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18 SMT Magazine • November 2014 nents must be removed safely at correct compo- nent temperatures. The pad area must now be prepared for a replacement component. When a faulty component is removed from a PCB, solder and epoxy are left as a byproduct (Figure 5). Residual solder and epoxy must be removed prior to component replacement. The most common way to remove residual glue is to scrape the epoxy using hand tools. Using this method greatly increases the risk of board dam- age. An alternative method is when the residu- al solder and epoxy removal operation can be done manually with a soldering iron, wicking braid, and flux (Figure 6). Solder is cleaned with wick or retinned with soldering iron hoof style tips to level pads, in most cases, and in some cases increase solder heights on corners to stop bridges in the cor- ners of a 0.4 mm PoP. This is because compo- nents are warped concavely in the middle as a new component, and might require this special technique to achieve success. The risks to the circuit board assembly are inconsistent solder removal resulting in poor adhesion, solder resist damage resulting in opens or shorts, and ther- mal damage to the PCBA. The alternative is less risk than the most common method. However, both manual processes add additional cost to the process due to the time and resources need to perform the pad cleaning operation and rep- resent a risk to the manufacturing in terms of rework cHaLLenges For smartPHones anD tabLets continues scrap costs. Cleaning residual solder from ultra- fine pitch 0.35 mm and 0.4 mm pitch compo- nents now require a better solution known as contactless solder removal. Commonly referred to as scavenging, vacuum and hot air are used to melt solder and vacuum up solder without touching the PCB. The advantage of a scaveng- ing site cleaning system is the equipment does not make contact with the circuit board reduc- ing the risk of damaging the solder resist and pads. Scavenging systems require the same ther- mal profile controls as other rework equipment and benefit from automating the task of mov- ing the vacuum collection nozzle over the site to provide consistent solder removal. With the solder pad cleaned, the operator is ready to re- place the component. Reattaching components to the PCB re- quires the use of either infrared or convection technologies to heat the board and a mechani- cal means of physically placing the component. The size of the components used in mobile devices provides a challenge to the operator. Processors have solder connections between 0.3 mm–0.5 mm (4–10 mils), micro-SMD com- ponent sizes range from 1 mm–6 mm square with 4 to 100 solder balls per package. Common solder ball sizes range from 0.1–0.2 mm (4–8 mils). Memory components are typically BGA feaTure figure 5: residual solder and epoxy. figure 6: Pad cleaning using company soldering system and blade cartridge.