Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


PCB washing

Guo Yingchun

#7646

PCB washing | 1 February, 2000

Hi,

I want to use ultrasonic machine to remove solder paste from PCB. Can I use water as the solvent? Will it do harm to the PCB?

Thanks in advance.

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Jason Bentley

#7647

Re: PCB washing | 1 February, 2000

This all depends on the mixture of your solder. If you are using water soluable flux then it will get clean. But not alcohol based flux. As for using the ultrasonic this might be a problem I personally have never heard of someone washing a board in a ultrasonic machine becasuse in theroy it uses ultrasonic waves. This in turn "if it is a true ultrasonic machine" will cause problems with the board. Ie. measing. bubbles.... and so on. This is going to be caused by the air that is traped between the layers of the board vibrating and in turn wanting to excape the PCB. But if you do decide to do it I would test it. If you have a solder sample or something that you know that you are not going to use in the future then try doing that first to make sure that you are not going to damage a board. "just to be on the safe side" If it does do damage to the board the you have your answer but don't forget to do it more then once because you know that there are some of those boards that you are going to get that are going to get washed over and over and over for some reason or another. Also if I does cause damage. A great factor of seeing the damage will be caused by the thickness of the board and how much air is really traped in between the layers.

As for getting clean I think that it will be clean if you are using the right mixture for the job.

Jason

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Mike Konrad

#7648

Re: PCB washing | 1 February, 2000

Your question may be answered in two parts. First, if you are using a water soluble (OA) paste, then water is an excellent �solvent�. Water, in one way or another, is used to clean all fluxes and pastes from PCB�s. In some cases, a chemical is added to the water. The fact that water is used poses no problem with the PCB, as long as the final water to contact the board is VERY clean (DI) and that the board is sufficiently dried.

Secondly, Jason is correct with regards to potential problems with air trapped in a multi-layer boards, although board damage is quite rare. A greater concern is that the board is properly rinsed. If your ultrasonic cleaner has provisions for rinsing, then be sure to utilize it. Keep in mind that your ultrasonic cleaner most likely does not filter or re-deionize the water in its tank. If this is true, then each board cleaned in the wash tank will be cleaned with dirtier water than the previous board. At some point, the water will require changing. In all instances, a good rinse (preferably with DI water) will be required.

Mike

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#7649

Re: PCB washing | 1 February, 2000

When cleaning water washable flux from PCBs using U/S, the final rinse can be with spray DI water for aesthetic purposes because the wash cycle is really just a long U/S rinse (no added chemistry). Fresh DI water should be continuously added to freshen the wash solution and it is best to add this water when the U/S are off. This will not only keep the wash solution fresh, it will also overflow the wash water to "skim" any floating contaminants off the surface.

However, when cleaning non-water washable contaminants, the most important factor is ALWAYS THE CHEMISTRY. U/S is just a mechanical scrubbing action (cavitation). If you do not identify the correct chemistry U/S will do little good. It is the same as trying to wash greasy hands without the correct soap, you can scrub all day long (mechanical action) but the grease will not come off without the right chemistry.

When using a chemistry, it is also important to use an U/S DI rinse. U/S will force the wash solution into tight tollerance areas and under components. If you feel that it is important to rinse the wash solution out from these areas, the only way to do so is with an U/S DI rinse. Spray rinses will not reach into these areas.

As far as damage, the EMPF Laboratory (Mil Spec) is recommending U/S for many applications. Dr. B.P. Richards of GEC Marconi has done the most research in this area. In a nut shell, the recommended parameters are: Use U/S of 40kHz or higher (the lower the frequency-the more aggressive the cleaning action)., Use cleaning cycles of 10 minutes or less, and use power input of 14 watts per liter or less, and, like any new cleaning process, TEST IT!

I have been working with U/S now for over 15 years for many different applications. When someone asks me if U/S will damage an SMT assembly, my response is "try putting your hand under the sprays of a high pressure inline washer-you could draw blood! If you put your hand into a 40kHz U/S bath, you can hardly feel it."

U/S got a bad reputation as a result of the 1950s experience of the U.S. Air Force. PCBs then were very delicate and U/S was a very new and uncontrollable cleaning technology using very low frequencies (20-25 kHz). Now SMT assemblies are more durable and U/S is the most controllable precision cleaning technology available. SMT and U/S are very compatible.

However, just as you would not buy or use a spray system without testing it, you should also test clean in the U/S system before implementing it.

Additional information is referenced on our Web Site: www.smartsonic.com click on the "Recommended Reading" button. Go to the article by Richard Clouther. Several articles are referenced by Mr. Clouthier regarding cleaning PCBs with U/S.

Feel free to call me if you have any other questions.

Bill Schreiber Smart Sonic Corporation Tel: 1(818) 909-6400 Fax: 1(818) 909-6409

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#7650

Re: PCB washing | 1 February, 2000

Guo:

You want to know:

1 Can I use water as the solvent in an ultrasonic machine to remove solder paste from PCB?

Continuing from Jason�s comments: Sure you can use water as the solvent to remove solder paste from PCB, but recognize that some fluxes were not intended to be water cleaned. So there may be a colored residue remaining after water washing.

2 Will using water as the solvent in an ultrasonic machine to remove solder paste from PCB do harm to the PCB?

Who knows? It depends on what you�re cleaning. You didn�t say.

For bare boards: No problems. For assembled boards: It depends on the construction of the components and design and use of the cleaner.

In support of this, J-STD-001 states:

"8.1.3.1 Ultrasonic Cleaning. Ultrasonic cleaning is permissible:

a. on bare boards or assemblies, provided only terminals or connectors without internal electronics are present, or b. on electronic assemblies with electrical components, provided the contractor has documentation available for review showing that the use of ultrasonics does not damage the mechanical or electrical performance of the product or components being cleaned."

Acceptance tests to determine the impact of ultrasonic cleaning from the IPC TM-650 Test Methods Manual are:

2.6.9.1 "Test to Determine Sensitivity of Electronic Assemblies to Ultrasonic Energy" 2.6.9.2 "Test to Determine Sensitivity of Electronic Components to Ultrasonic Energy"

Further, as with all cleaners, the solvent gets "dirtier" with use and it may be appropriate to monitor the cleanliness of your boards after cleaning.

For additional background, look in "Circuits Assembly" magazine, September 1999, for "Defluxing With Ultrasonics" by L. Hymes.

Good luck

Dave F

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#7651

Re: PCB washing | 5 February, 2000

I agree with all of the previous comments.

1) Here are a few things to keep in mind when cleaning with water. To effectively clean, any liquid must wet the surface. By itself, water has a surface tension of 70 dynes/square cm. This is relaively high. Most aqueous cleaning solutions like the one we use contain a surfactant which lowers the surface tension and promotes better wetting.

2) The water temperature has a large bearing on the effectiveness of the cleaning process. The rise of 10 deg. C will double the cleaning power of the bath. We have found 50-60 deg C about right. Temperature is also important in the rinse bath.

3) Ultrasonics are great if the board and components can accomodate it.

4) Many of the "cleaning" problems I have seen are directly caused by poor rinsing and drying processes. The drag-out from the cleaning bath must be removed from the rinse by either a reverse cascade or by periodically dumping the rinse bath.

5) The water quality of the rinse is important. Most stained gold plated components can be traced back to impurities in the rinse bath. For gold plated seal lids for ceramic packages, we recommend at least 16megohm D.I. water.

6) The rinse step should quickly follow the wash step. This especially true if any aqueous cleaner like AQ-1400 PakClean is used.

7) Drying is another issue. We developed the TVP drying process to cut the drying time in half. This is important to your solderability. Intermetallic growth of the metallization is a function of accumulated time at temperature.

If you have further questions, contact me.

Stu Leech Altos Engineering, Inc. (602) 843-8369 sleech@ix.netcom.com

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