SMT & Inspection | April 29, 2010

Flux Type Selection

The article is a continuation of paper ‘3 Steps to Successful Solder Paste Selection’. This part presents issues connected with choosing the right flux.
Both specifications and industry usage have contributed to current naming conventions for fluxes. Industry convention includes five basic categories of fluxes to choose from: R, RMA, RA, NC, and WS. The following summary is a short explanation of how specifications affected the categories.

There are four categories of flux to choose from as defined by Military Specification QQ-S-571E. The names are Rosin (R), Rosin or Resin Mildly Activated (RMA), and Rosin or Resin Activated (RA), and non-rosin or non-resin (AC). In most cases, the AC name has been replaced in industry with WS for Water Soluble. Each category of flux is available with a variety of activity levels, with the limits of each defined by applicable test results.

IPC also has a flux rating system. The system uses four characters to describe any flux as published in J-STD-004. These four characters describe the product in terms of material type, including but not limited to such categories as rosin and inorganic, along with an approximation of activity level and halide content. Example: A halide-free, rosin based solder paste with low activity is categorized as ROL0. RO stands for rosin, the L is for low activity and the number 0 is for no detectable halides.

In addition, the IPC standard added a category of flux called No Clean. This new category of flux is defined by non-tacky residue and compliance with a Surface Insulation Resistance (SIR) test to a value of 1x108Ω.

Each of the five flux categories can be described in brief by noting their activity, the physical qualities of their residue, and the cleaning methods that may be used in their removal.

Rosin (R) flux consists of rosin and solvent. Rosin flux has very low activity and is suited only to clean and easy-to-solder surfaces. IPC classification is usually ROL0. R residue is hard, non-corrosive, non-conductive and may be left on many types of assemblies. Residue may be removed with an appropriate solvent.

No-Clean (NC) flux consists of rosin, solvent, and a small amount of activator. NC flux typically has low-to-moderate activity and is suited to easily solderable surfaces. IPC classification is usually ROL0 or ROL1. NC residue is clear, hard, non-corrosive, non-conductive, and is designed to be left on many types of assemblies. Residue may be removed with an appropriate solvent. Some, but not all, NC fluxes are more difficult to remove than RMA fluxes.

Rosin mildly activated (RMA) flux consists of rosin, solvent, and a small amount of activator. Most RMA flux is fairly low in activity and best suited to easily solderable surfaces. IPC classification is usually ROL0, ROL1, ROM0, or ROM1. RMA flux residue is clear and soft. Most are non-corrosive and nonconductive. Cleaning requirements will be based on the activity of the flux and the product it is used on. Many RMA fluxes pass Surface Insulation Resistance (SIR) testing as a NC flux. Residue may be removed with an appropriate solvent.

Rosin activated (RA) flux consists of rosin, solvent, and aggressive activators. RA flux has similar and higher activity than RMA for moderately and highly oxidized surfaces. IPC classification is usually ROM0, ROM1, ROH0, or ROH1. RA flux residue is considered corrosive. Assemblies sensitive to corrosion or the possibility of electrical conduction through the residue should be cleaned as soon as possible after assembly. Residue may be removed with an appropriate solvent.

Water soluble (WS) flux consists of organic acids, thixotrope, and solvent. WS flux comes in a wide range of activity levels, from no activity to extremely high activity for soldering to even the most difficult surfaces, such as stainless steel. Because WS flux covers all activity ranges, the product specification must be referenced for corrosion and electrical conductivity hazards. IPC classification normally starts with OR for organic. They come in L, M, H activity levels and halide content of 0 or 1. By definition, residue may be removed with water.

The Flux Comparison Chart (Figure 1) is a graphical representation of the activity range each category of flux is typically available in and the activity relative to each other. As can be seen, there is quite a bit of overlap.

Figure 1: Flux Comparison Chart

The Solderability Matrix (Table 3) shows compatibility of the various flux types with common metals. For brass, bronze and stainless steel, there is enough variety in alloy composition that each alloy must be checked against the flux type you wish to use.

Table 1: Solderability Matrix

Author: John Vivari z EFD, Inc.
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