The state of copper: as designers push the performance envelope, copper foil reacts to the stresses of life over 50.

Copper foil has been used to form conductors on printed circuit boards for more than 50 years, ever since subtractively etched copper-clad laminates replaced labor-intensive point-to-point wiring. While advanced for the times, the boards of the Early Space Age were relatively unsophisticated, typically single- or double-sided, with large feature sizes and lower Tg dielectric substrate materials. In the early days, the major uses for the boards were in televisions, radios and mainframe computers.

The technical drivers for the foil were straightforward. High electrical conductivity was required to route electrical signals and power. The relatively thin copper sheets had to have sufficient strength to be handleable in wide widths. Finally, the foil needed to adhere to the substrate during processing and end use. Electrodeposited copper foil met the needs of the times and became the preferred material due to its wide width capability, surface cleanliness and matte surface for adhesion.

Copper foil still displays the same basic mechanical, electrical and thermal characteristics 50 years later. However, end-products have grown dramatically in number and morphed into displays, cellular telephones, personal computers, electronic games, digital cameras, medical electronics, avionics and automotive electronics. Today, more functionality, higher speed, smaller features and lighter weight are required. Greater uniformity and quality are demanded, all for lower cost. The pressure to improve copper foil to meet higher performance and quality standards has been constant.

Base Foil

Copper foil is composed of the base copper foil and treatment layers that are applied to the surfaces of the base foil. Base copper can be electrodeposited or rolled. The majority of PCBs are fabricated using electrodeposited copper foil. Electrodeposition (ED) produces high-purity copper, above 99.8% pure, which leads to high conductivity. The electrodeposition process parameters control the grain size and grain morphology which determine the strength and ductility of the copper. Process optimization has led to increases in both properties. Higher strength due to finer grain size has made even thinner foils handleable. Higher elongation at elevated temperature in the ED foil provides greater resistance to cracking in multilayers. Toughness improvements have enabled specially manufactured electrodeposited copper foils to be used in flexible circuits. Performance, handleability, quality and cost are improved by using a copper foil that has inherently high flexibility instead of a foil that has been annealed to provide expected flex cycle lifetimes.

The modern foil has small grain sizes, a couple of tenths of a micron or so on average. The grain size distribution is relatively narrow and the grains are equiaxed. The small, equiaxed grains can impart tensile strengths as high as 100 ksi (kip per square inch; one kip = 1,000 lbs.). Another feature of the modern foil is the relatively smooth surface topography.

Freestanding copper foils with thickness down to 7 [micro]m are available due to the improvements in strength, with 5 [micro]m freestanding in development. The thin copper, along with smooth surface topography, facilitates the fine-line etching of small features required to increase PCB functionality. Other approaches to achieving thin copper include carrier foils, panel etching of thick-copper laminate, and adhesiveless, direct deposited copper on flexible dielectrics.

Another base foil trend has been the emergence of thick ED copper foils. These foils can be as thick as 400 microns with the characteristic surface morphology and treatments of other ED foils. They have good adhesion to dielectrics, such as high Tg epoxies and polyimides, which reduces the likelihood of delamination and cracking during thermal cycling. The thicker copper foils provide greater conductivity and improved thermal transport for some automotive applications that have higher power requirements.

Treatment Layers

Treatment layers are thin films that improve adhesion of the base foil to dielectrics and add corrosion resistance.

Adhesion Layers

The adhesion at the interface between conductor and insulator must be robust during manufacturing, assembly and end-use. This interface is exposed to corrosive chemicals during processing and to high temperature, high humidity, cold, shock, vibration and shear stress during use. Technologies that optimize surface chemistry and surface area are utilized by foil manufacturers to promote and retain adhesion (1).

Several copper foil treatments promote adhesion of the dielectric resin to the conductor in PCBs. The primary adhesion promoting treatment is micro-roughness added in the form of microscopic granules of copper metal plated onto the copper foil surface (FIGURE 2). The nodules increase surface area and provide a mechanical interlock with the laminate resin. Different amounts and types of nodule treatments are available to tailor foil properties to specific performance requirements of the printed circuit board. Additional treatment is often used to enhance adhesion for higher Tg, more brittle dielectrics that have intrinsically poor adhesion. Less treatment is used to provide a smoother interface for etchability and electrical performance at high frequencies.

[FIGURE 2 OMITTED]

A chemical adhesion promoter is the last layer to be applied to the bond microroughened surface. This layer is often a silane coupling agent. The organic component of the silane is selected for compatability with the resin chemistry, while the silicon component interacts well with the metal oxide surface of the foil. The silane improves adhesion by acting as a bridge between the foil and the resin.

Barrier Layers

A barrier layer is another treatment applied to copper foil to promote adhesion. The barrier layer consists of a brass or zinc layer 800-1200 angstroms thick that is applied immediately after the nodule treatment. The barrier layer inhibits diffusion and contact of copper metal with certain dielectric components such as dicyandiamide. When the laminate is heated, dicyandiamide can interact with copper causing adhesion loss. The barrier layer inhibits the interaction and the associated adhesion loss.

Stabilization Layers

Stabilization treatments are typically applied to both surfaces of the copper foil. This layer, 25 to 50 angstroms thick, is typically an oxide of chromium or chromium alloys, such as zinc-chromium. RoHS compliance requires that any chromium present must be in the +3 valence state. Precision manufacturing ensures that this is the case.

Stabilizers, also referred to as anti-tarnish treatments, inhibit corrosion of the underlying copper. They help maintain a good cosmetic appearance and more importantly, maintain a clean, uniform, surface that is beneficial for subsequent processing and adhesion. Stabilizers are removed prior to photoresist application. A modern stabilizer is designed to be removed by etching in relatively weak etchants without mechanical brush or pumice scrub operations. The stabilizer also withstands the elevated temperature associated with higher Tg dielectrics and lead-free solder.

Topography

Copper foil surface topography plays a critical role in the performance of a PCB. Surface topography depends upon the base foil and treatments. The base foil roughness component is present on the side of the foil opposite the plating drumside surface. It is referred to as profile and is influenced by drum surface topography, additive adsorption, deposit defect structure, and mass transfer limitations (2). Thicker foils typically have greater profile. The treatment component is the nodule treatment described above and can be present on either or both foil surfaces.

Copper foil roughness affects signal integrity (3). Signal integrity impacts modeling, design and material selection. The high signaling frequencies in today's electronic designs cause the PCB trace to take on characteristics of a transmission line. At frequencies greater than ~2 GHz the skin depth approaches the value of the copper foil roughness (FIGURE 3). As a result, the measured conductor loss in PCB traces no longer conforms to the classical skin resistance models and signal integrity deteriorates.

[FIGURE 3 OMITTED]

Very-low-profile foils with the minimum treatment required for adhesion have been developed to address signal integrity issues. Additives are used to suppress profile growth during electrodeposition. Thinner foils, made possible by advances in strength, yield lower profiles since roughness has less time to develop. Very-low-profile foils can have treated surface roughness as low as 2 [micro]m [R.sub.z].

Reverse treat copper is used when minimum roughness is required. It is manufactured by applying the nodule treatment to the drum-side surface of the base foil. The treated side is laminated to the dielectric, and profile does not contribute to the interfacial roughness. One use for reverse treat is to minimize signal integrity issues. Reverse treat is also desirable for resin-based embedded capacitor applications. Dielectrics for these applications can be as thin as 8 microns. The smooth interface assures uniform dielectric thickness and maintains separation between conductive planes.

Minimizing interfacial roughness has several benefits, but adhesion can be a concern. The successful application of very-low-profile smooth foils has been facilitated by the adhesion treatment advances described above.

Advanced Technology Products

Today's OEM designers attempt to squeeze every ounce of performance and functionality out of the real estate available, and PCB fabricators try to add value to their products. A solid set of products to enable both objectives has become available. The improved utility of ED copper foils can be achieved when the foil carriers other layers, or is itself carried through the PCB manufacturing process.

Embedded Resistors

A thin resistive alloy layer can be deposited onto the treated or untreated side of copper foil (4). Processes used include vacuum deposition, electrodeposition and flame spray. Sheet resistivities ranging from 10 to 1000 ohms/square are commercially available today on the most common copper weights and laminates of all types. Embedding resistors into the laminate reduces the need for surface mount resistors, frees up surface real estate, and provides electrical and reliability advantages.

Design, cost/benefit analysis and manufacturing software tools have made integration of this technology much easier throughout the supply chain. Some embedded resistor technologies allow a fabricator having both cupric chloride and ammoniacal etchant to implement the technology with no additional capital equipment. Trace matched resistor configurations allow designers to place resistors in circuits with no "bump out" of the trace.

Laser Drilling

Significant improvement in direct laser drilling of copper clad laminates is afforded by special treatment of the copper foil surface (5). A treatment with fine surface roughness, black color and good absorption at the 10 lam wavelength of a C[O.sub.2] laser has a beneficial effect on laser drilling of copper. The treatment permits easy processing with higher throughput at lower laser energy by direct C[O.sub.2] laser drilling of standard thickness copper foils.

CAC 

CAC is designed for multilayer printed circuit boards. The untreated surfaces of each of two sheets of copper foil are bonded to an aluminum separator. The aluminum can be from 0.18 mm to 0.50 mm thick, and the copper can be from 9 pm to 140 [micro]m thick. Single-sided constructions with one sheet of copper are available.

The treated copper surfaces of CAC are laminated as the outer layer of a multilayer board. In a multiple book layup, the aluminum separates two books. The advantage of CAC is that pristine copper surfaces are sealed and protected by the aluminum. The surface of the copper remains free of airborne particles and resin dust. This improves yields, especially for high-density, fine-line boards. CAC also improves productivity, as less material handling and operator labor are required compared to conventional approaches.

Lead-free

The EU's RoHS Directive on lead content in electronics is of significant importance to the PCB community. Copper foil manufactured to high quality standards is free of lead. The higher temperature for lead-free soldering has been examined for its impact on foil. The higher Tg resins used to minimize dimensional change at the higher temperature present a greater difficulty to obtain high peel strength. Foil manufacturers and laminators have worked together to optimize adhesion with the next-generation high Tg halogen-free platforms. The performance requirements for lead-free assembly are already in place for copper foil.

In the 50-plus years since its introduction, subtractively etched copper foil continues to be the cost/performance interconnect of choice. The basic functions of the foil have not really changed. What have changed are the demands placed on the foil by inventive designers working on electronic devices that push the performance, quality and cost envelopes. In response, both the base copper foil and the treatment layers have changed significantly over the years. Copper foil manufacturers must continue to rise to the challenge by designing copper foil products with attributes that meet the more stringent requirements of the future.

[FIGURE 1 OMITTED]

ROCKY HILBURN is technical marketing manager for Gould Electronics; [e-mail]. JIANGTAO WANG is director of research & development; [e-mail]. SID CLOUSER is vice president of research & development; [e-mail]. TAD BERGSTRESSER is Senior Staff Scientist; [e-mail].

REFERENCES

(1.) L. Valette and R. Wiechmann, "High-performance Substrate From New Epoxy Resin and Enhanced Copper Foil" Circuit World, 30(4), 20 (2004).

(2.) T. Bergstresser and H. Merchant, "Surface Morphology of Electro-deposits" in Defect Structure, Morphology, and Properties of Deposits, ed. by Merchant, H.D., proceedings of TMS Materials Week Symposium, published by The Minerals, Metals, & Materials Society, Warrandale, PA., 1995.

(3.) G. Brist, S. Hall, S. Clouser, and T. Liang, "Non-Classical Conductor Lossed due to Copper Foil Roughness and Treatment" Electronic Circuits World Convention 10, Anaheim, February, 2005; $19-2.

(4.) J. Wang and S. Clouser, "Thin Film Embedded Resistors," IPC Printed Circuits EXPO 2001, Anaheim, CA, April, 2001; S08-1-1.

(5.) M. Sakamoto and S. Clouser, "Copper Foil Treatment for C[O.sub.2] Laser Direct Drilling." IPC Printed Circuits EXPO 2001, Anaheim, April, 2001; $18-1-5.