|Ask the Experts|
June 28, 2018
Water Washing Pressure
Is there any method to measure the washing water pressure exerted on the printed circuit board assembly surface by an in-line washing machine? We have some very delicate components on our assemblies and believe they may be damaged by excessive water pressure.
|Expert Panel Responses|
It's hard to answer this question without knowing what the delicate parts are, and what type of circuit card assembly (CCA) it is and its intended service environment, and the flux chemistry being used (RMA, no-clean, or water-soluble). There are spray pressure devices you could probably purchase that would give you the spray pressure reading as opposed to the supply pressure reading (gauges that display water pressure in PSI) that nearly all in-line cleaners have. But even if you could theoretically mount the pressure reader on a fixture, run it through the wash, and "profile" the wash so to speak, how would you know what pressure is going to be needed to not damage the components? The damage can also be due to variations in the water temperature for a given pressure. Partially clogged spray valves can give you variations in spray pressure also. I think the best way to determine this may be to get some scrap parts and solder them to some test boards. Are solder coupons or scrap PWBs available from the board fabricator? Keeping the water temperature at a given setting, you can vary the water pressure readings in the supply lines to the spray nozzles to determine what is the safest combination to use. Realize you will need to check all of the sprays, as most in-line cleaners have an initial spray, secondary spray, and final rinse spray. You may also want to document the air knife pressure also. Run the test boards at incrementally higher spray and air knife pressures and inspect for damage to the scrap or dummy components after each pass. Keep in mind that the pumps may need a minimal spray/flow rate to operate safely. Check the manual for the in-line cleaner you have, and talk to the machine supplier. They may also have some good advice regarding the spray force and volume requirements. Wash baskets with the right screen size inside of them may be enough to deflect the direct spray and protect the parts. For example, a wash basket with the right size screen above and below the sensitive parts, but not necessarily over the whole CCA. Another option is to not run the assembly through the in-line cleaner, but have an ESD-safe wash tub and adjustable spray wand set up using the de-ionized water in your supply, with a drain filter feeding the water back into the de-ionized water loop. This requires manual intervention, however, and having an operator cleaning the boards in the tub may add handling and labor cost issues. There are also batch-style "dishwashers" and solvent vapor degreasers that are gentler and can be loaded, set, and run without any further operator involvement. This allows you to not have to adjust the spray pressure on the in-line machine, where a higher pressure may be required for cleaning other more robust products. Again, the type of batch cleaner to be selected and the solvent choice depends on the flux chemistry being used. Realize that you may be able to use a different flux chemistry for this particular product that better suits the optimal cleaning process. No matter which safe cleaning option you come up with, be sure it gets the CCA clean. Run cleanliness tests such as Omegameter or Ionograph and monitor the results over time. If you do not have that option, you should at least send out some test boards to an outside contractor who can test for cleanliness. The level of cleanliness required is dependent on the type of product (Class 1, 2, or 3) and your customer requirements. Qualify the process carefully before using it in production, then monitor the production CCAs. Good luck.
Some machines give pressure / power ratings for their jets, but this is not always an accurate rating of what you can expect to see at board level. Also, the size and shape of the component may effect the pressure seen. The only guaranteed method would be to mount a component and place through your system and then test it. If it passes, then the likelihood is that the pressures are OK. To generate confidence, you could do multiple passes with the same component to determine how many exposures are required for it to fail. If the component is too expensive to test in this way, a pressure sensor mounted on the board (glued) in the same area will give you an indication of the pressure that the component will be exposed to. If either of the above mentioned methods is too costly, then you could consider placing the component post cleaning using a no clean process.
The factors influencing the impact pressure are liquid flowrate, spray angle, spray pressure, droplet size, spray distribution and spray nozzle distance from the board surface. The impact pressure is governed by the following formula: Impact = Mass flowrate x spray velocity For the technical paper including the step by step derivation of the final impact pressure formula please provide us your email address so that we could mail it to you as an attachment.
Application Technology Manager
You can investigate the products from Tekscan located in Boston, MA. We played with one of their testers about 3-plus years ago. This technology will come at a price so be prepared.
Technical Expert Sales Support
The best way I have found to measure maximum impactpressure at the board surface is to fabricate what we call a pressurepuck. This is normally made from plasticor metal resistant to the cleaning environment.The simplest is a small hole drilled in the upper surface, connected toa larger hole drilled from the side. The sensing hole should be no larger than the minimum dimension of the impacting jet or fluid spray pattern. A larger hole will not give accuratereading. A pressure gauge or sensor is then attached via atube. The pressure can then be accurately measured bypassing the sensor directly under the spray at then height of the component ofconcern.
Austin American Technology
You could apply a pressure indicating film to the pcb to measure the forces at the PCB, you'd still have to establish what is safe for your product. Run the board(s) through in multiple locations to determine pressure consistency within your machine.Mike Sewell, OAI Electronics
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