Solder vapor phase heating

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 plateability and hydrolytic stability advantages of polysulfone/ABS blends have been utilized to make selected appliance, plumbing, and sterilizable equipment parts, replacing polycarbonate. Polysulfone/ABS blend has also been evaluated for plateable components of electrical and electronic parts, circuit boards and connectors. However, the lack of adequate heat resistance (DTUL lower than polysulfone) for vapor phase solderability limited its use only in conjunction with other high heat resins as laminates. 

Uses Dielec, fluids (transformers, switchgear) heat-transfer fluid for vapor-phase soldering, electronics cooling systems leak testing fluid Manuf./Distrib. ABCR http //www.abcr.de,... 

Toxicology LD50 (oral, rat) >10 g/kg, (IV, mouse) 12 g/kg low toxicity by IV route irritating to eyes, skin, respiratory system may cause wt. loss TSCA listed Hazardous Decomp. Prods. Heated to decomp., emits very toxic fumes of F" and NOx Storage Hygroscopic Uses Heat-transfer fluid for vapor-phase soldering... 

Synonyms PFPE Formula [CF2CFOCF3]x Properties Colorless liq. lowm.w. dens. 1.82-1.84 kg/l (20 C) b.p. 95-240 C Uses Heat-transfer fluid for vapor phase soldering, electronic cooling systems leak testing fluid nonflamm. aircraft/aerospace lubricant... 

These unique combinations of transparency, hydrolytic, and heat resistance of polysulfones would be lost if other polymers are blended. Hence, there has been very little commercial incentive for blends of polysulfone. Nevertheless, in the early days of polysulfone commercialization, some blends with ABS and PET were developed primarily to lower the cost but also to improve selected properties such as plateability or chemical resistance. A PSU/ABS blend (Mindel A) was commercially offered for some time but is now no longer available. Additives such as phenoxy resins, styrene-maleic anhydride copolymers have been claimed to improve the compatibility and weld-line strength of the PSU/ABS blends (Robeson 1985). The blend was evaluated for selected appliance, plumbing, and sterilizable equipment plated parts. This blend lacked the heat resistance needed for the vapor phase solderability in electronic applications. 

Once SMDs have been placed in solder paste, the assembly will be reflow soldered. Thk can be done in either batch-type ovens, or conveyorized continuous-process ovens. The choice depends primarily on the board throughput/hour required. Whereas many early ovens were of the vapor phase type, most ovens today use IR heating, convection heating, or a combination of the two. The ovens are zoned to provide a thermal profile necessary for successful SMD soldering. An example of an oven thermal profile k shown in Fig. 11.26. 

The mass flow or reflow methods are suited for high-volume manufacturing. The entire board is heated and large numbers of components on the board are soldered simultaneously. The two most common of these methods are oven reflow soldering and wave soldering. A third technique, vapor phase reflow soldering, has dwindled in popularity due to environmental concerns regarding the use of the chlorofluorocarbon-based solvents that were key to this process. Now, however, perfluorocarbons are substituted and the technique is still in use. 

Although vapor phase reflow has the appearance of being a fast process, solder paste reflow recommendations must still be followed. If the paste is heated too rapidly, paste volatiles may boil, resulting in explosive solder ball formation. As in other soldering methods. 

The soft vapor phase (SVP) process maximizes control of heat rise. The temperature gradients can be adapted to the product by control of the immersion depth of the parts for soldering. IBL-Lottechnik GmbH holds patent rights to the SVP process. 

However, vapor-phase soldering is not suitable for class IB MID with concave structures. If the condensate is unable to drain off, heat transfer is obstructed and irregularities in temperature distribution occur. Vapor-phase soldering accounts for only about 10% of ordinary electronics production, but it is the standard technique for MID. 

As is the case with elaboration of the production process, prior to elaboration of the ACT process chain the compatibility between the detailed product and the assembly and connection technologies selected in the production concept is rechecked. For example, large, solid components act as heat sinks in the vapor phase and impair solderability. The product data are analyzed and compared with the solution concept for the production process. It is important to clarify the extent to which the choice of material and the design of the interconnect device and the components to be used are compatible with the selected assembly and connection technologies. The following are product properties that particularly require consideration in this context ... 

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. 

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