Inert atmospheres soldering

Martin Theriault Martin Theriault serves as Global Marketing Manager, Electronics Manufacturing with Air Liquide, Houston, Texas, U.S. A. Mr. Theriault currently oversees all of Air Liquide activities in electronics packaging, assembly, optoelectronics, and compound semiconductor markets. Mr. Theriault has worked extensively in the past 7 years with electronic assemblers in the field of inert atmosphere soldering and has written and presented several papers on that topic. Additionally, he is strongly involved in the development of new gas applications and products for the electronic assembly... 

The physical plant costs are generally similar for both a Pb-free assembly process and a Sn-Pb line. The two possible differences would be electricity usage, which may be greater with the Pb-free line due to the higher soldering temperatures, particularly with respect to a reflow process, and the need for an inert atmosphere capability for the soldering machine. 

When electronic products are constructed, individual components such as chips, condensers, or conductor boards must be connected to fully functional electronic devices. This crucial step is usually performed using reflow or wave soldering processes. Both processes can be performed either in air or in controlled and inert atmospheres (see Fig. 2.24). Using a controlled atmosphere, however, provides the following substantial benefits ... 

Furnace Soldering (FS) uniform heating takes place in an inert atmosphere or vacuum. 

Oxidation problems can result upon exposure to several soldering processes, particularly at elevated lead-free soldering temperature conditions. Utilizing an inert atmosphere and reformulating OSPs to withstand higher process temperatures will help [53]. 

The size of the particles in a solder paste determines the print characteristics, amongst other things. The particles are produced by a variety of methods, but they are most commonly made by dispersion of a stream of molten solder onto a rotating disk. The particles fall into a tank filled with an inert atmosphere where they solidify and then are collected at the bottom of the chamber. The collected particles are separated by size utilizing in a series of wire-mesh sieves. The mesh size is typically given in wires or holes per square inch. The Joint Industry Standard, J-STD-005, provides for solder paste particle size classification as listed in Table 1. The choice of solder-powder particle size for SMT applications is based on component pitch, part mix and pad arrangement. For example, the paste particle size required for an area array device is smaller compared to a peripheral-leaded device with the same pitch. A 0.5-mm pitch area array device may have 0.25-mm diameter pads which require a 0.25-mm to 0.3-mm stencil aperture to print a Type rv solder paste with an approximately at 60-80% transfer efficiency. Comparatively, a 0.5— mm pitch quad flat pack device would typically have an 0.2 mm to 0.2.5mm wide pad, but require a 0.15 to 0.2mm wide stencil aperture to print a Type III paste with approximately a 80 90% efficiency. 

Reichelt, G. Investigations about the Reliability of Atmospheric and Inert Wave-Soldered 1812-Capacitors at Various Footprint—Configurations, Proceedings of Nepcon West Anaheim, CA, 1994. 

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