We field a large variety of questions about assemblies, equipment, residue testing, cleaning procedures, and failures in the field, but, by far the largest group of questions regard fluxes. If none of these FAQs address your specific question, please contact us.
Q. What are the 3 primary types of flux technologies?
A. We consider the three major flux categories to be: high solids rosin (e.g. RMA); water soluble (or OA) fluxes; and low solids fluxes. A possible fourth category could be synthetic activated (SA), but this is a very specialized niche area.
Q. In general, what flux types are considered “dirty?”
A. High solids rosin fluxes have higher amounts of halide activators, but the rosin provides a certain amount of buffer protection. Water soluble fluxes are often more aggressive, but more easily cleanable. Low solids fluxes are lower in harmful residues, but excessive amounts can still cause problems.
Q. How is the electronics manufacturing industry transitioning?
A. A good answer to this would be to review the IPC Technology Roadmap. It addresses fabrication issues and assembly issues, as well as future technologies.
Q. I need to change manufacturing technologies. How do I choose materials and make the transition?
A. Another complex question. The primary consideration is the cleaning vs. no-clean question. If your company will retain the cleaning mindset (just as valid as any other process), then you are looking at an alternative cleaner for rosin, a water soluble flux, or a water cleanable low solids flux. The road to a true non-rosin, no-clean manufacturing process is a hazardous one, lined with ruffians and all manner of evils. You have hundreds of flux choices. Choose one that gives you adequate soldering performance first. Then choose one which gives the lowest overall residue levels. We recommend that you choose a halide-free flux formulation.
Q. What are the pros/cons of halide vs. halide-free fluxes?
A. The only advantage we see of halides is that they make the flux more active, or more aggressive. If you have boards or components that are received in an un-solderable condition, you need an aggressive flux to get decent wetting. It is our opinion that you need benign fluxes in assembly. Un-solderable components, necessitating an aggressive flux, is a problem your vendor should address.
The negatives of halides are many. On any circuit assembly, there is what we refer to as, “The Triad of Evil” – an electrical potential, moisture, and an ionic contaminant. Halides contribute to the ionic contaminant part of the equation. The greater the amount, or the more electro-active the material, the greater the risk of electrochemical failures. Chloride is the most common contaminant from halide-bearing fluxes. The only halide more electronegative in nature is fluoride.
Imagine a classic Venn diagram (three circles of equal size) with a central common area. This common area represents the risk for electrochemical failures: corrosion, metal migration, electrical leakage. Increase any one of the triad elements and you increase the size of that particular circle. An example is moisture. By increasing the moisture level in the operating environment, you increase the size of the moisture circle and the central area grows, indicating an increased threat of electrochemical failure – similarly for operating voltage, especially for halides. The more electro-active the ionic contamination, the larger the threat and the less moisture or voltage it takes to initiate and propagate a failure.
In terms of halide vs. non-halide fluxes, the advantage of a non-halide flux is the benign nature of the residue. The disadvantage is the inability to deal with solderability challenges.
Q. In a transition to low solids flux technology, how can Foresite assist/consult in flux evaluation/testing?
A. Our expertise is residue analysis and experience with fluxes and process qualification. We use our knowledge of residues in optimizing flux levels, detecting detrimental levels of halides at various steps of the process, testing to see that your components are clean enough for no-clean, etc. We have extensive contacts in the flux and equipment industries and can give you direction. When it comes time for you to put together a qualification package for your customers, to show them that your new system will be reliable, we can do that too. We take pride in the thoroughness of our investigations, and the level of support we provide during transitions.
Q. When should I transition from a water soluble flux to a no-clean process?
A. That depends on what is driving such a transition: economics, technology, politics, customers, environmental regulations, etc. You should not make such a transition without doing a great deal of homework because it requires a paradigm shift in your thinking. You need to examine the solderability condition of your incoming components because you will have less activity in your fluxes in a no-clean application. You must examine your bare board cleanliness because you won’t have any chance to remove HASL residues. You have to examine your ICT setup because you will have additional residues to punch through. The IPC has generated a document in this area that is helpful – Implementing a Low Solids Flux Process.
Q. What are the dangers associated with water soluble fluxes?
A. Water soluble fluxes (WSFs) are more aggressive in their ability to strip oxides. The degree of aggressiveness depends on the activators used. A WSF, as a corrosive material, does not know when to stop stripping oxide. Consequently, if you don’t clean the flux residues off, then it will continue to eat away the base metal. Electrical leakage currents and metal migration are also common failures with incomplete WSF removal. If you have an ineffective cleaning process, expect to see failures in accelerated testing and in the field.
Q. Can water soluble fluxes be used to produce high-reliability electronics?
A. Yes they can. Manufacturers have been doing so for at least 10 years. The move away from rosin fluxes and freon cleaning did a great deal to spur this activity. Like any flux, it must be intelligently used and must have a good cleaning process associated with it.
Spray vs. Foam
Q. I have a choice between a wave fluxer, a foam fluxer and a spray fluxer. Which should I use?
A. We recommend against the wave fluxer – it applies far too much flux. A foam fluxer puts on 2-3 times the amount of flux that a spray fluxer will. Since most flux manufacturers recommend using as little flux as you can, we recommend the spray fluxer. In many of our studies, well-controlled assembly processes use spray fluxing, while problem processes have used foam fluxing. A spray fluxer is a little harder to control consistently and is more susceptible to draft currents, but with adequate setup, this can be controlled.
Q. What is a good solder paste to use?
A. Whatever works for your product. In our view, there is no such thing as a “good” material or a “bad” material. You just have to find materials that have the right blend of properties for your hardware and your process. We have seen very good results with: Alpha Metals UP-78, Kester 245, Indium SMQ-92J, Multicore X33.Each paste manufacturer has some excellent offerings. It depends on your needs.
Q. What stencil thickness should I use in my solder paste evaluations?
A. That depends on the standards and if you are “qualifying” a material or process. In general, we recommend that you use the same stencil thickness that you will be using in production. Most standard test methods use an 8 mil stencil, but that puts on twice the amount of paste and flux as a 4 mil stencil that you might be using in production. If you are qualifying the paste and/or flux, use what the specification says. If you are doing a process or materials investigation, then use what you will use in production.
Q. What is a good cored wire solder?
A. Same answer as for solder paste. It depends on your application. For no-cleans, we like Alpha Reliacore, SMTCore Plus, Kester 245, and Multicore X34.