Will it be cooked if welded like this?

Let's take a look at some of the basic tools needed to solder some commonly used SMD components (see Figure 1).

Figure 1 Common tools used for manual soldering of SMD components

1. Electric
soldering iron This is definitely indispensable for manual soldering of components. Here I recommend the type with a sharper soldering iron tip, because when soldering SMD chips with dense pins, it can accurately and conveniently solder one or several pins.

An adjustable constant temperature soldering iron is your best choice (it can adjust the temperature and stabilize the temperature, and there are two types: single-handle temperature control type and soldering station).

It should be noted that the internal heating and external heating types mentioned above usually do not have power switches. They heat up when plugged in, and the power must be turned off when cooling is needed.

As the saying goes, a good horse needs a good saddle. Then the soldering iron head is the saddle of this horse (soldering iron).

The selection of soldering iron tip should be determined according to the contact surface of the object to be welded.

For example, for ordinary plug-in components, we often use horseshoe heads (large contact area); small SMD components can use pointed or curved tips (for welding dense components); and conventional chips can use knife heads (for convenient drag soldering).

Of course, a more advanced DIY method is to grind out a unique shape of soldering iron tip according to your own needs.

We can’t use the newly bought soldering iron right away. We need to tin it for the first time first.

2. Solder wire
Good solder wire is also very important for SMD soldering. If conditions permit, use thin solder wire as much as possible when soldering SMD components. This makes it easier to control the amount of solder, thus avoiding the trouble of wasting solder and sucking solder.

Chinese name: solder wire, solder line, tin wire, tin wire, English name: solderwire. Solder wire is composed of two parts: tin alloy and additives. The alloy components are divided into tin-lead and lead-free additives, which are evenly poured into the middle part of the tin alloy.

Different types of solder wires have different additives. The additives are used to improve the auxiliary heat conduction of the solder wire during welding, remove oxidation, reduce the surface tension of the material being welded, remove oil stains on the surface of the material being welded, and increase the welding area. The characteristic of solder wire is that it is a tin alloy wire with a certain length and diameter. It can be used in conjunction with an electric soldering iron or laser in the welding of electronic components.

The difference between leaded and lead-free solder wire:

1. The difference between lead-containing and lead-free solder wire is only the difference in content.

2. Lead-containing solder wire requires artificial addition of lead. The currently known best solder ratio is tin-lead solder wire (national standard: tin content 63%, lead content 37%).

3. Lead-free solder wire also contains very little lead. Currently, there is no completely pure metal product. Usually, lead-free solder wire is called lead-free solder wire. Lead-free does not mean that it is completely lead-free. Lead-free means that the lead content is relatively low, which can be roughly regarded as lead-free. The EU defines the lead-free standard as: lead content <1000PPm. Considering the possibility of further pollution during welding and subsequent processing, in order to ensure that the customer's finished product meets EU standards, the lead content of general solder wire will be much lower than this standard.

4. Both lead-containing and lead-free solder wires will corrode the soldering iron tip, because the lead-free soldering temperature is higher than that of lead-containing solder wire, and the alloy composition is different. Lead-free solder wire is more likely to corrode the soldering iron. Due to the lead-free requirements and corrosiveness, it is recommended to use a special lead-free soldering iron when soldering lead-free solder wire.

3. Tweezers
The main function of tweezers is to facilitate the picking up and placing of SMD components. For example, when soldering SMD resistors, you can use tweezers to hold the resistors and place them on the circuit board for soldering. Tweezers are required to have a pointed and flat front end to facilitate the clamping of components. In addition, for some chips that need to prevent static electricity, anti-static tweezers are required.

Anti-static tweezers are also called semiconductor tweezers and conductive tweezers. They can prevent static electricity. They are made of a mixture of carbon fiber and special plastic and have good elasticity.

It is light and durable, does not drop dust, is acid and alkali resistant, and is resistant to high temperatures. It can prevent traditional anti-static tweezers from contaminating products due to carbon black. It is suitable for the production of semiconductors, ICs and other precision electronic components, as well as for special uses.

Anti-static tweezers are made of special conductive plastic material, have good elasticity, are easy to use and can discharge static electricity, and are suitable for processing and installing components that are sensitive to static electricity.

Surface resistance: 1000KΩ—100000MΩ. Anti-static tweezers are suitable for precision electronic component production, semiconductor and computer head industries.

If you use anti-static tweezers made of a mixture of carbon fiber and special plastic, which will not drop dust, are resistant to acid and alkali, and high temperatures, you can avoid the contamination of products by traditional anti-static tweezers due to the presence of carbon black.

4. Soldering tape
When soldering SMD components, it is easy to have too much tin.
Especially when soldering dense multi-pin SMD chips, it is easy to cause the two or even multiple adjacent pins of the chip to be short-circuited by solder. At this time, the traditional soldering machine is useless, and the braided soldering tape is needed.

5. Rosin

Rosin is the most commonly used flux in welding because it can precipitate oxides in the solder, protect the solder from oxidation, and increase the fluidity of the solder.

When soldering plug-in components, if the components are rusty, first scrape them and put them on rosin with a soldering iron, then tin them. When soldering SMD components, rosin can be used as a soldering tape in addition to flux.

6. Solder paste
When soldering iron parts that are difficult to tin, solder paste can be used. It can remove oxides on the metal surface and is corrosive.

When soldering SMD components, you can sometimes use it to "eat" solder to make the solder joints shiny and firm.

7. Hot air gun
A hot air gun is a tool that uses the hot air blown out of its gun core to solder and disassemble components. The process requirements for its use are relatively high.
Hot air guns can be used from removing or installing small components to large integrated circuits. In different occasions, there are special requirements for the temperature and air volume of the hot air gun. Too low temperature will cause component cold soldering, and too high temperature will damage components and circuit boards. Too much air volume will blow away small components. For ordinary SMD soldering, hot air guns are not required, so I will not describe them in detail here.

8. Magnifying glass
For some SMD chips with very small and dense pins, after soldering, it is necessary to check whether the pins are soldered normally and whether there is any short circuit. At this time, it is very laborious to use human eyes, so a magnifying glass can be used to conveniently and reliably check the soldering status of each pin.

9. Alcohol
When using rosin as flux, it is easy to leave excess rosin on the circuit board. For the sake of appearance, you can use an alcohol cotton ball to clean the areas on the circuit board where there is residual rosin.

10. In addition to the above mentioned tools, there are some other common tools required for SMD welding, such as sponges, board cleaning water, hard bristle brushes, glue, etc.

Manual soldering steps for SMD components (soldering iron)

After understanding the patch welding tools, the welding steps are now explained in detail.
1. Clean and fix the PCB (printed circuit board)
Before welding, the PCB to be welded should be checked to ensure that it is clean (see Figure 2). The oily fingerprints and oxides on the surface should be removed to avoid affecting the tinning.

When manually soldering a PCB, if conditions permit, you can use a soldering station or something like that to fix it for easier soldering. Usually, you can just use your hands to fix it. It is important to avoid touching the pads on the PCB with your fingers to affect tinning.

Figure 2 A clean PCB

2. Fixing SMD components
Fixing SMD components is very important. According to the number of pins of SMD components, there are two ways to fix them: single-pin fixing method and multi-pin fixing method.

For SMD components with a small number of pins (usually 2-5), such as resistors, capacitors, diodes, transistors, etc., the single-pin fixing method is generally used, that is, tinning one of its pads on the board first (see Figure 3).

Figure 3: For components with fewer pins, tin one pin first.

Then use tweezers in your left hand to hold the component and place it in the installation position and lightly press against the circuit board. Use the soldering iron in your right hand to melt the solder close to the tinned pad and solder the pin (see Figure 4).

After soldering a pad, the component will not move, and the tweezers can be loosened. For SMD chips with many pins and multi-sided distribution, it is difficult to fix the chip with a single pin. In this case, multiple pins are needed to fix the chip. Generally, the pin-to-pin fixing method can be used (see Figure 5).

That is, after soldering and fixing one pin, solder and fix the pin opposite to it, so as to achieve the purpose of fixing the entire chip. It should be noted that for SMD chips with many and dense pins, accurate pin alignment with pads is particularly important and should be carefully checked, because the quality of soldering is determined by this premise.

Figure 4: Fixing and welding components with fewer pins

Figure 5: Soldering pins or multiple pins of components with many pins

It is worth emphasizing that the pins of the chip must be correctly judged.
For example, sometimes we carefully fix the chip and even complete the soldering, but when checking, we find that the pins are incorrectly matched - the pin that is not the first pin is soldered as the first pin! It's too late to regret! Therefore, these meticulous preliminary work must not be sloppy.

3. Solder the remaining pins
After the component is fixed, the remaining pins should be soldered. For components with fewer pins, you can hold the solder in your left hand and the soldering iron in your right hand, and spot solder them in turn.

For chips with many and densely packed pins, in addition to spot soldering, drag soldering can be used, that is, sufficient solder is applied to the pins on one side and then the solder is melted with a soldering iron and spread over the remaining pins on that side (see Figure 6). The molten solder can flow, so sometimes the board can be tilted appropriately to remove the excess solder.

It is worth noting that, whether spot soldering or drag soldering, it is easy to cause adjacent pins to be short-circuited by tin (see Figure 7). Don't worry about this, because it can be done. What you need to pay attention to is that all pins are well connected to the pads and there is no cold soldering.

Figure 6 Drag soldering of SMD chips with many pins

Figure 7: No need to worry about short circuits during soldering

4. Remove excess solder

In step 3, we mentioned the short circuit phenomenon caused by soldering. Now let’s talk about how to deal with the excess solder.

Generally speaking, you can use the tin-absorbing tape mentioned above to absorb the excess solder. The method of using tin-absorbing tape is very simple. Add an appropriate amount of flux (such as rosin) to the tin-absorbing tape and then place it close to the pad. Put a clean soldering iron tip on the tin-absorbing tape. After the tin-absorbing tape is heated to melt the solder on the pad, slowly press and drag it from one end of the pad to the other end, and the solder will be absorbed into the tape.

It should be noted that after the desoldering is completed, the soldering iron tip and the tin-absorbing tape that has absorbed tin should be removed from the pad at the same time. At this time, if the tin-absorbing tape is stuck to the pad, do not pull the tin-absorbing tape hard. Instead, add flux to the tin-absorbing tape or reheat it with the soldering iron tip and then gently pull the tin-absorbing tape to make it smoothly separate from the pad and prevent it from scalding the surrounding components.

If there is no special desoldering tape sold on the market, you can use the thin copper wire in the electrical wire to make your own desoldering tape (see Figure 8).

The homemade method is as follows: After peeling off the outer skin of the wire, the thin copper wire inside is exposed. At this time, use a soldering iron to melt some rosin on the copper wire.

The effect after removing excess solder is shown in Figure 9. In addition, if you are not satisfied with the soldering result, you can reuse the solder wick to remove the solder and solder the components again.

Figure 8 Using homemade desoldering tape to remove excess solder from the chip pins

Figure 9 Effect after removing excess solder from chip pins

5. Clean the soldering area
After soldering and removing excess solder, the chip is basically soldered. However, due to the use of rosin flux and tin-absorbing tape, some rosin remains around the chip pins on the board (see Figure 9). Although it does not affect the chip's operation and normal use, it is not beautiful and may cause inconvenience during inspection.

Because it is necessary to clean these residues. The common cleaning method is to use board washing water. Here, alcohol is used for cleaning. The cleaning tool can be a cotton swab, or tweezers holding toilet paper (see Figure 10).

When cleaning and erasing, you should pay attention to the use of an appropriate amount of alcohol, and its concentration should be higher so as to quickly dissolve residues such as rosin.

Secondly, the erasing force must be controlled well and not too strong, so as not to scratch the solder mask and damage the chip pins.

The cleaning result is shown in Figure 11. At this point, you can use a soldering iron or a hot air gun to properly heat the alcohol-cleaned area to quickly evaporate the residual alcohol. At this point, the chip soldering is complete.

Figure 10 Using alcohol to remove the rosin left over from welding

Figure 11 The effect of cleaning the welding position with alcohol

Manual soldering steps for SMD components

1. Preparation

1. Turn on the hot air gun and adjust the air volume and temperature to appropriate positions: feel the air volume and temperature of the air duct with your hand; observe whether the air volume and temperature of the air duct are unstable.

2. Observe the reddish color inside the air duct. Prevent overheating inside the air duct.
3. Use paper to observe the heat distribution. Find the temperature center.
4. Blow a resistor at the lowest temperature and remember the position of the lowest temperature knob that can blow down the resistor.
5. Adjust the air volume knob so that the steel ball of the air volume indicator is in the middle position.
6. Adjust the temperature control so that the temperature indication is around 380℃.

Note: When the heat gun is not used for a short period of time, it should be put into sleep mode (if there is a sleep switch on the handle, just press the switch; if there is no switch on the handle, the nozzle is facing down for working and the nozzle is facing up for sleep). If the heat gun is not used for more than 5 minutes, turn off the heat gun.

2. Desoldering Flat Package ICs using a hot air gun:

1) Steps for removing a flat package IC:
1. Check the direction of the IC before removing the component, and do not put it in reverse when reinstalling.

2. Check whether there are any heat-sensitive devices (such as LCD, plastic components, BGA IC with sealing glue, etc.) next to the IC and on the front and back. If so, cover them with a shielding cover or something like that.

3. Adding appropriate amount of rosin to the IC pins to be removed can make the PCB pads smooth after the components are removed. Otherwise, burrs will appear and it will be difficult to align when re-soldering.

4. Use the adjusted hot air gun to evenly preheat the area around the component at a distance of about 20 square centimeters (the nozzle is about 1 cm away from the PCB board, and moves at a relatively fast speed in the preheating position, and the temperature on the PCB board does not exceed 130-160°C)

1) Remove moisture from the PCB to avoid "bubbling" during rework.

2) Avoid stress warping and deformation between PCB pads caused by excessive temperature difference between the upper and lower sides due to rapid heating of one side (top) of the PCB board.

3) Reduce the thermal shock of parts in the welding area when the PCB is heated.

4) Prevent the adjacent IC from becoming desoldered and warped due to uneven heating.

5) Heating the circuit board and components: Keep the nozzle of the hot air gun about 1CM away from the IC, move slowly and evenly along the edge of the IC, and gently clamp the diagonal part of the IC with tweezers.

6) If the solder joint has been heated to the melting point, the hand holding the tweezers will feel it immediately. Be sure to wait until all the solder on the IC pins is melted before carefully lifting the component vertically from the board with "zero force". This can avoid damage to the PCB or IC, and also avoid short circuits caused by the solder left on the PCB board. Heating control is a key factor in rework. The solder must be completely melted to avoid damaging the pad when removing the component. At the same time, it is also necessary to prevent the board from being overheated, and the board should not be distorted due to heating. (For example: if conditions permit, 140℃-160℃ can be selected for preheating and bottom heating. The entire process of removing the IC should not exceed 250 seconds)

7) After removing the IC, check whether the solder joints on the PCB are short-circuited. If there is a short-circuit, use a hot air gun to reheat it. After the solder at the short-circuit is melted, use tweezers to gently scratch along the short-circuit, and the solder will naturally separate. Try not to use a soldering iron, because the soldering iron will take away the solder on the PCB. Less solder on the PCB will increase the possibility of cold soldering. It is not easy to tin the pads of small pins.

2) Steps for installing flat ICs:
1. Observe whether the IC pins to be installed are flat. If there is a short circuit in the IC pins, use solder suckers to deal with it. If the IC pins are not flat, place them on a flat plate and press them flat with the back of flat tweezers. If the IC pins are not straight, use a scalpel to correct the crooked parts.

2. Put an appropriate amount of flux on the pad. Too much flux will cause the IC to float away when heated, and too little flux will not have the desired effect. Cover and protect the surrounding heat-sensitive components.

3. Place the flat IC on the pad in its original direction, align the IC pins with the PCB pins, and look vertically downwards when aligning. The pins on all four sides should be aligned, and visually the pins on all four sides should be the same length, and the pins should be straight and not skewed. The IC can be glued by the adhesion of rosin when it is heated.

4. Use a hot air gun to preheat and heat the IC. Note that the hot air gun cannot stop moving during the whole process (if it stops moving, it will cause local temperature rise to be too high and damage). Observe the IC while heating. If the IC is found to move, use tweezers to gently adjust it without stopping heating. If there is no displacement, as long as the solder under the IC pins is melted, it must be discovered at the first time (if the solder is melted, the IC will sink slightly, the rosin will have light smoke, the solder will be shiny, etc. You can also use tweezers to gently touch the small components next to the IC. If the small components next to it are active, it means that the solder under the IC pins is also close to melting.) and stop heating immediately. Because the temperature set by the hot air gun is relatively high, the temperature on the IC and PCB board is continuously increasing. If it is not discovered early, the temperature rise will be too high and the IC or PCB board will be damaged. Therefore, the heating time must not be too long.

5. After the PCB board cools down, clean and dry the solder joints with thinner (or board cleaning water). Check for cold solder joints and short circuits.

6. If there is a cold solder joint, you can use a soldering iron to solder the pins one by one or use a hot air gun to remove the IC and re-solder it; if there is a short circuit, you can wipe the soldering iron tip clean with a damp heat-resistant sponge, dip it in rosin and gently scratch along the short-circuited pin to remove the solder at the short circuit. Or use tin-absorbing wire to handle it: use tweezers to pick out four tin-absorbing wires, dip them in a small amount of rosin, put them at the short circuit, and gently press the solder-absorbing wire with a soldering iron. The solder at the short circuit will melt and stick to the tin-absorbing wire, and the short circuit will be removed.

Alternatively: You can also use an electric soldering iron to solder the IC. After aligning the IC with the pad, dip the soldering iron in rosin and gently stroke it along the edge of the IC pins one by one. If the pin spacing of the IC is large, you can also add rosin and use a soldering iron with a tin ball to roll over all the pins for soldering.

3. Use a hot air gun to desolder heat-sensitive components

1) Component removal:
Generally, plastic components such as cable clips, inline sockets, sockets, SIM card holders, battery contacts, and tail plugs are easily deformed by heat. If they are really broken, you can just remove them like desoldering ordinary ICs. If you want to remove them and keep them intact, you need to handle them carefully. There is a kind of rotating air heat gun with uniform air volume and heat, which generally will not damage plastic components.

If you use an ordinary air gun, you can consider placing the PCB board on the edge of the table, and use the air gun to heat the front and back of the component from the bottom up, transfer the heat to the top through the PCB board, and remove it after the solder melts. You can also cover the heat-sensitive component with a used chip of the same size, and then use an air gun to heat the edge of the chip. After the solder below melts, you can remove the plastic component.

2) Install components:
Arrange the pads on the PCB board, dip a proper amount of flux on the component pins and place them close to the pads to allow them to receive a little heat.

Use a hot air gun to heat the PCB board. When the solder on the board turns shiny, it means it has melted. Quickly and accurately place the component on the pad. The air gun cannot stop moving and heating at this time. Use tweezers to adjust the position of the component in a short time and then evacuate the air gun immediately.

This method is also suitable for installing power amplifiers and power ICs with large heat dissipation areas. Some components can be easily soldered with a soldering iron (such as a SIM card holder), so don't use an air gun.

4. Desoldering small components such as resistors, capacitors, and transistors

1) Removing components:
1. Add appropriate amount of rosin on the components, gently clamp the components with tweezers, and use a hot air gun to evenly move and heat the small components (same as desoldering ICs). When the hand holding the tweezers feels that the solder has melted, you can remove the component.

2. Use a soldering iron to add some solder to the component, until the solder covers the solder joints on both sides of the component, place the tip of the soldering iron flat on the side of the component, and melt the newly added solder, then you can remove the component. If the component is large, add more tin to the solder joints, hold the component with tweezers, and use the soldering iron to quickly heat the two solder joints in turn until both solder joints are melted, then you can remove it.

II) Installing components:
1. Add an appropriate amount of rosin to the components, gently clamp the components with tweezers, align the components with the solder joints, and evenly move and heat the small components with a hot air gun. Wait until the solder under the components melts, then release the tweezers. (You can also place the components and heat them, wait until the solder melts, and then touch the components with tweezers to align them.)

2. Use tweezers to gently clamp the component, and use the soldering iron to touch each pin of the component to solder it. If there is less solder on the soldering point, you can put a small tin bead on the tip of the soldering iron and add it to the pin of the component.

5. Use a hot air gun to desolder the shield:

1) Remove the shielding cover:
Clamp the PCB with a clamp, clamp the shielding cover with tweezers, heat the entire shielding cover with a hot air gun, and lift it vertically after the solder melts.

Because removing the shielding cover requires a relatively high temperature, other components on the PCB board will also become loose. When removing the shielding cover, the main board must not move to avoid vibrating and shifting the components on the board. When removing the shielding cover, lift it vertically to avoid bumping and shifting the components inside the shielding cover.

You can also lift up the three sides of the shielding cover first, fold it back and forth a few times after it cools down, break the last side and remove the shielding cover.

2) Install the shielding cover:
Place the shielding cover on the PCB and heat it around with a hot air gun until the solder melts. You can also use a soldering iron to select a few spots and solder them on the PCB.

6. Add soldering to the cold soldered components:

1) Soldering with an air gun
Add a little rosin to the part of the PCB that needs to be soldered, and use an air gun to heat it evenly until the solder in the soldered part melts. You can also use tweezers to gently touch the suspected poorly soldered components when the solder is molten to enhance the soldering effect.

2) Soldering with an electric soldering iron
Used for soldering a small number of components. If you are soldering an IC, you can add a small amount of rosin to the IC pins and use a smooth soldering iron tip to solder along the pins one by one.

Be sure to wipe off the residual tin on the soldering iron tip, otherwise it will short-circuit the pins. If you are soldering small components such as resistors and transistors, just dip the tip of the soldering iron in rosin and solder the pins of the components. Sometimes, in order to increase the welding strength, you can also add a little solder to the pins of the components.

Summarize

To sum up, soldering SMD components is generally a process of fixing - soldering - cleaning.

The fixation of components is the prerequisite for good welding. You must be patient and ensure that each pin and its corresponding pad are accurately aligned.

When soldering multi-pin chips, don't worry about the pins being short-circuited by solder. You can use a wick to absorb the solder or just use a soldering iron to remove the excess solder by taking advantage of the flow of the melted solder. Of course, mastering these skills requires practice.

Due to space limitations, this article only demonstrates the welding of one type of multi-pin chip. For many other types of multi-pin SMD chips, the corresponding welding methods are basically the same under different circumstances for their pin density, mechanical strength, quantity, etc., except that the details are slightly different.

Therefore, if you want to become a master at soldering SMD components, you need to practice more to improve your proficiency. If conditions permit, you can use old circuit boards and old chips to improve your proficiency.

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