Chemicals are found throughout the electronic assembly and repair process, but no solvent is more common than isopropyl alcohol. It is universally used for cleaning and as a main constituent of fluxes, but how much do you really know about isopropyl alcohol?
Isopropyl alcohol (CAS #67-63-0) is also referred to as IPA, isopropanol, 2-propanol, and even rubbing alcohol (more on that later). It dissolves a wide range of polar and non-polar soils, is it is often used to dissolve and remove light oils, fingerprints, cutting fluids, flux residues, carbon deposits, and mold release. It is also readily miscible in water, so can be used as a drying agent as well.
For electronic printed circuit board (PCB) assembly, you will find isopropyl alcohol being used to clean flux residues from recently soldered circuit boards or in PCB repair and rework. IPA is also used to remove solder paste or adhesive from SMT stencils. Maintenance cleaning with isopropyl alcohol is common for removing caked-on and burnt-on flux from SMT reflow ovens, wave soldering fingers, selective soldering nozzles, pallets, and anywhere else flux tends to collect in automated soldering processes.
Material compatibility of Isopropyl Alcohol
Isopropyl alcohol is generally compatible with most materials used in electronics. As with any chemical product used, component compatibility must be determined on a non-critical area prior to use. The following are a list of common materials used in PCB assembly and compatibility rating:
| Material | Compatibility |
|---|---|
| Aluminum | Excellent |
| Buna-N | Good |
| CPVC | Excellent |
| Epoxy | Excellent |
| Neoprene | Good |
| Noryl | Excellent |
| Phenolic | Excellent |
| Cross-Linked PE | Good |
| Polycarbonate | Poor |
| Polypropylene | Good |
| Polystyrene | Poor |
| PVC | Fair |
| Teflon | Excellent |
| Tygon | Good |
| Viton | Excellent |
When a solvent is not compatible with a substrate, it may cause crazing (micro-cracks), soften the material, or in the case of seals or gaskets, cause them to swell, shrink, or become brittle. This video shows an extreme example of acetone on polycarbonate, which causes it to fog, soften, and scratch even from the plastic dropper.
Grades or Types of Isopropyl Alcohol
Isopropyl alcohol is available in a large number of varieties and grades, which are generally differentiated by the amount of water in solution and trace amounts of other chemicals or nonvolatile materials.
Isopropyl alcohol is hydrophilic or water miscible, so readily accepts water into an azeotropic solution. When you see on the label “70% isopropyl alcohol”, it refers to amount of alcohol in relationship to the dissolved water – i.e. 70% alcohol to 30% water. Water can be removed to form purer grades of isopropyl alcohol, forming what is called “anhydrous” isopropyl alcohol.
Whether IPA has 0.2% or 50% water has little effect on the look and smell of the material, but greatly impacts surface tension, or its ability to wet, and the dry time. The more water isopropyl alcohol contains, the greater the surface tension. This causes the solvent to bead up more as it dries rather than wet or sheet, which can lead to spotting. “Water spots” are particularly problematic with cleaning mirrors, lenses, and other optics.
The dry time is greatly increased as the percentage of water in the alcohol solution is increased. This can be an advantage or disadvantage depending on the application. For example, isopropyl alcohol is often used as a flux remover to remove flux residues from around solder joints. Technicians tend to prefer a fast dry time so they can clean the electronics and bring them back into service as quickly as possible. For breaking down and removing thick, gummy, or baked-on materials where more dwell or soak time is required, slowing down the dry time often improves the cleaning effectiveness. That is one of the reasons 70% pure isopropyl alcohol is commonly used for degreasing and SMT stencil cleaning. In the case of electrical contact cleaning, the faster evaporation helps ensure the flammable solvent is gone before energizing the equipment.
This video shows the comparative evaporation rate of 99.8% pure IPA, 70% IPA, and acetone.
The following are grade designations of isopropyl alcohol typically stated and specifications and operating procedures:
- ACS / reagent grade - meets or exceeds purity standards set by the American Chemical Society (ACS). This grade is specified for food, drug, or medicinal use and other applications that require strict quality specifications and purity over 95%.
- USP grade - meets or exceeds requirements of the United States Pharmacopeia (USP). For food, drug, or medicinal use in addition to most laboratory purposes.
- NF grade - meets or exceeds requirements of the National Formulary (NF). USP and NF have combined, so USP is generally what is referenced.
- Laboratory grade - most popular grade for use in educational applications, but exact amount of impurities are unknown.
- Purified grade - also called “pure” or “practical grade”, this meets no official standard and not pure enough for food, drug, or medicinal use.
- Technical grade - is used for commercial and industrial purposes but not pure enough to be offered for food, drug, or medicinal use of any kind.
The general term “rubbing alcohol” is often used for isopropyl alcohol, but users looking to a buy specific grade of alcohol should avoid products labeled that way. Rubbing alcohol does not refer to a specific grade, and may even be a different type of alcohol, like ethanol (also called ethyl alcohol, CAS #67-17-5). Consumer rubbing alcohol also can contain other ingredients like denaturant, fragrances, and colorants that are potential contaminants.
ASTM D770 (currently on revision 11(2019)) is the standard specification for isopropyl alcohol published by ASTM International. It covers the properties and requirements for isopropyl alcohol:
- D268 Guide for Sampling and Testing Volatile Solvents and Chemical Intermediates for Use in Paint and Related Coatings and Material
- D1078 Test Method for Distillation Range of Volatile Organic Liquids
- D1209 Test Method for Color of Clear Liquids (Platinum-Cobalt Scale)
- D1296 Test Method for Odor of Volatile Solvents and Diluents
- D1353 Test Method for Nonvolatile Matter in Volatile Solvents for Use in Paint, Varnish, Lacquer, and Related Products
- D1364 Test Method for Water in Volatile Solvents (Karl Fischer Reagent Titration Method)
- D1476 Test Method for Heptane Miscibility of Lacquer Solvents
- D1613 Test Method for Acidity in Volatile Solvents and Chemical Intermediates Used in Paint, Varnish, Lacquer, and Related Products
- D1722 Test Method for Water Miscibility of Water-Soluble Solvents
- D4052 Test Method for Density, Relative Density, and API Gravity of Liquids by Digital Density Meter
- D5386 Test Method for Color of Liquids Using Tristimulus Colorimetry
- E29 Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
- E300 Practice for Sampling Industrial Chemicals
(source: https://www.astm.org/Standards/D770.htm)
Application Methods of Cleaning with Isopropyl Alcohol
As mentioned above, isopropyl alcohol is also available is a different grades, or percentage of solvent to water, to fit is specific applications. When deciding on the best isopropyl product for a particular manufacturing or repair process, first decide on the grade of solvent, then the packaging.
Isopropyl alcohol is easily dispensed in a variety of packaging types to fit requirements for the various applications:
- Bulk containers: Isopropyl alcohol is packaged in gallons, 5-gallons, drums and even totes for automated dispensing. In most cases, a spigot can be attached to the 1 and 5 gallon containers for convenient dispensing for benchtop or workstation use. When transferring to another container, make sure the smaller bottle is properly marked with the chemical name and warning information.
- Small bottles / spray bottles: To avoid the risk and hassle of pouring and labelling smaller containers, isopropyl alcohol is also available in smaller plastic containers like pints and quarts. Some have a trigger spray attachment for convenient cleaning.
- Pump dispensers / dauber bottles: Empty bottles are available to make dispensing even more convenient. A wipe or swab can be saturated in the opening, which is filled with a valve when the user pushes down. This keeps the isopropyl alcohol sealed, which avoids cross contamination when a wipe is resaturated after use, and drawing in moisture from the ambient air. These bottles can come with a variety of features including ESD-safe materials and a selection of labels for standard chemicals like IPA.
- Aerosol cans: An aerosol can is a sealed container, so has some of the advantages of a pump dispenser. In addition, the propellant forces the solvent out of the can, creating agitation as it hits the substrate.
- Presaturated wipes: For the ideal mix of form and function, wipes are available that are already saturated with isopropyl alcohol. They can come in pop-up tubs where the wipes are pulled up from the top, plastic boxes or pouches that reduce linting caused by wipe perforations, and even individually wrapped, which are perfect for field maintenance because they fit perfectly in a tool box.
Safely Handling Isopropyl Alcohol
Although isopropyl alcohol is generally considered safe if handled properly, it is a flammable solvent, so precautions need to be taken to prevent a dangerous situation.
- Ventilation: the work area must be adequately ventilated to prevent an accumulation of vapors, which could combust. Ventilation should be explosion-proof.
- Storage: because isopropyl alcohol is very flammable, it needs to be stored in a closed container in a cool, well-ventilated area away from ignition sources like heat, sparks and flames. It has an NFPA rating of 3, so should be stored in a cabinet or room designed for that type of hazardous material. If solvent is transferred to another container, make sure it is marked with the chemical name and required warning information.
- Personal protection equipment: isopropyl alcohol has a tendency to defatten the skin, which means it draws out the protective oils from the skin and could lead to dermatitis. Gloves made of nitrile or other solvent-resistant materials should be worn. Glasses or even a face shield should be worn if the user is spraying or splashing could occur. Respirators can be used if ventilation is not adequate enough to avoid breathing in a high concentration of vapors.
- Disposal: isopropyl alcohol needs to be disposed of as a hazardous liquid according to state and local requirements. It certainly cannot be flushed down the drain, and additional disposal requirements may need to be followed depending on the soils dissolved in the waste material.
- Safety Data Sheets (SDS): The central resource for all safety requirements is the SDS, which should be readily available wherever isopropyl alcohol is used.
