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Solder Flux Basics

Contributed by Paul D. Stransky, Paul Stransky Associates


Abstract

A basic review of fluxes used for soldering electronic components to printed circuit boards is presented. Information is provided about flux types, uses, application, and residue cleaning.

Introduction

The purpose of flux is to clean surfaces that are going to be joined together to enhance wetting by solder in the molten state. To accomplish this the flux must be able to deoxidize metal surfaces at high temperatures without decomposing.

1. Types Of Fluxes

Fluxes fall into 2 catagories rosin based, and so called water soluble (the flux itself is not water soluble however the residue left after soldering is). There are 2 important specifications for fluxes used in electronics, ANSI/IPC-SF- 818 and MIL-F-14256E.

ANSI/IPC-SF818 spells out 3 assembly classes, class 1 consumer products, class 2 general industrial, class 3 high reliability and military electronics. Both specifications are similar in terms of tests and test methods required to characterize flux and flux performance (see table 1 ), although there are some language differences.

A. Rosin Fluxes

Rosin based fluxes are made from rosin which is extracted from pine sap. The purified product is known as "Water White Rosin". The active ingredient is an organic acid, abietic acid and may contain homologs such as dehydro abietic acid and leviopmaric acid (1,2). In addition to rosin other activators may be present at different levels to increase the ability to clean and deoxidize. Activators are compounds that decompose at soldering temperatures yielding ammonia or hydrochloric acid in the process. Flux activity is catagorized as R (rosin only) RMA (rosin mildly activated) and RA (rosin activated). A low boiling solvent such as isopropanol is used as the vehicle so they are flammable.

In addition to the rosin / activator ratio the solids content (specific gravity) of the flux can be varied. A higher solids content will be used for boards with a high density of connections, and visa versa.

Type R containing only rosin is the least active and is reccomended for surfaces very clean to start with. It leaves virtually no residue behind.

Type RMA contains a small amount of additional activator to enhance cleaning and deoxidation leaving only a minimum ammount of inert residue behind. A characteristic of RMA fluxes is that the remaining residue be noncorrosive, tack free, and exhibit a high degree of freedom from ionic contamination after cleaning.

Type RA are most active of the rosin fluxes, and leave the most residue, however the residues can be removed with appropriate flux cleaners. These 3 fluxes (R, RMA, RA) are the only ones specified for mil spec work (Mil-F-14256E , ANSI/IPC-SF-818 Class 3).

B. Water Soluble Fluxes

The so called water soluble fluxes are divided into two catagories, organic and inorganic based on composition. Organic fluxes are more active than RA rosin, and inorganic are the the most active of all.

2. Choice Of Fluxes

The following are factors that can effect the choice of flux for a given job:

A. Specifications such as ANSI/IPC-SF-818 or MIL-F-14256E may dictate the type of flux but other factors effect choice too.

B. Board Type Board type such as single or double sided, multilayer, connection density will influence flux solids content needed. A lower solids content for simpler boards higher for more complex.

C. Method Of Mounting Components Method of mounting components such as PTH or SMT can influence choice of flux. Because of the difficulty in cleaning flux residues from underneath SM components flux manufacturers have developed special products that leave very little corrosive or conductive residues behind if any. These do not require cleaning.

D. Solderability Of Metals To Be Joined Figure 1 is a graph of sorts that shows relative solderability of many metals versus flux activity (1). Most of the metals of concern in electronics fall within the first 2 catagories easy to solder and less easy. However it can be seen that metals which exhibit good corrosion resistance because of inherently tight oxides such as stainless steel require the strongest or most active flux.

E. Material compatibility With Activator There are halogen and ammonia free fluxes for materials that require active fluxes but are not compatible with halogens or ammonia fumes.

F. Soldering Method Most fluxes are formulated to be applied as a foam in automated systems prior to wave soldering. There are fluxes that can be applied by brushing or dipping for hand soldering. Solder creams and some solder preforms contain flux.

3. Post Soldering Cleanliness Considerations

It is important to remove any contamination from a completed board regardless of where it comes from, flux or other processes. Residues left on a board, especially when ionic, can cause electrical shorts or corrosion (6).

The bad actors in fluxes are the activators which are ionic in nature, although rosin fluxes (types R, RMA, RA,) may additionally leave behind rosin which must be removed also.

There are two routes for cleaning rosin flux residues, one is solvent cleaning the other aqueous cleaners. A blend of polar and non polar solvents has to be used so both rosin and ionic activators are dissolved and removed. Water based cleaning involves use of a biodegradable cleaner capable of saponifying rosin to form a soluble soap while ionics dissolve in the water (3).

Non rosin fluxes ( organic, inorganic ) require an appropriate water based cleaner. Flux vendors offer a variety of both types of cleaners. Cleaning will become more of an issue with the phase out of CFCs and chlorinated solvents.

4. Corrosion

Some thought should be given to possible corrosion from both fluxes (7,8) and aqueous based flux residue cleaners. For instance in the case of some microwave circuits, brass or aluminum are sometimes used for thick groundplanes. These may or may not be over-plated . Brass is subject to intergranular corrosion from sustained exposure to ammonia fumes and dezincification in acids. Aluminum is subject to attack from halogen acids and alkaline solutions.


References

   1. Kester Product literature
 
2. Alpha Metals Product Literature
 
3. C.F. Coombs, Jr., Printed circuits handbook, 3rd Ed., McGraw-Hill, pp. 25.1-8, New York, 1988.
 
4. ANSI-IPC-SF-818
 
5. MIL-F-14256E
 
6. E.Westerlaken, Electronic Packaging & Production, pp 118-124, Feb. 1985.
 
7. IEEE Trans. ON Parts, Hybrids, And Packaging, Vol.PHP-7, pp. 155-162, NO.4,Dec. 1971.
 
8. Microelectronics and Reliability, Vol.14, pp. 295-303, Pergamon Press, Great Britain, 1975.


Table 1: Comparison Of Tests And Methods For ANSI-IPC-SF-818 And MIL-F-14256E

  TEST METHODS
 PROPERTY               ANSI-IPC-SF-818          MIL-F-14256E
__________              _________________        ________________
 
  Solids Content          IPC-TM-650, 2.3.34       None
  Density                 ASTM 40, D1298           None
 Solder  Spread          IPC-S-804/805            Mil-F-14256E,4.7.5
  Activity                IPC-TM-650, 2.3.32       IPC-TM-650, 2.3.32
(Copper Mirror Test)
  Silver Chromate Test    IPC-TM-650, 2.3.33       IPC-TM-650, 2.3.33
  Halide Content          IPC-TM-650, 2.3.35       IPC-TM-650, 2.3.35
  Corrosion Test          IPC-TM-650, 2.6.15       None
  Surface Insulation      IPC-TM-650, 2.6.3.3      IPC-TM-650, 2.6.3.3 Resistance
 


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