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 without any risk If the solvent dries too quickly, the colour, excellent heat stability, post-reflow of corrosive attack. Activated fluxes are protective rosin film hardens and is not colour retention, resistance to oxidation similar to non-activated fluxes except that displaced correctly by the solder[1,2]. If it and low odour. These characteristics mean they contain an additional activating agent dries too slowly, the protective film will that post-reflow residue is exceptionally (activator) that is much more reactive contain some solvent, which evaporates light in colour. towards the passivation layer of metals abruptly in contact with the molten solder than rosin. The degree of flux activation and causes sputtering. The evaporation Activators depends on the nature and quantity of the process itself is a complex phenomenon Alkyl and aryl carboxylic acids are widely activator. Classically, types of activators and is affected by the nature of the solvent used as activators in ‘no-clean’ and water used include bromine salts of amino (chemical structure, hydrogen bonding washable electronic fluxes for wave compounds and carboxylic acids[1,2]. Solder paste is a mixture containing (a)solder in powder form, (b) resins (e.g. Table 1. Acid values of rosins rosins), (c) activator(s), (d) solvent, (e) Sample Acid Number (KOH mg equivalent) thickening agents and rhelogy aids, and (f) Tall oil 60 antioxidants. Upon heating, the solvent Portuguese 162 evaporates and the activators attack the Unirez 7104 251 metal surfaces, resulting in cleaning. On Dymerex 147 further heating, the solder powder particles melt and form a liquid mass, which makes Rosin H 154 the joint. American WW 158 www.globalsmt.net Global SMT & Packaging - December 2007 15 Fundamentals of Solder Paste Technology soldering and SMT reflow soldering. shear rate increase, followed by the paste soldering the function of the flux is to Examples include adipic, succinic and ‘yielding’ and then undergoing shear remove the passivating layers and then glutaric acids in no-clean and water thinning, i.e. decrease in viscosity with form a protecting blanket (e.g. the rosin washable fluxes[1,2]. Malic acid is used as increasing shear rate. melt in SMT reflow) to prevent further a component in activation systems of The above behaviour is desirable and oxidation during the joining process. no-clean and water washable fluxes[7]. necessary for satisfactory printing and anti- The ability to protect the metal surfaces Numerous activators have been reported slump prosperities. A paste is subjected to just prior to and during solder reflow is in the open literature and also stated in a wide range of shear rates during various important as it allows for more effective current and lapsed patents of several US phases of the printing process. These wetting. This in turns allows the solder and Japanese paste manufactures. Other phases are classified as mixing, rolling and paste formulator to use lower amounts types of activators that are know to be used screening. of activators thereby enhancing the in RMA, RSA and water washable fluxes The mixing phase refers to the process no-clean feature of the solder paste. In the include the following: of transferring the paste from its container electronics industry, it has been common • Organic halogen acid adducts to the screen surface. This process practice to use thin films of benzotriazole • Halogenated organics involves relatively low shear rates. During for the protection of copper surface from • Ammonium halides the printing process, the paste in front of oxidation. Paste manufacturers have • Halopyridines the squeegee tends to form a cylindrical extended these concepts to solder paste shape and appears to ‘roll’ as it is pushed formulations by incorporating anti- Rheology aids back and forth by the squeegee. As the oxidants. The rheological performance of a squeegee moves, the paste shear thins paste (i.e. its flow behaviour when and this is necessary for the formation Some considerations for subjected to a stress) is arguably the of a satisfactory ‘roll.’ Finally, the paste paste manufacture most important determinant of the is subjected to considerable shear as it Although, the idea of mixing metal powder quality of a solder paste. Rheology affects is forced through the stencil apertures in a thick emollient appears simple, a shelf-life (where rheological stability of and onto metal pads on the PCB. At large number of variables are involved in around several months is required), paste the instant, the paste is forced through paste manufacture[1,3]. The steps in paste printing performance, cold slump and the apertures, the viscosity reaches a manufacture are: hot slump[10,11,17]. Solder paste printing minimum and the shear rate is at a • Formulating the flux vehicle is a complex process with numerous maximum. At this instant the paste has • Producing, sizing and storing solder parameters that can affect the final lost all of its inherent internal structure. powder outcome. Furthermore, typical of a dense Once the squeegee has passed over suspension soft solid, the rheological the apertures, the stencil and PCB are behaviour of solder pastes is highly separated mechanically. The shear rate • Flux vehicle complex. In simplistic terms, a solder paste drops instantaneously and the paste must • Basic flux vehicle (e. g. rosin and exhibits non-Newtonian and thixotropic very rapidly re-build its internal structure solvent). behaviour when subjected to a shearing to avoid code slump. This is particularly • Polymer additives stress. The viscosity of a material can be important with current levels of fine pitch • Activation level defined as the ratio of shear force to shear assembly where the small gap between • Type(s) of activators(s) rate. For Newtonian liquids, this ratio pads maybe bridged if the paste deposits • Thickening agents has a constant value and plot of shear slump and touch. The ability of the paste • Secondary solvent force versus shear rate is a straight line to resist slump is also important in reflow • Solder powder through the origin. Materials made up of where hot slump can also lead to bridging. • Oxide layer thickness complex organic molecules with a range Here, the inherent ‘yield’ of the solder • Particle size distribution of organic functional groups are capable paste resulting from its internal molecular • Particle shape and uniformity of intermolecular interactions that lead and solder particle structure is important • Source alloy compositional to an inherent steady state structure in in resisting hot slump. uniformity the material. This phenomenon can also The chemicals present in a typical • Source alloy compositional control occur as a result of interactions between flux system have the ability to attain • Particle alloy uniformity particles, such as solder spheres. For such a structure at steady state that results • Mixing process materials, the thixotropic nature can be form molecular and particle interactions. • Insufficient material properties shown by a shear rate sweep experiment However, the above discussed rheological specified as product controls (for example, in a parallel plate geometry) properties of the paste can only be fully • Process procedure sequence in which the material is subjected to a attained through the use of rheological aids • Mixing rates, times and shear rate cycle of 0 to 100 per seconds (thixotropes and thickeners). Thixotropes temperatures and then 100 to 0 per second (called and thickening agents such as castor oil • Post-formulation property a rehogram). The thixotropy is then derivatives, cellulose derivatives, starch, adjustment procedures (e. g. equal to the area between the ascending ethoxylated amines have been reported. viscosity control, % metal control) and descending shear rate curves. It is • Absence of mixing atmosphere indicative of the degree of molecular and Anti-oxidants control (e. g. humidity) solder particle structure in the paste. Other It has been reported that anti-oxidants notable features of the rheogram are the such as benzotriazoles are effective Figure 1. Variables involved in paste initial increase in viscosity, as the as the oxidation inhibitors for various metals manufacture shear stress increases without significant including copper and its alloy[13,14]. In soft

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Fundamentals of Solder Paste Technology

during reflow. Additionally, it could lead to Nostrand Reinhold, New York, 1993, 1. Voids a catastrophic increase in the viscosity of ISBN 0442017529. 2. Solder balls the product, making dispensing or printing 3. G. R. Blackwell, The Electronic 3. Residues and cleanability very difficult or even impossible. Packaging Handbook, CRC Press, USA, 4. Corrosion Another issue is the type of the residue 2000, ISBN0849385911. 5. Screening quality-slump formed after reflow and its cleanability. 4. L.J Calbo, Handbook of Coatings 6. Screening quality-print detail The volume of flux in a paste is large Additives, Marcel Dekker Inc., USA, 7. Screening quality-reproducibility (up to 45%) and combined with the 1987, ISBN0824775619. 8. Stencil life trend towards finer pitch assemblies 5. Y. Okamoto, Applied Spectroscopy, 9. Shelf life-crusting makes cleanability or ‘no clean’ a serous Vol.33, No.3, 1979. 10. Shelf life-separation issue. A reported solution is to use a two 6. M. Nasta and H.C. Peebles, Circuit 11. Component hold before and component activation system, one part of World, Vol. 21, No.4, 1995, 10. during reflow which is specific to the powder and the 7. J. E. Sohn and U. Ray, Circuit World, 12. Reflow quality-detail other selected to flux the substrates and Vol.21, No.4, 1995, 22 13. Reflow quality-spatter leads. It has been reported that the two 8. Military Specification: Flux, Soldering, 14. Reflow quality-joints component activation system could be Liquid (Rosin Base), MIL-F-14256E present at a lower level than the broad- 9. R.P. Anjard, Powder Technology, No. Figure 2. Most commonly action single activator systems. This would 36, 1983, 189. encountered defects encountered lead to lower residues and may obviate the 10. J. Kevera, Int. J. Hubrid when using solder pastes need for cleaning. Microelectronics, Vol. 12, No.4, 1988. 11. J. Kevera and D. Mohoric, Printed Testing Circuit Assembly, Vol. 2, No.9, 1988. • Blending flux and powder to form the The quality assurance tests for solder pastes 12. M. Warwick, B.S. Chowdhary and N. solder paste and packaging. are well known and are documented in Stanton, Soldering and Surface Mount The variables involved in each of these several standards. All BizEsp products, for Technology, No. 20, 1995. steps can have a direct effect on product example, are tested according to industry 13. C. A. Mackay, Circuit World, Vol.13, quality. Additionally, there are interactions standard test methods. No. 3, 1987. between the variables that also have 14. S. Yoshida and H. Ishida, J. Mat. Sci., important effects on product quality. Recommendations for 19, 1984, 2323 and refs. therein. For example, it has been show that the formulation of solder pastes 15. L.J. Matinezo and R.R. Schaffer, J. thickness of the oxide film on powder A survey of patents and papers suggests Mat. Sci., 26, 1991, 787 spheres, which is critically dependent that similar or identical compounds 16. JP05228690A, 1993; GB2265156, on the conditions and method of solder are cited in most of the publications. 1993; US 5211764A, 1992; JP atomisation, interacts strongly with Furthermore, the quantity ranges cited for 04071797 A2; US5150832A; variables in blending such as blending formulation components are similar. The JP06262386 A2; JP06087090 A2: sequence and atmosphere. The result is a types of materials and ranges are shown in JP62187595 A2; JP 8921801; JP marked effect on the rate of ‘crusting’ and Table 2. These materials are blended and 01154897 A2; GB 86-28916. the length of storage life-time. used to make solder paste by using a ‘one- All the defects in Figure 2 can be stock’ or ‘two-stock’ approach. The flux Biographies traced to the three steps involved in solder product is then mixed with solder powder Dosten Baluch, BSc. Chemistry, Ph.D. paste manufacture, including blending and (88-91% by weight). Suppliers specify the Materials Science and MBA was educated packaging. temperature ranges and shear rates for the in UK and has 10 years ‘hands-on’ A consideration is air introduced incorporation of the various rheology aids. experience of solder powder production, during the mixing process that can solder flux and paste formulation and testing encourage solder balling. The mechanism References gained in USA and UK with international for this involves the reaction at room 1. G. Leonida, Handbook of Printed Circuit companies. He was involved with lead-free temperature during storage of activator and Design, Manufacture, Components & research at inception and has worked with surface oxide that produces a metal salt Assembly, Electrochemical Publications clients such as Motorola, AC Delco and (typically a halide or a carboxylate). This Limited, Scotland, 1981, ISBN AVX in USA and Celestica Europe. in turn reacts with carbon dioxide and 0901150096. Continued on page 26 water trapped in the mix to yield metal 2. F. G. Yost et al., The Mechanics of carbonate. Further reaction with carbon Solder Alloy Wetting and Spreading, Van dioxide and water produces insoluble basic carbonate. Further reaction with carbon dioxide and water produces insoluble basic carbonate deposits. These deposits Material % Weight both precipitate in the body of the solder Solvent 32-40 pate drying storage and also coat metal Rosin 49-52 particles. When this effect occurs to a Activators 0-4 significant degree, it causes a ‘crust’ to form Rheology 8-15 on the surface of the paste in a jar. At the same time, crusting throughout the body of Hydroxyamines 0-2 the solder paste encourages solder balling Anti-oxidants 0-2

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