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Our last chapter began a discussion of selective soldering, including how it works, some pros and cons, and several fluxing and soldering options to choose from. In this chapter, we’ll dive a little deeper into the mechanics of fluxing systems.
As covered in prior chapters on wave soldering, there are a variety of flux types and chemistries 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 manufacturer, 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 because they require little to no post-solder cleaning, 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, controlling the solder head to cover the area previously sprayed will burn off the flux and eliminate the necessity of cleaning the residue in a subsequent step.
Remember that solder types used for selective must be compatible with solder used on the top of the board, and this will likely have a material effect on the flux type used.
Through-hole penetration is the ultimate goal of any fluxing system, but there are a number of factors that affect the performance of a spray fluxing system.
Editor's Note: This article originally appeared in the May 2016 issue of SMT Magazine.