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HANDLING TO PROTECT THE PCBA
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The handling of PCBs both before and after the soldering operation can be very important in terms of inducing possible damage to the board or contaminating the board such that subsequent operations are affected. There are three subjects that should be well thought out and controlled in an assembly operation for PCBAs: electrostatic discharge (ESD) protection, contamination prevention, and physical damage prevention.
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PRINTED CIRCUITS HANDBOOK
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ESD Protection ESD is the rapid discharge of a voltage potential into an electronic assembly from a static energy source. The presence of a static discharge in the area of a sensitive device can damage the device even without actual contact. ESD-sensitive (ESDS) components found on the assembly and the amount of current generated by the discharge will determine whether immediate failure takes place. All types of ESD damage can be difficult to troubleshoot, but the most costly scenario is one in which the component is assembled and passes functional test only to fail later due to latent conditions resulting from ESD. This could cause the loss of a customer and even in some cases recall of the product. Some electronic devices are more sensitive to electrostatic overstress (EOS) damage than others. The degree of such sensitivity within a particular device is related directly to the manufacturing technology employed. The trend for electronic components is smaller size, wider band pass, and lower voltage. These are all contributors to the ESD protection problem and increased sensitivity of the component. Where sensitive components are handled, protective measures must be taken to prevent component damage. Improper and careless handling accounts for a significant portion of ESD damage to components and assemblies. Before handling ESD-sensitive components, equipment should be carefully tested to ensure that it does not generate damage-causing spikes. The preferred workstation to be utilized in electronic assembly is shown in Fig. 52.1.
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FIGURE 52.1 Target condition for EOS/ESD workstation (1) Personal wrist strap (2) EOS protective trays, shunts, etc. (3) EOS protective table top (4) EOS protective floor mat (5) Building floor (6) Common ground point (7) Ground. (IPC)
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Static charges are created when nonconductive materials are separated. Destructive static charges are often induced on nearby conductors, such as human skin, and discharged as sparks passing between conductors. This can happen when a person having a static charge potential touches a PCBA.The electronic assembly can be damaged as the discharge passes through the conductive pattern to a static-sensitive component. Static discharges may be too low to be felt by humans (less than 3,500 volts) and still damage ESD-sensitive components. Sensitive components and assemblies must be enclosed in conductive, static-shielding bags, boxes, or wraps when not being worked on, unless otherwise protected. ESD-sensitive items must be removed from protective enclosures only at static dissipative or antistatic workstations.
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ACCEPTABILITY OF PRINTED CIRCUIT BOARD ASSEMBLIES
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For ESD safety, a path to ground must be provided for static charges that would otherwise discharge on a device or board assembly. Provisions are made for grounding the worker, preferably via a wrist strap or a heel strap, provided conductive flooring is used.
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Contamination Prevention The key to any contamination problem is to prevent it from happening. The cost of subsequent operations required to clean a product or rework a product is orders of magnitude more than any cost associated with preventing the contamination from occurring.These contaminants can cause soldering and solder mask or conformal coating problems. There are any number of possibilities for inducing contamination in an assembly environment, such as dirt, dust, machine oils, and process residues; however, in many cases, contamination is caused by the human body, specifically the salts and oils on the skin. In a high percentage of assembly areas, good housekeeping will take care of contamination from the environment. It should be a common event on each shift to clean workstations, sweep floors, dust fixtures, empty trash, and so on. This not only prevents contamination, but it also helps to keep personnel morale at a higher level. After all, we all desire to work in a nice, clean environment. It will also serve as a positive point if visitors or possible customers visit the assembly area. To prevent contamination from the human body, every individual must be aware of the possibility of contaminating the PCBA. Printed circuits should be touched only on the edges away from any edge connectors prior to the soldering operation (see Fig. 52.2). Where mechanical assembly procedures require a firm grip on the board, gloves or finger cots should be worn. If the PCBA is to be conformal coated after the soldering operation, the handling of the assembly should be prevented. In some cases, using finger cots, gloves, or fixtures can reduce the contamination. Many producers utilize cleanliness testing to ensure that contamination levels remain within an acceptable range before the coating process, and to ensure adhesion and eliminate localized contaminants that could impact circuit operation and longterm performance. J-STD-001 provides levels for cleanliness acceptability.
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FIGURE 52.2 Appropriate method of handling board to avoid contamination and electrostatic discharger damage. Note position of fingers and the use of a grounding wrist strap.
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Physical Damage Prevention Improper handling can damage components and assemblies. Typical defects associated with handling are cracked, chipped, or broken components; bent or broken terminals; scratched
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