by Kerstin
October 20, 2022
PCB knowledge
(0)
11 mins

All you need to know about PCB SMD components soldering

PCB SMD components soldering is a real test of technology. Encountering dense chip pins and extremely small capacitors and resistors, false soldering, short circuits and other issues arise frequently and they are always extremely difficult to find, which requires high technique and patience. But more importantly, some tips and details about PCB SMD components soldering need to be paid more attention. This article introduces some skills for manual PCB SMD components soldering for you.

PCB SMD components soldering tools

soldering station
soldering iron
solder
sponge
flux

A few considerations on the use of soldering iron

Soldering iron head is vulnerable to damage, keep the soldering iron head hanging tin at all times.
Avoid knocking the soldering iron head with hard objects to prevent deformation.
The temperature is set to about 350°C when soldering patches.
Avoid long-term heating of the soldering iron, otherwise it is easy to burn (turn down the temperature when not in use for a long time).
Don’t add too much water to the sponge, wring it out to keep it soft.
Do not burn the iron to strange things, or there will be a strange smell.

Different PCB SMD components soldering

Chip

QFP package

For SOP packaged chips and QFPs with large pin spacing, it is generally enough to scrape them out with a cutter head and tin. Here we focus on the soldering of pin-dense QFP packages. First fix the chip, the steps are as follows:

Place the chip flat on the PCB, and the pads do not need to be tinned, otherwise the chip will not be placed flat, and it is very easy to solder.
The direction of one foot of the chip corresponds to the PCB (One foot is usually in a small circle or looking at the lower left corner of the silk screen).
Align the pins and pads on the four sides of the chip.
Soldering iron with tin to fix the edge of the side of a few pins, check the alignment of the four sides. If it is not aligned, use a soldering iron to re-adjust the chip position.
Soldering iron with tin to fix the opposite side of several pins.

This way the chip is fixed, then start soldering (many components are “first fixed, and then soldered”). The following points should be paid attention to when doing PCB SMD components soldering:

Select the side without solder fixation and start soldering.
Use a soldering iron dipped in flux to coat one side of the pins and pads.
Add tin from one side until a tin ball is formed.
Use the soldering iron to the other side of the diversion of solder, the pin wetted by tin was soldered on.
Remove the excess tin with the stickiness of the soldering iron tip.
The remaining three sides are the same as above.
Check the pins for adhesion and short circuit after all four sides are soldered, and make up the soldering if necessary.

In short, keep two points in mind when soldering the chip:

When soldering, add more tin first, it doesn’t matter if the pins are connected. Just slowly separate them after soldering.
More flux can make the solder flow better.

QFN package

For QFN chips without exposed pins (such as ESP8266), or 4-pin chip crystal oscillators with contact points on the bottom, it is difficult and unreliable to directly solder with a soldering iron. The correct method should be blown with a hot air gun, then how to blow it? The steps are as follows:

Thin tin on all pin pads, middle large pads.
Hold the chip with tweezers and use the bottom side to wipe the flux.
Place the chip on the pad and roughly align the position.
Wind gun temperature is also adjusted to about 350 ° C, and wind speed to the minimum.
Hold the air gun perpendicular to the PCB, with a height of about 8CM. First heat the periphery of the chip, and finally blow it against the chip.
Flux and solder melt and the chip automatically adsorbs to the correct position under liquid tension.
Turn off the air gun, check the chip connection.
If necessary, repair the surrounding area with a soldering iron.

Cautions:

Wind speed should not be too large, the height should be appropriate, otherwise the chip or peripheral small capacitors and resistors will be blown away.
If the chip is difficult to automatically adsorb, you can use tweezers to assist in adjustment, or press it.
Remember not to add too much tin to the middle fixed pad, otherwise the chip will extrude the tin outwards and cause a short circuit of the pins.
The heat gun is really hot, pay attention to safety.

Capacitor, resistor and diode

Capacitors resistors and other two-ended components are the most common. More commonly used soldering method is: First tin one side of the pad, use tweezers to solder one side to fix it, and then solder the other side. The biggest advantage of this method is that it is easy to adjust the position of the components (because there are tweezers to assist), so the soldered components look neat and beautiful.

Its disadvantage is also obvious – too inefficient (also because of the tweezers), especially when dealing with PCBs with a large number of components.

Now there is a fast soldering method-stick components with a soldering iron. This method does not require you to use tweezers to solder both feet of the component at the same time, doubling the efficiency. The steps are as follows:

Tin on both sides of the pads.
Shake off the excess tin from the soldering iron and keep the iron head covered with a thin layer of tin (the iron head has tin to be sticky).
Lay the component flat, and the soldering iron tip touches the component from the side to stick the component horizontally.
Put the soldering iron with components to the pad position, heat both ends at the same time, and fine tune the component position with the tip of the soldering iron.
Remove the soldering iron in the direction of the components.
Side by side components are soldered one by one from left to right.

One of the key steps is to use the soldering iron to stick the component, the trick is to slowly use the iron close to the side of the component, and gently touch. It is important to note that the tip of the soldering iron would be sticky only with tin. but not too much tin, otherwise the surface tension of the liquid will make it difficult to control the components.

How do you get the soldering iron tip covered with thin tin? First add tin, then shake a few times along the direction of the soldering iron. Actually sponge is rarely used in soldering. The excess tin are generally shaken off, so this “shake” action is very important (The sponge will wipe the tin too clean, which is not good. It is only used when there is dirt. Shake it off and it will naturally leave a thin layer of tin).

When using this method, you can’t blindly seek speed. While pursuing speed, quality must be ensured. Solder the components as neatly as possible, and do not solder falsely. In addition, the amount of tin should be well controlled, and it seems too much when both sides become a ball.

As for some of the larger and higher chip electrolytic capacitors and diodes, soldering iron can not heat both ends of them at the same time . Be careful not to put too much tin on the pad, otherwise the component will be high on one side and low on the other.

Socket

For sockets like USB, SD card, SIM card, etc., solder the pins first and then the fixed pins, because if the socket is fixed first, the position will not be adjusted, Be careful not to solder crooked. For sockets with fixed holes, like Micro-USB, after soldering the pins, turn the board over and add tin to the opposite side of the fixed holes, so that the solder flows all the way to the fixed side of the component.

The reason is that some Micro-USB sockets are not completely closed, and it is easy to get tin into the jack when soldering next to the component and block it, so that the plug cannot be inserted (in a word, the feet with holes are soldered on the back).

The last point is that since socket-type components need to be plugged and unplugged frequently, they must be soldered firmly. When soldering the fixed feet, the temperature of the soldering iron can be slightly increased, and the soldering time can be longer. Some large components (or large contact area with the pad) should also prolong the heating time and increase the temperature to ensure stable soldering.

Sequence of PCB SMD components soldering

To be more efficient and reliable to solder a board, certain principles (such as “small first, then large”) must be followed. Here is a general gprocessing process:

Print the PCB package diagram (that is, the pattern printed on the board), and mark the size of each component value, chip model, etc. on the paper with a red pen according to the circuit schematic diagram (if you have a small board and few components, you can skip this step).
Solder power supply part, including various voltage stabilizers and conversion circuits, and use a multimeter to check whether the potential of each point is normal after soldering (the small board can be skipped).
Solder major chips (MCU, Flash chips, device driver chips, etc.)
Solder small capacitors, resistors,diodes and other components.
Solder larger capacitors, diodes and other components.
Solder the peripheral components of the board such as switches, sockets, antennas, etc.
Power-on test.
Use board washing water and dust-free cloth for PCB cleaning.

In addition, for PCBs with complex circuits, the power supply circuit should be soldered first, and the voltage value of each point should be tested to be normal before soldering the digital circuit part. It is necessary to take the module as the unit, test while soldering, eliminate the problem in time, and ensure the correct connection of the circuit.

Of course, it does not have to completely doing soldering in this way. Specifically, you need to find out the most suitable sequence according to the differences between components and PCBs.

Common problems of PCB SMD components soldering

Solder joints are not smooth

Beginners often have the problem that the solder joints are not smooth, which is not only unsightly, but also causes virtual soldering and sharpening. There are two main reasons why the solder joints are not smooth. One is that the temperature of the soldering iron is not enough to completely melt the solder, and the temperature of the soldering iron can be increased.

The second is that the soldering time is too long, resulting in the complete volatilization of the flux, and the solder will lose its luster and fluidity. The solution is to add tin first, and then remove the excess tin with a soldering iron.

Pin bonding

When soldering components with dense pins such as chips, two or more pins are often soldered together and are difficult to separate. This is because the flux in the solder is volatilized due to the long soldering time, and the solder loses its fluidity. General tin wires are entrained with flux. At this time, tin can be added, and the excess tin can be removed in a short time. A more direct method is to directly add rosin or other flux to make the solder lubricate.

Unblocking of solder plug holes

It is really a headache that the solder holes of the plug-in are blocked by solder, and it can’t be sucked out with a solder suction device for a long time? Or drilled with something for a long time and it didn’t work? In fact, there is an easier way, that is – knock! Using the fluidity and tension of the liquid, coupled with Newton’s first law, can solve the problem perfectly. Let’s try it:

Add tin to the pad side of the solder hole (the more plugged, the more you add!).
Continuously heating the pile of tin that blocks the solder holes with the soldering iron.
Hold the board in your left hand and keep the iron in your right hand to continue heating.
Left and right hands lift and move at the same time to lift the board.
Move both hands at the same time, aim the board at the edge of the table and tap it down quickly.
Before the board collides with the table, the soldering iron stops and the board hits the edge of the table hard.
The molten solder is then “thrown” out of the solder hole by the impact.

Be careful not to move too violently, or it is easy to knock the board bad.

One application of this method is to remove the plug-in. In the past, it used a tin suction device to remove it one foot after another. Now, you can directly add a large lump of tin and heat it all together, then take off the whole component while it is hot, and then knock the solder hole. pass, convenient and fast.

Some tips for PCB SMD components soldering

The components should correspond to the size of the pad package. Don’t solder the 0805 components to the 0603 pads.
In order to prevent unnecessary places on the PCB (such as the company logo) from sticking to tin, it can be pasted with paper tape before soldering.
You can first take a bunch of commonly used components in a fixed location, so you don’t have to go through the components box every time.

Here is a complete guide for PCB SMD components soldering. Hope it can be helpful for you. For more information, just visit our website to know more!