Chemicals and materials for electronics production

Electronics Technical ArticlesSouth-East European INDUSTRIAL Мarket - issue 2/2021 • 20.07.2021

According to a recently published report by ResearchAndMarkets.com the electronic chemicals and materials market size is projected to grow from USD 58,8 billion in 2020 to USD 81,7 billion by 2025, at a compound annual growth rate (CAGR) of 6,8% during the forecast period. The major factor that is driving the market is the growth of the microelectronics industry, characterized by the emergence of new technologies. The silicon wafers segment is projected to register the highest CAGR during the forecast period. During the forecast period the semiconductor application segment is expected to hold a larger market share.

The fastest-growing electronic chemicals and materials market during the forecast period is estimated to be APAC due to the rapid growth of the global electronics industry, which has driven the demand for printed circuit boards in the region, ResearchAndMarkets.com´s report states. The printed circuit board industry´s growth is directly associated with the development in telecommunications and IT, smart cards, electronic gaming, and consumer goods applications.

 

Solder paste

The external interconnection features (such as leads, bumps, or balls) of surface mount devices (SMDs) are usually soldered onto a printed circuit board (PCB) through a board mounting process that consists of three basic steps – application of solder paste on specific locations on the PCB, or solder paste printing; positioning of the components on the board; and solder reflow.

Solder paste, which is used primarily as the attachment medium between the device interconnection features and the PCB itself, is a specially blended paste that consists of a flux medium containing highly-graded solder alloy powder particles. The components of a solder paste are designed to give it excellent printing and reflow characteristics. Reflow refers to the process of exposing the solder paste to elevated temperatures to melt the solder particles and allow the liquid solder to form a good and reliable connection with the board-mounted devices.

In addition to serving as the source of solder material that forms the mechanical and electrical connection between the SMDs and the board, the solder paste also keeps the components in place on the board prior to the reflow process, cleans the solder landing sites on the PCB as well as the external interconnections of the components and prevents these PCB solder lands and device interconnections from oxidizing until the soldering process is finished.

Experts believe that there are several factors to consider when choosing the right solder paste – the size of the solder alloy particles in the solder paste, the properties of the flux medium, the design of the stencil to be used, the paste printing parameters to be used, the tendency to form voids and other defects and, in sensitive devices, alpha particle emission rate.

The minimum size of the aperture openings of the stencil to be used in printing the solder paste over the board or substrate limit the particle size of the suitable solder paste. Large particles can easily clog the stencil apertures, resulting in poor printing quality and requiring frequent cleaning that slows down production. Particle size becomes more critical as the amount of solder to be deposited becomes smaller. According to specialists, the particle size of the solder paste should be no more than 12% the size of the smallest aperture opening of the stencil, i.e., at least 8 particles should be able to pass through the narrowest aperture gap at the same time.
The flux of the solder paste must have rheological properties that allow high-yield printing at very fine pitches. Naturally, the flux must also exhibit excellent chemical activity for removing the thin oxide films and other contaminants from the surfaces of the metals being soldered. The flux must be easy to activate thermally, but should not decompose easily. It must also form benign residues that are quickly removed by washing.

The stencil´s aperture size-to-spacing ratio affects the printability of the solder paste. The shape of the aperture can also affect the size of the deposited solder for the same pitch. The stencil must be thin but rigid enough to resist deformation.
With respect to the solder paste, the printing parameters must also be optimized. For instance, paste viscosity affects the speed at which printing can be done. Adequate fluidity is required to allow a good roll that fills up apertures properly. At the same time however, the paste also needs to exhibit enough stiffness to form a well-defined deposit when the stencil is separated from the board or substrate.
Pastes with the tendency to form excessive voids must be avoided. Voids must not be more than 5% of the solder in case total elimination is impossible. Lastly, experts warn about the possibility of the solder paste emitting alpha particles that can cause soft errors in memory devices, so this must be looked into if the process involves high-density memory devices.

Several types of evaluation tests are performed for solder pastes selection – solder balling test; wetting test; solder void potential test; shelf-life test; tack life test; stencil life and abandon time tests; slump tests. In-process evaluations must also look at the printability of the paste (relax/recovery properties, print speed, print durability), its component placement characteristics, and the quality of its solder joint/fillet formation. Solder joint reliability tests used for qualifying solder pastes include the temperature cycle test, the thermal shock test, the impact resistance test, the pressure cooker test, and the temperature-humidity-bias test.

 

Fluxes

Fluxes can be classified on the basis of three key characteristics. These attributes also govern whether boards need to be cleaned after soldering. However, the level of acceptability is not necessarily universal and depends on the requirements of the product. These three attributes are activity, solids content and material type.
Low-solids/no-clean fluxes typically have 2% to 8% solids content and can either be solvent based (with or without rosin/resin), or water based (VOC-free) containing no rosin or resin apart from rare exceptions. They exhibit low to medium activity, a short life (in-process), and may or may not require cleaning.
Rosin fluxes are full/high-solids rosins with 15% to 45% solids content, and are solvent based. Their activity may be low but is normally medium to high, they have a long in-process life, and they are typically always cleaned.
Water soluble fluxes generally have high solids content in the range of 11% to 35%, are usually solvent based, have a very long in-process life, are always highly active and always cleaned.

 

Cleaning agents

Within electronics production cleaning is meant to remove harmful contaminants such as flux, solder and adhesive residues, and other more general contaminants such as dust and debris present from other manufacturing processes and handling. It also impacts positively product lifetime by ensuring good surface resistance and by preventing current leakage leading to PCB failure.

Cleaning is required prior to stencilling and soldering in order to remove contaminants from the many previous production stages, after stencilling to remove excess adhesive, and after soldering to remove corrosive flux residues and any excess solder paste. Today, many manufacturers are applying “no-clean” processes, where the solids content of the flux is lower than traditional types, however they still contain rosin and activator which are not removed prior to the next process, such as coating or encapsulating of the PCB. Such residues, along with any other unwanted elements collected due to the missing cleaning stage, could cause issues with adhesion and possibly affect the performance of the protecting media applied.

There are two main categories of cleaner currently available, solvent based and water based. Traditionally, solvent based cleaners such as 1,1,1-trichloroethane and 1,1,3-trichlorotrifluoroethane dominated the market; however, due to their ozone depleting potential, they have been replaced by a more diverse range of solvent cleaners. This category is now typically divided into three sub-sections; flammable solvent cleaners, non-flammable solvent cleaners and non-flammable halogenated solvent cleaners such as HFCs and HFEs. All three types have their advantages and disadvantages but overall solvent cleaners can be described as fast evaporating, single stage cleaners. However, they require specialist equipment and extraction to protect against toxicity and other possible hazards.

Water-based cleaners were also developed to replace ozone depleting chemicals as well as offering a solution to reduce solvent emissions. Water-based cleaning has several advantages over solvent based cleaners including non-flammable properties, low odour, low/non-VOC and very low toxicity. There are many applications for cleaning, all of which depend on the type of equipment available. Whether it be ultrasonic, spray under immersion or dishwasher type application, identifying the correct water-based cleaner for the specific job is essential. Water-based cleaners tend to be much more complex than their solvent based counterparts. They utilise surfactant technology to assist the removal of contaminants from a PCB by reducing the interfacial tensions and suspending or emulsifying them in solution. Alternatively, water-based flux removers work by saponification, neutralising the flux acids. The only major disadvantage of water-based cleaners is that they require multiple stages to complete the cleaning process, including a two-stage rinse process and a final drying stage. There are also surfactant-free water based cleaners based on glycols. These cleaners combine the advantages of water based and solvent based cleaners with only minimal rinsing required.

 

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