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January 7, 2008

Resistivity and Cleaning

During the deflux cleaning process, along with water temperature and pressure, resistivity is one of the more important control parameters.

What is the minimum DI water resistivity requirement for deflux cleaning?

How do DI water trace metals (such as Ca, Na, Cl) affect the product quality?



Y. K. Lai

Experts Comments

Typical DI equipment tends to create DI water with resistivity of 2 to 5 megaohms. Resistivity greater than 10 megaohms can start to cause corrosion issues due to leaching.

The influence of trace metals is driven by conductivity and reactivity. Because of this, halides tend to be the most detrimental. Anions are also more detrimental than cations because they tend to form acidic solutions, which are more reactive with metals than basic solutions.

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Dr. Craig D. Hillman
CEO & Managing Partner
DfR Solutions
Dr. Hillman's specialties include best practices in Design for Reliability, strategies for transitioning to Pb-free, supplier qualification, passive component technology and printed board failure mechanisms.

To address the DI water resistivity question for proper defluxing with water the higher the resistivity the better (we currently start with 18 meg ohm), but will still clean with 4 meg ohm or better.

This typically takes a longer rinse time and elevated temperature. The effect of any ionic residues of 100 ppb or more in critical areas can lead to leakage and stray voltage problems. Chloride, fluoride, sulfate, sodium, calcium, iron, and ammonia all can have individual and collective impacts on the electrical performance.

This is why cleaning and rinsing with the best water quality available creates the best conditions for cleaning not only flux residues, but board and component fabrication residues. One last point on cleaning is the fact that surface tension of water is high and has limitations getting under some low standoff components.

Remember it is not the cleanliness of the entire board that causes the electrical failure but he amount of residue between two pads on a critical circuit the define the cleanliness of the assembly.

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Terry Munson
President/Senior Technical Consultant
Foresite
Mr. Munson, President and Founder of Foresite, has extensive electronics industry experience applying Ion Chromatography analytical techniques to a wide spectrum of manufacturing applications.

For defluxing cleaning applications, the recommended DI water quality should be 1 to 10 Microsiemens –cm. Any conductivity that is higher than 10 Microsiemens-cm is not recommended. The resistivity is the reciprocal of the conductivity. That afore mentioned range corresponds to 1 to 0.1 Megohm-cm. Any resistivity value below 0.1 Megohm-cm is not recommended either.

Ionic contamination on the substrate surface could increase dramatically which could later on impact the product quality due to possible formation of electrochemical migration (i.e. dendrite growth) or contamination induced leakage current if hard water – that contains Ca, Mg, Cl and other ions found in tap water - is used for rinsing purposes.

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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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