IPC-6013 is one of the primary specifications used when designing and manufacturing flex and rigid-flex printed circuit boards. The standard, formally titled "Qualification and Performance Specification for Flexible/Rigid-Flexible Printed Boards," establishes requirements for the classification, performance, plating, and quality acceptance criteria of flexible and rigid-flex circuits.
The current revision referenced in this article is Revision E, updated in September 2021. Across its 84 pages, IPC-6013 addresses a broad range of requirements that influence both design decisions and manufacturing outcomes.
Among the most important areas covered by the specification are circuit board classifications, plating requirements, and quality criteria specific to flexible circuit constructions.
How IPC-6013 Classifies Flexible and Rigid-Flex Circuit Boards
IPC-6013 classifies flexible and rigid-flex circuits in several ways. These classifications help define construction requirements and influence how designs are evaluated during manufacturing and inspection.
Performance Classifications
One classification method is the IPC performance class.
While IPC-6013 references performance classes, the formal definitions are provided within IPC-6011. IPC-6013 nevertheless applies performance class distinctions when defining both manufacturing requirements and quality acceptance criteria. The performance class selected for a design affects the level of quality stringency required for the finished product and reflects the importance of the intended application.
Printed Circuit Board Types
IPC-6013 also classifies flexible circuits according to board type. These classifications primarily reflect construction complexity.
Example of a double-sided flexible PCB.
Type 1: Single-sided flexible printed boards containing one conductive layer, with or without stiffeners.
Type 2: Double-sided flexible printed boards containing two conductive layers with PTHs, with or without stiffeners.
Type 3: Multilayer flexible printed boards containing three or more conductive layers with PTHs, with or without stiffeners.
Type 4: Multilayer rigid and flexible material combinations containing three or more conductive layers with PTHs.
Type 5: Flexible or rigid-flex printed boards containing two or more conductive layers without PTHs.
These circuit board-type definitions are foundational because many requirements within IPC-6013 are tied directly to the specific board type being manufactured.
Installation Use Classifications
In addition to performance class and board type, IPC-6013 classifies designs according to installation characteristics. These classifications address how the circuit is used, bent, or exposed to environmental conditions.
Use A: Capable of withstanding flex during installation.
Use B: Capable of withstanding continuous flexing for the number of cycles specified in the procurement documentation.
Use C: High temperature environment (over 105°C / 221°F).
Use D: UL Recognition. See UL 94 and UL 796F.
Although these classifications are used less frequently than board-type classifications, they can influence stack-up decisions and material selection.
Understanding IPC-6013 Plating Requirements
Plating requirements for flexible and rigid-flex circuits differ from those commonly associated with rigid PCB fabrication.
One notable difference is the use of button plating, also known as pad plating, for plated through holes. Rather than plating the entire circuit board surface, this process plates only the surface pad and the hole. Because of this construction approach, plating requirements for flexible and rigid-flex circuits differ from those applied to rigid circuit boards.
IPC-6013 defines minimum copper plating requirements for through-holes based on board type and construction.
Surface and Hole Copper Plating Minimum Requirements for Through-Holes
| Surface And Hole Copper Plating Minimum Requirements For Through-Holes | |||
|---|---|---|---|
| Copper - Average | Class 1 | Class 2 | Class 3 |
| Type 2 | 12 μm (472 μin) | 12 μm (472 μin) | 12 μm (472 μin) |
| Type 3 and 4 except as noted below | 25 μm (984 μin) | 25 μm (984 μin) | 25 μm (984 μin) |
| Type 3 and 4 greater than 1.5mm [0.059 in] thickness and has low Tg materials less than 110 °C in the areas of the board spanned by PTHs | 35 μm (1378 μin) | 35 μm (1378 μin) | 35 μm (1378 μin) |
| Copper (Min. Thin Areas) | |||
| Type 2 | 10 μm (394 μin) | 10 μm (394 μin) | 10 μm (394 μin) |
| Type 3 and 4 except as noted below | 20 μm (787 μin) | 20 μm (787 μin) | 20 μm (787 μin) |
| Type 3 and 4 greater than 1.5mm [0.059 in] thickness and has low Tg materials less than 110 °C in the areas of the board spanned by PTHs | 30 μm (1181 μin) | 30 μm (1181 μin) | 30 μm (1181 μin) |
| Minimum Wrap | AABUS | 5 μm (197 μin) | 12 μm (472 μin) |
| Minimum Wrap | AABUS | 5 μm (197 μin) | 5 μm (197 μin) |
Beyond through-hole plating requirements, IPC-6013 also includes separate requirements for blind vias, buried vias, microvias, and buried via cores. PCB designers working with these features should consult the applicable sections of the specification to ensure compliance with the appropriate plating criteria.
Quality Requirements Unique to Flexible and Rigid-Flex PCBs
Many quality requirements within IPC-6013 closely resemble those used for rigid PCBs.
Examples include requirements related to:
- PTH breakout
- Microvia breakout
- Internal plated-hole quality
- Conductor quality
- Copper weight tolerances
These areas maintain significant consistency between flexible and rigid board standards.
However, flexible and rigid-flex circuits also introduce unique considerations that are not present in conventional rigid PCB constructions.
Bow and Twist Considerations
Bow and twist are common measurements used to evaluate the circuit board flatness in rigid PCB manufacturing. Flexible materials, by their nature, are designed to bend, making these measurements less meaningful for flexible sections.
As a result, IPC-6013 applies bow and twist requirements only to the rigid portions of rigid-flex circuit boards.
Coverlay, Stiffener, and Adhesive Requirements
Flexible circuit constructions contain features that do not exist in traditional rigid PCBs, including:
Because these features introduce unique manufacturing conditions, IPC-6013 includes acceptance criteria specific to flexible circuits.
Examples mentioned within the specification include:
- Soda strawing, which is the lifting of coverlay around a trace and is considered an acceptable condition.
- Creases in coverlay films, which are considered acceptable provided delamination does not occur.
- Foreign material under stiffeners, which is considered acceptable if it does not cover more than 5% of the stiffener area.
These examples illustrate why flex-specific quality requirements must be evaluated differently from those applied to fully rigid printed circuit boards.
Why IPC-6013 Matters During Design and Manufacturing
IPC-6013 provides a framework for evaluating whether a flex or rigid-flex PCB design meets applicable manufacturing and quality requirements. Understanding board classifications, plating requirements, and flex-specific quality criteria can help ensure that design decisions align with the expectations defined by the specification.
At the same time, not every requirement within IPC-6013 applies equally to every design. The relevance of a particular requirement depends on the board type, intended use, construction details, and procurement requirements.
When questions arise regarding a specific IPC-6013 requirement or its applicability to a design, clarification from the manufacturer can help determine the appropriate interpretation for the intended application.
Summary
One of the most valuable tools for designing and manufacturing flexible and rigid-flex circuit boards lies with IPC-6013. Among the most important things to know are the classifications of circuits, the plating requirements, and the appropriate quality requirements that would impact a design. However, there are too many things to list that could impact a design within the document.
While IPC-6013 is an important tool, there are items within it that may not apply to all designs. In situations where there are questions about a requirement, it is best to contact the manufacturer for clarification if it applies to your design.
Key Takeaways
- IPC-6013 classifies flexible PCBs by type and installation usage: Flexible circuit boards are classified into five types based on layer complexity and construction, as well as four usage classes depending on flex requirements and environmental conditions.
- Plating requirements for flexible PCBs differ from rigid boards: Unlike rigid PCBs, flex circuits primarily use ‘button plating’ or ‘pad plating,’ where only the surface pad and plated through holes (PTHs) are coated rather than the entire board surface.
- Quality standards for flex PCBs share similarities with rigid PCBs but have key differences: While many requirements like conductor quality and copper weight tolerances are consistent, flex-specific elements such as coverlay integrity, stiffeners, and acceptable defects like soda strawing and creases are unique to IPC-6013.
- Bow and twist measurements apply only to rigid sections of rigid-flex designs: Since flex circuits naturally bend, traditional board warpage measurements are relevant only to the rigid portions of a rigid-flex PCB.
- Consulting IPC-6013 is crucial for designing flex and rigid-flex PCBs: While the specification provides essential guidelines on classifications, plating, and quality requirements, consulting a PCB manufacturer is recommended for clarifications on design-specific requirements.
