Factors Affecting the Adhesion of Thin Film Copper on Polyimide
September 6, 2016 | David Ciufo, Hsin-Yi Tsai and Michael J. Carmody, INTRINSIQ MATERIALSEstimated reading time: 2 minutes

Abstract
The use of copper foils laminated to polyimide (PI) as flexible printed circuit board precursor is a standard practice in the PCB industry. We have previously described an approach to very thin copper laminates of coating uniform layers of nano copper inks and converting them into conductive foils via photonic sintering with a multi-bulb conveyor system, which is consistent with roll-to-roll manufacturing. The copper thickness of these foils can be augmented by electroplating. Very thin copper layers enable etching fine lines in the flexible circuit. These films must adhere tenaciously to the polyimide substrate. In this paper, we investigate the factors which improve and inhibit adhesion. It was found that the ink composition, photonic sintering conditions, substrate pretreatment, and the inclusion of layers (metal and organic) intermediate between the copper and the polyimide are important.
Ink factors include the intensity of photonic sintering. Better sintering leads to better cohesive strength of the nano copper layer. The ink solvent and the dispersant used to suspend the nanoparticles are significant both for adhesion and the colloidal stability of the dispersion. Pretreatment of the substrate by plasma roughening did not improve adhesion. We describe the effects of chromium and nickel interlayers which are typically used in standard foil laminates. Finally, we describe the types of peel strength testing used to assess adhesion.
Introduction
The goal of our ultra-thin film project is to create a thin (1−2 micron) film of uniform copper on flexible polyimide for application to flex circuits. Ultra-thin films allow very narrow copper lines on patterns created by photolithography, plating, and etching. Figure 1 depicts the invented process.
A very thin layer of nano copper ink is created on polyimide by coating techniques. A picture of a coating made in a roll-to-roll (R2R) manner on a slot die coating machine is shown at left in Figure 2. After drying the ink (done by air impingement here), a film of copper nanoparticles in dispersant/binder of about 0.5−1.0 microns thick is produced. This film can be turned into a continuous film of conductive copper metal by photonic sintering with a flash lamp system. There have been developed special R2R machines with moving conveyors, multiple flash bulbs, and algorithms to control and interleave flashes to give uniform sintering of the coating[2]. On the other hand, the lab scale coatings are made by using an automatic Mayer bar coater (Figure 2, right), dried in a vacuum oven, and sintered by a single flash lamp unit it the lab. The resulting copper films are less uniform than the ones generated by the R2R process, and banding where the flashes overlap is usually observed. Alternatively, a uniform conductive copper film can be generated by sintering by immersing in a formic acid/argon reducing atmosphere at 190°C for an hour (Figure 3).
Editor's Note: This article originally appeared in the August 2016 issue of The PCB Magazine.
Suggested Items
Trouble in Your Tank: Can You Drill the Perfect Hole?
07/07/2025 | Michael Carano -- Column: Trouble in Your TankIn the movie “Friday Night Lights,” the head football coach (played by Billy Bob Thornton) addresses his high school football team on a hot day in August in West Texas. He asks his players one question: “Can you be perfect?” That is an interesting question, in football and the printed circuit board fabrication world, where being perfect is somewhat elusive. When it comes to mechanical drilling and via formation, can you drill the perfect hole time after time?
The Evolution of Picosecond Laser Drilling
06/19/2025 | Marcy LaRont, PCB007 MagazineIs it hard to imagine a single laser pulse reduced not only from nanoseconds to picoseconds in its pulse duration, but even to femtoseconds? Well, buckle up because it seems we are there. In this interview, Dr. Stefan Rung, technical director of laser machines at Schmoll Maschinen GmbH, traces the technology trajectory of the laser drill from the CO2 laser to cutting-edge picosecond and hybrid laser drilling systems, highlighting the benefits and limitations of each method, and demonstrating how laser innovations are shaping the future of PCB fabrication.
Day 2: More Cutting-edge Insights at the EIPC Summer Conference
06/18/2025 | Pete Starkey, I-Connect007The European Institute for the PCB Community (EIPC) summer conference took place this year in Edinburgh, Scotland, June 3-4. This is the third of three articles on the conference. The other two cover Day 1’s sessions and the opening keynote speech. Below is a recap of the second day’s sessions.
Day 1: Cutting Edge Insights at the EIPC Summer Conference
06/17/2025 | Pete Starkey, I-Connect007The European Institute for the PCB Community (EIPC) Summer Conference took place this year in Edinburgh, Scotland, June 3-4. This is the second of three articles on the conference. The other two cover the keynote speeches and Day 2 of the technical conference. Below is a recap of the first day’s sessions.
Preventing Surface Prep Defects and Ensuring Reliability
06/10/2025 | Marcy LaRont, PCB007 MagazineIn printed circuit board (PCB) fabrication, surface preparation is a critical process that ensures strong adhesion, reliable plating, and long-term product performance. Without proper surface treatment, manufacturers may encounter defects such as delamination, poor solder mask adhesion, and plating failures. This article examines key surface preparation techniques, common defects resulting from improper processes, and real-world case studies that illustrate best practices.