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Connect the Dots: The Future of Designing for Reality—Electroless Copper

On a recent episode of I-Connect007’s On the Line With… podcast, we discussed electroless copper deposition—depositing a thin copper layer into the through-holes and vias of what will eventually become a printed circuit board. Specifically, it is a chemically catalyzed deposition of copper, mainly to the epoxy inside the holes (as well as on the entire surface of the board). Electroless differs from electrolytic in that electrolytic deposition relies on electricity. 

Without proper electroless copper deposition, boards can wind up with voided copper holes, usually identified later in the manufacturing process. This preparatory step is critical, and it is important for the manufacturer to get it right. When done correctly, it creates a very fine layer of copper that makes the connection from the top to the bottom of the through-hole. Since this thin layer of copper is conductive, it allows a later electrolytic process to deposit a thicker, more substantial layer of copper.

When designing for manufacturing (DFM) ultra-high-density interconnect (UHDI) PCBs, the choices you make regarding electroless copper plating help ensure mechanical reliability. They also reduce the risk of issues with electrical and signal integrity. Designers should consider manufacturability, material properties, and their manufacturing partner’s process controls to maximize yield and ensure the board performs to specs. 

Thermal Stress and Board Defects
The higher density and smaller features of UHDI boards can amplify the effects of thermal stress. Designers should know how thermal stress can cause interconnect defects, copper cracking, and poor adhesion.

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 substrate during thermal cycling. Electroless and inner-layer copper can pull apart, especially during thermal 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 mismatch between the electroless copper and the laminate 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 impedance problems such as signal reflections, increased insertion loss, and higher bit error rates
• Insufficient or uneven plating resulting in voltage drops, power loss, and localized heating

Designers Can Reduce Risk With Electroless Copper

As a designer, you can mitigate these risks. Collaborate closely with your manufacturing partner to ensure every measure has been taken to ensure a quality result before the order is placed and production 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 identify 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 reduce 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. To learn more about designing for the reality of the PCB manufacturing process, check out more episodes of On the Line With… .

This column originally appeared in the November 2025 issue of Design007 Magazine.