Martha K. Raymond
Staff Editor
Manufacturing electronic products is no longer what it used to be. With so many new high-tech products being designed every day, and consumers demanding smaller, more powerful computers for cars and pocket telephones, it seems inevitable that everything laptop will become palmtop. To keep up with this demand for shrinking machines, engineers are finding clever ways to further miniaturize electronic components and stuff them into less space.
One component common to all these products and most responsible for product size is a printed-circuit board. Focusing design creativity and innovation on this platform is leading designers — though incrementally — to the next level of PCB manufacturing and assembly.
PCB DESIGN BASICS
PCB design starts with specifying the electronic components that perform the required functions for a product. Then, the challenge is to determine the most efficient and effective way to electrically connect those components.
Using computers with CAD software specifically engineered for PCB layout, the first step describes components on the board, a process called schematic capture. Then the physical layout process describes the board’s template using three basic tools: a list of the interconnections between devices on the board; a parts library of physical and electrical information about components on the board; and a mechanical drawing specifying the board’s shape and areas where components can’t be placed. The next stage in physical layout is locating all the devices for optimal electrical and mechanical performance.
FABRICATION
Designing a PCB for manufacturability not only includes a working physical board layout, but also involves such steps as fabrication and assembly. Economical designs consider low-cost PCB materials, fabrication methods, and assembly that meet the product’s performance needs. Some factors to consider when designing a PCB for fabrication include whether packaging a product on several smaller PCBs or one large board, placing components closer together to produce a smaller PCB, spacing components out to reduce the layer count, or just increasing layers is the optimal approach for that product.
Most advanced circuit boards consist of four or more layers. One reason for multiple layers is that products often contain small, high pin-count integrated circuits that require hundreds of signal interconnects. Many four-layer PCBs are built in panel sizes of 36 3 48 in. using a process called mass lamination, where tooling holes are drilled after lamination. When a PCB has six or more layers, they’re usually built on standard 18 X 24 in. panels using pin laminations, where tooling pins line up the layers.
One of the most critical fabrication steps is drilling the board for plated through holes or vias because drilling costs are second only to materials. To ensure accurate machining, layers are lined up using registration marks. These marks establish a printed wiring pattern’s position with respect to predetermined locations on adjacent layers.
Drilling data ensures the correct hole size and location in the PCB. Data include machine commands to change drill bits, vary spindle speed, and control the spindle feed rates. It also keeps track of the number of panels that can be simultaneously stacked and drilled.
After drilling, plating ensures the vias function as electrical conduits between conductive layers. An electroless, chemical plating process deposits as little as a 0.001-in. copper layer over the entire surface of the outer layers and on bare via walls.
ON THE BOARDS
When designing boards for automatic assembly, consider machines available in production to streamline the project. Basic assembly techniques include through-hole mounting, surface mounting, and a mixture of the two.
Axial, radial, dual in-line package (DIP), and odd-form components, such as certain inductors and large capacitors, are throughhole mounted. Axial components contain leads running axially through the center. The leads are sequentially taped to form a ribbon of components. During assembly, an insertion machine cuts the components from the tape, forms the lead in an upside down U-shape, inserts the component in the board, and clinches the leads below the board for secure attachment.
Radial lead components include such components as capacitors, potentiometers, resistor networks, and fuse holders.
While many DIP integrated circuits (ICs) are automatically inserted, they are large relative to other components on the board. Thus, most DIPs were converted to surface-mount components while the remaining few are treated as oddform components.
With advancements in robotics, most odd-form components can be automatically placed into a PCB. But because these components appear in small quantities and aren’t common from one board to another, manufacturing engineers and board designers work far in advance of assembly to eliminate one-of-a-kind components.
In surface-mount assembly, there are three methods of mounting components. The PCB is prepared first. The solder pattern and solder paste are screened on the PCB. Then, in the least-dense method, SMT parts are placed on top of the PCB. They are held in place by paste or an adhesive. The board goes under an infrared heater to reflow solder each connection. Then the board is turned over and through-hole components are installed. Then the board is flipped over again and goes through the wave solder machine. It takes two passes to solder both sides.
The second type is the moderate dense method where SMT components are placed on the top and held in place by adhesive. The adhesive is not cured. The through-hole components are installed on the opposite side, and the board goes through the wave solder machine. Both sides are soldered in one pass.
The most dense method uses SMT components on both sides. The SMT parts are glued and cured to one side of the board. The board is flipped over and the SMT components on the top are held in place by the solder paste. The board then goes through the wave solder machine, and both sides are soldered in one pass.
ASSEMBLY EQUIPMENT
Chipshooters quickly place tape-fed components because component carriers continuously move back and forth. Using a turret with as many as 12 heads that contain nozzles of different sizes, chipshooters handle various components. This machine performs assembly processes in parallel. For example, as one of the heads on the turret picks up a part, another head is placing a part in a board.
Another machine, a flexible/fine-pitch placer, runs at maximum speeds which are less than 20% of the maximum speed of a chipshooter, though they typically handle a complete range of components with more accuracy. For positioning, overhead gantries that move in the X and Y directions are mounted on placement heads that rotate and move in the Z direction. Heads with multiple spindles accommodate a variety of part feeding systems.
For bottom-side attachment, dispensers place adhesive dots at component locations. The required accuracy and dot size depends on component size. But because surfacemount components continue to shrink, adhesive dispensers need precise dot placement.
In adhesive technology, it’s the pump technology that drives the degree of accuracy. Positive-displacement pumps dispense consistent dot volume regardless of adhesive viscosity, compared to an air-over system that uses a syringe. A syringe dispenses various adhesive amounts depending on viscosity, which can change under environmental factors such as humidity and temperature. By using a piston, its stroke and diameter determine the amount of adhesive leaving the nozzle. Dot-size control is critical because if adhesive spreads from under the component, solder defects may surface down the line. On the other hand, too little adhesive won’t hold the components to the board through wave soldering.
ACROSS THE BOARD
After attachment components, PCBs usually sit in electronic housings and connect to peripheral devices. A couple of choices for connection include PCB circuit terminal blocks and Combicon plug-in screw connectors from Phoenix Contact.
Terminal blocks provide strain and bending relief of sensitive soldered connections by using sturdy, rigid housings. Sharp-edged solder pins anchor in bore holes and shrink fit into the PCB.
One type of terminal block, a spring terminal labeled ZFK4DS 1.5, has four-levels which allow assembly of compact connecting blocks for industrial PCBs. These interlocking single terminal blocks can be arranged in rows with any number of positions. The terminal block accepts up to #12-awg wires and is rated for a 10 A current at a voltage of 300 V. T
he other type of connector, the Combicon plug, accepts all types of conductors to 2.5 mm2. The structural design combines advantages of pluggability for easy maintenance with a sturdy screw connection, eliminating special tools.
The terminal screw, the cage, and the current- carrying contact system are made of alloys with a high copper content. Another advantage is the linear arrangement of the individual positions, which gives the impression of an interface terminal strip, but provides convenient and orderly wiring conditions at the same time.
In circuit-board applications, proper fastener selection can be key to easy assembly. Because most PCBs are densely populated, miniature fasteners as small as M0.5 (#000) meet the needs of electronics and other high-tech applications. Miniature Plastite fasteners from Camcar Textron have twin lead threads with a steep helix angle to let fasteners seat in half as many revolutions, reducing assembly time. The steep angle and fewer revolutions also reduce heat build-up from friction and offer greater resistance to stripout and pull-out in most thermoplastics.
Because PCBs are typically too thin to tap, Perma-Nut inserts offer permanent, threaded fittings. These fasteners combine the functions of a semi-tubular rivet and a nut without deforming the mating material. They use a roll-clinch design that fastens to the material for a secure joint. Also, rollclinches allow two-sided access so automatic or semiautomatic, multiple station equipment can efficiently install them. In addition, the underhead serrations resist rotation under torque.
PCB Assembly in the Real World In the configuration, different size pins are blown from the bowl feeder to a soldering location, where they’re soldered into a PCB by a Panasonic SoftBeam light-based soldering tool. The boards are fed into the system using a walking-beam-style transfer, and Lucent CAD data is input to specify the exact coordinates of the circuit boards and pins. The system’s main functions converge at the Spectra-designed end-effector. The end-effector uses five pneumatic lines to grip and rapidly and precisely move the wirefeeder, soldering tool, pins, and circuit boards. Using the SoftBeam light-based soldering tool lets Spectra cut the system’s soldering time to a fraction of a conventional soldering iron. The SoftBeam not only solders faster than conventional irons, it’s also more reliable and requires less maintenance because it never comes in physical contact with the soldering wire. Just as important, both the circuit boards and soldering wire are preheated to just below the solder melting point. The preheated solder retains enough mechanical strength to be rapidly positioned. Yet it requires less heating energy during the soldering process, which shaves more time from the soldering function. But, it is the Adept robot that increases system speed the most. “The workcell harnesses the speed of the Adept 550 in numerous areas, like moving the SoftBeam soldering tool to various soldering points on the board and controlling all pneumatic functions of the system,” says Victor Trotter Spectra electronics technical director. To further enhance productivity, the system provides in-process control capabilities. Prior to implementing the Spectra-designed workcell, Lucent’s manufacturing process required workers to manually place pins on circuit boards whose resistors, capacitors, and other components were held by solder paste. Because the paste had not solidified, the slightest bump could cause alignment errors, requiring a board to be reworked. The new system not only offers an impressive return on investment, it delivers better quality as well. |