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July 17, 2017

Is lead contamination during cleaning of PB-free stencils possible?

We have a stencil cleaner used for cleaning both Pb-free and SnPb stencils and misprint boards. We use ultrasonic type of cleaning and the solvent is VIGON SC200. Is lead contamination of PB-free stencils with this process possible?



H.N.

Experts Comments

In regards to the lead contamination in an ultrasonic cleaner, yes it is possible to cross contaminate the stencils because the particles of powder go to the bottom of the unit. As the unit vibrates the particles are moved through the bath and can pass through the stencil openings. Remember, the solvent cleans resides and fluxes only. The spheres of solder powder are left in the bottom of the bath.

I suggest 2 (two) separate units. One unit with a green sticker or tape on the unit indicating a lead-free only process and the other unit left as is for the Sn/Pb process.

Good Luck in you process!

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Ted Marek
President
SMT Sales Associates Inc
Mr. Marek is the Founder and President of SMT Sales Associates Inc. SMT is a distributor and manufacturer's representative organization in the PCB assembly marketplace. He has over 11 years experience helping customers select soldering materials and capital equipment.

The short answer is maybe.

We need a few more details. The cleaning agent you are using is generally used in one of two ways; rinsed with water or simple solvent air dry. This is an important distinction for reasons I'll explain later.

The cleaning agent is of course cleaning the soils on your stencils, that's why you're using it. With raw, un-reflowed solder paste as the cleaning agent does its job reacting with the paste, it free solder spheres, lead free or tin/lead into the solution. If the equipment employed does not have an effective filtration system, these solder spheres run a high, all but certain risk of being re-deposed onto any stencils cleaned in the machine. This is particularly true for those using the solvent air dry technique. If a water rinse is employed, these "loose" solder spheres should be rinsed off and not re-depose, but proving it would be a challenge.

Additionally, it is quite common for cleaning materials to absorb lead in stencil cleaning applications. After all, there is plenty of lead in raw paste, and while legacy technologies like the SC200 materials are not highly reactive the likelihood of absorbing some lead over an extended period of time is quite high. Once again, if you are employing the solvent air dry technique this absorbed lead will not evaporate and therefore will remain on your stencil. There is no where else for it to go. Once again, water rinsing should greatly alleviate this issue, but it may still be hard to prove.

Now let's consider the water you are using to rinse. If it is virgin, DI water used once and flushed down the drain, there is little chance of lead being transferred to your lead free stencils. If you recycle the water, it once again comes down to whether you have proper filtration and maintain that filtration. These solder spheres are not very large, and can be challenge to remove with 100% certainty.

I have mentioned the possibility of some process effluent going down the drain. One of the advantages of the move to lead free is the tight regulation of lead throughout the world. In the USA, an effluent stream with 5 ppm lead is considered a hazardous waste and is highly regulated. Often local regulations for lead in waste water start at the 0.5 ppm level. The lab test to check for lead levels in a solution is quite inexpensive, under $50. As you are evaluating your system, these are always good test to run and have on hand for future audits.

So if you have solid filtration that is maintained well you are in pretty good shape. If you have a 10 year old machine allowing the solvent to air dry, consider upgrading your system and maybe the cleaning agent. Mature, legacy products like SC200 can still be effective, but there are modern lower cost of ownership alternatives available as well.

Hope this was helpful.

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Tom Forsythe
Vice President
Kyzen Corporation
Mr. Forsythe is a recognized expert in cleaning chemistries and processes. Tom has a Bachelor's in Applied Mathematics & Engineering from the US Naval Academy. He is well published in both the industry trade magazines. Tom has spent the last 14 years with Kyzen Corporation.

There are several possibilities of lead contamination onto a lead-free stencil when using a single stencil cleaner for cleaning both lead and lead-free stencils.

Lead is very unstable when exposed to a liquid media. This is why lead pipes are no longer used to plumb water. Lead will begin ionization (dissolving) in liquids almost immediately. It can only take a few days to reach hazardous levels of 5 ppm. The filtration systems used in most stencil cleaners incorporate micron filters. Micron filters are very efficient for filtering solid particles. However, once the lead has ionized in solution, the lead ions will pass through a micron filter and contaminate the next stencil. This phenomenon will occur in both spray-in-air or ultrasonic stencil cleaners.

Another possibility depends on the type of stencil cleaner used. Stencil cleaners using spray-in-air technology will broadcast the solder paste throughout the process chamber. Paste (lead) will tend to coat the plumbing and walls of the process chamber and not reach the filtration system. The next time a stencil is cleaned; the fugitive paste from the previous cycle can dislodge and contaminate future stencils. 

Other factors to consider involve the chemistry used. If a solvent-based wash solution is filtered and used as a rinse agent, lead ion contamination can be expected. Solvent-based chemistries are usually difficult to rinse and require long rinse cycles and large amounts of DI water. Water-soluble chemistries (saponifiers and surfactants) usually rinse more effectively and are less likely to leave traces of lead contamination.

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Bill Schreiber
President
Smart Sonic Corporation
Mr. Schreiber developed the original ultrasonic stencil cleaning process in 1989. Obtained the only EPA Verification for specific parameters of Environmental Safety, User Safety and Cleaning Efficiency for a stencil cleaning process.

Cleaning processes consist of a wash, rinse and drying step. In your case, the wash process is cleaning the substrates with VIGON SC200. Most of the Pb & Pb-Free contaminants will be removed and trapped in this stage. As long as you do have a rinse stage in a separate tank either with VIGON SC200 or water, you completely eliminate this concern. Remember that VIGON SC 200 due to its unique formulation can also be used as a rinsing agent.

f your equipment doesn't have a separate rinsing tank available and in order to ensure that there is guaranteed zero cross contamination of Pb, there are two options:

1. Either the separation of the Pb and Pb-Free cleaning processes using two different cleaning machines or

2. Recirculate the cleaning bath through a suitable heavy metal adsorber which would take out all Sn/Pb or other heavy metals from the bath to avoid redeposition.

A detailed technical paper on this subject will be presented by Zestron America this September at Assembly Technology Show in Chicago. Upon request, our Technology department can provide you with a copy of this extensive study. This experimental study focuses on the feasibility of cleaning Pb & Pb-Free in the same process."

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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".
The short answer is "yes, it is possible to cross contaminate leaded and no-lead stencils when cleaning in a common stencil cleaner."

Cross contamination of lead to no-lead stencils can be transmitted either with solder balls already present in the wash moving to and sticking on a new stencil surface or by ions dissolved in the wash solution and dried on the stencil surface.

Re-deposition of lead containing solder balls can be minimized by filtering and sedimentation design elements in the cleaning system. Both ionic contamination and leaded solder balls can be minimized by proper rinsing.

The next question you should ask is "can you get enough lead on a no lead assembly to violate lead free standards by stencil transfer?  In my opinion a cleaner with a good clean rinse is at very, very, low risk of causing a problem.
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Steve Stach
President
Austin American Technology
Founder and President of AAT. Steve holds numerous patents and has authored numerous research papers and articles in cleaning and soldering. Steve is a founding member of the Central Texas Electronics Association and is a past Director of IMAPS. Steve is active on several IPC cleaning committees.
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