SMT: Soldering And Desoldering Techniques & Working With Multi-Lead Components

5. Soldering and desoldering techniques:

5.1 Using hot-air jets:

• Read manufacturers instruction manual before starting to work with the station.

• Use minimum air flow possible to reduce solder splatter and flying components.

• Use tweezers for removing the components and wooden stick for repositioning of small components.

• Add flux to the leads of the component to be removed with a fine brush or straight from flux dispensing bottle containing a needle tip.

• If leads have excess solder, use solder wick and the fine tip iron to remove most of the solder before applying hot air. Solder wick is more effective for removing solder from surface mount pad area that solder suckers and therefore preferred.

• Preheat the hot air jet and then attach the proper nozzle. This prevents the nozzle from falling off due to expansion of the nozzle holder.

• Preheat the flux and leads by blowing hot air on them. The air nozzle should be about 2” (50mm) from the leads. After preheating the joints for 10 to 15 seconds, slowly move the nozzle to within 1/2” or 1/4” from the lead. Hold the nozzle directly above the top of the package. This will minimize components from blowing off.

• When all the solder has melted, push the components to assure complete solder melt and then gently lift the component off the pads. Typical time is 20 to 30 sec.

• Use tweezers to lift the components. The tweezers can also be used to test for completeness of reflow by pushing or gently prying the component before lifting. This is to ensure that the lands and traces are not lifted accidentally due to incomplete solder melt.

• When working with multi-leaded parts it’s necessary to move or direct the hot air so each lead is exposed to the same amount of hot air.

• Special nozzles should be used for ICs that may be reusable. This assures uniform hot air on the leads while diverting the air away from the component body.

5.2 Solder irons:

• Long, fine tips work best for touching up individual leads and chip component terminations.

• A chisel tip works best for reflowing and planarizing multiple lead gull wing ICs.

• Always lay the soldering iron tips on the solder to be heated and do not push or force the tips into the board to avoid damage to the pad area. An elevation angle of 450 from the board surface is preferred.

• The small unglued ceramic chip components are usually easily removable when the solder iron tip is applied to one side. Use as low tip temperature as possible. Tweezers points and not the soldering iron tip are to be used to push or pry components or leads.

6. Working with multi-lead components:

• i. Using a short bristle brush or the flux dispensing bottle apply a small amount of flux to the solder joints pertaining to the component.

• ii. Remove all old solder from the land and component leads with suitable solder wick. Make sure with a tooth prick/ wooden stick that the lead is free from solder by gently pushing it.

• iii. Remove solder from other joints in the same fashion as given above.

• iv. Clean the pads using cotton swabs soaked with alcohol and dry.

• v. Solder coat the lands using a fine tip solder iron to melt and flow 0.015” solder (flux cored) wire. The amount of solder must be uniform and not excessive.

• vi. Dispense a small amount of flux on each lead and pad joint.

• vii. Place the component onto the pads (lands) using tweezers.

• viii. For high pin count ICs, use the iron to solder the corner leads. Check the alignment. If proper, proceed to solder all remaining leads and terminations.

• ix. Clean the soldered areas with alcohol soaked short bristle brush or cotton swab to dissolve the flux.

• x. Inspect for final workmanship.

• xi. A tally sheet giving fault description and number of faults occurred for any given product would be helpful in process improvement. A typical tally sheet is given below:

The information given in the format of a chart can be used for improving the soldering process to minimize the defects.