Rework is the repair of electronics that requires removing the soldered components via desoldering, and then re-soldering SMT components. This often occurs due to poor solder joints or misplaced or broken components that needs to be replaced.
Steps for Rework
Step 1: Preheating
In order to remove the original components from the board, the solder first needs to be heated and melted (aka reflow). A hot air tool is usually used to expose the targeted component/area of the board to high heat to melt the solder. However, sudden or prolonged exposure to high temperatures may potentially damage sensitive components on the PCB or even cause the board to warp or burn. Therefore, it is necessary to preheat the board to bring it slowly up to the temperature required to achieve reflow. This reduces the thermal stress and heat spikes, allowing for gradual, even heating.
Step 2: Reflow
Once the board has warmed up, it is time for reflow – increasing the temperature to melt the solder, separating the components from the board.
Small Surface Mount (SMT) components like 0603 resistor uses little solder and is attached only at 2 points – for such precise work, the solder can easily be melted using a soldering iron and removed with a solder wick. A soldering iron also allows for targeted reflow if you need to remove just 1 component on a crowded board.
Through Hole (PTH) components may benefit from using a desoldering tool to go over the pin and cover the pad to melt and remove solder at once. This removal method also helps prevent a mess when removing the PTH component.
On the other hand, larger components such as BGA chips with no exposed solder requires hot air rework to surround the component, heating it up and melting the solder underneath the chip. QFP packages that have many pins soldered onto the board can also use hot air to melt all the soldered points concurrently, instead of having to remove the solder in stretches. Hot air rework can also be used to melt a wider radius of the board to reflow all components in its path – allowing quick removal of multiple components.
Step 3: Removal of Components
After reflow, the components are released and can be removed. The easiest way is with a small precision tweezer (7SA). With hot air rework, the Hakko Hot Air Handpiece comes with a vacuum pick up function. Using a suction pad and vacuum, it picks up components only after it senses that the hot air has melted the solder joints. It also has a pickup indicator to let you know that the component has been picked up. This ensures safe removal of components without damaging it or the board.
Step 4: Attaching Components
Now that the original components has been removed, all that’s left is to attach the new components back onto the board in the correct location according to the necessary methods (e.g via manual, automated or wave soldering or using stencils for solder paste)