AN ALTERNATIVE TO THE SOLDER IRON
By Marv Cohen, Chip Quik®, Inc. / President
Solder / Desolder / and general rework offer an alternative choice other than the traditional solder iron. When applying heat to a solder joint, a certain amount of time is required to transfer the applied heat to the thermal load. The time is determined by the efficiency of the heating device and the thermal conduction of heat to the solder joint. A solder iron is the most efficient device because the heat makes direct contact with the solder joint. An alternative method is the use of hot air and PCB bottom preheat. Hot air convection does not have as good heat transfer as direct contact, but does offer some advantages for higher volume rework and thermal processing.
When soldering a PCB, a certain amount of time is required to bring the temperature of the joint from ambient temperature to reflow. The safe rework challenge is to apply the controlled heat only on the joint for as little time as possible. Longer heating periods will allow damaging heat to be transferred to undesired areas, causing board delamination and damage to adjacent and underside components. As you can see, the control of temperature in a specific location for a controlled amount of time will determine the quality of the rework and reduce the potential for inflicted damage. This controlled temperature process is called thermal profiling.
BOTTOM SIDE PCB PREHEAT
Bottom heating the PCB under the site where the rework is to take place will allow the temperature to be gradually raised to a safe point prior to reworking the SMD. This is done by placing the PCB in a horizontal bracket and applying focused hot air from a preheater directly under the site to be reworked. The distance below the PCB and the focused heat must be controlled to deliver a safe temperature just below reflow. For traditional boards the temperature is 300° F. By focusing hot air under the PCB, the heat will gradually transfer through the board, to the pads and to the SMD component to be reworked. This gradual rising temperature slope of controlled hot air will prevent damage to some of the more sensitive components. Now whatever rework that is done on the top side of the board will be enhanced. Soldering, de-soldering and solder removal will be safer, faster and more efficient.
USING CHIP QUIK® WITH PREHEAT
FOR SAFE SMD REMOVAL
Due to the low melting temperature of Chip Quik® removal alloy (136°F.) an SMD can be removed using bottom heat only. With the PCB bottom heated to a safe bottom temperature of 300° F., apply flux to all leads of the SMD. Apply Chip Quik low temperature alloy to all the pins. The removal alloy will easily become molten because all the pin pad connections are now preheated to 300° F. The applied alloy will melt and fuse into each individual pin /pad connection resulting in a new low temperature alloy with increased thermal mass. While in this molten state the SMD can easily be removed.
After the SMD is removed it is important that all pads be cleaned thoroughly. While pads are still being heated apply flux to the end of a swab and push all of the excess to an unused section oh the board. Clean and polish each individual pad. Clean up remaining residue with isopropyl alcohol. You are now ready to install the new SMD.