Electronic Design - PCBA welding principle and manual welding techniques

Soldering is a very important part of PCBA processing. Although the current automatic soldering process (SMT, Wave Soldeirng) has been very common, manual soldering (Toucu up) is still not completely avoided., Especially in the repair (Repair) operation can not be replaced by automatic welding.

Unskilled manual welding process will not only produce quality welding defects (cold welding, empty welding, false welding, bridging, short circuit), resulting in abnormal circuit operation, more serious, it may even damage electronic components during welding, or The components are burnt after power-on. Therefore, how to ensure that all personnel engaged in electronic manufacturing understand the principles and concepts of welding and implement them can effectively ensure and improve the welding quality of electronic products, reduce the probability of defective products, and even extend the life of the product.

Why do we need to weld?

The main function of soldering is to connect each independent electronic component to achieve the purpose of communicating electronic signals with each other. The second is to fix the electronic component on the circuit board, otherwise the electronic component will fall down if it is not moving, and it will not work.

The picture at the top of the article is a cross-sectional view of the circuit board. In the picture, we can see the relationship between the solder, the copper foil pads, and the pins of the parts. The component pins are connected to the copper foil pads of the PCB by soldering, and the copper foil pads are connected to other component pins, thus forming a complete electronic circuit.

What is welding?

Simply put, soldering is to add a metal solder with a lower melting point than the two metals between two metals. This solder must also interact with the two metals to produce a chemical reaction, and finally form an interface metal. Co-compound (IMC), which combines with each other.

The manual method of manual soldering is to use an "iron" to heat the two metals at the same time and add solder to melt them. The molten solder penetrates into the gaps between the pins of the electronic components and the copper foil pads on the circuit board and covers them. The two parties will finally combine and solidify the two parties together and serve as a relay medium for electrical signal transmission.

So there are three requirements for solder:

▪ The melting point should be low. The temperature should not be too high so as not to find materials that can be worked on, and the temperature should not be lower than the operating and storage temperature of electronic products.

▪ Have a certain electronic conductivity. Conductor or semiconductor.

▪ has a certain welding strength, and has a certain impact resistance.

Based on the above requirements, the current PCBA processing generally uses "tin" as the base alloy as the solder, so it is called "solder", while the pins and circuit boards of electronic components are almost all made of "copper" material and electroplated Metals such as nickel, tin, and silver are the most welded metals on the surface.

The use of "alloy" is to reduce the melting point of the solder, and can also meet the needs of various soldering. For example, adding a small amount of "silver" is to get better wetting, strengthen solder joint strength, and improve fatigue resistance. Adding a small amount of "copper" to the solder paste can improve the strength of the solder joint, and a small amount of copper can also reduce the corrosion of the solder on the soldering iron tip.

It’s actually a very interesting question as to why the eutectic point of two metals with a high melting point will be greatly reduced after being mixed in a certain ratio.

Principle of Welding

The principle of soldering has nothing to do with it. It is to use "temperature" to melt the solder and connect the electronic components to the PCB circuit board. When the temperature cools below the melting point, the solder will solidify and connect together.

However, manual soldering cannot be fully heated like an SMT reflow furnace, because humans cannot withstand such high temperatures, so an "electric soldering iron" single-point heat source is designed to heat the object to be soldered and the solder, which is efficient at this time. Conduction becomes very important.

Manual soldering uses the heat generated by an electric soldering iron to quickly heat the part pins and copper foil pads to a temperature above the melting point of the solder, but not so high as to melt the components and the circuit board, and then place the solder wire against the heated area, The original solid tin wire will naturally turn into liquid when it comes into contact with a temperature exceeding the melting point and will flow evenly between and around the entire solder joint and the pin. When the soldering iron is removed, the liquid solder without a heat source will naturally Cooling, when the temperature is lower than the melting point of the solder, it will solidify to complete the soldering work.

From the above description, there are several precautions that need special attention:

▪ The best electric soldering iron heating principle should be in the shortest time, with the lowest temperature to complete the best soldering quality. This is because most electronic components and circuit boards cannot withstand excessively high temperatures for a long time, otherwise it may damage the electronic components or cause the copper foil pads to peel off. Furthermore, heating the solder for a long time will also cause the flux in the tin wire to volatilize completely and fail, which will adversely affect the soldering quality.

▪ The heat capacity of the soldering iron must be sufficient. The heat energy of the soldering iron is converted from electricity, so its heat capacity is determined by the power wattage. The larger the wattage, the higher the heat capacity and the better the thermal compensation ability. When the soldering iron head rests on the originally cooled components and copper foil pads, the heat begins to overflow to the lower part. The soldering iron must continuously and steadily provide enough heat to meet the heating demand. If the wattage is too small, it will cause The heating temperature is unstable or insufficient, which causes the shortcomings of false welding and empty welding that cannot be welded.

Some PCB soldering pads are connected to a large area of copper foil and there is no [thermal relief] designed. Their heat absorption speed is even more terrifying. At this time, in addition to using a high-power soldering iron, you must also use a larger shape. Soldering iron tip (tip), such as a knife-type soldering iron tip, to increase its contact area and enhance the efficiency of heat conduction.

What is a good welding effect?

▪ Good soldering allows the solder to gradually spread.

▪ The solder and solder surface and component pins should present a natural arc.

Good welding effect

How to properly use an electric soldering iron to manually solder electronic components to the circuit board?

▪ When soldering with a soldering iron tip, it is recommended to use the soldering iron tip to heat the copper foil pads on the circuit board and the pins of the electronic components for about 1 to 2 seconds to heat up to a certain temperature. You can also feed a small amount of solder at the same time. The tip of the soldering iron tip can increase the contact area between the soldering iron tip and the object to be soldered, so that the heat energy can be more efficiently transferred to the copper foil pads and the pins of electronic components.

▪ Wait until the temperature of the object to be soldered rises before feeding a large amount of tin wire to the soldering iron tip, so as to ensure that the solder really wets between the copper foil pads and pins that need to be soldered, and even wraps them together.

▪ Do not feed all the solder to the soldering iron head at one go before the temperature of the copper foil pads and pins has risen. This will cause too much solder to overflow to some places where the temperature has not risen, which will easily cause soldering., False welding and other undesirable phenomena.

Manual PCB soldering skills

▪ When the soldering iron tip touches the pads of the circuit board and the pins of the components, be sure to complete the soldering in the shortest time, and then remove the heat source. The melting point of lead-free tin wire SAC305 is 217˚C, while the temperature of a general soldering iron will fall between 350～380˚C. The high temperature and long time local heating on the solder joints can easily damage the electronic components and circuit boards. It is not a metal material. Can't stand it, but plastic parts and glue can't stand it, and the coefficient of thermal expansion (CTE) of the material must be considered.

▪ After soldering with an electric soldering iron, the tip of the soldering iron must be quickly removed from the soldering place. If the removal is too slow, the solder tip will be formed. This is because the liquid solder will move with the high temperature soldering iron tip. When the soldering iron tip leaves the solder joint for a certain distance, the solder will be pulled off and form a wire drawing tip. Quickly remove the soldering iron tip When the solder is broken, it can promote the solder of the tip to spring back to the solder joint, and the residual temperature of the solder joint can absorb the rebound solder to form a beautiful arc.