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Hot-air soldering of surface-mount devices

Hand soldering of surface-mount devices (SMD) should be performed under microscope or illuminated magnifier lamp. We can use soldering wire or solder paste. Solder paste is little bit more expensive and have limited shelf life but it is excellent for very fine solder work. I usually fill 5CC syringe with solder paste and use it to solder tiny surface-mount components in sizes 0402 and 0603 or small ICs with very fine pitch. I use solder paste almost every day but if you do not use it for some time it will harden and will not come out of syringe. Less solder is required for soldering of surface-mount components than for soldering of through-hole components.
Using a pair of tweezers, pick up the component to be installed and place it over the appropriate pads. The leads on all SMDs are delicate, some more than others. The finer the pitch, the easier to damage the leads.
Apply solder paste to all leads of the component. Crucial part of soldering surface-mount devices is accurate positioning of that component before applying heat. We have to properly align all leads of component on the pads of circuit board and then apply some pressure to the top of the component with tweezers to hold component in desired place.
Then apply hot-air tool around 1/8 inch above the component. After 20-30 seconds the solder will start to melt (some large ICs with 50-60 leads will require 40-50 seconds). Make sure that the solder has flowed onto the pads as well as the component leads. If there seems to be insufficient solder, add a little more. Remove hot air and wait until the board cools down. If you have any doubt about the quality of the connections, apply more flux and reflow the joints until you are satisfied.
Clean flux residues with ethanol alcohol or some other solvent as soon as you can. The longer the flux remains on board, it will better stick on solder joint and then will be harder to remove it. Remains of flux on solder joint can cause electrical leakage problem in high-impedance analog circuits.