Seeking Alternatives to Solvent Cleaning
We are using a mix of trichloroethylene and IPA for cleaning in the ratio of 10 to 90. We are looking to eliminate the use of trichloroethylene.
And welcome to Board Talk. This is Jim Hall and Phil Zarrow, the Assembly Brothers. We are here to discuss process issues, problems, situations and whatever might be plaguing your mind in the electronic assembly arena.
Let's take a look at this one, Jim. P.N. writes, we are located in India. We manufacture and assemble circuit boards used in watches. We are currently using a mix of trichloroethylene and IPA for cleaning in the ratio of 10 to 90. e are looking to eliminate the use of trichloroethylene and request your input.
Fortunately for you P.N. you have about 25 years of experience of the rest of the world getting rid of trichloroethylene and Freon solvent.
For those of us who are old, like Phil and I, we remember those days when we cleaned with chlorinated solvents and did a great job. he Montreal protocol in 1989 made them illegal to use. Virtually everyone has either gone no-clean or water-soluble.
There are a few legacy products, which may include you, which are still using the old RMA fluxes. If that is the case, you will either have to find an alternative solvent, one of these non-ozone depleting advanced hydro-carbons or other exotic solvents. Or go to an aqueous cleaner with a strong saponifier which will dissolve the natural resins in your RMA paste.
If you are using a more recent solder paste, if it is no-clean you obviously don't have to clean and if its not there are a variety of very friendly, engineered aqueous materials that you can use in combination with water to remove the flux residues.
Yeah, chances are if you are making a consumer product like watches you may have a great application for no-clean. Wow, man this is like back to the future. Fond memories of trichloroethylene and leaning over a vapor degreaser and ingesting the fumes. But anyway, we digress.
It is actually rare that we see anybody using trichloroethylene. Actually it is rare that there are countries out there allowing the use of trichloroethylene. What can I say P.N., welcome to the future.
Jim, anything you want to add to that?
As with cleaning, or the soldering that comes before it, don't do it like my brother.
Oh, and please don't solder like my brother.
I agree with Rick that soldering experts are not necessarily cleaning experts; I hope there are no hard feelings from the Assembly Brothers. But their point is a great start for a healthy discussion: no matter what the application, getting fast, safe, convenient, consistent and affordable cleaning of your PCBs can be a challenge. |
The assertion that "Virtually everyone has either gone no-clean or water-soluble" is simply untrue. While no-clean fluxes and pastes are very popular (and have greatly improved over the years) many clients still clean their boards because there are more contamination on PCBs than just flux residues. Water-cleaning, in particular, is reaching the limit of its effectiveness on today's densely populated, high-performance PCBs soldered with high-temp lead-free materials. My company, as well as Rick's and Karl's, all have customers making PCBs and cleaning them with solvents because solvent cleaning offers important cost-saving advantages.
Now, both Rick and Karl have nicely summarized some of the concerns about which you should be concerned, but I would expand their list. First and always, start with the contamination. Everything begins with WHAT you're trying to remove. The Critical Cleaning Lab at my company has tested hundreds of solders and pastes and matched them to the optimal solvent; a mis-match between the solvent and the contamination can lead to white residues and ineffective cleaning.
The next step is to look at HOW you are cleaning. You did not specify the tools or process you are using for your PCB cleaning process. Rick said it nicely when he wrote there's "nothing like cleaning with hot solvent vapor..." However, that requires a specialized cleaning machine called a vapor degreaser. Vapor degreasers are great cleaning systems and actually pretty simple machines. They use a straightforward thermo-mechanical process — boiling a low-temperature solvent and then distilling it — to deliver high quality cleaning quickly, consistently, easily and inexpensively. Do a Google search for "What is a vapor degreaser" and you'll see dozens of listings from Wikipedia and elsewhere which can quickly bring you up to speed.
If you are cleaning your PCBs by hand, please don't use the ineffective "dip-and-brush" process. In general, everything gets cleaned using the same four steps: Wet, Scrub, Rinse and Dry. If you don't use all four steps you won't get clean PCBs. Since the dip-and-brush method doesn’t let techs rinse the board, the contamination stays on the board and the board is sticky. As we like to say, techs can MOVE the residues but cannot REMOVE them. So your techs waste a lot of time and solvent, which is expensive.
(In December 2007, there was a very interesting technical note in Circuits Assembly Magazine by Mr. Terry Munson who bills himself as “The Process Doctor.” Mr. Munson measured the contamination left on the boards after dip-and-brush cleaning using SIR testing, the most advanced form of board cleanliness testing. He concluded that rarely, if ever, would dip-and-brush cleaning achieve satisfactory results.)
Now, let’s talk about the solvent. Your home-made blend of alcohol and TCE has problems: at a 90:10 mixture, it's flammable, hygroscopic, smelly, and to some degree toxic. You are wise to try to upgrade your cleaning away from that brew. Here's some of the criteria I would suggest you consider:
Cleaning Strength. If the solvent cleans easily, your techs will produce a better result. Solvent strength is measured by "Kb Value." Look for Kb values of 50 or higher for stronger, faster cleaning. (TCE has a Kb value of 133 while IPA alcohol has a Kb value of about 15, so your blend probably has a Kb value of around 26.)
Surface Tension: Low surface tension leads to better wetting of the surfaces being cleaned.
Materials compatibility: Solvents which are too strong can damage components or remove inks. Find a balance between cleaning strength and "plastic safe" materials compatibility.
Smell. No aroma is the best aroma and minimizes the need to ventilate the work area.
Evaporation. Faster drying solvents simply are more convenient than slow-drying ones.
Fire Hazards. Nonflammable is always preferred; they're easier to use, transport and handle.
Health & Environment: TCE is an old-style chlorinated solvent with serious toxicity concerns, as Rick and Karl correctly described. Stabilized n-propyl bromide (also called nPB) is a strong solvent but also has health concerns; I would NOT recommend it for manual cleaning. The most common choices today are HFC and HFE solvent blends, which work great, are affordably priced and are nonflammable. However, to really look to the future, test the new HFO solvents which are available from Honeywell, MicroCare and a few other companies. These fluids clean great, are very safe for people, safe for components, nonflammable, fast-drying, ozone-safe, non VOCs, and have virtually zero global warming impact.
Technical support: A quality product should be backed by a quality lab, which can perform cleaning tests on your boards and your contamination, often for free. Don't settle for less than stellar tech support.
In short, in today's world you have some great new options that were not available even just a few years ago. Contact a cleaning expert and get help from people who really know the chemistries, the regulations and the economics.
Mike Jones, MicroCare Corp.
There are a number of alternative cleaning fluids as replacements to TCE. As stated above, they typically can be used as drop-in replacements in vapor degreasers. These new fluids don't have any of the environmental baggage of the legacy solvents yet clean just as well, if not better. The key is to find an alternative fluid with both a low surface tension and low viscosity. This will help to get the fluid into and under the tight spaces on the circuit board and clean more easily. Also look for a cleaning fluid with good materials compatibility to ensure you are not damaging any of the components.
Sheri Pear, MicroCare Corporation
Phil and Jim are probably not the best techies to answer this question as they have confused 1,1,1 trichloroethane with trichloroethylene (TCE). TCE has never been banned for its ozone depletion properties (there are none) and it is a solvent currently legal to use in the USA, Europe, and most of the world. TCE does have some issues with its cancer causing properties, so the exposure to the solvent must be controlled. |
Better yet, use one of the dozens of other safe, nonflammable, halogenated, solvent blends that are widely available. My favorite is an HFE / alcohol azeotrope with sky high polarity that cleans ionics, oils, etc. But for a lower priced product, check out stabilized n-propyl bromide and use it in a newish vapor degreaser to reduce solvent emissions and personnel exposure. Nothing like cleaning with hot solvent vapor to clean those type spaces in a watch.
Rick Perkins, ChemLogic
There are more sustainable solvent options for replacing TCE. Contrary to what is stated above, TCE was not regulated by the Montreal Protocol (TCE is not a significant ozone depleter, unlike its close cousin TCA, trichloroethane) and there are many companies still using this chemical. TCE is coming under scrutiny due more to its exposure guidelines and health effects. 3M, as well as other manufacturers, have new chemistry that have no ozone depletion potential (ODP), low global warming potential (GWP, a focus of the Kyoto Protocol) and much higher exposure guidelines to make them safer alternatives to TCE. These chemicals can be drop-in replacements for flux removal or other precision cleaning applications where vapor degreasing equipment is already in place.
Karl Manske, 3M Company