Almost all electronic equipment consists of printed circuit boards (PCBs), which are necessary for their operation. PCBs have copper-based conductors on which the electronic components are soldered. The welds / brazing between these tracks and the components themselves are generally carried out by the use of a solder paste.
The solder pastes are made by mixing a metal in the form of alloy powder (about 90% by weight) and a chemical part composed of organic elements (about 10% by weight). The chemical part is more commonly referred to as "flux" and is generally covered by a trade secret or by patents. The purpose of the flux is to give the solder paste its consistency and to allow the welding of the parts by eliminating the oxides that are located at the interface of them.
To use a solder paste under the best conditions, it is important to understand the concepts described below.
To achieve a good brazing, the metal surfaces to be soldered must be "cleaned" as they are oxidized upon exposure to the environment and can form compounds with oxygen, nitrogen, water and pollutants, such as sulfur, in the air. The degree of oxidation and the nature of the oxidizing chemical species determine the affinity between the metal atoms and these chemical species. For example, the copper surfaces form two types of oxides. Copper oxides interact with carbon dioxide and moisture in the air to form carbonates. Iron behaves like copper, while nickel produces a continuous thin film of oxide. Silver reacts with traces of hydrogen to form silver sulphide. Once a layer of these types of compounds forms on the surface of the metal, this leads to passivation of the metal surface resulting in poor wetting and consequent poor welding. Additionally, solder alloys also undergo surface oxidation, forming compounds on the surface with oxygen, water and nitrogen which also interfere with the solder. Thus, good welding requires the "cleaning" of the surfaces to be soldered by means of a particular chemistry. It is the flux that will play this role and that will be able to counteract the effects of passivation for example.
In the case of electronic soldering, the flux must have the following properties:
1. Remove the passivation layers and make the metal surfaces “wettable” by the solder alloy.
2. Protect the cleaned surfaces with a layer of chemical compound, usually rosin, that will make a barrier with air before applying molten solder.
3. Promote the wetting of the surfaces to be soldered by controlling the surface tensions which favor this wetting process.
4. Provide the correct rheology to the solder paste to ensure good printing of this solder paste and allow adhesion of the components among others.
Although the first function is the main function, good results can be obtained at the PCB manufacturer only if the flux also makes it possible to perform the other functions satisfactorily.
A solder paste must in fact have properties which enable it to cover the requirements described above and to be adapted to the manufacturing process of the electronic cards with the surface-mounted components. The characteristics to be taken into account are: chemical activation, temperature or window of activation, thermal stability, surface tension, wetting power, rheology, capability with respect to printing, toxicity and the nature & quantity of the residues.
Rosin is the base material for solder pastes. It is a natural resin that has been known for many years and comes from pine (Pinus Palustris in particular). It is a solid product at ambient temperature which is sometimes vitrous, sometimes lumpy, with colours varying from very light yellow to brown. Rosins are a mixture of organic compounds, including most terpene derivatives and hydrocarbons. Although the exact composition depends on the source, the most important compound is abietic acid - or sylvic acid (C20H30O2). In these rosins, pimaric acids are also present. Rosin is not soluble in water, but it is soluble in organic solvents such as alcohols, hydrocarbons, ethers ... A rosin of good quality consists of the following elements:
Abietic acid 80%-85%; Pimaric acid 12%-17%; Others 3%.
Rosin is widely used in fluxes because it has many advantages:
- It attacks the passivation layer of several metals, especially copper.
- It does not attack bare copper, even after prolonged contact.
- It can be dissolved in suitable solvents and applied to the surface to be welded. After evaporation of the solvent (function of the reflow furnace), it forms a thin layer which protects the molten metal and thus promotes wetting.
- It is a good vehicle for active compounds, such as certain amines.
- Rosin solder pastes have good tack to printed circuits and their rheology allows good printing while limiting the phenomenon of slump and stencil life.
Rosin fluxes can be divided into two main categories: activated and non-activated fluxes.
The activated fluxes can then be divided into 2 subclasses: RMA (Rosin Middly Activated) and strongly activated rosin (RA = Rosin activated).
Non-activated fluxes are used by manufacturers of aviation equipment, for example because residues can be left safely on soldered circuits without any risk of corrosive attack. The activated fluxes are similar to the non-activated fluxes, except that they contain an additional activation agent (Activator) which is much more reactive with the metal passivation layer than the rosin. The degree of flux activation depends on the nature and amount of activators. Conventionally, the types of activators used include bromine compounds and carboxylic acids.
A solder paste is therefore a mixture containing: (a) a powdered solder alloy, (b) resins (eg rosins), (c) activators, (d) solvent, (e) Thickeners and "rheological" adjuvants, and (f) antioxidants. During the first heating step, the solvent evaporates and the activators "attack" the metal surfaces, resulting in cleaning of the metal surfaces. Then, during the second step, the solder alloy powder melts and forms a liquid mass, which will constitute the solder joint.
The alloy powder (which is in the form of alloy balls), in order to obtain a high-performance solder paste, must meet the following criteria:
- Homogeneous chemical composition.
- Controlled level of impurities
- Size and shape of particles
- Controlled distribution of alloy ball size.
- Surface chemistry (oxide-free is correct).
These properties have important effects on the performance of the solder paste. For example, when users want to work with printed circuits with fine pitch, alloy powders with finer particle sizes are required to ensure good printing (type 4 or 5 required). Lower particle sizes can be achieved with direct consequences on the price of the solder paste (type 6 to type 7). Type 8 is used in very confidential applications and on products with very high added value.
In summary, the particle sizes, their shape and the distribution thereof have important consequences on the rheology of the solder paste and thus can influence printing and dispensing of the solder paste.
General solder paste flux formulation:
Generally, a chemical part of solder paste contains the following:
Solvents are much more than a "binder" that keeps the flux components in solution without precipitating them. They are of paramount importance and determine in particular the following parameters:
(A) the ease of application of the solder paste, whether in dispensing or in printing;
(B) the "drying" of the paste during reflow and the formation of a protective layer on the circuits;
(C) preventing the formation of sputtering and maintaining wettability when depositing molten filler metal on the substrate to be soldered.
If the solvent dries too quickly, the rosin protective film hardens and the rosin does not follow the application of the molten metal resulting in a risk of micro-cracks in the film composed of the residues.
If it dries too slowly, the protective film still contains solvent which will evaporate abruptly in contact with the molten solder and cause the phenomenon of sputtering ("spraying" of the molten alloy).
Control of the solvent evaporation process is a complex phenomenon which is dependent on the nature of the solvent (chemical structure, hydrogen bonding), Lewis characteristics (acid / base), number of solvents in the mixture, chemical interactions with the other components of the mixture, vapour pressure, surface / volume ratio etc. Other complications are also induced by the presence of moisture.
The solvent also affects the tackiness and viscoelastic properties of the solder paste.
The most important factor to consider is the Lewis acid / base index which will define the interactions between the solvent and the other components of the solder paste.
The solvents often belong to the following families: butyl carbitol, dibutyl carbitol, glycols and polyhydroxy aliphatic alcohols.
Conventionally, fluxes contain gum rosin, wood rosin (derived from pine) or modified rosin, which often have a minimum acid value of 100-150, dissolved in a solvent. The acid values are generally determined using a simple KOH titration. Rosin derivatives are also used, such as dimerized resins, saponified resins or rosin ester derivatives.
The most suitable resins will have the following characteristics: relatively high softening point, non-crystalline nature, oxidation resistance, excellent solvent release properties, light colour, thermal stability, low odour...
Carboxylic acids (with alkyl and aryl groups) are widely used as activators in NO CLEAN fluxes and in water-based fluxes for wave soldering and SMT reflow soldering.
Examples of activators are adipic, succinic and glutaric acid. Malic acid is also used. Numerous other activators are mentioned in the literature and derivatives are developed in connection with the advances of Chemistry.
When a greater activation is needed (to obtain fluxes type RA or RMA), one finds among others the following elements:
- Halogenated organic products
- Ammonium halides
- Halopyridines ...
The rheological performance of a solder paste (its behaviour when subjected to stress) is probably the most important criterion in its implementation. Rheology affects the shelf-life (where the rheological stability must be several months), printing performance, slump test results (cold and hot).
The printing properties of a solder paste result from a complex process with many parameters that can influence the final results. In a simplistic manner, a solder paste exhibits non-Newtonian and thixotropic behaviour when subjected to shear stresses. The viscosity of a material can be defined as the ratio of the shear stress to the velocity gradient. For materials made up of complex organic molecules with functional groups that are capable of interacting with one another, it is very difficult to predict all the phenomena.
A solder paste is subjected to a wide range of shear rates during the steps of the printing process. These steps are as follows: mixing of the solder paste, rolling on the screen and printing. The mixing phase refers to the process of transferring the solder paste from its container to the surface of the screen. This process involves relatively low shear rates. During the printing process, the solder paste, in front of the squeegee tends to form a cylinder and "rolls" when it is moved. Due to the movement of the squeegee, the shear rate of the solder paste decreases and this is necessary for the formation of a satisfactory "cylinder". Finally, the solder paste is subjected to very high shearing when it is forced to pass through the openings of the screen in order to be deposited on the printed circuit. At this time, when passing through the apertures, the viscosity reaches a minimum and the shear rate is maximum. Once the squeegee is passed over the openings of the screen, the screen and the PCB are mechanically separated. The shear rate decreases instantaneously and the structure of the solder paste must compensate for this phenomenon to avoid slump. This is especially important with technology moving towards increasingly fine pitch on PCBs. The ability of the solder paste to resist slump is also important during reflow where bridges can occur.
All the studies carried out have shown that the properties of the solder paste can only be obtained by the use of "rheological" agents (thixotropic and thickening agents). The compounds used are non-exhaustively castor oil, cellulose derivatives, derivatives of starch, specific amines, etc.
Antioxidants such as benzotriazoles are corrosion inhibitors effective for various metals including copper and its alloys. In soft soldering, the flux function is to remove the passivation layers and then form a protective cover (function of the rosin after reflow in the SMT applications) to prevent oxidation during the assembly process. The ability to protect metal surfaces just prior to and during reflow is important, as it also provides better wetting. As a result, optimized amounts of activators are required and this enhances the "NO CLEAN" character of the solder paste. In the electronics industry, benzotriazole and its derivatives are widely used for the protection of copper surfaces from oxidation. In fact, of course, the antioxidants have been incorporated into the chemical part formulas of the solder pastes.
SOLDER PASTE MANUFACTURING
Although the idea of mixing a metal powder with a chemical part seems simple, a lot of parameters have to be taken into account. The manufacturing steps of the soldering paste are:
- Formulation and manufacture of the chemical part
- Production of the alloy balls, sorting of them to obtain the desired types / classes and storage of the alloy powder
- Mixing the chemical part and the alloy powder to obtain the solder paste and packaging.
The parameters involved in each of these steps can have a direct effect on the final quality of the solder paste. For example, it has been demonstrated that the thickness of the oxide film on the alloy balls, which depends on the production conditions (during atomization), interacts strongly with the mixing process. The sequence and the atmosphere under which the mixture takes place will then have to be adapted. The consequences can be a "crust" effect on the solder paste and a reduction in shelf life.
One of the important points is to limit the introduction of air into the solder paste when the chemical part is mixed with the alloy powder because this can accentuate the phenomenon of solder balling. This mechanism is explained by the reaction of the activators with the surface oxides, which produces metal salts (typically a halide or a carboxylate) during storage at room temperature. These salts react in turn with carbon dioxide and water trapped in the mixture to produce carbonates. This eventually creates deposits of carbonates which are in the form of precipitates. These precipitates form, in the most unfavourable case, a "crust" on the surface of the solder paste in the plastic jar. This "crust" can lead to the phenomenon of solder balling during reflow. Moreover, this results in an increase in the viscosity of the solder paste, a high viscosity which will prevent dispensing or printing. Another point to consider is the type of residue formed after reflow and its cleanability. Indeed, the volume of flux in a solder paste is important (up to 45%) and the tendency to go on the PCB with increasingly fine steps makes the cleaning operations very complicated. A possible solution is to use an activation system with two components, one that is specific to the metal powder and another that serves to flux the tracks of the PCB. Such an activation system can be optimized in terms of quantity and therefore be present in the chemical part in less quantity than a "mono-activator" system. This leads to lower residues and can limit or cancel the need for cleaning.
In conclusion, the formulation of a solder paste is very complex because it involves many parameters that have interactions between them. Moreover, the evolutions of the technologies of solder paste deposit are increasingly "stressful" and imply rheological characteristics of very high level to maintain correct performances especially printing.