1.0 Introduction
Traditionally, soldering of boards and other electronic components has been done using rosin based fluxes and lead-based solders, even in automated manufacturing processes. In the past, Printed Wiring Boards (PWBs) were cleaned with chlorofluorocarbon (CFC) based solvents after soldering. In 1987, the Montreal Protocol gave international recognition to the detrimental effect of CFCs in depleting the stratospheric ozone layer. They agreed to a goal to a CFC-free world in 1996 and beyond. In order to reach this goal an investigation of alternative flux and cleaning technologies was required.
In the electronics industry, CFCs were widely used because of their cleaning ability, low toxicity and flash point and stability. The possible replacement solvents are hydrochlorofluorocarbons (HCFCs), hydroflurocarbons (HFCs), hydrofluoroethers (HFEs), terpene or hydro-carbon-based solvents and deionized water. The use of fluxes which require no cleaning is also being evaluated. The critical factors for selecting an appropriate substitute are its cleaning ability, toxicity, cost, material handling and waste disposal. Aqueous cleaning technology has distinct advantages over solvent-based cleaning methods since it eliminates hazards of working with toxic solvents and volatile organic compounds (VOCs) and also reduces waste disposal cost. However, aqueous cleaning tech-nology requires a change in the flux used for soldering and therefore, it requires a thorough evaluation and qualification of the soldering processes. Aqueous cleaning of printed wiring assemblies (PWAs) is becoming increasingly popular since aqueous cleansers are readily available and D.I. water is very inexpensive compared to other solvents.
When switching to the water soluble fluxes, the emphasis is not only on the cleaning performance of the system, but also on the effectiveness manufacturing practices and process controls so that the reliability of the final product is not jeopardized. With the numerous manufactures of the equipment and material suppliers with their own unique chemistries available, the evaluation of the overall technology becomes crucial.
2.0 Objective
This report summarizes a literature survey on water soluble flux based technology including process chemistry, flux characterization methods, lists of equipment/material suppliers and a bibliography.
3.0 Aqueous Cleaning Technology
3.1 Flux Chemistry. The main purpose of using flux in soldering processes is to remove metal oxides from the surfaces being soldered so that the solder will adhere. The chemical behavior of the flux is very important in the formation of defect free solder joints and to the long-term reliability of the interconnections.
A good solder flux must serve several functions. First, it must be able to remove the oxide film (may also include sulfides, carbonates, or other metal salts depending on the conditions of storage). The flux must coat the newly cleaned area to prevent reoxidation at soldering temperatures. The flux should also assist in thermal transfer so that the base metals rapidly heat as they come into contact with the molten solder. Finally, the flux must reduce surface tension between the liquid solder and the base metal. This promotes the wetting of the solder onto the base metal and permits the formation of a defect free solder joint. In addition to all of these factors, the flux and thermal degradation products must either be non-corrosive, or must be easily removable from the soldering areas. In the past, the fluxes used in the soldering of electronic components have been primarily rosin based. This is because rosin itself fulfills so many of the properties of the ideal flux. The ingredients of a typical flux include a vehicle, activator, solvent and other additives. The major components of the rosin vehicle include acids such as abietic acid, adipic acid, succinic acid etc. The activators in the flux formulation include amine, amine halides or organic halides. The halides are released during the preheat step to enhance the cleaning action on the metal surface. The solvents used in the flux are typically alcohols, glycol ethers, hydrocarbons or water. Their purpose is to dissolve the flux ingredients and to improve spreading across the surface to be soldered. The preheat step in the soldering process is designed to evaporate the solvent.
The soldering fluxes may also contain other chemicals such as surfactants which improve the foaming properties of the flux and lower the interfacial surface tension between the solder and the base metal. Solder pastes may also include thixotropic agents to increase viscosity. Fluxes used in flux-cored solder may include plasticizers to improve hardening properties.
3.1.1 Flux Types. There are several types of flux available in the electronic industry. The common flux types include: R (rosin based), RMA (mildly activated rosin), RA (activated rosin), SA (synthetic activator based) and OA (organic acid activated). Type R is the weakest containing only rosin without the presence of activators. Type RMA contains both rosin and activators. Type RA is a fully activated rosin or resin having higher activity than RMA types. SA type fluxes have been developed to provide higher fluxing activity and are soluble in solvents other than water. Type OA is an organic acid based flux which provides higher fluxing activity and is generally considered as corrosive.
Rosin based fluxes have been
preferred for military and space electronics applications for the following
reasons:
Water soluble flux (Type OA) formulations use di- or tri-functional organic acids such as succini acid, citric acid, adipic acid, etc. which are more soluble in polar solvents than the corresponding acids used in rosin flux formulations. Water soluble flux formulations also include other vehicles such as glycol, polyglycol, surfactants and glycerol.
The major difference between water soluble and rosin fluxes is their fluxing activity. However, incomplete removal of the water soluble flux residues can lead to corrosion of the metals used in electronic soldering. Unlike rosin fluxes, water soluble fluxes are aggressive not only at the elevated temperatures used during soldering operations but also at normal temperatures encountered in assembly processing. Therefore, it is critical to ensure complete removal of any water soluble flux residues.
Water, when used without any additives is not an efficient cleaning agent because it has a high surface tension. The effectiveness of water as a defluxing agent can be significantly increased with the use of additives, (such as surfactants) which decrease the surface tension.
Surfactants can also help to remove water insoluble residues. They serve to emulsify and in some cases dissolve oily soils and disperse water insoluble particulate soils. The cleaning performance of the aqueous system is also enhanced with the use of higher alkalinity (higher pH) since it prevents the formation of white residues as well as neutralizes any trace amount of residual acids.
3.1.2 Flux Characterization. Fluxes have traditionally been characterized by their chemical composition and their activity which can be measured by water extract resitivity and several other tests. In the early 1980’s Institute for Interconnecting and Packaging Electronic Circuits (IPC) developed a flux characterization criteria based on the flux and flux residue activity. Based on such criteria, L-type fluxes have low activity, M-type have medium activity and H-type fluxes have high activity.
The following test methods are commonly
used for flux characterization:
3.1.3 Qualification Criteria.
The criteria for the qualification of new water soluble flux based technology
includes "equal to or better" performance in the following process characteristics/categories:
Figure 1 Flow Chart of the typical Assembly Line (*indicates operations where water soluble flux technology is applicable)
3.2 Technology Status. The planned switch to a CFC-free world in 1996 has resulted in a significant increase in the total usage of water soluble flux based materials including solder paste, flux, cored-wire solder and aqueous detergent. Aqueous cleaning technology has demonstrated a higher level of fluxing activity resulting in higher first pass yields and improved reliability, even on difficult surface mount and mixed technology assemblies.
A number of military and aerospace contractors have evaluated and qualified water soluble flux technology even at a time of declining budgets due to increasing awareness and regulations of environmental issues. Many companies are evaluating water soluble flux technology either as a first choice or an interim step in the path to low residue flux (LRF) based technology.
During the 1990’s several committees and core groups comprised of technical representatives from EPA, military, DoD, Navy, industry, government agencies and research institutions have evaluated selected water soluble flux based technologies and approved those for electronic cleaning applications.
Today, highly cleanable water soluble based materials and associated equipment are available and aqueous cleaning technology has progressed to higher levels of effectiveness than those of CFC-based solvents. There is a wide variety of water based paste/flux materials and equipment currently available in the electronic market to meet users process, material and environmental compatibility requirements.
5.0 Summary and Recommendations
The literature data surveyed
for water soluble flux technology based applications shows encouraging results.
The advantages of this technology include:
When using water soluble flux technology, the emphasis is not only on the effectiveness of the system to thoroughly clean the assemblies, but also on monitoring and process control of soldering/cleaning processes utilizing the best manufacturing practices. Water soluble flux residues left on the assemblies will jeopardize the reliability of the product.
The recommendations for preliminary
qualification of water soluble fluxes for space flight applications include
the following:
