--- title: Thermal Expansion Challenges in Mixed-Material Rigid-Flex description: Understand thermal expansion challenges in rigid-flex PCBs, including CTE mismatch, via placement, stack-up balance, and material selection. image: https://blog.epectec.com/hubfs/blog/flex-pcb-stack-up-with-mixed-materials.jpg --- [![Epec Engineered Technologies](https://www.epectec.com/images/logo.png "Epec Engineered Technologies")](https://www.epectec.com/) [![Epec Engineered Technologies](https://www.epectec.com/images/logo.png "Epec Engineered Technologies")](https://www.epectec.com/) - [Articles](https://www.epectec.com/articles/) - [Blog](https://blog.epectec.com/) - [Webinars](https://www.epectec.com/webinar/) - [Videos](https://www.epectec.com/videos/) - [Case Studies](https://www.epectec.com/case-studies/) - [News & Events](https://www.epectec.com/news/) - [Customer Portal](https://system.netsuite.com/c.406760/site/NSlogin.html) - [About](https://www.epectec.com/about/overview/) - Back - [Company Overview](https://www.epectec.com/about/overview/) - 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[Tweet](https://twitter.com/share) By definition, [rigid-flex designs](https://www.epectec.com/flex/) are inherently ‘mixed-material’, as they contain both rigid and flexible components. These portions of the stack-up will have differing properties that can cause difficulties when attempting to create stack-ups and design functioning boards. Most notably, these materials act differently under heat due to differences in CTE, which can affect [flexible PCB design](https://www.epectec.com/flex/design/), manufacturing, and assembly. ## Why Thermal Expansion Presents a Challenge The coefficient of thermal expansion, commonly referred to as CTE, describes how much a material expands or contracts as its temperature changes. In any [printed circuit board](https://www.epectec.com/pcb/), controlling the effects of thermal expansion is important. With rigid-flex circuits, however, the challenge becomes more complicated because multiple materials with different mechanical and thermal properties must function as a single structure. The rigid sections of a rigid-flex PCB are typically constructed using [FR4](https://blog.epectec.com/differences-between-fr4-and-polyimide-materials-in-flex-circuits) or another rigid [laminate material](https://www.epectec.com/pcb/laminate-material.html). The flexible sections commonly use [polyimide materials](https://www.epectec.com/flex/material-properties.html), which respond differently to temperature changes. Adhesives and coverlay materials introduce additional variables because their rates of thermal expansion can differ significantly from the surrounding copper, polyimide, and rigid laminate materials. [![Free Ebook Download: Top 6 Quality Issues Seen in Flex & Rigid-Flex PCBs](https://no-cache.hubspot.com/cta/default/707152/f1da5027-aae4-46b7-ba59-65371917f1da.png)](https://cta-redirect.hubspot.com/cta/redirect/707152/f1da5027-aae4-46b7-ba59-65371917f1da) When these materials are heated during fabrication, assembly, or operation, they do not necessarily expand by the same amount or in the same direction. The resulting mechanical stress can concentrate around vias, transition areas, interfaces, and other locations where the construction changes. [Coverlay](https://blog.epectec.com/what-is-a-flexible-circuit-coverlay) and adhesive materials are particularly important considerations. These materials are necessary for protecting flexible circuitry and bonding portions of the construction together, but their thermal properties must be accounted for when establishing the stack-up. Heat distribution can also become a concern. Flexible sections are generally much thinner than rigid sections and have less material available to absorb and spread heat. Depending on the copper distribution, component placement, and application, this can contribute to localized hot spots and additional thermal stress. ## Accounting for Thermal Expansion During Design Many thermal expansion problems can be prevented by following established rigid-flex PCB design practices before the board reaches fabrication. One important rule involves [via placement](https://blog.epectec.com/overview-of-via-holes-in-flexible-circuit-boards-designs) near the transition between rigid and flexible sections. As a general design guideline, vias should remain a minimum of 0.050 inches away from the rigid-flex transition area. This clearance is necessary because adhesive and coverlay materials may overlap into the rigid portion of the PCB. If a plated hole is positioned too close to the transition, it may pass through one or more adhesive layers. The problem may not be immediately apparent. The circuit board could pass initial electrical inspection and function normally after fabrication. Once the assembly experiences repeated thermal excursions, however, the different expansion rates of the materials surrounding the plated hole can place mechanical stress on the copper barrel. Over time, this can lead to cracking and intermittent or complete electrical failure. Keeping vias away from these transition zones provides additional space for the materials to expand and contract without transferring unnecessary stress into critical electrical interconnects. ## Material Selection in the Flexible Sections Material selection is another critical part of controlling thermal expansion. Rigid-flex designs should utilize [adhesiveless polyimide-based cores](https://blog.epectec.com/adhesive-and-adhesiveless-polyimide-flex-core-material-types) within the flexible sections. Unlike adhesive-based flex materials, adhesiveless constructions eliminate an additional adhesive layer between the copper and polyimide substrate. Removing that adhesive reduces the number of materials with different thermal expansion properties within the stack-up. It also helps keep the flexible section thinner, which improves flexibility and mechanical reliability. This does not mean adhesives can be eliminated from every rigid-flex construction. Coverlays and bonding materials may still be necessary depending on the stack-up. The objective is to limit unnecessary adhesive layers and carefully control where required materials are located. The selected materials must also be compatible with the temperatures the circuit will experience during fabrication and assembly. A rigid-flex circuit may be subjected to multiple high-temperature processes before it ever reaches its final application, making thermal compatibility important from the beginning. ![Example of the materials used in a multi-layer rigid-flex](https://blog.epectec.com/hs-fs/hubfs/blog/flex-pcb-stack-up-with-mixed-materials.jpg?width=800&height=500&name=flex-pcb-stack-up-with-mixed-materials.jpg) *Example of the materials used in a multi-layer rigid-flex.* ## The Importance of a Symmetrical Stack-Up Stack-up balance is one of the most important considerations when designing a manufacturable rigid-flex circuit. [Rigid-flex stack-ups](https://www.epectec.com/flex/material-stackup.html) should generally be symmetrical around the central axis of the board. This means considering both the number of layers and their respective thicknesses. An unbalanced construction can react unevenly during lamination. As temperature and pressure are applied, one side of the board may expand or contract differently than the other. The resulting forces can pull the rigid sections out of plane and create bowing, twisting, or warpage. Once significant warpage develops, it can create problems throughout the remainder of manufacturing and assembly. Component placement may become more difficult, solder joints may experience uneven stress, and the finished circuit may not properly fit within its intended enclosure. Copper distribution should also be considered when balancing a construction. Even when dielectric materials are arranged symmetrically, major differences in copper density between opposing layers can contribute to uneven thermal and mechanical behavior. ## Managing Blind and Buried Via Complexity [Blind and buried vias](https://blog.epectec.com/advanced-via-technologies-in-rigid-flex-pcb-fabrication) can provide valuable routing space in complex rigid-flex designs, but they also introduce additional manufacturing considerations. Unlike a standard through-hole design, blind and buried via structures may require multiple sequential lamination cycles. Each additional cycle exposes the circuit materials to another combination of heat and pressure. In a conventional rigid PCB, this complexity can already create manufacturing challenges. In a mixed-material rigid-flex construction, the risks can be greater because the different materials repeatedly expand and contract throughout processing. For this reason, the number of blind and buried via sets may need to be reduced compared to an equivalent rigid PCB design. A technically possible stack-up is not always the most reliable or manufacturable option. When high-density routing is required, the PCB manufacturer should be involved early to determine how the via architecture can be simplified while still meeting the electrical requirements of the design. ## What Happens When Thermal Expansion Is Ignored? The consequences of ignoring thermal expansion can range from a localized electrical failure to complete board failure. One potential failure is a cracked via barrel near the rigid-flex transition. Because these failures can develop gradually, they may appear as intermittent electrical problems that are difficult to reproduce and diagnose. The more severe risk is delamination. When materials repeatedly expand and contract at different rates, stress can build between bonded layers. After one severe heat excursion or multiple thermal cycles, portions of the construction may begin to separate. Delamination is particularly concerning because failure may not always be immediately visible. A circuit can potentially pass initial testing and later fail after assembly, environmental testing, or field operation. Once delamination begins, the reliability of the entire structure is compromised. Preventing the conditions that cause it is far more practical than trying to address the problem after fabrication. ## Summary Thermal expansion is an unavoidable consideration in rigid-flex PCB design because the construction combines materials that naturally respond differently to temperature. The goal is not to eliminate expansion but to design the circuit so those differences do not create excessive mechanical stress. Via placement, adhesiveless flex materials, balanced stack-ups, controlled adhesive locations, and carefully planned blind and buried via structures can all help reduce thermal expansion risks. Manufacturer capabilities and design rules may vary, particularly when it comes to transition areas, via placement, material selection, and sequential lamination requirements. Working with the PCB manufacturer early can help identify these limitations before they become costly fabrication or reliability problems. Industry design standards should also be consulted when developing the construction and layout. Following established flex and rigid-flex design guidance, along with the specific requirements of the selected manufacturer, provides a stronger foundation for building circuits that can withstand fabrication, assembly, and repeated thermal excursions throughout their operating life. --- ## Key Takeaways - **Rigid-flex circuits combine materials** with different coefficients of thermal expansion, creating mechanical stresses that must be considered during stack-up development and PCB layout. - **Vias should generally remain** at least 0.050 inches away from rigid-flex transition areas to prevent plated holes from intersecting adhesive layers and experiencing excessive thermal stress. - **Adhesiveless polyimide cores** help reduce material complexity in flexible sections while supporting thinner and more reliable flex constructions. - **Symmetrical stack-ups** and balanced material thicknesses are critical for reducing bow, twist, and warpage during rigid-flex lamination and subsequent thermal processing. - **Complex blind and buried via structures** can require multiple lamination cycles, increasing thermal stress and making early collaboration with the PCB manufacturer especially important. - [Tweet](https://twitter.com/share) Topics: [Flex & Rigid-Flex PCB's](https://blog.epectec.com/topic/flex-rigid-flex-pcbs) --- Next | [All posts](https://blog.epectec.com/all) | [Previous](https://blog.epectec.com/designing-cable-assemblies-for-emi-sensitive-applications) [![New Call-to-action](https://no-cache.hubspot.com/cta/default/707152/ef3423e4-1633-48cb-93e6-cae03db5bdf4.png)](https://cta-redirect.hubspot.com/cta/redirect/707152/ef3423e4-1633-48cb-93e6-cae03db5bdf4) ## Leave a Comment --- ## Related Posts --- ### Recent Posts --- ### Posts by Topic - [Printed Circuit Boards (162)](https://blog.epectec.com/topic/printed-circuit-boards) - [Product Design (139)](https://blog.epectec.com/topic/product-design) - [Battery Packs (122)](https://blog.epectec.com/topic/battery-packs) - [Cable Assemblies (103)](https://blog.epectec.com/topic/cable-assemblies) - [Flex & Rigid-Flex PCB's (97)](https://blog.epectec.com/topic/flex-rigid-flex-pcbs) - [Electronics Industry (61)](https://blog.epectec.com/topic/electronics-industry) - [User Interfaces (55)](https://blog.epectec.com/topic/user-interfaces) - [Quality Solutions (33)](https://blog.epectec.com/topic/quality-solutions) - [Flexible Heaters (31)](https://blog.epectec.com/topic/flexible-heaters) - [Events, Training, and Support (9)](https://blog.epectec.com/topic/events-training-and-support) - [RF Products (7)](https://blog.epectec.com/topic/rf-products) - [CNC Machining (3)](https://blog.epectec.com/topic/cnc-machining) - [Fans & Motors (1)](https://blog.epectec.com/topic/fans-motors) See All --- --- ### Need Help with A Project? 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