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Surface mount devices (SMD) are soldered to PCB pads located on the surface of the board, rather than on the bottom of the board via through-hole connections. To solder SMDs to the surface of the board requires solder, flux applicator/pen, soldering tool (i.e., iron or pencil) and good soldering technique. This article will go through the steps needed to properly solder an SMD to a PCB pad.
Preparing the PCB Pad for the Soldering Operation
To ensure a good solder bond, the soldering tool should be heated to about 800oF, which typically takes about three minutes, but can depend upon the type of soldering tool you have. After the soldering tool achieves the correct temperature, touch the solder tip and the solder to the PCB pad. The solder will melt virtually instantaneously and will be drawn to the solder pad, resulting in a bead of solder solidifying on the pad. Then add rosin flux to the bead of solder with a solder flux applicator.
Properly Positioning the SMD on the PCB
Using a small tweezers, gently pick up the SMD and carefully put it on the target PCB pad. Then, with the soldering tool, apply heat to the solder bead, while at the same time pushing the SMD against the PCB pad. Using an adequately heated soldering tool, touch the bead of solder while simultaneously pressing down on the component on the pad. If you need to adjust the orientation of the SMD on the pad, this is the time to do it before the solder changes from a liquid to a solid state. Finally, remove the soldering tool from the bead and allow it to cool and properly bond.
Initial Check of the Soldering Operation
Once you have completed the soldering operation, you should verify that it has properly bonded the SMD device to the pad. The initial inspection should be to see if the SMD device has been oriented on the pad correctly. Don't worry about the quality of the solder joint just yet. To verify that the orientation is correct, you should see that the SMD's coils are touching the PCB itself and both ends are bonded to the pad. Finally, you should see that the SMD device is placed in the same vertical plane and is centered on the pads.
Completing the Soldering of the SMD to the Pad
Put on a healthy amount of fluid flux, utilizing the flux applicator, to the SMD's ends. The next step is to heat up the SMD's unsoldered end, as well as the near side PCB pad. Then, put on a bead of solder to connect the PCB pad and SMD together. It is critical to remember that you observe the bead of solder flow to both the pad the SMD's side in order to have a good solder bond.
After the solder bead has cooled down, you should check to make sure that the SMD is securely attached to the PCB pad. Let the pad and SMD cool for a few minutes after applying the bead of solder. Then, make a close visual inspection, using a magnifying glass, if necessary. You should see a smooth solder bond with no fractures and no cold solder joints. The SMD should be attached to the PCB pad in a secure manner. Utilizing a solder wick, clean up the soldering area by removing any extra solder.
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