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36 DESIGN007 MAGAZINE I NOVEMBER 2025 Interconnect Defects Thermal stress during assembly or operation can cause separations at the interfaces of plated vias, because PCB materials generally expand more than copper when heated. The thin electroless copper layer inside microvias can be vulnerable to stress from the expansion and contraction of the PCB sub- strate during thermal cycling. Electroless and inner- layer copper can pull apart, especially during ther- mal events like soldering. With UHDI designs, you must be more careful when choosing the low-profile laminate materials used in this type of board. A thermal expansion mis- match between the electroless copper and the lam- inate can increase the risk of interconnect defects, which can create signal distortion, transmission delays, and short circuits. Copper Cracking A thermal expansion mismatch between adjacent materials or insufficient plating thickness can cause copper cracking. This can lead to: • Solder joint fatigue, via failures, and disrupted traces • Board warpage leading to mechanical issues and component failures • Imperfect plating in microvias causing imped- ance problems such as signal reflections, incre- ased insertion loss, and higher bit error rates • Insufficient or uneven plating resulting in volt- age drops, power loss, and localized heating Designers Can Reduce Risk With Electroless Copper As a designer, you can mitigate these risks. Collab- orate closely with your manufacturing partner to ensure every measure has been taken to ensure a quality result before the order is placed and pro- duction begins. As you work on your design, use DFM and design for reliability (DFR) tools to ensure a smooth manufacturing process and that the board performs as needed for the expected lifetime of the electronic device it powers. Perform design rule checks (DRC) to catch potential short circuits and ensure proper trace thickness. Many simulation tools can perform thermal stress analysis to iden- tify potential thermal hotspots in your board design. Take these proactive design steps to reduce the risk of electroless copper plating issues: • To prevent copper-related interconnect defects, use copper balancing to ensure an even distribution of copper and prevent problems like warpage. • Ensure copper is evenly distributed by adding unattached copper to blank sections of the board. Known as copper thieving, this will also reduce the chances of warpage and minimize electromagnetic interference. • To prevent copper cracking, select materials with a similar CTE (coefficient of thermal expansion), which describes how much a material expands with temperature change. • When designing microvias, maintain a low aspect ratio. This will prevent voids and thin copper deposits that can lead to fractures during thermal cycling and reduce the board's reliability. • Work with your manufacturer to specify the drilling processes that will minimize surface imperfections. • In addition to choosing materials with similar expansion rates, specify laminate materials to improve microvia reliability. • Avoid design features that increase plating complexity and create more challenges for the manufacturer during the electroless copper process. The electroless copper process is a critical aspect of board production. Know the techniques for designing effectively for the process, and you can build sophisticated, resilient boards designed for manufacturability. DESIGN007 Matt Stevenson is vice president and general manager of ASC Sunstone Circuits. To read past columns, click here. Download Matt's book, The Printed Circuit Designer's Guide to… Designing for Reality and listen to the podcast here. C O N N ECT T H E D OTS