solder paste for smd which is better with solder

Solder Paste Reviews

Solder paste is a substance that is used in the soldering process instead of regular rosin; it contains solder powder, flux, some kind of binder and some other components.
The main selection parameters: • its composition – solders with alloying additives, with or without lead content, and so on; • the size of individual solder particles and shape, depending on this there are different dosages; • viscosity level - the method of application depends on this - using a stencil or dispenser; • solderability - determined by the contamination of the particles and the level of their oxidation.

In addition, such pastes may not be subject to corrosion - no-clean. Rust may form in soldering areas, since water-washable pastes contain organic substances. For high-quality soldering, you need to choose a paste depending on the metal with which it will be used. There are special compositions for chrome-nickel, copper, aluminum, gold, and silver.

I bought it because of the title “solder paste” and, accordingly, I expected to receive exactly solder paste in the classical sense - a mixture consisting of flux mixed with highly crushed solder particles, which is very convenient. When I opened the jar, it became clear that this was not the same paste, but only a flux, and very similar in appearance to the bad Chinese flux RMA-223. But after trying it at work, I was pleasantly surprised. However, there are some nuances that you need to be aware of. That's all we'll talk about in the review.

Today's review will be devoted to the MECHANIC XG-50 (XG-500) solder paste, which I purchased on eBay. I had a desire to acquire soldering paste for a long time, but since my tool assortment did not include a soldering gun, this purchase was constantly pushed into the background. But after I came across a budget technical hair dryer, I decided to purchase solder paste along with it. The choice was made spontaneously, mainly based on data from the seller’s pages, and fell on MECHANIC XG-50 paste.

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What is SMD and basic principles

The use of flux for soldering SMD components has its own characteristics, which make it possible to improve the connection of the surface of microcircuits and boards. General recommendation for using flux for soldering SMD is effective for chip resistors, as well as SOIC, LQFP, QFN and others. The application of the thinnest layer of material allows for production soldering without compromising quality. By the way, literally from English the meaning of SMD soldering paste is translated as “using components for surface soldering” (Surface Mounted Devices). As can be seen from the working name of the paste, it allows for sufficient assembly density of the connection compared to conventional technologies.

SMD component soldering process

Most craftsmen mistakenly believe that using SMD components is impractical at home. Most craftsmen believe that only TN technology can be needed at home, although the main problem is choosing the correct diameter of the soldering iron tip. Inexperienced craftsmen really do not know the intricacies of using SMD soldering with solder paste, since the result of the work is “staining” of SMD contacts on the printed circuit board with tin. To avoid typical mistakes, some parameters should be taken into account: capillary effect, which must have a fine structure, as well as surface tension and proper wetting of the surface being treated. Ignoring the tasks at hand will not be able to fully answer the difficult question of which flux is best for soldering SMDs at home or on an industrial scale.

High-quality contact with the legs of the microcircuit of the board with SMD components occurs for one simple reason, the effect begins to be exerted by the general tension force, which forms separate independent drops of tin formation on the surface of the board.”

As can be seen from the general description, the wizard’s actions are minimized and the flux for soldering SMD components only heats the legs of the microparts used. Remember, when working with very small components and parts, seizing (unexpected connection) of technological elements to the tip of a working hot soldering iron may occur, which negatively affects the further operation of the microcircuit.

Features of technology in factory conditions

For industrial production, soldering paste for SMD components is adapted to a group system, which uses an electronic system for applying flux to the surface of the microcircuit. Fine application technology using silk-screen printing is used on the surface of contact working areas. Thus, in its technology and consistency, the material somewhat reminds us of the usual toothpaste. The substance includes solder powder, as well as flux components. The entire substance is mixed and applied to the surface of the microcircuit using a conveyor method.

Appearance of paste for SMD

An automated system carefully turns over the boards that need to be soldered, then the microcircuits are moved to a temperature cabinet, where the mass spreads, followed by solder. In the oven, under the influence of the required temperature, a conditional flow occurs around the technological contact legs of the SMD components, and as a result, a fairly strong connection is obtained. After the temperature cabinet, the chip is again moved to a natural environment, where cooling occurs.

Is it possible to solder with SMD paste yourself?

Theoretically, yes, but in practice, quite a lot of experience is needed to carry out this technological operation. For work we will need the following tools and preparations:

The flux should always be in a liquid state, this way you completely disinfect the surface of the microcircuit. In addition, the drug removes the formation of oxides on the surface of the board during operation. Remember that an alcohol solution together with rosin cannot ensure the quality of soldering, and their use is only permissible if a suitable soldering compound is not available.

Soldering iron selection

To work, you need to select a special soldering iron that has an adjustable heating range. To work with the microcircuit, a soldering iron is suitable, which has an operating heating temperature of no more than +250...+300 C. If you do not have such a soldering iron at hand, you can use a device with a power of 20 to 30 W and no more than 12-36 Volts.

A soldering iron with a voltage of 220 Volts will not be able to ensure the quality of soldering, where it is very difficult to regulate the required heating temperature of the flux.

Soldering iron for soldering SMD components

We do not recommend using a soldering iron with a cone-type tip; this will lead to damage to the surface being processed. The most optimal tip is the “microwave” type. A soldering iron with a voltage of 220 Volts not only heats up quickly, but also leads to volatilization of components during the soldering process. For efficient operation of the soldering iron, we recommend using the thinnest wire to ensure interaction between the tip, flux and solder.

But, for a microcircuit, the soldering procedure is slightly different from the above:

In some cases, it is possible to use a special soldering gun for soldering, but for this it is necessary to create appropriate working conditions. Remember that the hair dryer can only be heated to a temperature of +250 C, no more (in rare cases, up to +300 C).

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Types of solders for body repair

• There are different types of solders. For body repairs, soft (low-melting) solders are often used. They are sold in the form of rods, 45 cm long, of different thicknesses. In general, they are classified as lead free or lead free solders. The first type of solder was used for many years due to its ease of use. However, it has been banned in some countries for use in mass production due to health and environmental concerns. However, lead solder is still used by individuals and can be found commercially. Many craftsmen prefer to use lead solder.
• The ratio of its components (tin, lead) is indicated on the solder. Traditional body solder is 30% tin and 70% lead. May contain an additional component, for example, 74% lead, 25% tin and 1% antimony. Lead is hazardous to health. To use it, protective equipment is required and safety precautions must be taken into account. However, it is easier to use. It remains soft longer after heating (in the range of 180 to 260 degrees Celsius). This makes it easier to apply and level. This solder is easily applied to vertical and horizontal surfaces. Once lead solder has hardened, it is not recommended to process it with a grinder, as very toxic dust is generated. So, it is usually processed with a special body file, and at the final stage with a block of coarse sanding paper by hand. If you still use a grinding machine, then you need to use only coarse abrasive so that there is no suspension of fine dust. Lead residues after grinding must be disposed of immediately.

Solder containing lead (Pb 74%), tin (Sn 25%) and antimony (1%).

• Solder can have different lead to tin ratios (70/30, 60/40 or 50/50). Solder with lower lead content is also used in auto body repairs, but has a low ductility range (183ºC to 188ºC) so it is more difficult to use. Solders with this ratio are usually used on horizontal surfaces, since they quickly become liquid and can flow. The adhesion of all lead solders is approximately the same and depends on proper surface preparation.
• As an alternative to lead solder, safer lead-free solder has been developed. In all lead-free solders, tin is the main component. The other component may be silver, copper, indium or bismuth. Most lead-free solders have either a higher or lower melting point than lead solder. For body repairs, the most common lead-free solder is a mixture of tin and silver (94% tin and 6% silver or 96% tin and 4% silver). It has a higher cost. Lead-free solder is approximately equal in performance to traditional lead solder with a 50/50 (lead/tin) ratio. This solder has a higher melting point (221°C). Once heated, it remains soft for less time than lead solder, making it more difficult to align. When hardened, it becomes harder and more brittle. It is more difficult to process with a file. The advantage is that it can be sanded as it does not produce toxic dust. Although, do not forget about the respirator. An additional benefit of lead-free solder is higher tensile strength.
• Solder used in electronics typically consists of 60% tin and 40% lead. It may contain flux in the center. Rosin flux, often used with this solder, does not promote adhesion to steel, but acid flux may work well. This solder is best used only for soldering small holes, since its ductility range is very small.
• To work with aluminum bodies, a different type of solder (containing tin and zinc) is used.

Solder paste and its properties

Initially, such compositions were used only in SMT technologies. Now their area of ​​use has increased. Solder paste for SMD contains the following ingredients:

It is possible to achieve high-quality soldering only in a situation where all conditions and shelf life of the solder mass have been met. Most of the ingredients in such formulations have a shelf life of no more than six months. They should be stored at a temperature of +2…+10°C. The room should not be too cold or hot. The air humidity level should not exceed 80%. Before using the composition, it must be warmed to room temperature and only then open the jar or remove the substance from the syringe. In some cases, it takes about 5-6 hours to warm up.

It is also necessary to take into account that over time the solder mixture loses its properties. It is necessary to select materials for soldering with tin or other materials taking into account the following requirements:

In addition, solder paste materials must be easy to dispense and suitable for screen printing.

Important Specifications

Solder mass must be selected taking into account its physical and chemical properties. These characteristics are directly dependent on the ingredients of the composition. These properties are:

In addition, no-clean pastes do not provoke the formation of corrosion, while water-washable pastes can lead to such problems at the soldering site, since they contain a number of organic substances.

Making at home

Sometimes ready-made solder paste is not available, so it is useful to learn about how to make it. For this purpose, it is necessary to prepare soldering fat and a solder rod made of lead and tin. If the first material is not available, it can be replaced with Vaseline LTI-120.

The solder must be crushed using a needle file or a special drill attachment. The finished crumb should consist of small particles. It is put into a container into which Vaseline is added in a 1:1 ratio and a small amount of flux.

All components of the homemade paste are thoroughly mixed and heated in a water bath. The finished composition can be stored in a large syringe. With the help of it, a paste-like substance will be applied to the boards.

On mineral oil

It is convenient to solder small parts using homemade flux paste consisting of two components. To prepare it yourself, you should take any mineral oil (a petroleum product) in the amount of 900 g.

It should not contain acidic impurities. Gradually add 100 g of ammonium chloride into the oil, rubbing the mixture until smooth. Immediately place the finished product into a sealed container.

Some craftsmen like viscous solutions to work with. For such situations, it is recommended to dissolve ammonium chloride in the specified proportion in kerosene.

Application benefits

Solder pastes are used not only in everyday life, but also in industry. Such widespread distribution of this material is explained by its following advantages:

If there is no opportunity or desire to make the mixture yourself, you can purchase a ready-made version. But you need to remember that high-quality solder paste costs about 10 US dollars per 50 g. Cheaper options are of poor quality and may not only not provide sufficient fixation reliability, but also lead to additional problems during subsequent use of the electronic product. To avoid problems, it is better to buy such materials in certified stores or directly from manufacturers.

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Advantages of tinning and soldering the body

1. Solder sticks to the surface better than putty. The tensile strength (the force required to separate the solder from the metal it is applied to) is 423 bar. This is a very high figure.
2. Even a thick layer of solder does not shrink, unlike putty.
3. Solder has better elasticity and strength than automotive putty. Lead bends well together with the metal on which it is applied, so it does not crack.
4. The solder will not crack or peel off on impact, as happens with putty.
5. Solder is waterproof. Putty cannot be called completely waterproof.
6. Solder can withstand high temperatures, so it can be used for repairs and subsequent powder coating.
7. Another big advantage of solder over putty is that you don't have to worry about the hardening time. You can add additional solder directly onto the already applied layer. You just need to heat the surface and new solder and add it. There is no need to completely melt the already applied layer.

Solder paste for smd which is better with solder

Mikhail Nizhnik

, General Director, METTATRON Group LLC

The author summarizes information about the properties and behavior of solder pastes during soldering, based on extensive experience working with solder pastes. The article will be of interest to a technologist working on a surface mounting line.

Types of solder pastes

Pastes are classified according to the type of flux (see Fig. 1).

“Water-soluble” solder paste (flux residues after soldering are dissolved with water), which requires mandatory cleaning due to the content of active flux (see Table 1), is washed sequentially with ordinary, distilled and deionized water, and at each stage jet cleaning or ultrasound is used. For “water-soluble” pastes that do not require mandatory cleaning, the process is limited to distilled water.

Rice. 1. Classification of solder pastes

Table 1. Classification of fluxes

With pastes that require cleaning with special liquids, the situation is different. Regardless of the presence of halogens in the composition, such pastes are based on rosin fluxes, therefore, to clean them after soldering, it is recommended to use a solvent such as HCFC and a saponifying reagent. Then the cleaning liquids, in turn, are washed with distilled and then deionized water.

However, many halogen-free solder pastes are difficult to clean and leave a whitish flux residue on the surface of the boards. In this case, resistance to sediment is considered more important than washability.

Most no-clean solder pastes eliminate this process from production. The fluxes of such pastes protect the soldered joint from corrosion like varnish. Let's focus on pastes that do not require cleaning: they are the most technologically advanced.

Rice. 2. Composition of solder pastes

It is often said that no-clean pastes must be halogen-free. It must be clearly understood that if the documentation for the paste states “Requires washing,” then it must be washed, and if there is no such marking, then the issue is resolved based on additional requirements for the product: appearance, application of varnish.

In Japan, for example, halogen-containing pastes (0.2%) in processes without cleaning after soldering are much more popular than halogen-free ones. Halogen-containing solder pastes are comparatively more technologically advanced, for example, in terms of solderability, but are often inferior to halogen-free pastes in terms of reliability, which is manifested in a decrease in the insulation resistance of the finished installation. This is explained by the higher chemical activity of flux residues. Thus, solderability and reliability, in most cases, are mutually exclusive factors.

Rice. 3. Main characteristics taken into account when developing or selecting solder pastes

Ideally, for no-clean soldering you need a halogen-free paste, but with the solderability of a halogen-containing paste.

The difficulty lies in increasing the chemical activity of halogen-free no-clean pastes. In most of these pastes, organic acids are used as an activator instead of halogen-containing compounds, and the lower the molecular weight of the acid, the greater the activation ability. Since the activating effect of organic acids is much weaker than that of halogen-containing components, they try to introduce a couple of dozen relatively active organic acids into the flux system.

At the same time, such highly active organic acids absorb moisture. This is fraught: the acid remaining in the flux residues on the surface of the substrate is ionized when interacting with water, which reduces the surface insulation resistance and leads to electromigration.

Activation systems in solder pastes (here the author relies on technical data on pastes) use less hygroscopic organic acids and a specially developed non-ionic activator. This special system does not dissociate into ions, its electrical properties are stable, and its activating ability is not inferior to halogens. Due to the high activation temperature, the non-ionic activator in combination with carefully selected organic acids makes the activation at the reflow stage longer. As a result, solderability is improved without sacrificing reliability.

Here are examples of popular types of pastes:

Composition of solder pastes

Solder pastes consist of solder and flux (see Fig. 2). When choosing a solder + flux complex for solder paste, take into account the characteristics shown in Fig. 3.

Gas and centrifugal atomization methods are used to produce solder powder. Features of the gas atomization method:

— obtaining small particles;

— ease of control of the process of formation of an oxide film on the surface of particles;

— low level of oxidation of solder particles.

The resulting solder powder particles range in size from 1 to 100 microns. The size distribution of solder particles and their diameter are influenced by solder feed speed, spindle speed and oxygen content.

Rice. 4. Obtaining solder powder by gas atomization

The powder is obtained in a container about 5 m high and 3 m in diameter, which is filled with nitrogen and oxygen of very low density (see Fig. 4). The solder ingots are melted in a crucible located at the top of the tank. Molten solder drips down onto the spindle, which rotates at high speed. When drops of solder hit the spindle, the solder splashes towards the walls of the reservoir, causing the solder to become spherical and solidify before the particles reach the wall of the reservoir.

Rice. 5. The degree of oxidation of solder particles depending on their size

The solder powder then goes to the sorting screen, where it is best to use the double sorting method of solder powder. At the first stage, the powder is sorted by a stream of nitrogen from a blower. In this case, particles with sizes smaller than the required size are eliminated. Then the powder goes to a sieve, where particles with sizes exceeding the specified values ​​are retained.

Solder pastes with a particle size of 20–38 microns are used for mounting printed circuit boards with a stencil aperture pitch of up to 0.4 mm, and with a particle size of 20–50 microns for pitches of 0.5 mm.

The quality of powders is influenced by two factors.

Particle size distribution affects solder paste rheology, printability, flow, stencil release behavior, and paste slump performance. The minimum size of the stencil apertures depends on the minimum size of the pads on the printed circuit board, with the maximum size of the aperture being less than or equal to the size of the pad. Select the required particle size on the basis that at least 5 particles of solder should be guaranteed to fit into the smallest aperture of the stencil, as shown in Fig. 12.

The second component of solder paste is flux. The role of flux in solder pastes is the same as in wave soldering, or selective soldering. The flux should:

— remove the oxide film and prevent re-oxidation during the soldering process. Metal surfaces at high temperatures during melting quickly oxidize. At these temperatures, the solid components of the flux soften and turn into a liquid state, covering and protecting the soldered surfaces from re-oxidation. Flux restores metal and removes oxide film from the surface of contacts of electronic components, the final coating of the printed circuit board and the surface of the solder powder;

- remove dirt. However, flux will not cope with a large number of sweat and grease marks, so it is better to handle the board with gloves;

— ensure the stability of the paste viscosity required during printing and reflow.

The main fluxing components and their role are indicated in Table 3.

Table 3. Main fluxing components and their role

Let us now consider the factors affecting print quality.

Rice. 6. Factors affecting print quality

With vegetable oils

Many people recommend using palm kernel oil for solder flux paste.
It itself already has a suitable consistency and allows for the thinnest soldering when carrying out electrical work. The activity of such solder paste can be increased by adding ammonium chloride. The concentration of the additive varies from 5% to 10%. Some home craftsmen consider it necessary to add aniline hydrochloride to the soldering mixture.

When performing delicate work, soldering flux paste, made at home with your own hands from the most affordable means, has proven itself well. You need to take 100 g of regular vegetable oil.

It is advisable to use a refined product. It is free of impurities and will not foam the solder paste when the temperature rises. You will need another 300 g of pure beef fat. It is better to take melted fat that does not contain any fiber or muscle tissue.

The main component of homemade solder paste is rosin. You will need 500 g of it. All this must be combined in a porcelain cup, mixed carefully, heating until completely melted.

Pour 100 g of finely ground ammonium chloride into a homogeneous mixture, stirring until dissolved. Hot DIY solder paste should be immediately placed in a jar for later storage. It is not recommended to leave the composition in the cup.