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July 21, 2008

Clean No Clean PCBAs

How do I remove flux residue and white residue from a no clean PCBA?

Once this has been done are their any reliability tests I should perform?



A. B.

Experts Comments

As a manufacturer of automatic defluxing systems for more than 16 years, I can tell you that the majority of fluxes being removed are no-clean.

The white residue is actually metal salts. This residue may be removed using modern defluxing equipment and a defluxing chemical (normally an aqueous-based chemical diluted in water).

The most common defluxing method is aqueous-based. This technology is available in batch and inline configurations (batch being more popular).

A typical automated defluxing process provides a wash, rinse, cleanliness test, and dry cycle.

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Mike Konrad
President
Aqueous Technologies
Mr. Konrad has been in the electronic assembly equipment industry since 1985. He is founder and CEO of Aqueous Technologies Corporation, a manufacturer of automatic de-fluxing equipment, chemicals, and cleanliness testing systems.

Thanks for your question about cleaning "no clean" circuit boards. This is probably the #1 question we get about PCBs today - the flux companies say their products don't require cleaning, but the QC and Production people feel it's essential. Who's right? What's the best process? It really depends. It's NOT just cosmetics; cleaning no-cleans can make boards more reliable and have a longer service life. An unofficial estimate from the sales team at MicroCare suggests that about 50% of our business now stems from helping people clean "no cleans". Here's why...

The Ten Most Common Reasons to Clean Circuit Boards Made with "No Clean" Fluxes and Pastes

  1. Uncured flux residues can cause corrosion, shorts, dendrite growth and premature failure of the boards
  2. Flux residues are not the only contamination being removed (ink, salts and solder balls, fingerprints)
  3. Flux residues make conformal coatings fail (this is a BIG item)
  4. Flux residues make visual QC inspections nearly impossible
  5. Flux residues cause automated visual alignment systems to fail
  6. Flux residues cause "bed of nails" test systems to fail
  7. Flux residues cannot be tolerated on high-voltage systems, and they add noise to analog-to-digital conversion circuits
  8. Flux residues make troubleshooting field repairs failures very difficult
  9. Flux residues make R&D work, like troubleshooting new designs, difficult
  10. Flux residues detract from the cosmetic appearance of the boards

So you can feel comfortable that there's lots of people cleaning lots of PCBs, even those made with the "no-clean" fluxes. But what's the best way to clean the boards?

CLEANING PROCEDURE

Let's talk solvents first. In general, aqueous cleaners aren't getting it done today. It dries too slowly to be effective at the workbench, and for batch cleaning aqueous cleaners use too much electricity and too much floor space to be the best choice today. So your best choice will be a modern, safe, effective solvent of some sort.

But which solvent? The most common choice is a type of alcohol. But alcohol has many problems. In general: it's flammable, it's hygroscopic, it's smelly, and it's a weak cleaner. Technicians try to make IPA work. But they don't like it. They spend a lot of time (and your money) trying to clean with a old-fashioned solvent and an inappropriate process. No wonder they get inconsistent results.

Most techs care about the following characteristics of their solvent, in this order of importance:

  • Smell. No smell is better than a nice smell, which is better than a bad one
  • Evaporation Speed. The faster the better
  • Cleaning Strength. If it cleans fast and easily, that makes their job easier
  • Safety. Nonflammable is better than flammable

While many companies make solvents, most MicroCare customers find either Flux Remover C (#MCC-FRC), PowerClean or VeriClean (#MCC-DC1) to work on "no clean" materials. They meet the "benchtop criteria" above wonderfully:

Flux Remover C is an excellent choice. It is based on HFC solvents, it cleans great, is generally plastic safe, has a modest aroma, fast drying, and a very competitive price. It can be used in aerosol, dip tanks, or even in high-speed vapor degreasers if you need greater through-put.

If all you're doing is benchtop cleaning, then I would recommend PowerClean (#MCC-PW2). PowerClean is similar to Flux Remover C but is even stronger (not plastic safe!) and works best on lead-free soldering materials. It has been specifically formulated to eliminate the white-residues so often found on lead-free PCBs. However, because PowerClean is not an azeotrope, it cannot be used in a vapor
degreaser; but other than that it's practically perfect.

VeriClean is another quick-drying cleaner with almost no aroma; it's widely used in areas where VOCs are a concern.

But it's not just the solvent, it's the PROCESS of cleaning that's important. Most people clean their boards using a little jar of solvent and a brush. It's not very sophisticated. So why do engineers like dip-and-brush cleaning so much? Because engineers THINK it works fine and it appears to be inexpensive. However, the person who really knows how to get the board clean is the benchtop technician, and know they're leaving sticky residues behind. In fact there are three things wrong with dip-and-brush cleaning: the process, the tool and the solvent.

Basically, dip-and-brush cleaning doesn't work because it doesn't use the universal four steps to cleaning. In the real world, everything gets cleaned using the same four steps: Wet, Scrub, Rinse and Dry. But since dip-and-brush method doesn't allow techs to rinse the board, the contamination stays on the board. The techs can move it around, but most of the contamination stays on the PCB. So they waste a lot of time, which is expensive.

In December 2007, there was a very interesting technical note in Circuits Assembly Magazine by Terry Munson, the "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.

It's simple. Most techs would love to have a better cleaning tool, if one was available, and the Trigger Grip is the benchtop cleaning tool your techs need. The Trigger Grip is a small, inexpensive dispensing tool that fits on the aerosol cans of solvents. The Trigger Grip forces each tech to use all four cleaning steps -- wet, scrub, rinse and dry. The Trigger Grip is ideal for cleaning circuit boards, mechanical parts and stencils. When connected to a can of MicroCare solvent, this system removes fluxes, oils, tape residues, grease, and even white residues without damage to electronics. It also makes each can of solvent last longer, saving money. Importantly, it exceeds the ESD requirements of MIL-STD-2000, so Trigger Grips can be used safely in static sensitive environments.

Our recommendation: give everybody a can of Flux Remover C (#MCC-FRC) or PowerClean (#MCC-PW2), a Trigger Grip and a Bench Mounting Kit. It's an inexpensive little system that works great, is priced right and solves all of the technician's worries.

CLEANLINESS TESTING

The last part of your question was about ways to determine how clean is clean enough. There are three common ways to check for cleanliness. The first is a simple visual inspection - if there are no visible flux materials and no white residues, most people just call it "clean enough." Not very sophisticated, but it works.

At the other end of the scale is Surface Insulation Resistance testing. A full SIR test takes 28 days, a big machine and serious system calibration; one of the industry leaders is Thermotron. Most users simply farm out the test to a research lab. The result of an SIR test is a measure of the resistance of electrical flow across the traces on a PCB caused by conductive contamination, such as the ionics from fluxes. High resistance is good and means the board is clean.

A less precise but faster and far more user-friendly measurement of cleanliness is ionograph testing. This process requires the board to be immersed in a bath of deionized water for 15 minutes or so. After the water has had a few moments to circulate around the components, the water automatically is tested for ionic contamination. Since the DI water will dissolve ionics from the boards, any change in resistivity of the water would indicate the relative dirtiness or cleanliness of the board.

The Omegameter 600SMD is the industry standard for ionic testing. The system provides an accurate, repeatable and rapid method for determining cleanliness and complies with industrial specifications including MIL-STD-2000A, IPC-TM-650 and ANSI/J-STD-001B. A quick Google search or a visit to TechNet will help clarify these options.

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Mike Jones
Vice President
Micro Care
Mr. Jones is an electronics cleaning and stencil printing specialist. Averaging over one hundred days a year on the road, Mike visits SMT production sites and circuit board repair facilities in every corner of the globe, helping engineers and technicians work through the complex trade-offs today's demanding electronics require.

There are two types of defluxing approaches in the electronics manufacturing industry, namely the automated and manual cleaning applications. With no-clean flux removal, the most commonly used approach is the automated defluxing process. Companies either use spray-in-air, spray-under-immersion or ultrasonic technologies. In contrary to water soluble flux residues, RMA/RA, no-clean, and synthetic residues require the use of water-based or solvent-based defluxing agent.

White powderish residues are typically the sign of partially cleaned flux residues being the result of inefficient cleaning processes.

Reliability tests could be listed as:

  • Visual Inspection
  • Ionic contamination Testing (Omegameter, Ionograph, etc. )
  • Ionic Chromatography
  • Flux Test Kit
  • Resin Test Kit
  • Halide Test Kit
  • Impedance Spectroscopy
  • Core Test

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Umut Tosun
Application Technology Manager
Zestron America
Mr. Tosun has published numerous technical articles. As an active member of the SMTA and IPC organizations, Mr. Tosun has presented a variety of papers and studies on topics such as "Lead-Free Cleaning" and "Climatic Reliability".
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