Wave-solder flux

Certain no-clean fluxes are not compatible with each other. Their chemical interaction could result in corrosion on the PCA. To avoid corrosion issues, solder paste flux, wave solder flux, and repair flux should be tested separately and in combination for SIR and ECM with the board surface finish. 

Cleaning and Cleanliness. Improper handling procedures and improper selection and application of solder paste and wave-solder fluxes and their associated cleaning processes can cause ionic residues to be left on the board that result in low surface insulation resistance. Low SIR values can cause failures in and of themselves for some sensitive circuits and in other cases set up the conditions for further corrosion that eventually result in short circuits. Sodium and potassium ions and halide ions are the most commonly quoted culprits for these failures. The major source of sodium and potassium ions is handling, i.e., fingerprints. The primary sources of halide ions are soldering fluxes. 

Studies have shown that reliable lead-free solder joints, with proper grain structures and in-termetallics formation, can be produced using appropriate rework processes. Care must be taken to minimize any potential negative impact of the rework process on the reliability of the components and the PWB. Surface insulation resistance (SIR) tests must be performed to ensure the compatibility between the reflow/wave solder flux and the rework flux, i.e., to ensure that the rework flux and any products of reaction between the reflow/wave solder flux and the rework flux do not pose any unacceptable risk for electromigration and dendritic growth for noclean applications. 

Should wave-solder flux and no-clean solder paste be compatible ... 

This short duration test method allows the corrosivity of liquid flux residues after wave soldering to be assessed and employed as a screening test prior to employment of the more time-consuming procedures. 

The cable test piece consists of ten untwisted strands of btu e copper wire moimted on a glass fiber-filled epoxy substrate with holes to allow flux residues to pass from the underside of the board to contaminate the cables on the topside during wave soldering. A DC power supply poleuizes the test pieces anodically with respect to the copper conductor track on the laminate. 

The impact of Pb-free technology on wave soldering has largely occurred in the equipment performance. It has been determined that the same solder bath temperatures that are used for Sn-Pb processes (250 to 270°C) are suitable for the Sn-Ag-XCu Pb-free alloys. Therefore, excessive dross formation and flux residue removal have not become a significant problem during equipment operation. The lack of shiny fillets with the Sn-Ag-XCu alloys has been addressed by modified alloys having Ni and Ge additions that alter the solidification process, which leads to shinier fillet surfaces. 

In the 1970s, telecommunication companies in the United States and Europe predominantly used rosin fluxes for their wave-soldering requirements. These companies had their own internal set of test methods for seleding noncorrosive rosin flux formulations. Using their selection criteria, they beUeved the fluxes to be sufficiently safe. For their applications, they cleaned only the bottom side of the assembly, where rosin residues needed to be removed to ensure good electrical contact during bed-of-naik testing. 

FIGURE 46.5 ICT probe noncontact (a) If there is flux on a test pad, it will harden after soldering and may prevent the test probe from making electrical contact to the test pad. The test result may be interpreted as an open circuit, (b) As the board is fluxed in preparation for the wave solder process, the liquid flux may be drawn by capillary action between the wave solder pallet and the PWB. When this occurs, the in-circuit test pad (the target for bed-of-nails testing) may become fouled with solder flux. The flux residue inhibits probe contact. 

Use of N2for Increased Fluxing Effectiveness. The soldering environment is an important variable. Obviously the most common and least expensive soldering ambient is air, but many board assemblers introduce nitrogen to their reflow ovens, wave soldering... 

R. Iman et al. have demonstrated formic and acetic to be especially effective for fluxing in the wave-soldering process in conjunction with a tight application of adipic acid, a common food preservative and also a known fluxing agent. However, a water rinse was reqnired to remove the adipic residue. 

One attribute of this process is the speed at which solder joints are formed. Much quicker than oven reflow soldering, the wave-solder process allows little time for preheating, fluxing, and solder-joint formation, which also explains the variabiUty of this process. 

There are five basic subsystems of a wave soldering machine, including conveyors, fluxers, preheaters, solder pots with pumps and heaters, and ventilation. If alcohol-based fluxes are being used, it is wise to install a fire suppression system also. 

Flux quantity may also be of concern in the wave-soldering process for yet another reason fire hazard. Flux-laden boards are preheated going into the wave. If the flux application is too heavy, the flux may drip onto preheater elements. This may cause the flux to volatilize rapidly, combine with oxygen in the atmosphere, and provide the right conditions for flame initiation. Even if there is not direct exposure of the liquid flux to preheaters, if the quantity of volatile, flammable components is high enough to be an ignition source, then an explosive condition may develop. With the advent of more eco-friendly, water-based fluxes, fire hazard is less of a concern. 

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