Cleaning R.F. Circuits - Aqueous or Vapor?

Phil
Welcome to Board Talk with Jim Hall and Phil Zarrow, the Assembly Brothers pick and place. Coming to you high atop Mount Rialto in New Hampshire at the Board Talk headquarters.

Jim
This came from B.B. It's a process question.

We are a contract house using no-clean paste and wire solder. We clean assemblies by hand dipping using Insolve and want to get away from that. A third of our customers' products are R.F. Which would be better for R.F., vapor degreasing or aqueous cleaning?

Phil
To be honest with you, regardless of you using R.F. or whatever you are trying to clean, the basics of cleaning apply.  There are two key elements you have to have here. You have to have a solvent be it in vapor form or a engineered aqueous in this case or whatever your solvent is, that is suitable for the residue that you are trying to remove and that is matched to the flux residue that of the solder that you are using.

Whether it's solder paste, wire solder, flux, wave or selective soldering, it's got to be matched. So that's the chemical aspect of it.

The mechanical energy aspect is the other important thing and that's going to depend largely on what you have on your board in terms of clearances and board geography.

You have to have ample mechanical energy to spread that chemical solvent, as well as rinse and dry, depending on what you are using. So regardless of whether you are doing R.F. or anything else, it's really more determined upon what you are cleaning. Try to clean, doing it right.

Jim
I'm no expert on R.F., but I would be more concerned with the board and solder mass materials' compatibility with whichever cleaning chemistry I'm using in terms of compatibility because you don't want to mess up the surface chemistry at all. So you want to make sure that the solvent, be it aqueous or vapor, is compatible with your board materials and solder mass.

Phil
Well one of the other important things with regard to the R.F. situation and I don't know what frequencies you are going, if you are going up to the UHF spectrum, at least you are cleaning your boards.

There is a lot of debate as far as using ultra-high frequencies and above, you start getting up above the 15 GHz - 20 GHz range if no-clean will suffice to stay on the board or whether it should be cleaned.

We don't know if you are getting up anywhere near there, but again, it's good to keep track of what papers are coming out and work is being done with clean versus no-clean, but in your case, you are cleaning it so almost becomes a non-issue now.

But you've got to have proper cleaning and that's the most important thing. As Jim is saying you don't want to do anything detrimental to anything else on the board and we've seen that happen a lot too.

Jim
And big is how it is initially a no-clean that you are trying to clean. You have to make absolutely certain that whatever solvent again, aqueous or vapor, is compatible with the specific paste formulation that you are dealing with.

Phil
So hopefully that answers your question, and doesn't provide you with further confusion.

Jim
Don't solder like my brother --

Phil
And please don't solder like my brother.

Of the two methods, I would greatly favor aqueous cleaning as you get the dynamic action as well as the chemical action. Mike Konrad, Aqueous Technologies, gave an excellent presentation this morning on how residues can impact RF performance. I would recommend having some cleaned assemblies benchmarked for residues using ion chromatography.

"Insolve" was mentioned as the cleaning solvent. Did the person who asked the question mean "Ensolv?" If so, and if the main solvent is n-propyl bromide, I am very concerned that, with an exposure limit of 0.1 ppm, it is being used for hand cleaning, unless proper PPE and exposure controls are being used. And I agree with Mike Jones' comments and recommendations about solvent cleaning in a vapor degreaser usually being faster and more affordable, especially for RF components.

I really think the question revolves around 2 ideas - what equipment you have available, aqueous or vapor phase and how DRY do the boards have to be. This is the customer's responsibility to answer the dryness question since they're the experts on what affects their circuit. Does the board absorb water? FR4 acts like a sponge and may require a post-wash bake before they could be used. Then you get into "how dry is dry enough", and would require monitoring of the drying process. The higher in frequency you go the more this effects the circuit operation. I would go back and work with the customer on whether or not water absorption in the board and components is an issue, it may lead you to vapor phase. Odds are they haven't given that much thought before you ask.

Here at MicroCare, we would agree with Phil and Jim that your #1 priority is identifying the contamination. It's ALL about the contamination, not the cleaning fluid or machinery.

That said, the next step is to understand the constraints under which you'll be operating. Are there soft plastics or elastomers on the boards? Conformal coatings? Inks which might be removed? Special insurance or local regulatory issues? These factors may affect the solvent or surfactant choices.

At this point, it usually is very helpful to send parts to a vendor's cleaning lab and have them test for cleanliness and compatibility. The better vendors, like MicroCare, also will prescribe a cleaning process specifying times and temperatures.

Assuming your cleaning issues can be resolved by either aqueous or solvent choices, now you need to move into the economic analysis. Are electrical costs high? Is floor space available? Do I need a chemist or analytical lab to support the aqueous option (usually not with solvent)? How do I handle the waste products? What do the cleaning machines cost? What's their throughput and economic life span?

Your ultimate goal is to produce parts of satisfactory quality at the lowest cost-per-part-cleaned. That's the key: it's not the price per gallon, it's the price per cleaned component. In my experience (30+ years of critical cleaning) solvent cleaning usually is faster, better, simpler and more affordable.

I look forward to the contradictory comments from all my buddies in the aqueous cleaning world!