What are the Pros and Cons of Cleaning No-Clean?

Phil
Welcome to Board Talk! This is Phil Zarrow and Jim Hall of ITM Consulting. What do we have today, Jim?

Jim
A question from A.S. about the pros and cons of cleaning no-clean.

"We are working with our contract manufacturer to develop the best process for our boards. With today's newer chemistries, what are the pros and cons of cleaning circuit boards assembled using a no-clean process?"

Phil
A very good question. Of course, the first thing is why are we cleaning no-clean in the first place? It sounds like an oxymoron, but we know better, and we know there are situations in circuit board applications where we do have to clean the no-clean. These include very high frequency microwave products where the residue will degrade the circuit performance.

In cases where certain conformable coatings won't adhere to the no-clean residue.

Jim
And some people with high rail products just want to clean their boards. Another reason might be poor reflow or wave processing that didn't fully deactivate the no-clean such that the residues are not safe to leave on the board. In that case you certainly would want to clean. Although it would beg the question, why don't you clean up your process and solder correctly leaving you with a safe residue?

I think we talked about this before when somebody asked about IPA. No-clean is not one thing. It is a very wide-ranging family of materials that use all different kinds of chemistries. The one bottom line is that if processed properly, you should be able to leave the residues on the board for most applications.

If, for any of the reasons that we talked up before, you need to clean the residue, make certain that you use a cleaning chemistry and a cleaning process that will remove the no-clean and leave no dangerous chemistries on the board.

Phil
One of the biggest mistakes we see is where people have not matched the cleaning solvent with the residues. There is no universal solvent. There are some that work with quite a wide range; some that work with a narrow range. If you haven't done the due diligence, you're going to be in trouble.

Jim
You need a cleaning system that's going to get completely under all of your components and around them. So you're going to look at your low clearance or low standoff components. QFNs jump out as being one of the toughest to clean. Remember, one of the worse things you can do is to partially clean a no-clean.

All no-clean formulations produce a safe residue through some level of encapsulation. That means that there are certain active chemicals on the board, but if properly processed they are encapsulated in a resin system. That resin system is not water-soluble. That's why pure water will never clean a no-clean.

And the worse thing is to put an improper solvent that partially dissolves the resin and potentially exposes some of the previously encapsulated active materials. So you end up with a board that's more dangerous, that's dirtier from a chemical standpoint than when you started.

Phil
The predominant methodology being used for cleaning no-cleans today is what's known as engineered aqueous, and there's been a long evolution that's led up to this. What we highly recommend is that you consult the supplier of your particular engineered aqueous solvent. In this case the two leading suppliers are Kyzen and Zestron. They have generally done very good due diligence matching up their particular formulations with the various residues from no-clean fluxes.

Jim
Look seriously at your product. Will it perform correctly and have proper reliability if there is a safely-processed no-clean residue remaining on it? Cleaning is just another expense. It's just another process. There's no reason to do it if you can avoid it. Many of the products that we use every day are processed with no-clean materials and not cleaned. If you have to clean for whatever reason make sure you get the right solvent and the right process to completely clean it.

Phil
Hopefully that answers that question and provides some general guidance.

Jim
And whatever you do ladies and gentleman out there in assembly land, don't solder like my brother!

Phil
And don't solder like my brother.

One other consideration to clean no clean is ICT probing issues.

You might check the recently published IPC-CH-65B Cleaning Handbook. The late Dr. Bill Kenyon and I wrote a section on why OEMs clean no-clean fluxes and give some history as well.

Another reason to clean the no-clean is when you need to apply conformal coating to the board assembly. We did match a cleaning solution and process to the specific no-clean used.

Surely, a flux that is formulated to be easily cleaned and then cleaned to qualified standards is preferred. It will offer a large margin of security with minor process variations. A flux that is formulated as to be relatively "safe" if not cleaned may leave "unsafe" residues if not cleaned optimally. However, even if a cleaning process is qualified as OK with one given cleaning process, it may offer little in the way of tolerance. In other words, a small variation in either the soldering/reflow or cleaning processes may take you outside the margin of safety.

Where cleaning is necessary, I prefer the extra safety of fluxes designed for the job.