Choice of solder paste can make or break an assembly process

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Solder paste is used for connecting the leads of integrated chip (IC) packages to attachment points (lands) in the circuit patterns on a printed circuit board (PCB). The paste is applied to the lands using a stencil to ‘print’ the paste, although other methods like dispensing are also used. Usually, solder paste is used with surface mount devices (SMD).

By Richa Chakravarty

Thursday, January 12, 2012: In printing solder paste, the substrate is placed on the work holder mechanically or by vacuum and aligned with tooling pins. To achieve good printing results, a combination of right paste material (of the right viscosity, metal content, largest powder size and lowest flux activity), right tools (printer, stencil and squeegee blade) and right process (clean sweep) is necessary. Typically, solder paste accounts for 0.05 per cent of a circuit board’s final cost.

Most defects in PCB assembly are caused due to issues in the solder paste printing process or due to defects in the solder paste. Therefore, an electronics manufacturer needs to be experienced in the printing process, specifically about the paste characteristics, to avoid costly rework on the assemblies.

A solder paste is a powdered metal solder suspended in a thick medium called flux. Flux is added to act as a temporary adhesive, holding the components until the soldering process melts the solder and makes a stronger physical connection. Thus, choosing the right solder paste for a particular application can help one achieve the highest process consistency and solder joint quality. The paste’s physical characteristics, like viscosity and flux levels, need to be monitored periodically by performing inhouse tests.

Choosing the right solder paste

Dr SK Khanna, owner, Khanna Traders & Engineers

The goals of manufacturing are to maximise quality and throughput while controlling costs. So choosing the right solder paste can achieve the highest process consistency and solder joint quality. The quality of manufacturing can be enhanced by choosing the paste that performs well with the materials, geometry and heating processes used for manufacturing a product. The throughput can be maximised by choosing a solder product that accommodates the optimal deposition and heating methods.

There are various types of solder products with differences in wetting characteristics, void control, flux residue, alloy strength, alloy flexibility and other performance measures that can play a significant role in achieving quality, throughput and cost goals. Therefore, to choose the right solder paste, one must look for the following characteristics.

The alloy requirements must be evaluated so that they can meet all product requirements. The flux types should be properly identified. In this process, fluxes with unacceptable criteria are removed from consideration. Next, issues like difficult-to-solder surfaces, rapid reflow conditions, cleaning options and solder joint voiding concerns should be considered before choosing a paste.

When you choose a solder alloy, you should look at its lead content, melting temperature, alloy powder particle size and tensile strength.

Types of solder pastes

Watanabe Daisuke, managing director, Nihon Superior (Singapore) Pte Ltd

Solder pastes can be classified into three types based on the type of flux:

Rosin (R) based pastes are made of rosin, a natural extract from pine trees. These fluxes need to be cleaned after the soldering process using chlorofluorocarbons (CFCs). However, due to ban on CFCs, rosin fluxes are no longer predominant.

Water soluble fluxes are made up of organic materials and glycol bases.

No-clean fluxes are made with resins and various levels of solid residues. No-clean pastes save not only cleaning costs, but also capital expenditure and floor space.

Earlier, pastes for electronic assembly applications were comparatively crude, but as technology has developed considerably over the past few years, innovations are taking place in the composition of solder pastes, particularly to meet the requirements of surface mount technology (SMT). Changes in solder pastes are taking place particularly to reduce the activity of the flux and the potential for harmful residues. This can improve print characteristics and reflow characteristics, and increase the metal content.

Advances in solder paste technology

Solder paste technology has evolved from being water soluble to the no-clean variant, and from leaded to lead free. This technology evolution is driven by miniaturisation, cost reduction, and legislation mandating the use of lead free and halogen free materials. The industry is also gaining more experience with lead free alloys with lower silver content, lower costs and better

Richard Puthota, director, business development, Cookson India Pvt Ltd

performance.

Most recently, the halogen free buzz has resulted in a big change in solder paste technology. Halogens are associated with activators in paste flux chemistry, and halogen-free activators are not as aggressive as their halogenated counterparts. The objective is to get the same or better wetting and coalescence characteristics but with halogen free activators.

With the ban on the use of CFC, the industry moved on to no-clean solder paste system that comprises minimum activators. The latest shift in the industry has been towards green technologies and green processes. This includes compliance with Restriction of Hazardous Substances (RoHS) and Waste from Electrical and Electronics Equipment (WEEE) directives, which require manufacturers to use lead-free soldering.

Shares Dr SK Khanna, owner, Khanna Traders & Engineers, “Today, the electronics industry is moving towards SMT and compact designs, and hence, towards lead free solder pastes. By using alloys with low melting points, energy will be saved during the manufacturing processes.”

What players offer

Khanna Traders & Engineers offers a wide variety of solder pastes, including the regular tin, lead, silver and lead free solder pastes (tin/silver/copper), low melting point ( Sn/Ag/Bi) available in T-3 and T-4 versions. “To meet the mass production demands in the electronics assembly, Khanna Traders & Engineers offers solder paste that has excellent stability during

Pankaj Saxena, director, Mectronics Marketing Services

continuous printing, has a high durability against pre-heat slumping and the tackiness of solder paste is obtained till 24 hours of printing. Our Cosmo series solder paste can be stored at 35°C for one month,” says SK Khanna.

Khanna Traders & Engineers also offers a low melting point solder paste Sn/Ag/Bi having melting point 140°C. This paste prevents the components and pcbs from thermal shock and is very useful for both side SMT process. Also, emerging alloys will add more cost benefits to the production process.

Keeping the latest green trend in mind, Nihon Superior offers its SN100C P500, which is a highly reliable no-clean lead free general-purpose solder paste. Due to its high fluidity close to its melting point and fast wetting, SN100C P500 can be used as a drop-in replacement for SAC solder paste in reflow profiles that peak around 240°C. It delivers excellent printability, long stencil life, good tack, excellent reflow and wetting on all substrates with no solder balls and minimal clear residue. It brings to reflow soldering the advantages that have made SN100C such a popular choice in wave soldering, that is, smooth shiny fillets with no shrinkage defects, high ductility and stable inter-metallic compound layer resulting in superior performance under vibration, and impact loading.

Says Watanabe Daisuke, managing director, Nihon Superior (Singapore) Pte Ltd, “Nihon Superior has always introduced next generation products that are completely halogen free. SN100C has grown to become one of the most popular lead free wave soldering alloys across the globe. The SN100C was initially developed to address the need for an economical wave solder; it has since been found that its properties also make SN100C an ideal choice for reflow, hand soldering as well as for component packaging. Nihon is doing continuous R&D to meet and support its

Nihon Superior’s SN 100C solder paste

customer requirements.” The company is among leading players delivering advanced soldering and brazing materials to the global market. It has been manufacturing the patented silver free, eutectic SN100C soldering alloy.

Cookson Electronics introduced its Alpha CVP-520 paste, which is a low temperature lead free solder paste (that is, <170°C peak reflow temperature). This paste offers three significant process advantages—elimination of a wave or selective soldering process step while preventing damage to temperature sensitive components and connectors, reduced energy consumption, and a significant reduction in reflow process cycle time. Informs Richard Puthota, director, business development, Cookson Electronics, “The carefully selected Sn/Bi/Ag alloy in Alpha CVP-520 well give the lowest melting point, lowest pasty range during melting and re-solidification, along with a very fine grain structure, offering maximum resistance to thermal cycle based fatigue. The alloy also yields very low voiding BGA solder joints, even when a traditional SAC alloy sphere is used.”

Cookson Electronics has also introduced a several low silver containing SAC alloys like SACX0307 Plus (0.3 per cent Ag), SACX0807 Plus (0.8 per cent Ag) in the Indian market as

Low melting lead free solder paste from Khanna Traders & Engineers

replacements for high silver containing SAC305 alloys, where the silver is at 3 per cent which directly impacts 30-35 per cent of the lead free solder paste cost. Cookson’s lower silver containing as well as low temperature lead free solder pastes have significantly reduced costs, added phenomenal value and saved thousands of dollars for OEMs as well as contract manufactures in India.

Indium Corporation manufactures low oxide spherical powder, composed of a variety of lead free alloys, which covers a broad range of melting temperatures. Indium 8.9 is an air or nitrogen reflow no-clean solder paste, specifically formulated to accommodate the higher processing temperatures required by the tin/silver/copper alloy (SAC), tin/silver and other alloy systems favoured by the electronics industry to replace conventional lead bearing solders. This paste offers unprecedented stencil print transfer efficiency while working in the broadest range of processes. In addition, the high probe testability of Indium 8.9 also minimises false failures in in-circuit tests (ICT).

Henkel Adhesive Technologies has developed Multicore LF620, a new lead free solder paste formulated with a new activator chemistry. It exhibits extremely low voiding, good coalescence and excellent solderability over a wide range of surface finishes including nickel/gold,

F 640 solder paste from Mectronics Marketing Services

immersion tin, immersion silver and OSP copper. In addition, the material’s versatility delivers outstanding printability at low or high speeds, with brick like print definition, yielding vast improvements in throughput while also ensuring consistency of print deposits. According to a report, Multicore LF620 is an idela choice for manufacturers as for multi-functional, complex populated boards and for smaller handheld devices that dictate finer pitches (0.4 mm and above), reduced board sizes and increased functionality, Multicore LF620 offers unmatched performance and processability.

Mectronics Marketing Services has introduced the F 640 solder paste series. This is a state of the art lead free no-clean solder paste that promotes wetting and minimises soldering defects. The F 640 flux system is specifically optimised for SAC alloy soldering. This formula not only provides superior performance on a variety of surfaces and leaves behind a clear residue, but also exhibits minimal slump and has excellent print-after-wait performance. Shares Pankaj Saxena, director, Mectronics Marketing Services, “Extensive testing at customer locations has proven this paste to be capable of defect free performance in the production environment.”

Alpha CPV 520 lead free solder paste from Cookson Electronics

Solder manufacturers today understand the demands of their customers and offer a gamut of solder pastes that can be availed at reasonable prices to help the industry manufacture highly efficient and durable electronic items. Only the best quality of soldering can help the Indian electronics industry make products that meet industrial standards.

Key players

Company Website
Bharat Products  
Ciba Speciality Chemicals  
Cookson Electronics www.alpha.cooksoneelectronics.com
Choice Trading Corporation Pvt Ltd www.choicegroup.in
Dow Corning Corporation www.dowcorning.in
Electrolube www.electrolube.com
Henkel Adhesive Technologies www.henkel.com
Jamex Smelting  
Khanna Traders & Engineers www.bondsolder.in
Kuraray Co Ltd www.kuraray.co.jp/en/
Mectronics Marketing Services www.mectronics.in
Nihon Superior (Singapore) Pte Ltd www.nihonsuperior.co
SCB Interconnection Solutions Pvt Ltd www.scbispl.com
Senju Metal Industry Co Ltd www.senju-m.co
Soni Alloy Industries  
Sumitron Exports Pvt Ltd www.sumitron.com
Usha Commercial Corporation  

Buying tips

Solder paste, which serves primarily as an attachment medium between the device interconnection features and the PCB, is critical to product yield in an SMT line. Poor solder paste printability results in a large number of defects. The components of a solder paste are designed to give it excellent printing and reflow characteristics. Says SK Khanna, “While choosing a solder paste, it is important for the buyer to ensure that the paste offers long hours of printability, has the right particle size for the components’ pitch and is of suitable flux type as per the assembly application.”

According to experts, there are several things to consider when choosing the right solder paste. These selection criteria include:

Size of solder alloy particles in solder paste: The appropriate particle size of the alloy particles in a solder paste for a given application is based on the minimum size of the aperture openings of the stencil to be used in printing the solder paste over the board or substrate. Excessively large particles can easily clog the stencil apertures, resulting in poor printing quality which requires frequent cleaning that slows down production. Hence, particle size gets more critical as the amount of solder to be deposited becomes smaller.

Properties of flux medium: The flux should exhibit excellent chemical activity for removing the thin oxide films and other contaminants from the surface of the metals being soldered. The flux must be easy to activate thermally, but should not decompose easily.

Design of stencil to be used: The stencil design also impacts the effectiveness of the solder paste. 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.

Paste printing parameters: The printing parameters must also be optimised with respect to the solder paste. Viscosity of the paste affects the speed at which printing can be done, where adequate fluidity is required to allow a good roll that fills up apertures properly. It should be noted that the paste also needs to exhibit enough stiffness to form a well-defined deposit when the stencil is separated from the board or substrate. Thus, check that the viscosity of the paste is neither too free flowing nor thick.

Electronics Bazaar, South Asia’s No.1 Electronics B2B magazine 

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