Almost all electronic equipment consists of printed circuit boards (PCBs), which are necessary for its operation. PCBs have conductive copper tracks onto which the electronic components are soldered. The soldering/brazing between these tracks and the components themselves is usually achieved using soldering paste.
Soldering pastes are made by mixing a metal in the form of alloy powder (approximately 90% by weight) and a chemical component made up of organic elements (approximately 10% by weight). The chemical part is more commonly known as the « flux » and is generally covered by trade secrets or patents. The purpose of the flux is to give the soldering paste its consistency and to enable the parts to be soldered properly by eliminating the oxides at their interface. To use soldering paste in the best conditions, it is important to understand the concepts described below.
To make a good solder joint, the metal surfaces to be soldered must be « cleaned », as they undergo oxidation when exposed to the environment and can form compounds with oxygen, nitrogen, water and pollutants, such as sulphur, in the air. The degree of oxidation and the nature of the oxidising chemical species determine the affinity between the metal atoms and these chemical species.
For example, copper surfaces form 2 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 consequently poor welding.
In addition, welding alloys also undergo surface oxidation, forming compounds on the surface with oxygen, water and nitrogen that also interfere with the weld. Good soldering therefore requires the surfaces to be soldered to be « cleaned » using a particular chemistry. The flux will play this role and will be able to counteract the effects of passivation, for example.
In the case of electronic soldering, the flux must have the following properties :
- Remove passivation layers and make metal surfaces wettable by the welding alloy
- Protect the cleaned surfaces with a layer of chemical compound, usually rosin, which will act as an air barrier before the molten solder is applied
- Encourage wetting of the surfaces to be soldered by controlling the surface tensions that promote this wetting process
- Providing the right rheology for the soldering paste to ensure good solder paste printing of the paste and to enable components to adhere, among other things. Although the first function is the main one, good results for the PCB manufacturer can only be achieved if the flux also enables the other functions to be carried out satisfactorily
A soldering paste must have properties that enable it to meet the requirements described above and be suitable for the manufacturing process of boards with surface-mounted components. The characteristics to be taken into account are :
- chemical activation
- temperature or activation window
- thermal stability
- surface tension
- wetting power
- solder paste printing capability
- quantity of residues
Rosin is the base material for soldering pastes. It is a natural resin that has been known for many years and comes from the pine tree (Pinus Palustris in particular). It is a solid product at room temperature that is sometimes vitreous, sometimes lumpy, with colours ranging from very light yellow to brown. Rosins are a mixture of organic compounds, most of which are terpene derivatives and hydrocarbons. Although the exact composition depends on the source, the most important compound is abiatic acid or sylvic acid (C20H30O2). Pimaric acids are also present in these resins. Rosin is not soluble in water, but it is soluble in organic solvents such as alcohols, hydrocarbons, ethers, etc. A good quality rosin resin is made up of the following elements :
- Abietic acid 85%
- Pimaric acid 12%
- Others 3%
Rosin is widely used in fluxes because it offers a number of advantages :
- It attacks the passivation layer of several metals, particularly copper
- It does not attack bare copper, even after prolonged contact
- It can be dissolved in appropriate solvents and applied to the surface to be soldered. Once the solvent has evaporated (depending on the reflow oven), it forms a thin layer that protects the molten metal, thereby promoting wetting
- It is a good vehicle for « melting » compounds such as certain amines
- Rosin-based soldering pastes adhere well to printed circuit boards and their rheology allows them to be solder paste printed while limiting slumping and stencil life
Rosin-based fluxes can be divided into two main categories : activated and non-activated.
Activated fluxes can be further divided into 2 sub-classes : rosin moderately activated (RMA = Rosin Middly Activated) and rosin highly activated (RA = Rosin Activated).
Non-activated fluxes are used by manufacturers of aviation equipment, for example, because the residues can be left safely on soldered circuits without any risk of corrosive attack. Activated fluxes are similar to non-activated fluxes, except that they contain an additional activating agent (Activator) which is much more reactive with the metal passivation layer than rosin. The degree of flux activation depends on the nature and quantity of the activators. Typically, the types of activators used include bromine-based compounds and carboxylic acids.
A soldering paste is therefore a mixture containing :
- A welding alloy in powder form
- Resins (e.g. rosins)
- Thickeners and « rheological » additives
During the first reflow stage, the solvent evaporates and the activators « attack » the metal surfaces, cleaning them. Then, in the second stage, the solder alloy powder particles melt to form a liquid mass, which forms the solder joint.
To obtain a high-performance soldering paste, the alloy powder (in the form of alloy balls) must meet the following criteria :
- Homogeneous chemical composition
- Controlled level of impurities
- Size and shape of alloy beads conforme
- Controlled distribution of alloy bead sizes
- Correct surface chemistry (oxide-free beads)
These properties have important effects on the performance of the soldering paste. For example, when users want to work with printed circuit boards with fine pitches, alloy powders with finer bead sizes are required to ensure good solder paste printing (type 4 or 5 required). Smaller sizes can be achieved with direct consequences on the price of the soldering paste (type 6 see type 7). Type 8 is used in very confidential applications and on products with very high added value.
To sum up, the size, shape and distribution of the beads have a major impact on the rheology of the soldering paste, and can therefore influence the way the paste is solder paste printed and dispensed.
General formulation of soldering paste fluxes :
Generally, a chemical part of soldering paste contains the following elements :
- « Rheological » agents
Solvents are much more than a « binder » that keeps the components of the flux in solution without them precipitating. They are of vital importance and determine the following parameters in particular :
- Soldering paste is easy to apply, whether for dispensing or solder paste printing
- The « drying » of the paste during reflow and the formation of a protective film on the circuits
- Preventing the formation of micro-beads and maintaining wettability when depositing molten filler metal on the substrate to be welded
If the solvent dries too quickly, the rosin protective film hardens and does not follow the application of the molten metal, resulting in the risk of micro-cracks in the film formed by the residues.
If it dries too slowly, the protective film will still contain solvent, which will evaporate abruptly on contact with the molten solder, causing the phenomenon of microbeads (« spraying » of the molten alloy).
Controlling the solvent evaporation process is a complex phenomenon which depends on the nature of the solvent (chemical structure, hydrogen bonding), the Lewis characteristics (acid/base), the 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 tack and viscoelastic properties of the soldering paste.
The most important factor to consider is the Lewis acid / base number, which will define the interactions between the solvent and the other components of the soldering paste.
Solvents often belong to the following families: butylcarbitol, dibutyl carbitol, glycols and aliphatic polyhydroxy alcohols and alcohols with aryl groups.
Typically, fluxes contain gum rosin, pinewood rosin or modified rosin, which often have a minimum acid number of 100-150, dissolved in a solvent. Acid numbers are generally determined using a simple KOH titration. Rosin derivatives are also used, such as dimerised resins, saponified resins or rosin base ester derivatives (also known as « gum ester »).
The most suitable resins will have the following characteristics :
- Rather 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 soldering applications.
Examples of activators are adipic, succinic and glutaric acid. Malic acid is also used. Many other activators are mentioned in the literature and derivatives are developed in line with advances in chemistry.
When greater activation is required (to obtain RA or RMA type fluxes), the following elements are used, among others :
- Halogenated organic products
- Ammonium halides
« Rheological » agents
The rheological performance of a soldering paste (i.e. its behaviour when subjected to stress) is undoubtedly the most important criterion when it comes to processing. Rheology affects shelf life (where rheological stability must be several months), solder paste printing performance and slump test results.
The solder paste printing properties of a soldering paste are the result of a complex process involving numerous parameters that can influence the final results. Simplistically, soldering paste exhibits non-Newtonian and thixotropic behaviour when subjected to shear stress. 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 each other, it is very difficult to predict all the phenomena.
A soldering paste is subjected to a wide range of shear rates during the phases of the solder paste printing process. These phases are as follows :
Mixing of the soldering paste, paste « rolling » across the solder paste printing screen and printing of the tracks (solder paste printing itself). The mixing phase refers to the process of transferring the paste from its container to the surface of the screen. This process involves relatively low shear rates.
During the solder paste printing process, the paste in front of the squeegee tends to form a cylinder and « rolls » as it is moved back and forth by the squeegee. As the squeegee moves, the shear rate of the paste decreases and this is necessary to form a satisfactory « roll ».
Finally, the soldering paste is subjected to very high shear when it is forced through the openings in the « stencil » (the screen) and onto the printed circuit board. At this point, as it passes through the openings, the viscosity reaches a minimum and the shear rate is at a maximum.
Once the squeegee has passed over the screen openings, the « stencil » and the PCB are mechanically separated. The shear rate decreases instantaneously and the structure of the paste must compensate for this phenomenon to prevent the covered tracks from collapsing.
This is particularly important with technology that is moving towards ever finer pitches on PCBs. So the slightest sag will result in a « link » between the tracks. The ability of the paste to resist sagging is also important for reflow, where bridges can occur when hot.
Studies have shown that the properties of soldering paste can only be obtained through the use of « rheological » agents (thixotropic and thickening agents). The compounds used include, but are not limited to, castor oil, cellulose derivatives, starch derivatives, specific amines, etc.
Anti-oxidants such as benzotriazoles are effective corrosion inhibitors for a variety of metals including copper and copper alloys.
In soldering, the function of the flux is to remove passivation layers and then form a protective blanket (the function of rosin after reflow in SMT soldering) to prevent oxidation during the joining process.
The ability to protect metal surfaces just before and during reflow is an important consideration, as it also enables better wetting. As a result, optimised quantities of activators are required, reinforcing the ‘no clean’ character of the soldering paste.
In the electronics industry, benzotriazole and its derivatives are widely used to protect copper surfaces against oxidation. In fact, anti-oxidants have naturally been incorporated into the chemical part formulas of soldering pastes.
MAKING SOLDERING PASTE
Although the idea of mixing a metal powder with a chemical component seems simple, there are a large number of parameters to take into account. The steps involved in making soldering paste are as follows :
- Formulation and manufacture of the chemical part
- Production of alloy beads, sorting of beads to obtain the required types / classes and storage of alloy powder
- Mixing the chemical part and the alloy powder to obtain the soldering paste and packaging it
The parameters involved in each of these stages can have a direct effect on the final quality of the soldering paste. For example, it has been shown that the thickness of the oxide film on the alloy beads, which depends on their production conditions (during atomisation), strongly interacts with the mixing process. The sequence and atmosphere in which the mixing takes place must therefore be adapted. The consequences can be a « crusting » effect on the soldering paste and a reduction in shelf life.
One of the important points is to limit the introduction of air into the soldering paste when mixing the chemical part with the alloy powder, as this can accentuate the microbead phenomenon. This mechanism is explained, during storage at room temperature, by the reaction of the activators with the surface oxides, which produces metal salts (typically a halide or a carboxylate). These salts in turn react with the carbon dioxide and water trapped in the mixture to produce carbonates. This eventually creates carbonate deposits in the form of precipitates. In the worst case, these precipitates form a « crust » on the surface of the soldering paste in the pot. This « crust » can lead to the formation of microbeads during reflow. In addition, this leads to an increase in the viscosity of the paste, a high viscosity that will prevent dispensing or solder paste printing.
Another point to consider is the type of residue formed after reflow and its suitability for cleaning. The volume of flux in a soldering paste is high (up to 45%) and the tendency to go onto PCBs with increasingly fine pitches makes cleaning operations very complicated. One possible solution is to use an activation system with two components, one that is specific to the metal powder and another that is used to flux the PCB tracks. Such an activation system can be optimised in terms of quantity and therefore be present in the chemical part in smaller quantities than a « mono-activator » system. This leads to lower residues and can limit or even eliminate the need for cleaning.
In conclusion, the formulation of a soldering paste is highly complex, involving a large number of interacting parameters. What’s more, developments in paste application technologies are putting more and more « stress » on the paste, requiring very high-level rheological characteristics to maintain correct performance, particularly in solder paste printing.