Definition of electronic auxiliary materials: auxiliary materials used in electronic assembly processes.

Scope: fluxes, solder bars, solder wire, solder paste, adhesives, cleaning agents, etc.

Role: the quality of these materials has an extremely important influence on the quality and reliability of electronic products.

What is flux?

In PCBA soldering processes, flux is a chemical substance that helps and promotes the soldering process while also protecting against oxidation. It is an indispensable auxiliary material in soldering and its role is extremely important. The technical indicators of the product must therefore be carefully analysed in order to select the right flux, and to analyse soldering failures quickly and accurately and find ways to resolve poor soldering, so that production can proceed continuously and smoothly.

Basic requirements for flux

· A certain fluxing ability or chemical activity (ensuring the ability to remove the oxide layer)

· Good thermal stability (ensuring no decomposition or failure at higher soldering temperatures)

· Good wettability, promoting the spread of solder (ensuring a good soldering result)

· Little flux residue left on the board, non-corrosive to the material after soldering, no effect on the electrical properties of the material and good electrical insulation

· Good cleanability

What are the main performance indicators of fluxes used in electronic soldering?

Appearance, physical stability, density, viscosity, solids content (non-volatile content), solderability (expressed as spreading rate or wetting force), halogen content, resistivity of aqueous extract, copper mirror corrosion, copper plate corrosion, surface insulation resistance and acid value. The relevant tests carried out in our laboratory are briefly explained below to aid understanding.

Appearance testing

The flux must first be free of foreign matter and uniform; liquid fluxes must be transparent. Any foreign matter or layering will cause soldering defects.

Physical stability testing

The product is generally required to remain stable within a certain temperature range (usually 5-45 degrees C); otherwise it cannot be used normally in hot summers or severe cold.

Viscosity testing

Viscosity is an important process selection and control parameter for flow and coating during soldering. Reference viscosity data is essential: a suitable flux viscosity can improve soldering efficiency and quality and strongly support high-quality soldering, so rational use of flux is what ensures the stability and reliability of the soldering process.

Solids content testing

This indicates the non-solvent portion of the flux. In practice it differs in meaning, and in value, from non-volatile content, which is a testing concept and corresponds to some extent, though not uniquely, to the residue after soldering.

Solderability testing

This indicator is also critical as it expresses the fluxing effect. Expressed as spreading rate, a larger value is better in isolation, but corrosivity also increases, so to ensure good post-solder reliability the spreading rate is generally between 80% and 92%.

Halogen content testing

Adding activators containing halide ions (F, Cl, Br, I) to flux can significantly improve solderability and the soldering result, but excessive content brings a series of corrosion problems. For example, when much halogen residue remains after soldering, solder joints blacken and white powder forms as the joints corrode cyclically, so this content is a very important technical indicator. It is expressed as the chloride equivalent of the total ionic fluorine, chlorine, bromine and iodine, and may carry different meanings under different test standards. For example, the current IPC standard uses the solid part of the flux as the denominator; because the solid part (non-volatile content) is usually less than 10% of the liquid flux, the value appears relatively large, whereas GB or the old JIS (Japanese Industrial Standard) standards use the mass of the whole flux as the denominator, giving a relatively smaller value.

Resistivity of aqueous extract testing

This indicator reflects the level of conductive ions in the flux: the lower the resistivity, the more ions present and the greater the effect on electrical properties after soldering. Under current requirements for resin-type fluxes, most low-solids or organic acid fluxes cannot meet the requirements for L0, M0 and H0 type products specified in JIS Z 3283-2006 and GB/T 9491-2021. As fluxes move towards low-solids no-clean formulations, the latest IPC J-STD-004C standard has dropped this indicator but tightened the requirement for surface insulation resistance.

Acid value testing

The acid value of a flux describes its acid-base character. Fluxes are generally divided into acidic, neutral and alkaline types. The acid value has an important influence on the soldering process and quality: under the same soldering conditions, fluxes with different acid values produce different soldering results. Generally speaking, acidic fluxes give better soldering results but are more corrosive to metals and require prompt cleaning to avoid affecting soldering quality.

Neutral fluxes both ensure the soldering result and reduce corrosion of metals, and are the most widely used type. Alkaline fluxes have lower acidity and lower corrosivity, but their soldering result may be slightly inferior to acidic and neutral fluxes. When choosing a flux, it is therefore necessary to select the appropriate flux type and acid value according to the specific soldering process and the desired result. At the same time, the amount of flux used and the cleaning cycle must be strictly controlled during soldering to ensure stable and reliable soldering quality.

Corrosivity testing

Because of the solderability requirement, flux inevitably brings some corrosivity to the PCB or solder joints. To measure the extent of corrosivity, various standards specify corrosion measurement methods. Copper mirror corrosion tests the corrosivity at the time of use, while copper plate corrosion reflects the corrosivity of the residue after soldering and indicates reliability; each has its own emphasis. For electronic products with high quality and reliability requirements, these tests must be carried out, and the environmental test period needs 10 days (generally 7-10 days).

Copper mirror corrosion testing:

Copper plate corrosion testing:

Electrical performance testing

The most important electrical indicators are surface insulation resistance (SIR) and electrochemical migration (ECM). All standards set strict requirements for SIR and ECM before and after soldering, because they greatly affect the electrical performance of the electronic products assembled with the flux; in severe cases they can cause signal disturbance and malfunction.

Under GB or JIS standards, SIR must be no less than 10^10 ohm, whereas IPC J-STD-004C requires SIR of no less than 10^8 ohm. Because the test methods differ, the two values are not comparable, and for some products the requirements are higher.

Comparison of the main test items in IPC, GB and JIS standards