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Fundamentals of Solder Paste Technology Fundamentalsfundamentals Ofof Soldersolder Pastepaste Technologytechnology Fundamentals of Solder Paste Technology FundamentalsFundamentals ofof soldersolder pastepaste technologytechnology Solder pastes are key materials Background water and nitrogen that also interfere with in surface mount technology Almost all electronics equipment contains soldering. (SMT) for assembly of printed circuit boards (PCBs), which are So, effective soldering requires that metal printed circuit boards (PCBs). critically important for satisfactory functioning surfaces are clean of passivating surface chemical Introduction of lead-free has placed new demands of the equipment. PCBs have patterns of layers through the use of a technique called on materials and processes electrically conducting copper tracks to which fluxing, which involves the application of a in SMT, requiring materials electronic components are joined. The joints chemical cream to the metal surface. The flux and process engineers to between the tracks and the components are must have the right chemistry to destroy surface adopt to lead free whilst made by soldering the components to the copper passivation layers. Fluxes for soldering electronic ensuring process yields stay tracks, generally by using a material known as PCBs require several features: at the highest possible levels. Key is the solder paste, a solder paste. 1. To remove the passivation layers and make material of great complexity Solder pastes are made by mixing a metal the metal surfaces active and wettable by the involving engineering sciences, alloy powder (around 90% by weight) with a solder alloy. metallurgy, chemistry cream-like material made of organic chemicals 2. To protect cleaned surfaces with a layer of and physics. This article (around 10% by weight). The organic chemicals some substance, usually rosin, which prevents helps those working with cream is referred to as ‘flux’ and is generally contact with air prior to application of the solder pastes improve their understanding of this key a trade secret and/or covered by patents. The molten solder. material. purpose of flux is to give the solder paste its 3. To promote wetting of the surfaces to be cream-like texture and to enable formation of joined by controlling the surface forces that metal joints by ensuring that the metal surfaces determine the wetting process. Dosten Baluch & Gerard Minogue, are ‘clean’ of oxides at the time the metal joint 4. To provide the right rheology for excellent BizEsp Limited, Abingdon, are formed. paste printability, tack and slump Oxfordshire, UK To achieve satisfactory process yields, it characteristics. is important to understand the design and Although the first function is the main Keywords: Printed Circuit engineering of the solder paste in order to select one, good results can not be obtained for Board, Surface Mount the right materials and use correct process the PCB manufacturer if the flux does not Technology, Lead-Free conditions. perform the other functions satisfactorily. To be Solder Paste, Soldering Metal surfaces need ‘cleaning’ because commercially viable, a flux must have all the they undergo oxidation on exposure to the properties that allow it to be used in common environment and form compounds with oxygen, PCB manufacturing operations, such as SMT. nitrogen, water and pollutants, such as oxides of The properties that control requirements (1) to sulphur, in the air. The degree of oxidation and (4) above include chemical activity, activation the nature of the chemical bond between the temperature or window, thermal stability, metal surface and the oxidising chemical species surface tension, wetting power, rheology, print determines the affinity between the metal atoms performance (for SMT), toxicity and nature/ and the oxidising chemical species. For example, amount of residues. copper surfaces form copper(I) and copper(II) Rosin is the base material for many flux oxides. The copper oxides interact with carbon systems. It is a natural resin that has been know dioxide and moisture in air to form carbonates or for many years. Basically, it is colophony (named hydroxyl carbonates. Iron behaves like copper, after Colophon, an Ancient City in Asia whereas nickel produces a continuous thin film Minor or the Greek pitch that was used as an of oxide. Silver reacts with traces of hydrogen incendiary weapon by the Ancient Greeks). It is sulphide to form silver sulphide. Once a layer of a natural organic compound extruded by trees of these kinds of compounds forms on the surface of the genus Pinus and in particular Pinus Palustris. the metal, it causes passivity of the metal surface. It is a solid product that is sometimes vitreous The metal surface then becomes solder-repellent, and sometimes crumbly, with colour ranging which leads to poor wettability and poor from light yellow to brown. Rosins are a complex soldering of joints on the PCB. Furthermore, mixture of organic compounds, most of which solder alloys also undergo surface oxidation, are terpeneoids and hydrocarbons. Although the forming compounds on the surface with oxygen, exact composition depends on the source, the 14 Global SMT & Packaging - December 2007 www.globalsmt.net Fundamentals of Solder Paste Technology most important compound is abietic – or The important properties of solder and Lewis acid/base characteristics), sylvic – acid (C20H30O2). Also present in powder, from a solder paste performance the number of solvents in the mixture, rosins are d- and l-pimaric acids. objective, include the following: chemical interactions with other Rosin is not soluble in water, but • Consistency of chemical composition. components in the mixture temperature, it is soluble in organic solvents such • Level of impurities. vapour pressure, surface/volume ratio, and as alcohols, hydrocarbons, ethers and • Particle size and shape. convection. Further complications arise halocarbons. A typical high grade water • Particle size distribution. from the presence and pick-up of moisture. white rosin is comprised of the following: • Surface chemistry. The solvent also affects tack and the These properties have a number of viscoelastic properties of the solder paste. Abietic Acid 85% important effects on paste performance[9, The most important factor to consider Pimaric Acid 12% 10,11,13]. For example, as users move to here is Lewis acid: base type interactions Others 2% finer pitch applications, smaller particle between the solvent and other paste size powders are required for acceptable components that lead to molecular or Rosin is widely used in fluxes because it print performance. This includes Type 5 solder particle structuring in the paste. offers many advantages: to Type 7 solder powders made by BizEsp. Solvents include butyl carbitol, dibutyl • It attacks the passivation layer of The surface area of the powder, of course, carbitol, glycols and polyhydroxy aliphatic several metals, especially that of copper- increases with decreasing particle, and and aryl alcohols. producing copper abietate special manufacturing techniques are • It does not attack bulk copper, even required to achieve acceptable quality Rosins after prolonged contact when manufacturing Type 5 -7 powders. Classically, fluxes contain gum rosin, • It can be dissolved in suitable solvents Particle size, shape and distribution have wood rosin or tall oil rosin, having a and applied to surface to be joined. important implications in paste rheology minimum acid value of 130, dissolved in After solvent evaporation, for example and affect print and syringe dispensing a solvent. The approximate acid values as in SMT reflow, it forms a thin performance[9,10]. of typical rosins are shown in Table 1. coating that protects the fluxed surface These values can usually be determined and promotes wetting by the molten General flux formulations using a simple KOH titration. Derivatives solder. Typical flux systems have been described in of rosins are also used, such as dimerized • It forms a good vehicle for more active several studies. resin, saponified resin or rosin-derived fluxing compounds, such as amine ester resin (also know as ‘ester gums’). hydrobromides. Solvents Dymerex is a modified resin and is sold by • Rosin solder pastes generally have good The solvents is much more than just a manufactures as a particularly suitable resin tack and rheological performance with vehicle for holding the flux components for soldering flux due to its outstanding respect to printability, slump and shelf- in solution without precipitation. It is of characteristics: high softening point, non- life, relative to non-rosin pastes. fundamental importance and determines crystalline nature, resistance to oxidation Rosin fluxes can be divided into two the following: and excellent solvent release properties. main classes: activated and non-activated. (i) ease of application, such as syringe Dymerex has also been recommended in Activated fluxes can be further divided dispensing and printing; several texts, for example Yost et al. Foral into rosin mildly activated (RMA) and (ii) drying and formation of a protective AX-E is an acidic resin that is produced rosin strongly activated (RSA). Non- film; by hydrogenating rosin to an exceptionally activated fluxes (MIL-F-14256) are used (iii) absence of sputtering and wettability high degree. It is recommended by a major by manufactures of avionics equipment promotion when the boards contact supplier (Hercules) as a replacement for because the residues can be safely left the molten solder. tall oil rosin due to its exceptionally pale on the soldered boards
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