yes. Revision E (IPC-SM-840E, December 2010) added requirements for flexible cover materials (coverlays) used as dielectric protective layers over conductors on flexible printed circuits. These materials are classified as Class TF (telecommunication flex) or Class HF (high reliability flex) and must meet additional flex testing requirements beyond rigid solder mask testing.

Conclusion

IPC-SM-840 provides the foundation for solder mask qualification and specification in printed circuit board manufacturing. Understanding the difference between Class T and Class H ensures you specify the right performance level for your application—avoiding both over-specification (unnecessary cost) and under-specification (potential field failures).

When creating fabrication drawings, include the complete solder mask callout: standard reference, class designation, color, and finish. If you’re specifying IPC-6012 compliance, remember that solder mask requirements are automatically invoked, but explicit specification prevents ambiguity.

For detailed application guidance beyond what IPC-SM-840 covers, the companion document IPC-HDBK-840 provides practical information on material selection, process optimization, and troubleshooting. Together, these documents give you the tools to specify and verify solder mask that will protect your PCBs throughout their service life.

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Primary (158 characters): IPC-SM-840 defines solder mask qualification for PCBs. Learn Class T vs Class H requirements, test specifications, and how to properly specify solder mask on drawings.

Alternative 1 (156 characters): Complete guide to IPC-SM-840 solder mask requirements. Understand Class T and Class H differences, qualification tests, and specification best practices for PCBs.

Alternative 2 (160 characters): IPC-SM-840 explained: solder mask qualification standard covering Class T, Class H, adhesion, flammability, and dielectric requirements. Includes specification examples.