In this description, the fabrication of rigid multilayer PWBs is divided into nine process steps. These steps to form a generic process flow, with many processes and potential alternative processes within each function. Each process is described, identifying the most common processes, common alternatives, and the general technology trends. The nine process steps are:

• Circuit design/data acquisition
• Preparation of PWB Laminate
• Inner-layer image transfer
• Laminate layers
• Drill hole pattern on board
• Clean holes (desmear)
• Make holes conductive
• Outer-layer image transfer
• Surface finish
• Final fabrication

Circuit Design/Data Acquisition. Nearly all PWB design and layout is performed with computer aided design (CAD) software that is specifically manufactured for this purpose. The files created by this software are transferred to PWB manufacturing facilities on magnetic media, via modem or via the Internet. The computer aided manufacturing (CAM) department at the PWB facilities transforms CAD generated data into customized tools for the manufacture of the part. These tools are refereed to as photo-tools, drill files, and profile routing files. These files (native CAD files or, more often, "Gerber" files, so named for the company that created the format for its vector plotters) are then transferred to photoplotters for film imaging. Laser photoplotters are used where silver-based high-contrast film, secured on a flat bed or a rotating drum, is passed under a laser source and the image created by the CAM software is reproduced on film. Developing is performed in a three- or four-chambered conveyorized developer that includes developer, fix, rinse and drying steps. The film generated in this step usually serves as the photo-tool for the image transfer process.

Preparation of PWB Laminate. The PWB base material consists of a dielectric core that has been coated or impregnated with resin. The dielectric material is usually woven glass fibers or paper. Different combinations of these two materials and the substitution of various resin systems can alter the electrical, physical, performance, and cost characteristics of the material. The type of material employed for a specific part depends on the function of the PWB, design requirements, and how it will be manufactured. Some materials perform better in certain environments (e.g., extreme heat or high humidity), others are more suitable for a particular manufacturing process (e.g., punching), while others are chosen for their electrical properties (e.g., dielectric constant). FR4 is the designation given to the most widely used material for the printed wiring board industry. It is constructed of multiple plies of resin impregnated woven glass cloth. GI type material, also known as polyimide, is an example of a high temperature type material. Its resin system allows it to sustain temperatures of 200°C vs. FR4’s 120-135°C.

Copper foil is rolled or electrolytically deposited on the base laminate. PWB facilities generally purchase sheets of copper-clad base laminate in sizes of 3x4 feet or larger. The core material is sheared to panel size then cleaned mechanically, chemically, or by a combination of both. The purpose of this cleaning step, referred to as "pre-clean" or "chem-clean," is to remove surface contamination, including any anti-tarnish coating present and to condition the surface copper topography to promote the subsequent adhesion of photoresist.

Inner layer Image Transfer. The purpose of this process step is to transfer a circuit image to the copper-coated base laminate of the PWB. Two basic strategies exist: subtractive and additive. The predominate method is subtractive which is accomplished through a series of steps known collectively as "print-and-etch."

Print-and-etch is a series of process steps that accomplish the goal of image transfer from the photo-tool to the copper foil layer of the base material. The "print" step includes the coating of the copper-foil-clad base material with a light-sensitive, organic photoresist. The photoresist (so named because in addition to being light sensitive, the coating will subsequently "resist" the etchant during a later step in the print-and-etch process) polymerizes when exposed to a light source of appropriate energy. The phototool, placed over the photoresist, acts to allow only an image of the circuit to be exposed, protecting the other areas of the photoresist layer. After exposure, the photoresist layer is developed; the exposed, polymerized areas remain, the unexposed areas (those which were under opaque areas of the photo-tool) are washed away, revealing the copper layer underneath.

The "etch" portion of print-and-etch removes exposed copper areas selectively from the panel, but cannot attack the copper residing under the photoresist. Thus, the image of the circuit is transferred from the photo-tool to the copper layer. Etching is performed with conveyorized equipment that typically includes a main spray chamber, an etchant flood rinse, and several cascading water rinses. Long conveyorized units that include developing, etching, and film stripping are common only in large production shops. Acidic cupric chloride and alkaline ammoniacal are the most common etchants (sulfuric-peroxide, a common microetchant, is also employed as a primary etchant).

Lamination. During the lamination process the thin-core innerlayers are subjected to heat and pressure and compressed into a laminated panel. Sheets of material consisting of glass fibers impregnated with epoxy resin, known as pre-preg or b-stage, are slipped between the layers and bond the layers together. Pre-preg is available in different styles with varying amounts of resin and glass fibers, which allows the manufacturer to control the thickness between layers and to provide the appropriate amount of resin flow between circuitry.

Lamination steps are fairly consistent among manufacturers, although substitute lamination materials are available. All of the materials, including the innerlayers and pre-preg, are tooled to the same registration system and are held in place by tooling pins. Several panels can be pressed together in one set of heavy plates, creating what is known as a "book." Next to each copper outer layer is some sort of protective coversheet. Sheets of aluminum or a thin plastic sheet such as Paco-Thane^® and a steel separator plate are used for this purpose.

Drilling. Holes are drilled through the PWB to interconnect circuitry on different layers and to allow the insertion of components (Exhibit 3-10). The etched innerlayer pattern will extend to the barrel of the hole and therefore will be interconnected with the other layers when the hole barrel is made conductive in a later step. Most drilling is performed with computer numerical control (CNC) equipment, but as hole sizes less than .012" have become more common, other methods of making small holes are increasing in popularity. Two alternate methods are punching and laser processing.

Hole Cleaning. Hole cleaning generally refers to a process called desmear and/or the closely related process of etchback. Desmear removes the melted resin smear that results from the friction of the drill bit cutting through the material. If the smear covers the copper that extends to the barrel of the hole, it would prevent interconnection between it and the subsequently metallized hole. During etchback, in addition to removing resin smear, glass fibers are etched. The result is that the copper on the innerlayers protrudes out into the barrel of the hole. This allows for what is known as a "3-point" connection after metallization.

Deburring and scrubbing are performed immediately before or after desmear or etchback. During drilling, copper burrs may be raised on both sides of the panel by the action of the drill entering and exiting the material. The burrs are sanded smooth on a deburring machine, which consists of a sanding wheel and a conveyor. In wet deburrers, copper dust is carried off in a waste stream. Dry machines usually are outfitted with vacuum units. Deburring is more correctly considered a surface preparation step rather than hole cleaning. Scrubbing is performed as a surface preparation step prior to electroless copper (and during other stages, such as before solder mask). Scrubbing may be performed similarly to deburring, except a much less aggressive surface abrasion occurs. Pumice or aluminum oxide scrubbers, which direct a high-pressure spray of abrasive particles at the PWB, are also used for surface preparation.

Making Holes Conductive. To provide for the intended interconnection between layers, the holes must be coated or plated with a conductive substance. The PWB substrate itself is not conductive, so a non-electrolytic deposition method is required. Afterwards, electroplating is performed to plate the copper to the specified thickness.

Until recently, electroless copper has been used almost exclusively to metallize the holes. Direct metallization (DM) processes were introduced in the 1970s, but reports of higher costs and inconsistent quality kept manufacturers from experimenting with an unproven process. New interest in alternatives to electroless copper was ignited in 1992, when OSHA amended a standard for occupational exposure to formaldehyde, a known carcinogen. With few exceptions, electroless copper uses formaldehyde as the reducing agent.

Alternatives to electroless copper now include palladium-based systems, carbon/graphite-based systems, electroless nickel, conductive polymer, and non-formaldehyde-based electroless copper. Based on the survey results, however, it is apparent that the electroless copper process is still entrenched as the predominant method of making holes conductive, although its use appears to be declining. Seventy-seven percent of all respondents reported using electroless copper for through-hole metallization, with the remainder split evenly among graphite-, palladium-, and carbon-based systems. One user reported testing an electroless nickel-based system on a small percentage of their product.

Electroless copper baths can be divided into two types: heavy deposition baths (designed to produce 75 to 125 micro-inches of copper) and light deposition baths (20 to 40 micro-inches). Light deposition must be followed immediately by electrolytic copper plating. The more common heavy deposition can survive the outer layer imaging process, and copper electroplating occurs thereafter. The main constituents of the electroless copper chemistry are sodium hydroxide, formaldehyde, EDTA (or other chelater), and a copper salt. In the complex reaction, catalyzed by palladium, formaldehyde reduces the copper ion to metallic copper. Formaldehyde (which is oxidized), sodium hydroxide (which is broken down), and copper (which is deposited) must be replenished frequently.

Most heavy deposition baths have automatic replenishment schemes based on in-tank colorimeters. Light deposition formulations may be controlled by analysis. Formaldehyde is present in light deposition baths in a concentration of 3 to 5 grams/liter and as high as 10 grams/liter in heavy deposition baths.

When light deposition is applied, the next process step must be electrolytic copper plate. This is either a full panel plate (the typical 1 mil is plated in the holes and on the surface) or a "flash" panel plate, designed only to add enough copper to the hole barrels to survive the imaging process. Flash-plated panels return to copper electroplating after imaging to be plated up to the required thickness. This double plating step has made heavy deposition the more common electroless copper process.

Outer Layer Image Transfer. The majority of manufacturers in the U.S., Southeast Asia, and Europe use the print, pattern plate, and etch sequence for outer layer image transfer. This method is similar to the inner layer imaging described previously.

Exposing may be done with first-generation photoplotted phototools or with diazo, a reddish transparent film that allows for manual registration. When pattern plating is to follow, outer layer phototools are positive images of the circuit. The circuit image is developed away exposing the underlying copper. The photoresist remaining on the panel is the plating resist for the pattern plate process.

Pattern plating is so named because only the circuit pattern and hole barrels are plated. Only the thin electroless copper layer has been deposited in the hole barrels up to this point in the process and it is far short of the typical 0.001-inch specification for copper thickness. None of the copper plated during this process is etched away, but rather, remains on the circuit and is part of the finished product. The copper is protected from the etchant by a metallic etch resist that is plated on during the next process step. The ordinary outer layer will have about 33% of the panel plated to a thickness of 1.5 mils of copper. This 1.5 mil target is to ensure a minimum thickness of 1 mil in the holes. The result is 0.6866 ounces of copper being plated per square foot of product run for a typical panel. This result does not account for the copper contained in the solution that is dragged with the panel.

Although a few copper electroplating chemistries exist, nearly all PWB facilities use basically the same copper sulfate bath composition. The bath is typically made with 10 ounces/gallon of copper sulfate, 25 to 40 ounces/gallon of sulfuric acid, and a small amount of hydrochloric acid to provide a chloride concentration of 30 to 90 mg/l. This bath has an extremely long life (measured in years) and is generally easy to maintain and control. Proprietary organic additives, usually referred to as brighteners, distinguish one vendor’s bath from another. The pre-plate line consists of an acid cleaner (dilute phosphoric acid is a common constituent), a microetch, and a sulfuric pre-dip.

High-performance copper plating is reliably performed at a current density range of 20 to 35 amperes/ft². Manufacturers generally plate from 0.0013 to 0.0017 inches of copper to ensure that all hole barrels meet the minimum of 0.001 inches in all areas of the panel. Dwell times depend on current density and target thickness, but generally range from 30 minutes to somewhat more than one hour.

Surface Finish. For most parts, the functions of the surface finish are to prevent copper oxidation, facilitate solderability, and prevent defects during the assembly process. A number of metallic alternatives exist along with organic solderability preservatives (also known as OSPs or pre-fluxes). A variety of deposition techniques exist, including hot air leveling, electroplating, immersion, and electroless plating. The shelf life of immersion, electroless plated, and OSP coating alternatives are less than that of leveled or tin-lead reflowed boards. Other surface finishes are dictated by the environment in which the part will reside or by specific performance criteria.

Solder-mask-over-bare-copper (SMOBC) with hot-air-solder-leveling (HASL) has been the preferred surface finish for over 15 years (ref. 38). Nickel-gold, another popular finish, can be applied electrolytically as an etch resist, replacing tin and tin-lead, or electrolessly as a substitute for HASL. Other electrolytic plating metals include rhodium, palladium, palladium-nickel alloys, and ruthenium. Non-electrolytic deposition processes include tin immersion, tin-lead displacement plating, electroless nickel, electroless gold, immersion gold, immersion silver, immersion bismuth, and the previously mentioned OSPs. The following describes the SMOBC/HASL processes.

This method predominates for several reasons. Copper is a surface that lends itself to rigorous cleaning, which is essential for solder mask adhesion. If the solder mask were placed over tin-lead traces, the tin-lead would liquefy during soldering and may cause the mask to blister and peel. The hot air solder leveling process generally produces less wastewater and introduces less lead into the wastewater stream than tin-lead plating and reflow. The overall process begins with a solder-mask pre-clean, usually a mechanical or pumice scrub. Solder mask is then applied, followed by hot air solder leveling, nomenclature screening, and finally, gold edge plating if necessary.

Increasing circuit density, however, is causing some to look for alternatives to this difficult to control process. Maintaining planarity of fine-pitch surface mount pads across both sides of the panel is a challenge. Another is solder bridging on fine pitch pads. This tends to happen when pads are lined up in the same direction of travel as the panel as it is pulled from the solder pot. Increasing the pressure of the air knives may blow off the solder bridges, but it also reduces the thickness of the solder coat on all of the other features.

The purpose of solder mask is to physically and electrically insulate those portions of the circuit to which no solder or soldering is required. Increasing density and surface mount technology have increased the need for solder mask to the point that, with the exception of "pads only" designs, nearly all parts require it. Manufacturers have had some autonomy in selecting masks. Many specifications do not call out a specific product or product type, and this has allowed the manufacturer to choose masks based on processing as well as performance issues.

Three basic types of masks are commonly applied: thermally cured screen-printed masks, dry film, and liquid photoimageable (LPI). Thermal masks have predominated for decades but are gradually being replaced by LPI, despite being the lowest cost alternative. Dry film has some specific advantages, such as ease of application, but its use seems poised to decline as well in the face of improving LPI formulations.

The HASL process consists of a pre-clean, fluxing, hot air leveling, and a post-clean. Pre-cleaning is usually done with a micro-etch. However, the usual persulfate or peroxide micro-etch is not common in the process. Dilute ferric chloride or a hydrochloric-based chemistry is favored for compatibility with the fluxes that are applied in the next step.

Fluxes perform the following functions:

Higher viscosity fluxes provide better oxidation protection and more uniform solder leveling, but reduce overall heat transfer and require a longer dwell time or higher temperature. A balance in flux use must be struck between better protection with high viscosity fluxes and superior heat transfer with lower viscosity fluxes.

Hot air level machines consist of a transport mechanism that carries the panel into a reservoir of molten solder (460°F, 237°C), then rapidly past jets of hot air. All areas of exposed copper are coated with solder and masked areas remain solder-free. Boards are then cleaned in hot water, the only step in the SMOBC process where lead may enter the wastewater stream, albeit in very small quantities. Once cleaned, the panels may again enter the screening area for optional nomenclature screening, or proceed directly to the routing process.

Copper, flu, and other impurities build in concentration in the solder pot as panels are processed through the hot air leveler. These impurities can be removed to some degree by performing a procedure known as drossing. From the hot operating temperature, the temperature is reduced to 385°F (196°C) and the machine sits idle for 8 to 12 hours. The impurities will float to the surface of the solder where they are scooped out and placed in a dross bucket. This material can be returned to the vendor for reclamation of the metals. Some manufacturers go for years without changing the solder; they dross and make additions. When the time comes to change over the solder, vendors will issue credit on the purchase of new solder as long as the old solder is returned to them for processing.

The acid pre-clean will have some copper in solution and can be treated conventionally. The waste flux is collected and is sent off-site for treatment.

Final Fabrication. Non-plated features are added to the board during the final fabrication process. These may include tooling holes, cutouts, and countersink holes. Numeric controlled routers run profiling programs that are output from the CAM systems with all of the features needed according to specifications.

Finally, the circuit is either completely routed from the panel or partially depanelized. Partial depanelization is common with production lots or when board assembly will be performed by machine. Most of the circuit is routed out of the panel, but tabs remain to hold the circuit in place. This allows the assembly machine to populate multiple boards at once. Afterwards, the circuits can be snapped or broken out of the panel. Such panels are often referred to as "breakaways," "snaps," or "arrays."

An alternative to breakaways is to have the panel V-scored. This allows more circuits to be placed on a panel since no spacing is necessary for the routing bit. An array of circuits is routed as if it were a single image. The V-scoring machine has a thin rotating scoring blade that will rout across the top of the panel 30-40% of the thickness of the panel. This is repeated on the bottom side. Some scoring machines have two blades and both sides can be scored in one pass. The remaining 20-40% of the panel not routed will hold the panel together through assembly and can then be easily broken apart.