Ionic contamination

Edmund: Responding to your questions:

1 What is purpose of using DI water to wash a PCBA?

Compared to tap water, DI water is less likely to leave: � Metal cations on sensitive components. � Salts of Ca, Mg, and other stuff on metals on the assembly.

Water treatment is the most important application of ion exchange resins. The resins in deionizers reduce the amount of all ions (positively-charged cations and negatively-charged anions) to very low levels by exchanging sodium ions for hardness minerals (Ca++ and Mg++). Ion exchange materials: � Most common: Those based on styrene. � Once common, but used less frequently today: Zeolites � Cellulose ion exchangers, often used in chromatographic separations.

Rinsing with DI water requires less time, energy, and about a third of the amount of water to reach the same cleanliness level as tap water.

It provides process control, because tap water resistivity varies by: � Geography � Temperature of the water, because the specific resistance of water drops dramatically when heated. For instance: When 0.06 microsiemens-cm water at 23�C is heated to 70�C it drops to 0.33 microsiemens-cm. � Vagaries of Publicly Owned Treatment Work (POTW) processing. {On this latter point: I recall a thread on the variation in tap water resistivities from POTW posted by Steve Zeva on SMTnet. (Although, as JAX says, I could be wrong.)(If you can�t find it, I�ll make-up a story about what I remember him saying.)}

2 If the board is not washed using DI water, what will happen?

Who knows? It depend on the residues that on your boards when shipped. The issue is not whether you use DI water or not. What matters is the cleanliness of your product when your customer removes it from your packaging.

3 What should be the range of resistivity reading?

The range of resistivity reading should be the level is necessary to meet you board cleanliness requirements. Beware that cleanliness depends on the particular approach used to measure the residues. J-001 recommends methods for measuring and standards for cleanliness.

Following on, the best sources of information are: � Mixed bed packaged industrial deionizer technical sales types from: - Culligan - USFilter - Ionpure

� Product sales types of exchange resin offered by: - American L. B. Sci. & Tech. (www.americanlb.com/company.html) - Boehringer Ingelheim (www.bi-bioproducts.de/products/knowledge/000110.shtml) - Dow Chemical (DOWEX) (www.dow.com/liquidseps/about/resin.html) - Purolite (www.coins.co.uk/purolite/product.htm) - ResinTech (www.resintech.com/products.htm) - Rohm and Haas (www.rohmhaas.com/businesses.dir/IonEx.html) - Sybron Chemicals (www.ionac.com/ionexchange/tables.htm) - Thermax Chemicals (www.thermaxchem.com/prod1.htm)

� Equipment suppliers, such as: - Remco Engineering modular ion exchange systems - Advanced Separation Technologies continuous ion exchange systems - Hydromatix developed a regeneration process for rejuvenating ion exchange resins in rinse water recycling.

� Books on cleaning technology are have problems, including: - Written in the early 1980s: "Cleaning And Contamination Of Electronics Components And Assemblies" Ellis, BN; Electrochemical Publications; ISBN 0901150207 - Less dated: "Handbook Of Aqueous Cleaning Technology For Electronic Assemblies" Cala, FR & Winston, AE; Electrochemical Publications; ISBN 0901150312 - Farly cuurent, but highly commercially oriented: "The Aqueous Cleaning Handbook" McLaughlin, MC & Zisman, AS; Morris Lee Publishing; ISBN 096453367X

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