Electronics solder paste

Table 4.4 lists commercial cleaning solutions currently used in the electronics industry for cleaning printed-circuit boards, assemblies, ceramic printed circuits, thick-film interconnect substrates, electronic components, wiring harnesses, and stencils and screens (used in applying adhesives and solder pastes). 

Surface mounting. Surface mounting of components onto PWBs is commonly referred to in the industry as SMT or surface-mount technology. It consists of the attachment of electronic components by automated dispensing or screen printing of solder paste and adhesive onto a PWB, placement of the components, then curing the adhesive and reflowing the... 

Surface Mount Applications. Tin-lead solders (Sn-Pb) are the standard materials used to interconnect electronic components on PCBs. The most common reflow soldering process is SMT which uses tin-lead solder pastes (58-61). The pressure to reduce the industrial use of lead is growing, particularly in Europe, since it poses as a hazard to human health (62). 

Although commonly used solder paste for both THT and SMT production contains 63-37 eutectic tin-lead solder, other metal formulations are available, including 96-4 tin-silver (silver solder). The fluxes available are also similar, with typical choices being made between RMA, water-soluble, and no-clean fluxes. The correct decision rests as much on the choice of flux as it does on the proper metal mixture. A solder paste supplier can best advise on solder pastes for specific needs. Many studies are in process to determine a no-lead replacement for lead-based solder in commercial electronic assemblies. The design should investigate the current status of these studies as well as the status of no-lead legislation as part of the decision making process. 

Hollomon, J.K., Jr. 1995. Surface Mount Technology for PC Board Design. Prompt, Indianapolis, IN. Holmes, J.G. 1993. Surface mount solution for power devices. Surface Mount Technology 7(9) 18-20. Hwang, J.S. 1989. Solder Paste in Electronics Packaging. Van Nostrand Reinhold, New York. 

Isopropanol is specified to clean electronic devices such as contact pins in connectors as well as the connectors, magnetic tape and disk heads, lenses of lasers in optical disc drives, and LCD/LED and glass computer monitor screens. Isopropanol is also used for removal of thermal (solder) paste from integrated circuit packages, and restoration of vinyl records. 

Component Placement. The purpose of the component placement machine—also called the pick-and-place machine—is to select the proper component, orient it correctly, and then place it on the circuit board, all with degrees of accuracy and precision that minimize defects on the finished product. In addition, the component must be placed on the printed solder paste, the dispensed adhesive, or a combination of the two deposits with a controlled pressure or release distance that does not excessively spread out either material or damages the component package. Moreover, the placement machine must execute these tasks as quickly as possible in order to maximize the production volume. Lastly, the equipment must be sufficiently versatile to address continually changing electronic packages, specifically dimensions and I/O configurations. 

It is beyond the scope of this chapter to provide a detailed explanation of SPC. Rather, there will be a qualitative discussion of process control that addresses those factors affecting circuit board assembly, beginning with defect types in electronic solder joints. Then the discussion will turn to process control as it pertains to the basic assembly steps (e.g., dispensing, pick-and-place, etc.). The various material sets (e.g., circuit board, solder paste, flux, etc.) will be incorporated in the discussion. Also, the use of Pb-free soldering technology will be addressed where applicable. 

There are a few other materials that need to be covered for this discussion. The first is solder paste. This is a mixture of minute solder beads, flux, and other materials to give it specific rheological characteristics for dispensing and chemical agents for metal surface preparation. For surface-mount applications, it is typically stenciled onto PWB bonding pads, and then the electronic component is placed upon the solder paste deposit. The paste holds the component in place during the reflow process.The second is flux, which, as mentioned previously, is a key component of solder paste. The flux is a heat-activated chemical agent used to clean solder-able surfaces. Both paste and flux will be covered in subsequent sections of this chapter. 

J-STD-005, Requirements for Soldering Paste." This standard prescribes general requirements for the characterization and testing of the solderpastes used to make high-quality electronic interconnections. 

J-STD-005 General Requirements and Test Methods for Electronic Grade Solder Paste... 

Jackson, G.J., Durairaj, R. Ekere, N.N. (2002). Characterisation of Lead-Free Solder Pastes for Low-Cost Flip-Chip Bumping. Proceedings of the 27 Annual lEEE/CPMJ/SEMI/ International Electronics Manufacturing Technology (lEMT) Symposium, San Jose, CA, USA, July 2002, pp. 223-228. 

Study of the Rheological Behaviours of Sn-Ag-Cu Solder Pastes and their Correlation with Printing Performance. Proceedings of the 11 Electronics Packaging Technology Conference, Singapore, December 2009, pp. 869-874. 

Nguty, T.A. Ekere, N.N. (2000). Modeling the Effects of Temperature on the Rheology of Solder Pastes and Flux System. Journal of Materials Science Materials in Electronics, Vol. 11, (July 1999), p>p. 39-43. 

Evaluation of Sn-Zn Solder Paste, Proceedings of IPC/JEDEC 2004 International Conference on Lead-Free Electronic Assemblies and Components, March 2004... 

M. Arra, D. Shangguan, E. Ristolainen, and T. Lepisto, Solder Balling of Lead-Eree Solder Paste, J. Electron. Mater, Vol 31 (No. 11), 2002, p 1130-1138... 

The conventional method using screen printed solder pastes or isotropic conductive adhesives is drflicult to apply when the distance between adjacent connections is less than 0.5 mm. Thus, the main impetus for developing anisotropic conductive adhesives has been the fine pitch interconnection of electronic devices... 

A decade ago, PCAs comprised little more than a through-hole PCB with wave-soldered components. Today, modest electronics products can involve an array of surface mount, chip-scale, through-hole, and electromechanical technologies. Screen-printed solder paste and reflow/wave ovens no longer are sufficient to complete the build alone. This is where benchtop dispensing comes in. 

In the electronics industry, the list of commonly dispensed fluids includes adhesives (anaerobic, cyanoacrylates, epoxy, and UV) conformal coatings flux (liquid and paste) RTV silicone (room-temperature vulcanization — curing without heat) solder mask, and solder paste. Given the vast differences in the properties of these fluids, in particular their viscosities and typical dispensed volume, dispensing... 

The processes generally employed to fix electronic components on three-dimensional circuit carriers are reflow soldering and electrically conductive-adhesive gluing. The first step in reflow soldering is to print solder paste on to the pads of the circuit trace (Fig. 4.1). The electronic components are subsequently placed in these solder depots. The module, complete with its electronic components, is then heated in a reflow oven, the solder particles of the paste melt, and the molten solder flows over... 

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