Solder pastes: how to use? DIY solder paste

Category: All about soldering, About materials and tools, Articles about everything Published 03/02/2020 · Comments: · Reading time: 2 min · Views: Post Views: 1,290

Solder paste is a mixture of solder and flux. Widely used for soldering SMD and planar chips.

It differs from conventional solder wire in its paste-like form.

The paste is soft and contains lead microballs.

Therefore, it should not be applied with your fingers. It is better to use toothpicks or spatulas. And cotton swabs are ineffective because they absorb a lot.

The paste is also sold in syringes. In this form, using a piston, you can precisely dose the amount of solder onto the board. Use a large diameter needle.

After applying the paste, be sure to close it, regardless of the time. Otherwise, the balls will harden in the dried flux, and it will lose its properties.

General information

Solder paste is a pasty mass that consists of many small particles of spherical solder, flux and various additives. Why is it needed and what to do with it?

Solder pastes are used for surface mounting of electronic components by soldering on printed circuit boards, hybrid integrated circuits, and ceramic substrates. After application to the surface, the composition remains active for several hours. Scope of application: industry.

Characteristics

Shape and dimensions of solder particles

The characteristics of the solder particles determine how solder paste will be applied to the surface. Compositions with small particles are much less prone to oxidation. In addition, if the soldering substance has large particles of irregular shape, this threatens to clog the stencil, and therefore the application procedure will fail.

Specific gravity of metal in the composition

This indicator determines the thickness of the melted solder; the degree of precipitation and spreading of the soldering substance depends on it. The thickness of the connection after reflow is directly dependent on the specific gravity of the metal in the paste: the higher its percentage, the greater the thickness of the connection after the solder paste has been reflowed. The choice of application method also depends on the metal concentration. So, if the solder paste contains it in a volume of 80%, it should be applied using a stencil method, if it contains 90%, it should be applied by dosing.

Type of flux in the paste

Affects the level of activity of the substance and the need for washing. Depending on the method of removing flux residues, three groups of fluxes are distinguished:

• Rosin. The main component is purified natural resin, which is extracted from pine wood. Rosin fluxes are divided into non-activated, moderately activated and activated slightly corrosive. The former are characterized by minimal activity indicators, the latter are quite easy to clean, provide good wetting and spreadability of the solder, and the third are characterized by the highest activity indicators and a low level of demand.
• Water-washable. Contains organic acids. The use of water-washable active flux guarantees a good soldering result, but there is a need for washing with deionized water at a temperature of 55-65 degrees.
• No laundering. Does not require washing. Manufactured from natural and synthetic resins. The specific gravity of the resin in the composition of such fluxes is 35-45%. They exhibit moderate activity, their solder residues are non-corrosive and non-conductive, and the concentration of solid residues can reach a maximum of 2%.

For plumbing systems

A completely separate group consists of paste-like compositions intended for installation of fittings made of copper and its alloys in water supply systems with a soldering iron. These compositions are subject to special requirements, which are strictly regulated by GOST.

None of the paste components can be toxic. The flux must completely prevent oxidation of the seam and the penetration of corrosion products into the water.

Water supply pastes are completely unsuitable for working with electronic circuits for many reasons, in particular because copper or silver is often added to them to increase the strength of the connection. Such compositions are not used in electronics.

G5-SM800, G4(A)-SM833, G5(A)-SM833

Pastes with flux that does not require cleaning, consisting of RMA flux and solder powder, slightly susceptible to oxidation processes and with a uniform distribution of particles of a constant, strictly spherical shape.

The flux used is a product of the latest generation that does not require cleaning. The flux used does not contain halogens. This makes it possible to improve not only the technological properties of the solder, but also significantly increase the reliability of products. Consumers of our paste noted very good spread of solder on inversion gold (like pastes with active flux) and improved soldering of elements made using lead-free technologies. This is especially important during the period when some of the elements are made using old technologies (using lead), and the terminals of another part of the elements no longer contain lead, for example, from a silver-palladium alloy.

• Alloys used: Sn63/Pb37; Sn62/Pb36/Ag2
• Does not require cleaning - after soldering, flux residues do not contribute to corrosion and other processes that cause deterioration in the electronic characteristics of the product.
• High wettability during the reflow process. Provides high-quality removal of oxide films from the surfaces of soldered metals.
• High reliability of the formed solder connections.
• Does not cause the formation of solder balls near pads.
• Suitable for components with small lead pitches
• Do not contribute to the formation of jumpers between component leads after soldering due to sudden deposition.
• Long shelf life with minimal change in viscosity.

https://fr4.ru/upload/fr4/paste/reflow_profile.pdf

Properties

Viscosity

This is nothing more than the thickness of the solder paste substance. The paste is endowed with the ability to change the degree of its viscosity when exposed to mechanical loads. It can be determined using special instruments: Brookfield and Malcolm viscometers. As a rule, this indicator is indicated by the marking method.

Draft

Solder pastes have the ability to expand in size after the imprint is applied to the surface. The considered indicator should be at a low level, since a significant increase in the size of the solder paste print causes the formation of jumpers.

Properties saving time

This is reflected in indicators such as the longest residence time of the substance on the stencil before or after application, which does not entail degradation of properties. In most cases, the value of the first parameter is within 8-48 hours, the second - 72 hours. These indicators are recorded by the manufacturer on the packaging. Moreover, either one parameter (either of the two) or both can be specified.

Stickiness

Identifies the ability of solder paste to hold SMD components in place after installing them on the surface and before the soldering procedure. The degree of stickiness indicates the “viability” of the paste and determines its shelf life. It is calculated by implementing a special test, which uses a traditional tester capable of measuring the force required to move an element of certain weight parameters from an area of ​​a paste-like substance of certain sizes.

The presence of adhesiveness and its level depend on the type of solder paste. On average, the retention time is in the range of 4-8 hours, while the maximum value, which is typical for a number of pastes, can reach 24 hours or more.

Solder paste: how to use

The operating rules can be divided into three blocks:

1. General terms of use:

• the room where soldering work is carried out must be clean and not be a source or place of concentration of dust or any other contaminants;
• For personal protection, use eye protection and hand gloves;
• To clean the already applied paste from the surface of the board, use isopropyl alcohol or other solvents.

2. Before opening the packaging:

• place the paste in a room where the temperature is between 22-28 degrees and the humidity is 30-60%;
• before opening the package, keep the paste at room temperature for at least a couple of hours, while resorting to artificial methods of heating the substance is strictly prohibited;
• During operation, the soldering agent should be stirred regularly.

3. After opening the packaging:

• apply approximately 2/3 of the total amount of soldering agent to the stencil fabric;

• given the speed of the production process, as the applied paste is used, it is necessary to add the remaining soldering agent in small portions;
• If the printing work is completed, and the surface mount solder paste remains, in no case should it be mixed with new one, but should be stored in a separate container in refrigeration equipment;
• if the soldering work is not completed, to continue it, you can use both the started solder paste and a new one, the main condition here is to mix the previously opened paste in a 1:2 ratio with the new one and mix thoroughly to obtain a homogeneous mass;
• the installation of components on a printed circuit board should be done as quickly as possible; in fact, it must then be soldered at the same accelerated pace;
• in the event of a forced pause in work that exceeds 60 minutes, remove the remaining solder paste (that is, the paste that was applied to the stencil but not completely used) into a special container and close it tightly;
• In order to obtain high-quality screen printing, it is recommended to clean the screen fabric every 40-45 minutes.

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.

To increase the efficiency of the solder mass, you can add a little ammonium chloride to it. In the finished mixture, the concentration of this ingredient should not exceed 5-10%. You can replace ammonium with aniline hydrochloride. Substances must be stored in sealed containers and away from pets and children.

Drop-jet method

Dispenser printing is a method of applying a soldering agent by “shooting” it at almost room temperature (about 30 degrees) from a cartridge through an ejector onto a printed circuit board exactly in the place where the paste should be applied, based on the board diagram. The cartridge is in constant motion, following the ordinate and abscissa above the surface of the printed circuit board. The correct application of the solder layer depends on it. The cartridge stops exactly where you want it, exactly when you want it, thanks to a well-functioning drive system. At home, you can use not an ejector and a cartridge, but another solder paste dispenser - a syringe.

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:

• possibility of use in the production of circuit boards containing the smallest elements;
• You can use solder paste even without a soldering iron, but in this case you will need a special station or hair dryer to heat the product and evenly distribute the ingredients over the surface of the board;
• You can use soldering paste in situations where it is not possible to use classical tools.

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.

Stencil method

It is most popular and involves applying paste to the soldering surface by pressing it through apertures in the stencil with a specially designed tool - a squeegee. In this case, the squeegee makes moving movements along the surface of the stencil in a horizontal position.

Step-by-step instructions for the stencil method:

• Step 1. Fix the soldering surface (board) in the work area.
• Step 2. Align the soldering board and the stencil with absolute precision.
• Step 3. Squeeze or apply the required amount of solder paste onto the stencil fabric.
• Step 4: Apply the paste through the stencil using a squeegee.

• Step 5. Check the quality characteristics of the soldering agent application.
• Step 6: Remove the soldering surface.
• Step 7. Clean the stencil.

Rheological features

Important characteristics of surface mount solder pastes are viscosity, adhesiveness, durability, and the ability to create a three-dimensional connection on the board.

Knowledge of quantitative indicators of rheological properties allows you to choose the right printer for applying solder paste, which can rationally dispense portions.

The paste is applied taking into account the tendency to increase the viscosity of the paste mass. A decrease in viscosity occurs with increasing temperature. To successfully solder with solder paste, you need to periodically add new portions to the mass and monitor the temperature readings in the working area. This can be easily done using screen printing machines equipped with thermal sensors.

Many packages of imported pastes indicate “lifetime”. The value determines the time interval from the moment the can is unsealed until the end of soldering, during which the rheological properties will remain unchanged.

If the indicator is low, you will have to work quickly to obtain a high-quality connection. Now there are mixtures on sale with a “lifetime” of 72 hours. You can work slowly with such tools.

An important characteristic is the stickiness of the solder paste, which reflects the ability of the part to stay on the board before starting work.

Some pastes can fix electronic components for more than a day, which is convenient when installing large boards. Compositions with low adhesiveness are capable of holding the element for 4 hours.

There is a wide range of solder pastes on sale, some of which are sold in a syringe for manual or automatic dispensing, others in cans or cartridges.

Products in cans are intended for screen printing machines. They are made of metal sheets with great scrupulousness, which makes it possible to cut out cells on the board for applying solder paste with an accuracy of 0.1 mm.

Special types of stencils can regulate the thickness of the paste-like mass. The machines can operate in both manual and automatic modes. Expensive models are additionally equipped with a stencil cleaning system, which significantly increases work productivity.

Storage conditions

Solder pastes require not only compliance with operating rules, but also special storage conditions, the main ones being the following:

• the temperature of the room where the substance is stored should not exceed 25°C;
• the temperature range for storing paste depends on the manufacturer and can range from 4-10°C;
• the shelf life of the paste when stored under recommended conditions for compositions with water-washable fluxes is 3-6 months, and with indelible fluxes - 6-12 months;
• Soldering with solder paste should be carried out in a room where the relative humidity is 60-80%.
• started pastes can be stored for no more than 12 hours;
• in order to reduce the degree of degradation that remained from the previous soldering process, the paste can be used in new soldering works, but only with the addition of a new one;
• jars, cartridges and syringes with soldering agent should be stored only in a vertical position, with the syringe tips down to avoid the possibility of delamination.

How to choose?

Solder pastes are produced in very different compositions - this causes differences in the scope of use of such compositions and dictates certain requirements when choosing a particular substance. The choice is based on the metal for which soldering paste is required.

If you plan to work with nickel, then for such alloys it is better to give preference to compositions containing chromium or nickel alloyed with beryllium, silicon or boron - such alloys are classified as hard. The maximum effect can be achieved when soldering in a vacuum or in an argon atmosphere.

Copper - copper-silver alloys are relevant here, which can be additionally alloyed with lead or tin to reduce the melting point.

When fusing aluminum components, zinc- and tin-based solders are required.

Gold and silver, as well as many other alloys coated with them - here you should take a universal paste made of silver with admixtures of zinc and copper.

Temperature

Solder pastes are sensitive to significantly low and high temperatures. Considering that the base contains two materials of different densities (flux and solder), the natural process of delamination of the flux and other components of the soldering substance, as well as the appearance of a thin layer of flux above the surface, is considered possible. Exposing the paste to high temperatures for a long time leads to significant separation of the flux and the remaining paste, causing the formation of a thick surface layer of flux. What is the result? But it turns out that the solder paste loses its properties, and, therefore, its application to the surface will be defective. Temperatures above 30°C will completely provoke chemical decomposition of the soldering agent.

When exposed to low temperatures, the paste loses its wetting ability, since the flux activators partially or completely turn into sediment. Compositions from some manufacturers can still be stored at temperatures from –20 to +5°C.

What is it and what is it for?

Soldering paste is a viscous structure that contains solder, flux, various volatile solvents, and special adhesive components. Depending on the chemical composition of the flux and solder, the melting temperature, operating technique, and options for cleaning the printed circuit board may vary.

It is known that soldering to connect elements is allowed when using any materials whose melting temperature is several levels lower than the melting point of these same parts. That is why, for the simplest household circuits at home, solder is most often used along with flux or acid. Solder pastes contain two components at once, as well as their various impurities, due to which the soldering process is accelerated many times over. In addition, such pastes have found wide application in the manufacture of electrical appliances.

Alloys with lead, as well as tin or silver are selected as base materials for solder, but lead-free solder paste has gained maximum popularity.

The flux in the structure acts as a degreaser. In addition, to successfully complete the work, you will need an adhesive adhesive component - it greatly facilitates the fixation of SMD elements on working boards. Moreover, the larger the dimensions of the board, the more saturated the elementary density will be, and the more relevant the use of paste for soldering.

Pastes have found widespread use in production. They are used for tinning car bodies, surface mounting, repairing wires and soldering LEDs.

We can say that solder paste has become a profitable and effective substitute for traditional solders, of course, if its brand and the flux included in its base were selected correctly.

The main advantages of the material include ease of application, as well as cleanliness of the printed circuit board, which is achieved through precisely dosed application of the composition. The only disadvantage of such pastes is their short shelf life, on average it does not exceed six months. After this, the consistency of the paste begins to separate into phases, and the product becomes unsuitable for use.

Exposure to moisture

The most detrimental effect on solder paste is not low and high temperatures, but moisture. If humidity levels are elevated, the solder balls in the paste begin to oxidize at a rapid rate, which results in flux activators being wasted on the balls instead of on the solder surfaces as they should be. When moisture enters, the paste spreads, bridges and solder balls are formed, flux/solder is sprayed, electronic components are displaced during the soldering process, and the retention time of electronic components is reduced.

Solder in coils and bars

• tubular solder
  from Cookson Electronics Assembly Material's in various sections with high-quality flux that does not require cleaning.
• High-quality solder in bars
  of the SoldECO® brand, produced at an enterprise focused on fulfilling defense orders from France and the EU.

Radio amateurs have long chosen such an innovation as solder paste. It was originally invented for soldering SMD components during machine assembly of boards. But now many people use this paste for ordinary manual soldering of parts, wires, metals, etc. It’s understandable - everything in one is at hand. After all, solder paste is almost actually a mixture of flux and solder.

In fact, to make solder paste for the needs of radio amateurs, it does not take much effort, time and ingredients. To make solder paste we need:

1. Medical Vaseline. Used as a thickener;
2. Flux LTI-120 or other liquid.

I will make from these components. Ideally, it’s better to take:

1. Tin-lead solder rod;
2. Soldering fat. And if you find “active fat,” it’s absolutely beautiful.