A High-Reliability, Stress-free Copper Deposit for FPC, Polyimide and Rigid-Flex

Estimated reading time: 2 minutes

Today’s wide variety of laminate materials and specialized dielectric choices pose a challenge for process engineering. In particular, smooth surfaces, such as polyimide, flex circuit substrates and rigid-flex constructions with window cut-outs, can be a challenge for electroless copper and plating processes. Conventional electroless copper systems often required pretreatments with hazardous chemicals or have a small process window to achieve a uniform coverage without blistering. To overcome the challenge of metallizing smooth surfaces, a new stress-free electroless copper was developed to serve this important sector of the printed circuit industry.

Introduction

As the thermal, physical, chemical and electrical properties of PCBs have advanced, so too have the substrates of construction. With a wide variety of substrates available, it is becoming increasingly difficult to accommodate these new substrates in current manufacturing processes[1]. Polyimide resins (PI), for example, provide exceptional thermal and chemical stability but remain challenging with industry standard processes.

In particular, electroless copper deposition, the most commonly used method of metallizing a nonconductive substrate, is susceptible to blistering or peeling due to the low adhesion of the copper film to the substrate. Typically, electroless copper films require mechanical anchoring to provide adhesion to a substrate to prevent blistering. A roughened surface is commonly created with a chemical or plasma etch process to help create anchoring sites.

Conventional chemical etches, which were primarily designed for epoxy substrates, are generally ineffective at activating PI substrates. Plasma etching, which is effective at etching PI, is still insufficient to prevent peeling and blistering. Some manufacturing processes have resorted to using an alkaline solution containing hydrazine. While this can be effective at improving adhesion of electroless copper films, hydrazine is extremely hazardous and challenging to handle safely. In addition, many material types, such as those that contain adhesive bonding layers, are incompatible with strong alkaline solutions.

Due to the fact that most surface treatments are ineffective, or not practical or compatible in some situations, it is critical that the electroless copper process provides a significantly wide processing window to alleviate blistering defects and accommodate a variety of substrate types.

The most common commercially available electroless copper plating solutions are not designed to meet these requirements. It is known that blistering and peeling of the copper deposit is also a function of the internal stress and strain of the deposit and that additives can be included in an electroless copper solution that affect the properties of the resulting electroless copper deposits (5–7). However, inclusion of additives may affect PCB reliability and careful selection is necessary. In this study, we evaluate select additives in an electroless copper system for their influence on the deposit stress and, ultimately, their effect on the reliability of a PCB by thermal shock and interconnect stress test (IST).

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Editor's Note: This article originally appeared in the June 2015 issue of The PCB Magazine.