Collaborative cleaning process innovations from managing experience and learning curves
bump pitches reduce, less flux is available minor ingredients to improve materials between oil and water by adsorbing at the
to remove oxides needed to join the flip compatibility and control foam liquid-liquid interface. When micelles
chip die to the substrate. To address this propagation under high pressure. Cleaning form in the cleaning solution, their tails
concern, high temperature stable flux materials come in three families: solvent, encapsulate an oil droplet and their ionic
materials with increased activation are semi-aqueous and aqueous designs. polar heads form an outer shell that
needed. Circuit designers find improved Solvency: Oxygenated organic maintains favorable contact with water.
performance from removing the flux materials dissolve rosin and resin (solute) Wetting agents improve penetration under
residues post soldering. structures naturally present in many flux the Z-axis to remove flux residues under
IPC-CH-65A provides guidelines types. The interaction of resin/rosin with components.
for cleaning printed circuit boards and solvent(s) increases the dissolution rate. Minor ingredients: This category of
assemblies
.9
Conventional printed circuit Solvents are selected on the basis of “like materials address two important functions:
assemblies used larger components. The dissolves like” commonly referred to as 1. Control wash bath foam when
standoff heights on many conventional the solvated state, whereby organic resin processing in high-pressure equipment
printed circuit board components are flux residues are dissolved by oxygenated and 2. Decrease the rate of metal alloy
greater than 10 mils off the board. As solvent molecules. Solvent dissolution corrosion. Foam is a substance that is
this research notes, leading edge printed is a kinetic process and is quantified by formed by trapping many gas bubbles
circuit boards now use components that its rate. The rate of dissolution depends at the liquid interface. Rapid turnover
have standoff heights less than 2 mils, on the solvent and solute, temperature, of the wash tank and high-pressure jets
and in the case of flip chip, chip cap impingement pressure and interfacial create a condition to trap and grow
and QFN components, flux becomes surface tension. gaseous tight foam. To break or retard
trapped and shielded from the cleaning For aqueous and semi-aqueous foam, antifoaming agents are added to the
process. To accomplish this demanding electronic assembly defluxing, the solvent engineered composition to inhibit foam
cleaning job, customers, cleaning material, materials interact with the polar solvent, formation.
solder paste and cleaning equipment water. Solvation involves different types The second class of minor ingredients
companies have collaborated to develop of intermolecular interaction: hydrogen includes materials that decrease the
new products and process optimization to bonding, ion-dipole and dipole-dipole corrosion rate of tin, lead, aluminum
meet these demanding cleaning challenges. attractions. The hydrogen bonding, ion- and yellow metals. Alkaline cleaning
The cause/effect diagram in Figure 2 dipole and dipole-dipole interactions occur materials chemically react with soft metals.
summarizes some of the effects that cause from the water and water’s interaction Corrosion inhibitors form a passivation
product failure and created the need for with oxygenated solvents. The oxygenated layer—a thin film on the surface of the
improved cleaning processes. solvents interact with water to improve alloy(s) that stops access of the corrosive
dissolution of rosin, resin, and organic substance to the metal. Properly designed
cleaning material innovations acid ions necessary to leave an ionically inhibition packages reduce oxidation
Electronic assembly cleaning materials are clean assembly. and reduction reactions. Solder joints,
designed to remove a broad array of flux Builders: For aqueous cleaning aluminum heat sinks, anodized aluminum
technologies including organic acid, rosin, fluid designs, mild alkalinity provides and copper are protected from exposure to
resin and polymeric structures from mixed two important functions: 1. Improve the the cleaning media.
technology circuit boards. The building cleaning rate and 2. Maintain a consistent Using the building blocks to improve
blocks (Figure 3) used to formulate these pH by forming a strong buffer. Rosin, aqueous cleaning performance under low
cleaning materials are solvency to dissolve commonly used in flux compositions, is standoff components: Aqueous cleaning
resin structures, reactive agents to buffer a solid form of resin obtained from pine materials are engineered concentrated
and saponify soils, wetting agents to lower trees and some other plants. To improve fluids that dissolve with water. Aqueous
surface tension and improve penetration the cleaning rate, mild alkaline materials cleaning materials are non-flammable
under low standoff components, and are used to react (saponify) with the rosin/ and processed in high-energy machines.
resin to increase the rate of dissolution. Aqueous concentrated products work
Additionally, the alkaline source is used to based on the “Cleaning Rate Theory”
react with a weak acid to form a buffer that that holds: The static cleaning rate (rate
keeps the pH at a nearly constant value. at which the cleaning material dissolves
An optimal pH range of 9-11 prevents the flux residue at its temperature
redeposition of flux soils and ionic and concentration in the absence of
constituents onto the circuit assembly after impingement energy) plus the dynamic rate
the cleaning process. (energy and time in the cleaning machine)
Wetting: Wetting agents lower the equals the process cleaning rate.
11
surface tension of the cleaning fluid,
by reducing the droplet size, improving Aqueous cleaning materials fit four
spreading, and lowering the interfacial classifications (Figure 4):
surface tension. Surface active agents form
1. Aqueous high reactivity: Product
micelles that contain a lipophilic end to
contains greater than 30% active
dissolve oily soils and hydrophilic ends
free amine (Note: there are several
 
to hydrogen bond with water. Wetting
amine structures commonly used by
agents reduce surface tension of water by
Figure 3. Electronic assembly cleaning material formulators) reactive materials that
building blocks.
adsorbing at the liquid-gas interface. These
saponify the flux residue to improve
materials reduce the interfacial tension
26 – Global SMT & Packaging – October 2009 www.globalsmt.net
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