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In the previous column, I discussed flux functions and general considerations in their selection. In my next three columns, I will review various types of fluxes.
Fluxes are classified based on their activity and constituents (which determine activity). Flux activity, in turn, is an indication of its effectiveness in removing surface contaminants. Fluxes are generally classified as inorganic acid, organic acid (OA), rosin and resin (no-clean). J-STD-004 classifies fluxes by the letters A through Y.
The general classification of flux as rosin, resin, organic and inorganic is further subdivided. For example, the full description of flux A will be rosin or RO-H0 to indicate it is a halide-free rosin flux. Other halide-free fluxes in this category will be RO-M0 and RO-H0, but they are considered relatively more active than RO-L0. The halide content alone is not an indication of the activity level because other constituents may be substituted for halides. The J-STD classification describes both flux activity and flux residue activity as follows:
- L = Low or no flux/flux residue activity
- M = Moderate flux/flux residue activity
- H = High flux/flux residue activity.
In each category, there are three flux activity or corrosiveness levels: low, medium and high. In each of these subcategories, there are further classifications indicated by the numbers 0 and 1. Zero indicates halide-free flux, while 1 indicates less than 0.5 percent halide in the low-activity flux category, 0.5 to 2 percent halide in the moderate-activity flux category, and more than 2 percent halide in the high-activity flux category.
There are various tests, such as copper mirror, halide content, corrosion, wetting balance and spread, that these fluxes must pass to be classified in a particular category (refer to J-STD-004 for details2). The fluxes to be classified as L and M must pass more tests than fluxes with H prefixes. Similarly, fluxes with 0 suffixes in the L, M or H category must pass more tests than fluxes with 1 suffixes in the same category.
Generally, fluxes can be classified as highly corrosive (inorganic acid fluxes), corrosive (OA), mildly corrosive (rosin based) and noncorrosive (no-clean or low-residue fluxes). However, there are different levels of corrosiveness in any flux category.
The highly corrosive inorganic acid fluxes of any category are seldom used in the electronics industry, while mildly corrosive fluxes are generally used only in commercial electronics. Rosin and resin fluxes in the mildly corrosive category have activity comparable to that of OA fluxes and are designed for solvent cleaning, whereas OA fluxes are meant for aqueous cleaning. Resin fluxes in the low-residue and no-clean category will have very low activity, however.
Rosin fluxes are also referred to as rosin (R), rosin mildly activated (RMA) and rosin activated (RA). Rosin fluxes can be cleaned by either aqueous or solvent methods. RA fluxes are rarely used in solder paste. For reflow, in addition to rosin, both OA and no-clean pastes also are used. However, for wave soldering, RMA, RA, OA and no-clean fluxes are used. No matter which category of flux is used, it must provide a good balance between the activity level needed for the job and board cleanliness requirements. My next column will discuss inorganic, organic, rosin and no-clean fluxes in more detail.
This column was adapted from Chapter 13 of Ray Prasad`s 1997 textbook, Surface Mount Technology: Principles and Practice.
1 Ray Prasad, SMT: Principles and Practice, Walter Kluwer Academic Publishers, 2nd Edition, ISBN 0-412-12921-3.
2 J-STD-004, Requirements for Soldering Fluxes, available from IPC, Northbrook, Ill.