In fact, every welding operation will not be a perfect assembly. Even the highest quality components can fail from time to time. This is why desoldering is very important for engineers and technicians who manufacture, maintain or repair printed circuit boards (PCBs).

 

 

    The challenge in the desoldering process is to quickly remove excess solder without damaging the circuit board. That's why in this article, we will introduce you to the best cases and best techniques for desoldering that we have encountered in the long history of the electronics industry. These techniques mainly focus on the removal of components and their advantages with the help of suction braid (also known as desoldering wire or core). It is portable, easy to use, is one of the most commonly used tools for PCB repair, and does not require continuous maintenance like other tools.

       1. Keep the soldering iron tip clean and tin plated to achieve effective heat transfer

    This may seem effortless, but it is often overlooked and is essential for effective desoldering. If the tip of the soldering iron is covered with baking flux and oxide, the tip of the soldering iron will not be wetted (adsorbed solder) and the thermal conductivity will be poor. The clean tin-plated soldering head can better transfer heat through the tin suction braid and start the tin suction faster.

1) Before use, tin-plate the solder joints by adding new solder wires to it.

2) If the soldering iron tip does not absorb excess solder, use a soldering iron tip cleaner (usually called "Tip Tinner") to clean the soldering iron tip. The Plato brand Tip Tinner (part number TT-95) is a halogen-free solid sticky substance that can be quickly and safely retinned and cleaned the oxide tip. Roll the tip of the hot soldering iron tip in the compound until the tip of the soldering iron tip becomes shiny.

3) Clean other residues on the tip with a welding wire, and then wipe it with a wet cellulose sponge or brass wire tip cleaner.

4) Finally, apply solder to the tip again to prevent oxidation. Whenever your soldering iron is placed for a long time, or after soldering is completed, please "paint" new solder on the soldering head to prevent oxidation.

 

      2. Minimize the time that circuit boards and components are exposed to high temperatures

   Exposing the circuit board or its components to a high temperature environment for a long time will damage the circuit board or components, will cause the solder joints to be fragile, and may reduce the reliability of the product.

1) Set the soldering iron at a reasonable temperature. I know that keeping the soldering iron tip high temperature for a long time will improve your work efficiency, but it will affect the performance of the components. Even if lead-free solder is used, any temperature exceeding 700ºF (371ºC) will risk the component's thermal damage.

2) If there are multiple components on a single circuit board that need to be replaced, or a component is sensitive to heat, a PCB preheater can be used. The preheater allows you to increase the temperature of the circuit board and maintain that temperature while working. Although the preheating temperature will be much lower than the melting point of the solder, since rapid heating is not required, the preheater can minimize the effect of thermal shock.

 

       3. Match the width of the soldering braid to the pad

    Suction braid usually has several different widths, so you can choose the appropriate suction braid to match the pad. Too narrow suction braid usually cannot remove all the solder at one time, and it needs to repeat the operation over and over again. Too wide suction braid takes longer to heat up and may affect other components on the circuit board.

Choose the width of the soldering braid that closely matches the size of the welding area. This will ensure that you use proper heat transfer and you will not remove areas that do not need to be desoldered. The width of the absorbing braid is specified by the number 1 to 6 or the color code, which is the industry standard.

-#1 / White weave is the smallest (width less than 1mm), mainly used for SMD and microcircuits.

-Most people will find that #2/yellow, #3/green and #4/blue are the most common models.

-#5/brown is the best choice for removing large pads, #6/red is best for desoldering BGA pads or terminals.

-In the work area, three or four different width suction tin braids are often reserved to cope with all applications.

-The braid can be folded or trimmed at a certain angle to better fit the unsoldering area.

         

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       4. Match the soldering iron tip to the width of the suction braid for more precise application

    Use a soldering iron tip with approximately the same suction braid and contact area width. A tip that is too small will require a longer contact time. A tip that is too large, such as in a dense component area, will affect the risk of other components. The matching soldering iron tip allows you to melt the unwanted solder faster and minimize the heating time. When you need to desolder a large area, you can use a blade or a knife tip, similar to a BGA pad.

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       5. Note: When moving and desoldering, move the soldering iron tip on the soldering braid instead of moving the soldering braid on the pad

    Cover the pad with a copper braid, such as when desoldering a BGA pad, if too much pressure is applied, the OSP coating or even the pad itself may be scratched. It is best to cover the braid with suction, and then place the tip of the soldering iron on the braid to desolder.

 

       6. Cut off the used suction braid

    However, it is best to cut off the heat source at the end of the operation. As the soldering braid reaches the soldering temperature, the flux is fully activated, so the part will not absorb more solder. The used tin suction braid is used as a radiator, which delays your process flow.

 

       7. Avoid novice mistakes: keep the soldering iron tip and the suction braid away from the pad at the same time

    This is by far the most common mistake made by inexperienced operators. After removing the solder, be sure to keep the soldering iron tip and the soldering braid away from the pad at the same time. Otherwise, if you solder the absorbing braid to the unsoldering area, there will be a risk of bringing the pads together.

 

       8. The type of flux is matched to the cleaning process

    According to your cleaning process and other requirements, Kentron suction braid can provide various types of flux.

1) Rosin-Rosin flux braiding has the fastest tin absorption effect, but will leave rosin residues that need to be thoroughly cleaned.

2) No-clean-No-clean flux-absorbing braid is the best choice for subsequent no-cleaning processes. After unsoldering, the remaining material is a non-ionic residue. For special work occasions, when thorough cleaning is more difficult, this type of suction braid is recommended.

3) Flux-free type-In the production or maintenance environment where the flux is specified and the flux cannot be replaced, or when water-containing flux is needed, you can add your own flux to this kind of absorbing braid. Unless flux is added, the unmelted braid will not absorb the solder. Pen packaging provides different types of flux, which is very suitable for self-coating braid.

 

       9. Choose static dissipative packaging for static sensitive applications

    When using near electrostatic sensitive components, please ensure that the spool of the tin suction braid is electrostatically dissipative (or ESD safe). We usually see operators in the high-cost ESD safe working environment, anti-static mats and grounding, but are troubled by an insulated spool. The packaging of most static dissipative shafts can be identified by its blue color. Even if the spool is black, it does not necessarily meet the S20.20 standard.

 

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      10. Add solder in the tight gap to make it easy to be sucked out

    A small amount of solder in a tight crack may be difficult to remove, but a larger uniform solder joint will be easily absorbed, that is, adding more solder to the crack will help the soldering iron tip to contact more solder.

 

       11. Use high-quality flux cleaner to protect your circuit board from corrosion

    Flux residues can cause dendritic growth and corrosion of PCB components, so please make sure to use the best solder braid and thoroughly clean the circuit board. After replacing all components and removing excess solder, please complete the following:

1) Use high-quality flux cleaner to clean the area thoroughly

2) Adjust the angle of the plate to make the cleaning agent and residue flow out

3) If necessary, use a brush or lint-free wipe to gently scrub the PCB

4) Then rinse.

5) If you use a wiping cloth, please make sure it does not leave fiber/lint on the PCB, which may cause problems in future applications.

However, for dense boards or high-voltage boards, the use of no-clean tin-absorbing braid is still an ideal choice. If a conformal coating is subsequently used, it must be cleaned regardless of the type of flux.

 

      12. Follow our recommended desoldering process to get the best results

    Finally, we hope to end this article with best practices for desoldering processes:

1) Put the soldering braid on the solder that needs to be cleaned, preferably at the place where the solder is most accumulated, so that the contact surface between the soldering braid and the surface area of ​​the solder can be maximized.

2) Next, put your soldering iron tip on the 45-degree soldering braid to let the heat transfer to the mat. The molten solder will be absorbed into the braid.

3) Move the soldering iron tip and braid as needed to remove all solder at once. Be careful not to drag the braid onto the table mat, otherwise it will be scratched.

4) Once the solder braid is full of solder, you must trim the used part and move to a new braid to pull out more solder. Remove the soldering iron tip and the braid at the same time to avoid soldering the braid to the circuit board.

    So with these tips, you can use industry-tested best practices to effectively remove solder.