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106 SMT Magazine • July 2017 inants left behind can potentially cause elec- tromigration of these soils, which can cause reliability problems later in the life of the as- sembly. Conversely, the high-viscosity gel flux- es, when dispensed or stencil printed, stay in the area where they were applied. One must be careful of the application of the gel flux as too much flux volume can make the lead or ball float off the pad during the soldering process. What does paste flux do? Like all fluxes, the paste fluxes used in re - work serve two basic functions. The first job of the flux is to make sure that the oxides are cleaned up from the area to be soldered. It should form an oxidation barrier which allows the BGA ball or paste to coalesce and form a homogenous solder joint. Its other function is to make sure that proper wetting is achieved so that the solder fillet can be formed properly to meet the acceptability requirements. How does paste flux get applied? There are a multiple paste flux application methods for the rework of BGAs and other complex components. In some cases, the sim - plest form of application of the tacky flux is by using a flux dip of the part into a flux bath. This bath is a reservoir slightly oversized com- pared to the package outline dimensions. The depth of the reservoir is approximately 60% of the ball diameter which allows more than half of the ball to be covered in paste flux when the device bottoms out in the fixture. If the reser - voir is kept full of fresh paste flux, the process works well. In another application method, paste flux is simply applied to the board area where the device rework location resides. This can be accomplished with a brush, gloved-fin - ger or even selectively applied via a miniature stencil. The stencil has the added benefit of not having excess flux residues on the PCB to clean off post reflow. Alternatively, dispensing paste flux on to the pads of the device area being re - worked has benefits similar to that of stencil printing. Finally , the part may be selectively printed with the paste flux by using the proper - ly-sized rework fixture. In this paste flux appli- cation method, the part is held in to an invert- ed fixture which holds and aligns the part. The bottom side of the component is then stencil printed with the paste flux such that the sol- der balls, component leads or pads are printed with paste flux, By using the right type of paste flux in both area array and leadless device rework, consis- tent, acceptable results can be achieved. SMT Bob Wettermann is the principal of BEST Inc., a contract rework and repair facility in Chicago. USING PASTE FLUX FOR REWORK Figure 1: Solder paste dipping of POP package into dipping well for paste flux application. Figure 2: Dispensing of paste flux onto BGA for rework.