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Issue link: https://iconnect007.uberflip.com/i/1543955
76 I-CONNECT007 MAGAZINE I MARCH 2026 said, it is formally recognized by key federal bod- ies, including OSHA, the FCC, and the Consumer Product Safety Commission. UL certification is important for Canada and Mexico, making it es- sential for products entering the North American market. While UL may not be government-run, it is effectively mandatory for many products. One of the most important technical discussions in the webinar centers on UL's shift, implemented in January 2022, from solder temperature limits to soldering cycle testing. Historically, UL certifica- tions relied on short-duration temperature limits, such as a maximum solder temperature for a specified number of seconds, paired with a mini- mum operating temperature. That approach no longer reflects modern assembly processes, where boards often experience multiple reflow cycles with elevated temperatures sustained for minutes rather than seconds. UL now defaults to six simulated soldering cycles using a thermal stress assembly simulation. This change significantly raises the bar for materials and constructions and dramatically increases the cost, duration, and risk associated with UL testing. Pedersen noted that older solder-limit approvals remain valid, but they do not provide the same level of thermal assurance as newer solder-cycle- based certifications. The webinar addressed the core question many engineers are asking: What makes UL certification for flex-rigid boards so difficult and costly? Accord- ing to Pedersen, who started in the PCB industry in 1973, the primary driver is material complexity and compatibility. Flex-rigid boards combine rigid lami- nates, flexible films, adhesives, and bonding layers, each with very different mechanical and thermal properties. These materials must survive repeated thermal cycling, remain bonded without delamina- tion, maintain electrical insulation, and withstand mechanical bending over time. UL evaluates the entire construction, including rigid-to-flex transitions and flex window areas. The challenge is not simply heat or mechanical stress alone, but the interaction between materials that behave very differently under the same conditions. This complexity drives longer test times, limited testing capacity, and higher certification costs, sometimes resulting in months-long delays. One of the most eye-opening takeaways from the webinar is the limitation on flex layers. For UL- approved flex-rigid stackups, NCAB is limited to a maximum of four flex layers. Designs with more than four flex layers can still be manufactured, but they cannot carry UL stackup approval. While this limit still covers the majority of designs, it is a con- straint that engineers must understand early in the design process. UL approval is never generic. Every approved flex-rigid construction is tied to a specific stackup, a specific UL file number, and a specific factory. A single factory may hold multiple UL certifica- tions, including older solder-limit approvals and newer solder-cycle approvals. Even stackups that appear nearly identical can have very different UL constraints. Assuming a design is already UL- approved without verification is one of the most common and costly mistakes. Pedersen shared several real-world stackups, including balanced and unbalanced builds, HDI-ca- pable designs, RF stackups, and selective solder- ing scenarios. These examples demonstrate how narrowly UL approvals are defined and how easily a design can fall outside the approved window. What made this webinar particularly valuable was its practicality. Rather than focusing on stan- dards language alone, Pedersen showed actual UL stackups, factory constraints, and tradeoffs based on production experience. NCAB currently sup- ports roughly 35 UL-approved flex-rigid stackups, yet even with that breadth, early engagement with the fabricator remains critical to ensure your mate- rials, thickness, HDI features, or soldering methods are, in fact, on the approved list. For engineers working with flex or flex-rigid designs, this webinar is not just informative: it is preventative. It explains why UL certification feels so complicated, where the real constraints lie, and how early collaboration can save significant time, cost, and frustration. If UL compliance is part of your product's reality, this is a webinar worth watching. I-CONNECT007 Click here to view the webinar.