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74 SMT Magazine • May 2016 Our last chapter began a discussion of selec- tive soldering, including how it works, some pros and cons, and several fluxing and solder- ing options to choose from. In this chapter, we'll dive a little deeper into the mechanics of fluxing systems. Flux Materials As covered in prior chapters on wave solder- ing, there are a variety of flux types and chem- istries available, including low pH, high-solids content, and water soluble, alcohol-based, and others. Selection of a particular type of flux is frequently not an option for the contract man- ufacturer, since it is usually determined by the end-user's application, so they must be able to accommodate work using many types of flux. No-clean fluxes are generally preferred be- cause they require little to no post-solder clean- ing, except for a visible residue that should be removed. If the user opts not to use a no-clean flux, it is very important to control the amount of flux applied to the board. In most cases, con- trolling the solder head to cover the area previ- ously sprayed will burn off the flux and elimi- nate the necessity of cleaning the residue in a subsequent step. Remember that solder types used for selec- tive must be compatible with solder used on the top of the board, and this will likely have a ma- terial effect on the flux type used. Spray Fluxing Through-hole penetration is the ultimate goal of any fluxing system, but there are a num- ber of factors that affect the performance of a spray fluxing system. Factors to consider for spray flux deposition include: 1. Compatibility of materials of construction: Make sure the application technique is compatible with your flux chemistry. 2. Motion and speed of the spray head: The faster or slower the spray head moves, the leaner or heavier the deposition will be. 3. Flux flow rate delivered to the spray head: Will affect the quantity of flux deposited on the board. 4. Pitch pattern between spray strokes: Determines whether consistent coverage will be achieved over a wide area. 5. Spray pattern: Check for controls that will help minimize overspray and waste while targeting the critical areas to be covered. In a spray fluxing system, flux can either be delivered by ultrasonic energy or by pumping from a canister in a housing below the machine by Robert voigt DDM NoVastar Selecting a Selective Soldering System, Part 2 SMt Quick-tiPS Figure 1: Drop-jet fluxer nozzle (right) and standard nozzle (left).