|Ask the Experts|
October 30, 2017
What is the Proper DI Water Resistivity?
What the proper resistivity reading for DI water when cleaning electronic assemblies and why? Is there a specification that supports this reading?
|Expert Panel Responses|
Deionized water is often used in precision cleaning. It's a tried-and-true process, but with a number of hidden pitfalls. In general, there are three general grades of water: tap water, distilled water and deionized (DI) water. In terms of precision cleaning, neither tap water nor distilled water are sufficiently pure to handle the job as both are contaminated, to greater or lesser degrees, with minerals and organics. So you must jump to DI-water. The quality of DI water usually is measured by the water's resistance to electric current (in OHM-cm). Quoting from Finishing.com: "Deionized water quality depends on a variety of factors, including raw water composition, ion exchange types and quantities, and the number of resin tanks in the system. Two-bed deionizers use separate tanks, one containing cation resin, the other containing anion resin. A two-bed weak vase deionizer typically produces water with electrical resistance of about 50 kOHM-cm. A two-bed strong base deionizer typically produces water with electrical resistance of about 200 kOHM-cm." Now, how pure is that?
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.
For specifications, please refer to Resys Inc. or Myron L Company websites where most cleaning equipment manufacturers work with when it comes water treatment systems.
For printed circuit board cleaning applications, the brand new installed systems provide a resitivity level up to 18 MegOhm. As the DI water beds wear out and the resistivity drops down to 1 MegOhm, the beds are normally replaced with brand new ones.
Application Technology Manager
The resistivity of DI water is higher as 108 Ohm. With a bubble equipment (additional carbonic acid) you can achieve a value smaller as 107 or 106 Ohm. The problem is, do you use normal DI water, you can produce electrostatic charge on the PCB or electronic device (ESDS). This electrostatic charge can damage your electronic device shortly or later in the final product. You cannot control the damage or degradation of ESDS.
The maximum resistivity of treated water should be 18,3 M-Ohms. Classification for Electronic assembly water resistivity does exist. Should be able to find this on the internet for confirmation. I believe you will find three classes and grades if memory serves me right.
Resisitivity of a rinse water is a function of surface area being cleaned. We rinse to a resisitivity of 200 Mohm's b/c that is the recommendation from our equipment supplier. We also, checkfor active material in areas of the PCBA that are specific to damage. Example, we will spot check that there is no ionic activity in on or near QFPs and other areas that may have residual flux left behind.
Capital Equipment Operations Manager
Specialty Coating Systems
Ionic contamination testing is required as ionic residues remaining on both the PCB manufacturing process and the soldering process may affect the reliability of a finished assembly. A Contaminometer measures ionic contamination by essentially immersing a sample in a test solution of propanol and de-ionized water to dissolve the contaminants. The dissolved ionic substances cause a change in conductivity of the test solution; this change is precisely measured and converted into a contamination value expressed as ug/cm2 NaCl equivalence. In practice, it's impossible to completely remove all contaminants, so maximum threshold levels such as 0.2ug/cm2 NaCl equivalence are commonly accepted for modern densely packed assemblies.
Electronic Technology Corporation
Using high resistivity water has been the most effective cleaning with both batch and in line cleaning. We use 18.2 meg ohm water typically but change the mixed beds on recirculation at 10 meg ohm. We clean 6 days a week with two shifts and we clean finished assemblies, with no clean flux, bare boards prior to any assembly and components prior to assembly. We have found under proper cleaning conditions (time and temperature) the use of 18.2 meg ohm DI water will not damage, BGA, QFP, DIP, flip chip, bare boards with OSP, Tin Lead, SAC LEAD FREE HASL, SAC assemblies, tin plated parts. I have a lot of data on cleaning if you need additional information please contact me directly.
President/Senior Technical Consultant
There is not a published specification applied to removing flux from populated assemblies. As a practical suggestion, it is important to consider why DI water is used. Solvency: DI water, with its near absence of ions is hungry. It wants to absorb ionic content. Absence of solids: Tap (drinking) water normally contains minerals. These minerals can leave residues behind as the water evaporates (like spots on a car). DI water may be mechanically removed via airknifes or rapidly evaporated (a common technique in batch-format defluxing systems) without concern for solids left behind. Contrast: Some automatic batch-format defluxing systems are equipped with real-time cleanliness testing capabilities. In this case, one must use DI water in the rinse cycles. Because DI water is non highly ionic (it does not conduct electricity well), it makes a very effective contrast agent when compared to defluxing chemicals. Most defluxing chemicals are highly ionic and are easily detected in DI water. Because modern batch-format defluxing systems test the electrical conductivity of each rinse cycle, even small trace quantities of defluxing chemical can be detected, forcing additional rinse cycles until there is an absence of detected chemicals in the rinse water. Recommended Quality of DI Water: DI water is available at qualities between 1 M-Ohm and 18 M-Ohm. While higher qualities (above 10 M-Ohm are common in the semiconductor industry, a quality of about 5 M-Ohm is adequate for all defluxing applications. The cost of DI water averages about 6 cents per gallon. Based on this low cost, I would always recommend the use of DI water.
Appreciate the concise and yet in-depth comments on DI water resistivity.Sita Bhamidipati, Western Digital
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