Soldering process reflow

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). 

The lead-free solder adopted by mannfactnrer Texas Instrnments is a nickel-palladium-gold (NiPdAu) formulation which is claimed to be backward compatible with existing reflow soldering processes. It is also claimed to be free of the whisker-artifact problems which have been experienced when nsing snch alternatives as matted tin. A Texas Instrnments spokesman stated that the move to pnre tin would necessitate the increase of the reflow solder temperature to obtain the same reliable solder contacts. He claimed that Texas Instruments had shipped more than 30 million lead-free units which confirmed the suitability of its NiPdAu solution. 

The need to solder components calls for the material to have high thermal stability since the temperatnre of solder baths is around 270 °C and the reflow soldering process involves brief exposnre to high temperatures. 

The use of nitrogen in the reflow soldering process also raises many questions. The first is whether or not nitrogen is required. This is not an easy question to answer. As with any process design, all process details must be considered ... 

Printability of lead-free solder paste will not change, but its spread during reflow will, which may require tightening of the stendl-printing process. One possible issue is print accuracy, or the alignment of the printed solder paste onto the PCB pad. Because lead-free alloys do not spread or wet as well as tin/lead, any solder paste that is not accurately printed onto the PCB will stay close to where it was printed after the reflow soldering process. Figures 3 and 4 depict the same deposits before and after reflow for QFPs and passives. 

The proportion of water in the polymer matrix can be as high as several percent, depending on ambient temperature and type of plastic. This has an effect on the electrical properties of the plastic and also on mechanical properties such as the modulus of elasticity E. Consequently, the interconnect device has to be dried to prevent evaporation of this entrained moisture in the reflow soldering process. This precaution helps ensure uniform quality of the modules. Moisture absorption in use, moreover, causes volume expansion of the substrate. This in turn can create mechanical stresses in the metallization and at contact points to electronic components mounted on the device. 

There is some debate associated with the toxicity of elemental Sb, but a considerable toxicity issue associated with its salts, oxides (particularly antimony trioxide), and organometallic compounds. However, these products are typically not formed during standard reflow soldering processes. The standard, IPC-J-STD006, allows solder alloys to contain up to 0.6% Sb to avoid leaching Ag or Cu into ground water [78]. 

A limited environmental life cycle impact study of lead-free wave and reflow soldering processes... 

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. 

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