IPC-4203 Explained: Complete Guide to Coverlay and Bonding Materials for Flex PCBs

A flex circuit failure during thermal cycling traced back to coverlay delamination. The adhesive thickness was insufficient to encapsulate the copper traces, leaving voids that allowed moisture ingress. The specification simply called out “polyimide coverlay” without any reference to adhesive thickness, adhesive type, or an actual material standard. This ambiguity cost weeks of debugging and a complete production restart.

IPC-4203 prevents these problems by establishing standardized specifications for coverlay and bonding materials used in flexible printed circuits. This standard covers adhesive-coated dielectric films for outer layer protection (coverlay) and adhesive bonding films used to laminate inner layers together (bondply). Understanding IPC-4203 ensures proper material specification and reliable flex circuit performance.

This guide explains what IPC-4203 covers, the differences between coverlay and flexible solder mask, how to select the right materials, and how to specify them correctly.

What Is IPC-4203? Understanding the Coverlay and Bondply Standard

IPC-4203, officially titled “Cover and Bonding Material for Flexible Printed Circuitry,” establishes the classification system, qualification requirements, and quality conformance standards for adhesive-coated dielectric films used in flex circuit fabrication. The current version is IPC-4203B, released in March 2018 as a 38-page document.

The standard covers two primary material categories: coverlay films that protect outer circuit layers, and bondply films that bond inner layers together in multilayer constructions. Both material types consist of flexible dielectric films coated with adhesive on one or both sides.

IPC-4203 Standard Overview

Attribute	Details
Current Version	IPC-4203B
Release Date	March 2018
Pages	38
Supersedes	IPC-FC-232C and IPC-FC-233A
Scope	Coverlay, bondply, adhesive bonding films
Excludes	Liquid solder mask (IPC-SM-840), rigid board adhesives (IPC-4101)

What IPC-4203 Covers

Material Type	Description
Coverlay	Dielectric film with adhesive on one side for outer layer protection
Bondply	Dielectric film with adhesive on both sides for inner layer bonding
Adhesive bonding films	Unsupported adhesive films for lamination
Supported adhesive films	Carrier-backed adhesive films

IPC-4203 Coverlay vs Flexible Solder Mask

One of the most common questions in flex circuit design is when to use coverlay versus flexible solder mask. Both protect outer circuit layers, but they differ significantly in composition, capabilities, and design rules.

Material Composition Differences

Property	Coverlay (IPC-4203)	Flexible Solder Mask (IPC-SM-840)
Form	Solid film + adhesive	Liquid coating
Base material	Polyimide film	Epoxy-based with flex agent
Adhesive	Acrylic or epoxy	Not applicable
Application method	Laminated under heat/pressure	Screen printed or sprayed
Opening formation	Mechanically cut (drill, laser, punch)	Photo-imaged

Design Rule Comparison

Design Parameter	Coverlay	Flexible Solder Mask
Minimum dam width	10 mils (0.25 mm)	4 mils (0.1 mm)
Minimum opening size	0.6 mm typical	Much smaller possible
Feature resolution	Limited by mechanical cutting	High (photo-imaging)
Annular ring	Larger required	Standard
Fine-pitch capability	Limited	Good

When to Use Coverlay vs Solder Mask

Application	Recommended Material	Reason
Dynamic flex zones	Coverlay	Superior flex life and durability
Tight bend radius	Coverlay	Better mechanical properties
Fine-pitch SMT areas	Flexible solder mask	Better feature resolution
Rigidized component areas	Flexible solder mask	Finer design rules
EMI shield attachment	Coverlay	Adhesive bonds to shield material
ZIF connector fingers	Coverlay	Required for stiffener adhesion

Many rigid-flex designs use both materials: coverlay in flexible areas where bend reliability is critical, and flexible solder mask in rigidized component areas where fine-pitch SMT features require tighter design rules.

IPC-4203 Coverlay Material Types

Coverlay materials consist of a polyimide film layer bonded to an adhesive layer. The adhesive type and thickness significantly affect performance and processing.

Adhesive Types in IPC-4203

Adhesive Type	Properties	Best For
Acrylic	Flexible, reworkable, good peel strength	General-purpose flex, dynamic applications
Epoxy	Higher bond strength, more rigid	High-reliability, harsh environments
Modified acrylic	Enhanced temperature resistance	Lead-free reflow compatible

Common IPC-4203 Coverlay Constructions

Film Thickness	Adhesive Thickness	Total Thickness	Application
0.5 mil (12.5 µm)	0.5 mil (12.5 µm)	1.0 mil (25 µm)	Ultra-thin, tight bend
1.0 mil (25 µm)	1.0 mil (25 µm)	2.0 mil (50 µm)	Standard (most common)
1.0 mil (25 µm)	2.0 mil (50 µm)	3.0 mil (75 µm)	Heavy copper (2 oz)
2.0 mil (50 µm)	1.0 mil (25 µm)	3.0 mil (75 µm)	Higher dielectric strength

The most common coverlay construction is 1 mil polyimide film with 1 mil adhesive (1:1 ratio), providing a good balance of flexibility, protection, and cost.

IPC-4203 Bondply Materials for Multilayer Flex

Bondply materials bond multiple flex layers together in multilayer flexible and rigid-flex constructions. These materials have adhesive on both sides of a dielectric carrier film.

Bondply Construction

Component	Description
Dielectric film	Polyimide carrier (typically 0.5-1.0 mil)
Adhesive (top)	B-staged acrylic or epoxy
Adhesive (bottom)	B-staged acrylic or epoxy
Function	Bonds conductive layers from different cores

Bondply vs Prepreg

Property	Bondply (IPC-4203)	Prepreg (IPC-4101)
Flexibility	High	Low (glass reinforced)
Use in flex areas	Yes	No
Use in rigid areas	Limited	Yes
Material	Polyimide + adhesive	Glass + resin

Sheet Adhesive (Unsupported)

IPC-4203 also covers unsupported adhesive films—pure adhesive without a dielectric carrier. These are used for applications like stiffener attachment where a dielectric layer is not needed.

Thickness	Application
0.5 mil (12.5 µm)	Thin bonding applications
1.0 mil (25 µm)	Standard stiffener attachment
2.0 mil (50 µm)	Heavy copper encapsulation
3.0 mil (75 µm)	Gap filling, uneven surfaces

IPC-4203 Slash Sheet System: How to Specify Materials

Like other IPC material standards, IPC-4203 uses a slash sheet system to identify specific material types. Each slash sheet defines requirements for a particular combination of dielectric and adhesive materials.

Understanding IPC-4203 Slash Sheet Designations

The nonspecific designation format for design drawings:

IPC-4203/S

Where S is the specification sheet number.

Slash Sheet	Material Type
IPC-4203/1	Polyimide coverlay with acrylic adhesive
IPC-4203/18	Unsupported acrylic adhesive bonding film
IPC-4203/26	PTFE/ceramic coverlay with adhesive (high-frequency)

IPC-4203 Designation Example

A typical nonspecific designation on a master drawing:

IPC-4203/1

This indicates polyimide coverlay with acrylic adhesive. The fabricator then selects a specific material (such as DuPont Pyralux FR or Pyralux LF) that conforms to this specification.

Specific Designation Format

For purchase orders and detailed fabrication specifications, IPC-4203 provides an extended designation format:

IPC-4203/S – C1E2M3/3

Parameter	Description
/S	Specification sheet number
C1	Dielectric type designation
E2	Dielectric thickness code
M3	Adhesive type designation
/3	Adhesive thickness code

Commercial Materials Mapped to IPC-4203

Commercial Product	IPC-4203 Slash Sheet	Description
DuPont Pyralux FR Coverlay	IPC-4203/1	Flame retardant polyimide coverlay
DuPont Pyralux LF Coverlay	IPC-4203/1	Low-flow polyimide coverlay
DuPont Pyralux LF Sheet Adhesive	IPC-4203/18	Unsupported acrylic adhesive

IPC-4203 Coverlay Thickness Selection Guide

Selecting the correct coverlay thickness requires matching the adhesive thickness to copper weight and considering bend radius requirements.

Adhesive Thickness Rule

The fundamental rule for coverlay adhesive selection:

Minimum 1 mil (25 µm) of adhesive per 1 oz (35 µm) of copper

Finished Copper Weight	Minimum Adhesive Thickness
0.5 oz (17.5 µm)	0.5 mil (12.5 µm)
1.0 oz (35 µm)	1.0 mil (25 µm)
2.0 oz (70 µm)	2.0 mil (50 µm)

Insufficient adhesive results in incomplete encapsulation, voids, and potential delamination. Excessive adhesive causes squeeze-out into openings, creating solderability issues.

Bend Radius Considerations

Application	Coverlay Selection
Static bend (install once)	Standard 1 mil/1 mil
Dynamic flex (repeated bending)	Thinner film preferred
Tight bend radius (<1 mm)	0.5 mil/0.5 mil
Controlled impedance	Consider dielectric thickness impact

Post-Lamination Thickness

During lamination, adhesive flows to fill spaces between traces. The finished thickness is less than the sum of individual material thicknesses because adhesive is consumed during encapsulation. Account for this when calculating final stack-up thickness.

IPC-4203 Design Considerations for Coverlay Openings

Coverlay openings for pads and vias are created mechanically, which imposes design constraints different from photo-imaged solder mask.

Opening Formation Methods

Method	Application	Typical Feature Size
NC drilling	Round openings	>0.6 mm
Routing	Complex shapes	>0.8 mm
Laser cutting	Fine features	>0.3 mm
Punch and die	High volume	>0.6 mm

IPC-4203 Coverlay Design Rules

Parameter	Minimum Value	Notes
Minimum dam width	10 mils (0.25 mm)	Prevents web breakage
Minimum opening	0.6 mm	Depends on method
Annular ring	Larger than solder mask	Allows for squeeze-out
Overlap on traces	0.4 mm typical	Ensures encapsulation

Ganged Openings

For fine-pitch components where individual pad openings would violate minimum web requirements, multiple openings are combined into larger “ganged” openings.

Situation	Solution
Fine-pitch QFP/QFN	Gang all pads into single opening
Close-spaced vias	Combine into oval or slot opening
BGA arrays	Large rectangular opening

Adhesive Squeeze-Out

During lamination, some adhesive flows into openings. Design rules must account for this:

Consideration	Guideline
Pad size allowance	Add 0.1-0.15 mm to opening size
Isolated features	Not possible (would fall out after cutting)
Island features	Must be connected to main coverlay area

IPC-4203 vs IPC-4202 vs IPC-4204: Related Flex Standards

Understanding how IPC-4203 relates to other flexible circuit material standards ensures complete and correct material specification.

IPC Flexible Circuit Material Standards Comparison

Standard	Covers	Primary Use
IPC-4202	Base dielectrics	Unclad flexible films
IPC-4203	Coverlay and bondply	Cover films, adhesive bonding
IPC-4204	Metal-clad dielectrics	Copper-clad flex laminates
IPC-SM-840	Solder mask	Liquid photo-imageable coatings

When to Reference Each Standard

Design Element	Applicable Standard
Base film (unclad)	IPC-4202
Copper-clad laminate	IPC-4204
Outer layer protection (film)	IPC-4203
Outer layer protection (liquid)	IPC-SM-840
Inner layer bonding (flex areas)	IPC-4203
Inner layer bonding (rigid areas)	IPC-4101

Useful Resources for IPC-4203 Implementation

Official IPC Standards:

• IPC-4203B Cover and Bonding Material for Flexible Printed Circuitry (shop.ipc.org)
• IPC-2223 Sectional Design Standard for Flexible Printed Boards
• IPC-6013 Qualification and Performance Specification for Flexible Printed Boards
• IPC-SM-840 Qualification and Performance of Permanent Solder Mask

Material Manufacturer Resources:

• DuPont Pyralux FR Coverlay Technical Data Sheet
• DuPont Pyralux LF Coverlay Technical Data Sheet
• DuPont Pyralux LF Sheet Adhesive Data Sheet
• Panasonic FELIOS Coverlay Product Guide
• ThinFlex Coverlay Materials Catalog

Related Test Methods:

• IPC-TM-650 Test Methods Manual
• IPC-TM-650 2.4.9 Peel Strength
• IPC-TM-650 2.3.17.1 Adhesive Flow

Frequently Asked Questions About IPC-4203

What is the difference between coverlay and bondply in IPC-4203?

Coverlay and bondply serve different functions in flex circuit construction. Coverlay has adhesive on one side and is used to protect outer circuit layers—it is the flex circuit equivalent of solder mask. Bondply has adhesive on both sides and is used between inner layers to bond multiple flex cores together in multilayer constructions. Both materials are specified under IPC-4203, but they use different slash sheets. DuPont Pyralux FR Coverlay conforms to IPC-4203/1, while Pyralux LF Sheet Adhesive (used as bondply) conforms to IPC-4203/18.

How do I determine the correct coverlay adhesive thickness?

The fundamental rule is 1 mil (25 µm) of adhesive per 1 oz of copper. This ensures sufficient adhesive to completely encapsulate the copper traces without leaving voids. For 1 oz copper, use 1 mil adhesive minimum. For 2 oz copper, use 2 mil adhesive minimum. Insufficient adhesive leads to delamination and reliability problems; excessive adhesive causes squeeze-out into pad openings. The most common coverlay construction is 1 mil polyimide with 1 mil adhesive, suitable for circuits with 1 oz or lighter copper.

Can I use flexible solder mask instead of coverlay for all flex circuits?

No. Flexible solder mask works well for rigidized component areas where fine-pitch features require tight design rules, but coverlay is strongly preferred for flexible areas that will bend. Coverlay provides superior flex life, better mechanical protection, and higher dielectric strength. Additionally, some materials like EMI shields, polyimide stiffeners, and ZIF connector reinforcements require coverlay because they do not adhere properly to solder mask. Many designs use both: coverlay in bend areas and solder mask in component areas.

Why does coverlay have larger minimum dam widths than solder mask?

Coverlay openings are created mechanically (drilling, routing, laser cutting) rather than photo-imaging. This limits feature resolution and minimum web thickness. Thin webs between coverlay openings are easily damaged during processing, and insufficient adhesive in thin web areas may not create proper lamination bonds. The 10 mil (0.25 mm) minimum dam width ensures structural integrity and reliable adhesive bonding. Photo-imaged solder mask can achieve 4 mil dams because the liquid material is patterned in place without mechanical cutting.

How do I specify IPC-4203 coverlay on my drawing?

Use the nonspecific designation “IPC-4203/1” for standard polyimide coverlay with acrylic adhesive. Add notes specifying polyimide thickness, adhesive thickness, and any special requirements. For example: “Coverlay per IPC-4203/1, 1 mil polyimide, 1 mil adhesive.” Do not specify coverlay by brand name alone. The fabricator will select a material (Pyralux FR, Panasonic FELIOS, etc.) that meets the IPC-4203/1 requirements. For sheet adhesive used as bondply, specify IPC-4203/18 with the required adhesive thickness.

Applying IPC-4203 in Your Flex Circuit Designs

Proper coverlay and bondply specification is essential for flex circuit reliability. IPC-4203 provides the framework for clearly communicating material requirements, ensuring that protective films perform correctly throughout the product lifecycle.

For most flex circuits, polyimide coverlay per IPC-4203/1 with 1 mil film and 1 mil adhesive provides the best balance of protection, flexibility, and cost. Adjust adhesive thickness upward for heavier copper or downward for tighter bend requirements. Use flexible solder mask in rigidized areas where fine-pitch components require tighter design rules, but always use coverlay in dynamic flex zones.

Remember the key design rules: 10 mil minimum dam width, gang fine-pitch features into larger openings, and ensure sufficient overlap on all traces. Account for adhesive squeeze-out when sizing openings, and specify at least 1 mil of adhesive per ounce of copper to ensure complete encapsulation.

Understanding IPC-4203 means understanding the complete flex circuit material system. Combined with IPC-4202 for base dielectrics, IPC-4204 for copper-clad laminates, and IPC-2223 for design standards, these specifications create reliable flexible circuits for demanding applications.