What is the best solder paste. Soldering paste: what are the types of this composition and their features. What should be

Solder pastes are specific substances of a pasty consistency that are used to fix parts on. They have certain advantages, disadvantages and application features.

Advantages and varieties of the presented material

Consider the advantages that solder pastes have:

The possibility of their use for the manufacture of boards with very small parts;

They do not require a soldering iron, however, to work with this material, you will need a special hair dryer or station, thanks to which the product warms up;

Such a substance can be used in cases where it is not possible to work with conventional tools.

Solder pastes come in many varieties. First of all, they are classified according to the method of removing excess substance after work: wash and no-clean. The second option is safer, as it does not lead to corrosion of the board. The first type of pastes can be washed off with water, so they contain components that can harm the radio device.

It should also be noted that the substance can be made with or without lead. The second type of paste is environmentally friendly.

How to choose and store the material?

In order for the job to be done efficiently and accurately, you need to purchase the "right" solder pastes. First of all, you should pay attention to the technical characteristics of the substance: humidity and air temperature, storage features.

The presented material may lose its qualities depending on aging. The choice depends on the type of paste and its scope. Naturally, an important factor is the cost of the material. Solder paste, which costs about $10 for 50 grams or more, is purchased only from certified retail outlets.

As for the storage of the material, it is necessary to put it in the refrigerator, the temperature in which does not rise above 4 degrees. During its use, the room should be warm, but not hot (no more than 25 0 C). Humidity should not reach 80%. Before using the substance, it is necessary to warm it to room temperature and only then open the jar. Sometimes it can take up to 6 hours.

Features of the use of the material

Pasta has its own nuances. For example, the surface on which the substance will be applied must be absolutely clean, dry and free of grease. The board for the duration of the work should be fixed as firmly as possible in a horizontal position.

The place to be soldered must be completely covered with the substance. After that, try to place all the necessary details on the board very accurately. Now you can start heating the product with a hair dryer. The jet should not be too strong. Its temperature should be about 150 degrees until all the flux has evaporated from the paste. After that, the jet can be made hotter (200-250 0 C).

After all work is completed, the board will need to be cooled and cleaned of the remnants of the substance. This procedure depends on the type of paste.

Soldering with solder paste is the closest method to how boards are soldered in production. All boards in your phones, laptops, TVs and other devices are soldered in this way. I must say that the process is very bewitching and looks like magic :)

Of course, at home in artisanal conditions, the technical process is somewhat different from what is used in factories, but the general principle is the same.

First we need a stencil for paste. In industry, stencils are made from thin stainless steel by cutting holes in them with a laser or chemical etching. You can order the manufacture of this for yourself, but it is quite expensive. I ordered Kapton stencils, which these guys make very cheaply. One such stencil cost about 230 rubles. Probably in the future I will use steel, but for the time being, for the sake of testing, I decided to stop at this option.

To attach the stencil, I built a simple "device". The printed circuit boards in this photo are needed solely in order to accurately position the board that we will solder. I used them because their thickness exactly matches the thickness of the target board.

We put the board that we are going to solder into the device. In this case, all holes in the stencil exactly fall opposite the contact pads of the parts.

And here is the paste itself, bought from eBay. The paste is a mixture of small balls of solder and flux. Paste these million kinds and they say picking up a good whole is a problem. This paste is lead, there are lead-free ones - they are worse in many respects, but not so harmful. Probably in the future I will switch to lead-free.

The paste was quite liquid. After a couple of attempts, I managed to apply it so that it hit all the contact pads. Of course, some dexterity is needed here. The paste is applied with a plastic card, although it may make sense to use something softer.

Raise the stencil and take out the board. A picture from the category "how it really looks", because ideally it should look a little neater :)

Next, we arrange the components. I did not solder the entire board, but simply placed a few not the most valuable parts for testing. The arrangement turned out to be very simple and after a couple of details the process went very quickly and accurately. only later I noticed that I put the capacitor on a size larger than necessary, but oh well.

After that, the board needs to be heated. In production, this is done in special furnaces, where the heating and cooling process is very accurately controlled by the thermal profile. This is not a simple matter, there are a lot of subtleties, different boards, pastes, components need different thermal profiles. Such stoves exist for "home" use, and some even manage to make them from ordinary cooking stoves and toasters. For lack of all this, I warmed it with a blow dryer.

The paste conversion process looks very funny, I didn’t even regret it and then soldered another board, filming it on video (at the end of the post).

Visually, the soldering quality is almost factory-made. In any case, it looks much neater than when soldering with a soldering iron. At the same time, of course, soldering takes many times less time, because. no need to manually solder each contact. Particularly pleased with the quality of soldering contacts with a good heat sink. It is almost impossible to solder them with a soldering iron neatly.

Since the paste is dosed quite accurately, all soldering points are the same and neat. Due to the forces of surface tension, the components themselves take an even position, even if they were originally set a little crooked.

And here is the promised short video!

Summary: the method is definitely worth using if you need to solder more than 1-2-3-10 boards. Time spent -10, accuracy +20 :)

P.S. The paste can be applied without a stencil from a syringe, but this is not very convenient. There are special pneumatic dispensers, but these are not very cheap. Also, for placement, you can use not tweezers, but vacuum manipulators. I just have one coming from eBay, as soon as it arrives, I'll test it and write.

P.S.2. I remind you that lead is harmful, and especially in the form of such a paste. In no case should you eat, drink, smoke while working with the paste. Any pollution of a workplace - to remove. Vapors - do not inhale and generally check. The same goes for working with regular solder.

When solder paste was brought to the only normal store in the city, almost to order, I was the first in line for it :)
I have long wanted to completely switch to SMD, as the laziest technology - I was too lazy to drill holes and there was a LINKO 850 soldering station, a Chinese clone I don’t know what (Well, judging by the style of writing the logo, they mow everything under HAKKO =) A kind of Adibas =) approx. DI HALT), so far used only for dismantling. Picking out mosfets from motherboards is a sweet thing. I had pasta BAKU BK-30G(I have the same mud. Nasty thing, but it's fun to solder. approx. DI HALT)

We develop the payment as usual.

Wiring Tips for SMD Mounting

• Two sites side by side - never merge them! On the contrary, stretch and connect with a thin conductor, so they do not stick together (which makes the board sloppy) and allows you to visually check the presence of a track between them (just because there are two resistors nearby, or there is a conductor).
• Don't chase the size! Make the pads slightly larger than the component, and leave enough space between them. If you are limited in size, take a larger case, or make a double-sided board. At first he suffered such garbage. While the resolution is enough - I put it as close to each other as possible, now there are a bunch of small boards with 1206 components stuck in a checkerboard pattern - the board and conductors are not visible behind them.

After that, we poison as usual, but there are problems with tinning:
I puddle with a rose alloy, followed by removing the excess layer with a hot rubber scraper (right in the same pan / jar where the board was tinned) - it turns out flat conductors with almost a mirror shine :)

If you don’t have it, you can apply the following hint - we wind a braid for removing solder on a low-power soldering iron, tin it, and draw it along the tracks pre-coated with flux. If this does not work out, but tin with a sting, leave as thin a layer of tin as possible on the contact pads.
On flat tracks, parts are practically “glued” to solder paste, and they are installed worse on a convex tin layer. Well, if this is still a resistor, it will still be dragged into place by the surface tension of the solder (the main air pressure is at a minimum so as not to be blown away).

But mikruha (for example, the notorious FT232RL) on a convex surface, oh, how difficult it is to install evenly, everything strives to fall into the hole between the tracks, and if it gets up, the air flow, even at a small degree, will blow it into that very hole, after which the solder will spoil the legs , and contacts, turning the conclusions into a monolith ;-), and the flux will almost completely evaporate in a minute, after which it will be almost impossible to move it normally without first spoiling the conclusions with some kind of rosin-gel.

In short, as a result, we should get a board with FLAT contact pads (the flux there is weak, it clings to pink copper and alloy with a bang, but not so much to crap copper).

After that, having thoroughly mixed the paste, carefully, avoiding air bubbles, we tighten the semi-liquid paste (this paste, by the way, tends to dry out even when tightly closed. You can soak it by adding alcohol to it approx. DI HALT) into a regular insulin syringe, we put it on and break it off (it’s convenient for anyone, I first broke off the needle, leaving a centimeter, then spat and broke off at the root) the needle.

Now, having washed it well and dried it even more well (: the board, we smear a little paste on each platform. How much, you can see in the photo, but after two or three times you will understand for yourself, after which we seat the crumble with tweezers.

Installation Tips

• Install tall and large components last. First capacitors 0603, then resistors 1206, high LEDs, and then mikruhi.
• Each size has its own tweezers. (or is it already bourgeois?) Usually two are enough - a trifle and mikruha. You can’t take the same 2313 with small tweezers, and large ones don’t work so neatly to plant resistors as small ones - hands are shaking, chtoli. (And I always had enough. Approx. DI HALT)

Due to the fact that the temperature of the station floats a little, I had to learn how to determine the degree of frying by ... the smell ^_^ When the flux heats up to the operating temperature, it starts to smell something like vanilla ;-), and when it starts to smell like burnt hair, it means again I turned the temperature knob with my elbow and I had to go and buy 5 LEDs instead of the fried ones. (I prefer to fry at about 290 degrees at the exit temperature of the hair dryer. The board will have 10 degrees less, just right. And the air flow to a minimum. approx. DI HALT).

Mikhail Nizhnik, General Director, METTATRON Group LLC

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

TYPES OF SOLDER PASTES

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

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

Rice. 1. Classification of solder pastes

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

However, many halogen-free solder pastes are difficult to clean off and leave a whitish flux residue on board surfaces. At the same time, sediment resistance is considered more important than launderability.

Most solder pastes that do not require cleaning free production from this technological process. The fluxes of such pastes protect the solder joint from corrosion like varnish. Let's focus on the 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 should not contain halogens. It must be clearly understood that if the documentation for the paste indicates “Requires cleaning”, then it is necessary to wash, and if there is no such marking, then the issue is resolved based on additional requirements for the product: appearance, varnishing.

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 relatively more technologically advanced, for example, in terms of solderability, but are often inferior to halogen-free solder pastes in terms of reliability, which manifests itself in a decrease in the insulation resistance of the finished assembly. This is due to the higher chemical activity of flux residues. Thus, solderability and reliability are, in most cases, mutually exclusive factors.

Rice. 3. Key features to consider when designing or selecting solder pastes

Ideally, no-clean soldering paste would require 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.

However, such highly active organic acids absorb moisture. This is fraught: the acid remaining in the flux residues on the substrate surface 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 the technical data for KOKI 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 that of halogens. Due to the high activation temperature, the non-ionic activator in combination with carefully selected organic acids makes the activation in the reflow stage longer. As a result, solderability improves without sacrificing reliability.

Here are examples of popular types of pastes:

• solder paste for high-speed printing;
• solder paste with high wetting ability;
• solder paste for automatic in-circuit testing;
• An all-purpose paste with an extremely long screen life.

COMPOSITION OF SOLDERING PASTES

Solder pastes are made up of solder and flux (see Figure 2). When choosing a solder + flux complex for solder paste, the characteristics shown in fig. 3.

solder powder

For the production of solder powder, gas and centrifugal atomization methods are used. Features of the gas spray method:

Obtaining particles of small size;

Ease of control of the formation of an oxide film on the surface of the particles;

Low oxidation of solder particles.

The resulting solder powder particles are 1–100 µm in size. Solder particle size distribution and particle diameter are affected by solder feed rate, spindle speed and oxygen content.

Rice. 4. Preparation of solder powder by gas spraying

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). Solder ingots are melted in a crucible located at the top of the tank. Molten solder drips down onto a spindle rotating at high speed. When drops of solder hit the spindle, the solder is sprayed towards the walls of the tank, while the solder becomes spherical and solidifies before these particles reach the wall of the tank.

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

The solder powder then enters the sorting sieve, where it is best to use the solder powder double sorting method. In the first stage, the powder is sorted by a nitrogen jet from a blower. This eliminates particles with sizes smaller than desired. 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 size of 20-50 microns - for a pitch of 0.5 mm.

Two factors influence the quality of powders.

Particle size distribution affects solder paste rheology, printing, flow, de-stencil behavior, and paste slump performance. The minimum size of the stencil apertures depends on the minimum size of pads on the printed circuit board, while the maximum size of the aperture is less than or equal to the size of the pad. Select the desired particle size on the basis that at least 5 solder particles must fit into the smallest aperture of the stencil, as shown in Fig. 12.

Flux

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

Remove the oxide film and prevent re-oxidation during the soldering process. Metal surfaces at high temperatures during melting quickly oxidize. The solid components of the flux at these temperatures soften and become liquid, covering and protecting the soldered surfaces from re-oxidation. Flux restores metal and removes oxide film from the contact surface of electronic components, the PCB finish and the surface of the solder powder;

Remove dirt. However, the flux will not cope with a large number of sweat-fat prints, so it is better to take the board in your hands with gloves;

Ensure the viscosity stability of the paste required for printing and reflow.

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

Let us now consider the factors that affect print quality.

Rice. 6. Factors affecting print quality

PRINTERS

The electronic industry is developing, and the density of components on the printed circuit board is increasing, and the size of the components is decreasing. Because of this, the requirements for the characteristics and quality of solder pastes are being tightened.

A critical factor when assembling high-density printed circuit boards is the choice of equipment and printing parameters, as well as the quality and characteristics of solder pastes. This means that even if a potentially very good solder paste is selected, the result can be depressing just because of the incorrect setting of the printer operating parameters or the poor selection of the squeegee and the method of making the stencil.

The factors that determine print quality are listed in Figure 6. Let's take a closer look at them.

stencils

Methods for making stencils (see Fig. 7):

Chemical etching;

laser cutting;

Electrotype.

Previously, stencils obtained by chemical etching were used due to their relative cheapness. However, the shape of the apertures of such stencils does not make it possible to obtain high-quality printing with an aperture size of less than 0.5 mm.

Laser-cut stencils have smaller apertures, but scale remains on the walls of the apertures, resulting from the melting of the metal. Without additional processing, such stencils cannot be used for apertures less than 0.4 mm wide or for BGA packages with pad diameters of 0.25–0.3 mm. This problem is easily solved by electropolishing stencils, which removes roughness from the aperture walls, which makes it possible to use such stencils with aperture sizes up to 0.2 mm.

The third method - electroforming - produces stencils with apertures up to 0.1 mm. It is used extremely rarely, because this size of apertures is practically not used, and the cost of production is high.

The thickness of the stencil is determined by the minimum dimensions and the spacing between the apertures. The thinner the stencil, the better the printing results, since thin stencils cause less shear stress in the paste when it is released from the substrate (see Figure 8).

Rice. 8. The thinner the template, the less the paste shifts when separated from the substrate

It is desirable that the aperture size be slightly smaller than the PCB pad to compensate for stencil stretch, alignment tolerances, and solder paste slump. An example of the aperture for the contact pad of the output of the QFP package (0.5 mm pitch) is shown in Figure 9.

Rice. 11. In holes with rounded corners, the adhesion between the paste and the walls of the holes is less

Rice. 12. The smallest hole in the stencil should fit 4 to 5 of the largest solder balls.

The geometric shape of the apertures strongly affects the number of soldering defects. Therefore, the manufacture of stencils must be approached very responsibly, both at the design stage and at the manufacturing stage.

Figure 10 illustrates the rules for calculating the size of apertures. Figure 11 shows that when using apertures with rounded corners, the adhesion between the paste and the walls of the apertures decreases when the stencil is separated from the substrate, which reduces the distortion of the print.

As for the minimum size of the apertures, at least 5 of the largest solder balls must fit into the smallest aperture along its smaller side (see Fig. 12).

Rakely

Squeegees are rubber and metal. Rubber doctor blades are divided in shape into square, flat and reciprocating (see Fig. 13). It is impossible to say which of the squeegees is better: the spreadability of the paste depends on the working angle of the squeegee, and good spreadability ensures proper filling of each aperture with solder paste.

The working angle of the reciprocating squeegee is 70–80°. Since the downward force is relatively small, this squeegee is more suitable for low viscosity pastes.

A square squeegee has a working angle of 45°. It exerts high pressure on the solder paste, so it is best used for high viscosity pastes. If you work with this squeegee with low-viscosity pastes, then the paste will flow under the stencil (see Fig. 14).

The working angle of the flat squeegee is 50–60°. By changing the angle of inclination, it is possible to work with pastes of various viscosities.

When working with rubber squeegees, you must constantly ensure that the working edge is always sharp. When the edge wears, you have to increase the pressure to avoid smearing the paste. At the same time, the pressure under which the apertures are filled with paste also increases, which increases the friction between solder particles and adversely affects the separability of the paste from the walls of the apertures.

Unlike rubber squeegees, rigid metal squeegees do not wear out, last a long time and do not capture paste from holes.