Electronics solder paste materials

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). 

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. 

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. 

Suitable connection mediums matched to the process and to the substrate material are essential for the dependability of mechanical and electrical connection. Solder alloys are the most widely used in standard electronics production. Viable alternatives for MID technology along with solder pastes are conductive adhesives for low thermal loading during the connection process, for example. Nonconductive adhesives can also be a good choice in certain circumstances. 

Bosch is involved with lead-free activities including solder paste alternatives. The likely candidates are alloys within the Sn Ag Cu or Sn-Ag Bi solder systems. Some automotive electronics products at Bosch are aheady lead-free. According to material suppliers, Bosch introduced conductive adhesives in its hybrid devices to a significant extent. 

Extensive activities are underway in both industry and academia to identify and implement alternatives to Pb for the various aspects of Sn-Pb solder electronic assemblies. In the area of Pb-free solder pastes, a ternary Sn-Ag-Cu alloy has been generally identified as the most promising candidate and so for the solder paste portion of electronics assemblies, the selection of a Pb-free material appears to have a clear direction. 

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. 

There are ongoing changes in the makenp of the snppUer base of electronic materials, including solder mask. Many acquisitions have taken place and several products eliminated in the recent past, so users must have confidence that they will have a continued source of supply of solder mask that is the same lot to lot for the future. International Organization for Standardization (ISO) qualification for the manufacturing operation as well as for environmental considerations are good indicators of a snppUer with a long-term view. 

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