Solder heating methods

F. Heating Methods Used for Assembly with Pb-Sn Solder... 

The second process used in the assembly of printed wiring boards with Pb-Sn eutectic or neareutectic solder is referred to as vapor phase or condensation soldering. This method uses the heat of vaporization released when a vapor condenses into its liquid phase, to heat the assembly and reflow the Pb n solder. Vapor phase reflow is a batch process [5]. The Pb-Sn solder is printed as a paste on the circuit board the components are placed on the solder paste deposits then, the assembly is placed into the equipment. A preheat step is performed on the assembly, typically in a separate chamber. Entry of the assembly into the vapor results in relatively fast heating rates for the solder joints, approximately 7° to 10°C/sec. Similarly, the joints experience fast cooling rates upon withdrawal from the fluid. The working fluid vapor is not a very efficient barrier against O2. 

The heat transfer mechanisms utilized for the mass reflow soldering of electronic assemblies include convection, radiation (infrared sources), and condensation (vapor phase). The most popular solder reflow method is based on forced convection. 

Examination of the Solder from the Tin.—The solder to be examined is that found in the inner part of the tin. It is either cut from the tin and then melted or heated by means of a bunsen flame until it drops into an unglazed porcelain crucible, in which it is fused to a homogeneous button. In this the lead is determined by one of the following method ... 

This method is the simplest of all the methods and is capable of detecting surface flaws such as corrosion, contamination, surface finish and surface discontinuities on joints.47 The discontinuities on joints such as welds, seals, solder connections and adhesive bonds can be detected. General corrosion, qualitative pitting corrosion, stress-corrosion cracking, weld-heat-affected zone attack, erosion corrosion and other type of degradation can be observed by visual examination aided by microscopes with sufficient magnification. Degradation of plastics can also be detected by visual examination. Visual examination is also used in conjunction with other techniques, such as powerful microscopes. 

The backmetal schemes investigated primarily remove heat via the conduction method. Both copper and silver possess excellent thermal conductivity properties, which are beneficial for heat dissipation. Device solderability to a package, which also strongly influences the heat dissipation, was not effected by the Ta/Cu metal scheme. The three key parametric areas examined, which included packaging, Vsd and Rdson all showed that the Ta/Cu metal scheme is a viable alternative without altering the current packaging process. 

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