REWORK ON A PCB WITH
A PREHEATER & HOT AIR PENCIL
By Marv Cohen Chip Quik® , President
With multilayer PCB’s and increased density of surrounding components near the SMD’s, there is a heat sink affect that draws heat away from the pads. The incorrect way of overcoming this problem is by increasing the applied temperature, resulting in an increased potential for damage to the PCB board and components. When soldering in these dense areas, bottom side preheat has many advantages that will overcome this problem.
SETTING UP THE PCB FOR
RESOLDERING THE SMD
Place the PCB in a horizontal fixture that allows the preheater to be placed under the PCB rework area at the correct height. The preheater will blow focused hot air under the PCB.
The heater is set at the correct temperature to maintain the bottom of the PCB at a constant 300°F. At this safe low temperature the risk of thermal damage has been eliminated, including the bottom side components directly under the heat. The heat will be transferred through the board, to the pin / pads and to the SMD. This safe temperature is maintained on the board throughout the rework process.
PREPARING FOR THE SOLDER PROCESS
Check pads on PCB board for coplanarity (equal height) and clean tinned pad surfaces with a good cleaning solvent. Using a syringe apply a thin bead of solder paste 63/37 w/flux along all of the pads. The amount of paste dispensed will depend on the size of the nozzle. It will take some practice to judge just how much paste to apply for different size SMD’s. Carefully place the SMD directly into the paste on the pads. Use magnification to insure accurate pin / pad placement on all sides. Once the SMD is set in place, be careful not to move it.
USING A HOT AIR PENCIL AND BOTTOM HEAT
Apply bottom heat and wait until solder paste starts to activate. This will be visible when the flux in the solder paste starts to smoke. Now set the air pencil for low air velocity and a temperature of 500°F. With the solder pencil slowly move from pin to pin and apply heat. Watch as each individual pin / pad solder joint as it is formed and then move on to the next. If a solder bridge is formed, apply more past flux and heat again. If this does not work, use solder braid while heating with the hot air pencil. Clean all connections with an acid brush and flux cleaner. Check with magnification for accuracy. Place preheater on cool air and allow rework site to slowly go back to room temperature. This method works quite well with fine pitch components.