Solder controlled atmospheres

Several DTA instruments have been described by Barrall et al. (94, 95). A DTA calorimeter cell in which the AT-sensing thermocouples are attached to the sample container is shown in Figure 6.35. In this apparatus, copper-constantan thermocouples are soldered to a 4-mm-OD copper cup fitted with a copper lid. The thermocouples and sample cups are supported on ceramic insulator tubes which are attached to a metal base. All the cups were heated by thermal radiation received from the blackened copper radiation shield this prevented radiation hot spots due to furnace windings. The entire DTA cell was enclosed by a glass bell jar which provided a controlled atmosphere from reduced pressures to about 2 atm. 

The adhesion of specimens subjected to environmental exposure tests was evaluated prior to and subsequent to the contact. Wet thermal shock testing consisted of five cycles each for IS minutes in boiling water and 2 minutes in ice water. The maximum transfer time between the two baths was 30 seconds. Thermal and temperature/humidity exposures were performed in controlled atmosphere chambers for 200 hours. Simulated solder tests comprised immersing the test. specimen in a silicone oil followed by solder flotation. The solder temperature varied from 232 to 288 C and the contact time was S or 10 seconds. In some cases, two flotations were performed on the same sample. The effect of a heat treatment at 135 C prior to solder testing was examined the heat treatment time varied from 0 to 16 hours. ... 

Such a positrol log contains complex steps, each of which might well be kept in its own log. For example, the reflow soldering operation could have a number of what steps, such as belt speed furnace zone temperatures for preheat soldering zone cool down zone atmosphere chemistry control atmosphere flow speed visual inspection for missing, misaligned, or tombstoned parts solder bridges and solder opens. 

Hwang, X S., Controlled-Atmosphere Soldering Principle and Practice, Printed Circuit Assembly, July, 1990, pp. 30-38. 

Arslancan,A. N., IR Solder Reflow in Controlled Atmosphere of Air and tiitrogen, Proceedings of NEPCONWesf 90, Anaheim, 1990. 

Reflow. The reflow process attaches SMT and some TH components to the PCB by melting solder paste to form solder joints using an oven with a controlled thermal profile (see Fig. 57.17) and, in some cases, a controlled atmosphere (often N2). Reliability problems that can arise due to improper reflow parameters can be gronped into three categories damaged components, poor solder joints, and, for no-clean assemblies, cleanliness issnes. 

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 ... 

Nitrogen is preferred for controlled atmosphere soldering [46,47]. It is the least expensive gas to produce. Nevertheless, the use of a N2 atmosphere extols a significant cost penalty on the assembly process. Specially equipped furnaces are required to support a N2 reflow environment. Additional plant costs are incurred for providing the N2 supply as well as to transport N2 gas to the furnace location. Residual O2 levels less than 100 to 200 parts per million (ppm) and, preferably, in the range of 20 to 50 ppm, can significantly reduce Pb-Sn solder defects that occur during assembly. 

Of course, the controlled atmosphere that most effectively excludes O2 from the process is a vacuum. However, the use of vacuum furnaces for large-volume soldering assembly processes is... 

An alternative approach to reactive, controlled atmospheres for Pb-Sn solder processes has been to mix acid vapors with an inert carrier gas (N2) [48-50]. The acid vapors provide the fluxing action that reduces oxides on the base metal and molten solder surfaces. The inert gas also prevents reoxidation of those new, pristine surfaces. Gas mixtures of 5% formic acid or 3% acetic acid in Ar have been shown to provide adequate solderability of 37Pb 3Sn solder on Cu with little remaining residue. Although residues are considerably less than those that remain after the use of conventional PbASn solder pastes, cleanliness tests should be performed to assure that residue levels meet product acceptance criteria. 

Reactions of the hydroperoxides eatalytic decomposition were carried out in glass-soldered ampoules in argon atmosphere. To control the proceeding of hydroperoxides thermolysis and their deeomposition in the presence of Et NBr the iodometric titration with potentiometrie fixation of the equivalent point was used. 

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