The best-expected manual insertion rate of through-hole components is 200 to 400 components per hour. Automation has been a prime objective for printed circuit board assemblers since the electronics industry moved to manufacturing quantities that justified automated processes that ensure consistent quality and reduce manufacturing costs. There are areas where component reliability can be compromised during automatic handling and PCB insertion. Two formats of leaded-component taping are necessary for automated handling. The Axial format is more popular, and is supplied either in width, ‘W’, 52 mm or 26mm.
The component leads are used as spokes and position locators for the automated insertion tool (component registration). When the component is in position, the pick-up pliers grip the component leads on either side and, simultaneously, the leads are cropped to separate the component from the paper tape. Arms are then lowered to form the leads into a ‘goal-post’ format to a pitch that meets the PCB plated through-hole pitch. The component is then lowered to the board with the leads passing through it. Some machines also include automatic lead cropping and clinching prior to the soldering process. The radial component format is used where the board area is limited and the components placed upright on the PCB. Again, there are several radial taping formats, but the key parameters are component pitch, ‘P’, and lead pitch, ‘F’, typically 12.7 mm and 5mm respectively. Radial taping also utilises sprocket holes in the paper tape to ensure accurate component registration on the placement tool.
With both formats, it is important to ensure that the component body does not experience stress during lead forming and clinching operations. With the axial component, the lead forming or ‘knee’ must not be formed too close to the component body to ensure that mechanical stress is not transferred into the component, particularly in the case of glass body diodes. Clinching the leads too close to the PCB also introduces mechanical stress into the component.
Surface mount technology has accelerated the drive for high-speed automation to a position where it is now possible, on a medium volume placement machine, to place 20,000 to 30,000 components per hour, and on a high volume machine greater than 50,000 components per hour. The speed of placement is normally given as an average speed, as larger components take longer to pick and place compared with small discrete and passive components. To facilitate automation and high-speed component placement, the component must be delivered to the placement machine in a manner where it can be automatically offered to the equipment pick-up tool or nozzle. To achieve this, immediately after the final manufacturing and test stage of the component, it is incorporated into a tape and reel system. Figure 8-2, illustrates a taping system that is now industry standard and must comply with international standards with respect to dimensions, materials and mechanical durability. British Standard BS EN 60286-3 – 1998 (Packaging of components for automatic handling) is a taping standard, which is also matched with similar standards in the USA and Japan.
A surface mount tape and reel system comprises of a compressed polystyrene reel, a component carrier tape, either of laminated paper or polyester, depending on the component type and weight. Almost all resistors and capacitors will use paper tape carrier. Plastic carrier tape is generally referred to as ‘embossed’ tape. Finally, a polyester top cover tape is glued to the carrier tape to hold the component in the tape pocket. All parts of the taping system will be treated to be static dissipative with a resistivity of less than 10-12ohms. A static dissipative material will charge to some electrical voltage level if placed in an electrostatic field or rubbed against other materials (the tribo-electric effect). If the reel materials are inadvertently charged, they will discharge if given a discharge path. Being static dissipative does not mean that the materials will not charge. Manufacturers of PCB assemblies must take great care to minimise the possibilities for electrostatic charge build-up on the component packaging during handling and storage. Equipment should also be grounded in accordance with good ESD safe handling practices to eliminate any possibility of static voltage build up. Static can have a fatal effect on electronic components in the PCB assembly process. Static charge can cause miniature components to stick to the top tape as it is peeled back, presenting the component pickup nozzle with an empty carrier tape pocket and causing machine downtime.
