Update: Composite PCBs

Small printed circuit boards are big consumers of composite materials.
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Printed circuit boards (PCBs), the foundation of virtually all electronic devices in use today, represent a significant market for composite materials. According to industry sources, nearly two-thirds of all woven textile-grade electrical or E-glass reinforcements produced today are consumed by PCB laminators. Until the year 2000, the U.S. was the largest PCB producer, followed by Europe and Japan. In the last five years, however, the majority of PCB manufacturing has migrated to Asian fabricating firms. According to one source, China now accounts for 55 percent of PCBs produced, mainly for commodity consumer electronics like desktop computers and appliances, while U.S. manufacturers produce just 15 percent. Total PCB market size has been pegged at more than $35 billion (USD), according to one manufacturer.

Many styles of PCBs are available, including single-sided or double-sided PCBs that consist of a single layer or "core" of woven prepreg material combined with one or two outer skins of thin conductive copper. Multilayer boards, as the name suggests, are made up of many interlayered prepreg and copper layers, connected by "vias" or pathways that are drilled during board fabrication. PCBs are compression molded to form rigid "substrates" that support the silicon chips and interconnect the other electrical components in a wide range of electronic devices. ("Flexible" PCBs are made on flexible polyimide film substrate and are not composites.) While some low-end PCBs are still fabricated with phenolic paper composite, higher-end rigid boards are made with E-glass laminates.

Key Composite Structures

The majority of PCBs are made with E-glass/epoxy prepregs -- the PCB industry's traditional workhorse material, designated "FR-4," is an E-glass/epoxy material -- although other reinforcing fibers, including aramid and quartz, are sometimes used for specialty applications. Resin alternatives include vinyl ester and polyester, for commodity boards, and cyanate ester, polyimide and bismaleimide triazine (BT) for more demanding, elevated-temperature applications. The value chain consists of E-glass fiber manufacturers who sell to weavers who produce the cloth; weavers sell both to custom prepreggers who impregnate the cloth and to board laminators, some of whom prepreg in-house, who form the basic single- or multilayer boards. Laminators, in turn, sell to PCB manufacturers, who etch and drill the boards, create the electrical connections and attach the components according to OEM specifications for specific devices.

Major E-glass suppliers to the PCB sector include AGY (Aiken, S.C.), PPG Industries Inc. (Pittsburgh, Pa.), Saint-Gobain Vetrotex (Courbevoie, France) and Nittobo (Tokyo, Japan). Their glass recipes are very similar for each specific PCB "style," says AGY's Doug Lyle, because the fabric and the glass fibers must meet specific performance standards -- such as the number of ends per inch in the warp direction of the fabric, the number of pics per inch in the fill direction, the fabric weight tolerance per unit area, and guidelines on fabric thickness. Glass fiber yarn suppliers must produce a yarn that meets industry-standard E-glass composition requirements, at the designated filament diameter required for the specific fabric being produced. Yarns have to hold up during high speed weaving to produce fabrics with as little damage (broken filaments) as possible. Many of these performance standards, based originally on several military (MIL) specifications, are overseen by industry group IPC (Bannockburn, Ill.). Originally founded in 1957 as the Institute for Printed Circuits, it later became the Interconnecting and Packaging Electronic Circuits group but now goes by IPC.

"Heavier yarns are used for less technically demanding products," says AGY's PCB yarn product manager Jean-Marc Bajoit. "AGY is focusing on niche applications that require highly consistent and high-quality, fine yarns." AGY's PCB products include 5μ, 6μ and 7μ (0.00021-inch, 0.00024-inch and 0.00029-inch) filaments, in contrast to the typical 9u (0.00036-inch) reinforcement filaments in commodity reinforcements. An important issue for PCB glass yarn makers is "seed count." "Seeds" are the occasional, hollow air spaces within filaments, created during the fiber drawing process, which can become natural break points. They are highly undesirable in a PCB, because any discontinuity in the glass can create an electrical short circuit. While a seed count of zero is not possible, manufacturers typically monitor their manufacturing processes and certify no more than a certain percentage of seeds in their products destined for PCBs.

Major U.S.-based weavers that supply the PCB market include BGF Industries Inc. (Greensboro, N.C.) and Hexcel Schwebel (Anderson, S.C.) among others. BGF's VP of sales and marketing Robby Dunnagan says that a basic plain weave, with alternating warp and fill yarns, is the best for PCBs. "Since the parts being created are flat boards," he explains, "drape or formability isn't an issue, and a plain weave provides the best dimensional stability in a finished PCB." Yarns come from the manufacturer with a standard sizing or coating; weavers typically add another to provide more abrasion resistance for easier processing. Once the fabric is woven and inspected, it goes through a high temperature heat-cleaning procedure that removes all organic content from the fabric, says Dunnagan, adding that the fabric is finished with a "topical treatment," a chemical coating or size, compatible with the customer's intended requirements. Dimensionally correct, high-quality weaves with consistent thickness are critical, he notes.

Polyclad Laminates Inc. (Franklin, N.H.), Isola Laminate Systems (Chandler, Ariz., Polyclad's parent company as of April 2006), Park/Nelco (Newburgh, N.Y.) and Grace Electron Corp. (Guangzhou, China) are perhaps the best known of the many worldwide laminate producers. After converting the woven E-glass into B-staged prepreg with either solution or hot-melt methods, laminators produce multiboard panels by laying up prepreg plies with copper foil sheets in steel caul plates, in accordance with the predetermined details for each board style. A completed layup, called a "book," is placed in a vacuum compression press for an approximately two-hour cycle, to ensure full wetout and cure. Subsequently, these are cut into smaller, individual boards. Primary suppliers of the epoxy laminating resins most widely used to make PCBs include Dow Chemical Co. (Midland, Mich.), Huntsman Advanced Materials (The Woodlands, Texas) and Hexion Specialty Chemicals (Columbus, Ohio, now the source for EPON epoxy resins).

The "bare boards" are turned into finished PCB products by literally thousands of manufacturers, with some commodity producers promising finished boards within a few hours of receiving a design. A complex series of steps are carried out in cleanroom environments, including creating the pattern of desired copper traces (e.g., screen printing the pattern, then etching away the excess copper); bonding together multiple separately etched boards to form multilayer boards; drilling and plating of vias through the multiple layers to create electrical pathways; soldering the electrical components and chips and connecting the layers; and testing/validation.

Ongoing Evolution

PCB design complexity has increased as electronic devices have become smaller and endowed with more functions, which is pushing suppliers to create thinner laminates for higher layer counts and greater circuit density. Glass manufacturers are experimenting with lower dielectric constant glass recipes and methods for "flattening" filaments that take up less room for finer weaves. AGY's Bajoit explains that by decreasing the amount of yarn twist, the filaments bundles tend to flatten and create a flatter fabric (and hence thinner prepreg).

Consultant Carl Zweben (Zweben Consulting, Devon, Pa.), a specialist in electronic thermal management, says that the thermal stresses generated within the PCB can become problematic: "The coefficient of thermal expansion (CTE) of silicon memory chips is 2.5 ppm/°C and the CTE of a fiberglass laminate can range from 14 ppm/°C to 24 ppm/°C. High temperature swings during PCB processing as well as shipment can cause the two materials to expand at different rates, causing fatigue cracks in solder joints as well as damage to the chips themselves," Zweben says. "Heat management is an industry-wide problem that several groups are taking a look at."

ThermalWorks Inc. (Huntington Beach, Calif.) is addressing this issue with its relatively new family of STABLCOR carbon fiber/epoxy laminates. The prepregs are produced with 1K or 3K tows made from PAN- or pitch-based fiber, depending on the application. Kris Vasoya, executive VP of ThermalWorks and one of the company's founders, explains that a carbon laminate, while it needs to be insulated with a dielectric (nonconductive) layer because of its conductivity, provides a much better CTE match with attached chips and components because of its low negative CTE, thus minimizing thermal stresses. Further, explains Vasoya, its electrical and thermal conductivity permit it to be configured as the ground layer of a PCB, eliminating the traditional copper layer normally used. It also can serve as a heat sink -- a structure that pulls heat away from a memory chip -- at a significantly reduced weight compared to copper layers. Hunter Technology Corp. (Santa Clara, Calif.) is one of several PCB producers who have licensed STABLCOR.

Nonwoven aramid fiber mat, wet out with epoxy or polyimide, is an alternative laminate substrate material for high-performance applications, such as avionics and ultra-thin cell phones. An advantage of aramid is its low negative CTE, which reduces thermal stress, as well as its low dielectric constant of 4, compared to 6.2 for E-glass. The aramid can be combined with E-glass in multilayer PCBs. DuPont Advanced Fiber Systems (Richmond, Va.) is the supplier of Thermount aramid laminate and prepreg materials.