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Flux residue?

#24366

Flux residue? | 30 April, 2003

Hello all, We have received reports from two customers about electrical leakage on pcb assemblies that used Kester paste #HM531 (63/37) and washed with Kester 'Bio-kleen' saponifier and DI water. The leakage is such that when the boards receive power, low levels of voltage will leak and cause LEDs to illuminate and current sense circuits to be innacurate. As time passes, the voltage accually increases until in the case of the LED, it is completely illuminated. The really embarassing part came when one customer put the pcbs in his dishwasher at home, threw in some Cascade, and the boards came out with no leakage. (Apparently, Kenmore makes a better pcb washer than Hollis!) We've checked with Kester and appear to be following all of the rules regarding reflow and wash temperatures, our the resistivity of our water is good, and there were no fine pitch devices on the boards where flux could be trapped. I can't believe that we would need to hand-scrub pcbs to remove all of the flux residue. Has anyone else had this type of experience?

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Mike Konrad

#24369

Flux residue? | 30 April, 2003

In all likelihood, you may not be leaving flux on the board rather you may be leaving saponifier on the board. Improper rinsing is a leading cause of cleaning related board failures.

The problem with some inline cleaning systems (especially old ones) is that they lack proper rinsing. The final rinse manifold (the last water that comes into contact with the board) is normally powered by incoming water pressure as opposed to a pump. The �sprinkle� of final rinse water may not be enough to displace dirtier pre-rinse water. The only thing worse than leaving flux on a board is leaving saponifier on a board. Saponifiers are much more ionic than flux and therefore are much more conductive. Additionally, a saponifier may continue to �attack� a board after the cleaning process if not thoroughly removed. The fact that you have enough leakage that can �light an L.E.D.� leads me to believe that there is much more than flux on the board.

If it is truly flux residue then consider how a saponifier works. The process of saponification works when two things occur:

1. Contact between flux and saponifier. 2. Exchange of energy (heat, pressure) that forces the chemical reaction that transforms rosin flux into rosin soap.

If your old Hollis lacks the ability to produce the energy required to convert the flux to soap then there will be flux residue on the board.

Check the following:

1. Wash pump pressure (what is the HP rating and PSI?).

2. Clogged nozzles.

3. Is the wash solution loaded (spent)?

4. Is wash solution at correct concentration? Use titration test to determine proper percentage.

5. How�s your reflow profile? Too hot may burn flux on the board and not allow proper removal.

6. What wash temperature are you running? 150F � 160F works best for saponifiers (this is a problem with many polypropylene machines).

You may also want to try a different wash chemical. New wash chemicals utilize a saponifier / aqueous-solvent combination. They rely less on saponification and generally work better on �weaker� machines.

Chemicals I would recommend are:

PCB-Wash (ours) http://www.aqueoustech.com Vigon A200 (Zestron) http://www.zestron.com Kyzen 5020 (Kyzen) http://www.kyzen.com

If nothing is wrong (defective) with your old Hollis and profiles are in order and better chemicals do not solve the problem then you may want to consider purchasing a new cleaning system.

Mike Konrad www.aqueoustech.com konrad@aqueoustech.com

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#24370

Flux residue? | 30 April, 2003

Haaaa!!!!

Yes, we�ve seen things like that. We found that a high input impedance amplifier worked fine in a room with a controlled environment and worked poorly in the sweat-shop portion of our plant. We were using good ol� WS609, straight water washed [open loop] with a monitored DI rise.

You obviously are not cleaning your boards properly. Tell us how you know: * Washer is cleaning boards properly. * Saponifer concentration is monitored properly. * Saponifer concentration is measured properly. * Cleaning after third ops is ineffective. [Don�t tell us your operators use squeeze bottles for their flux.]

Just to suave your wounds, Cascade does a GREAT job cleaning, but it�s very expensive and highly ionic.

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#24388

Flux residue? | 1 May, 2003

Did you get any dulling of the solder joints at all or importantly was the resist discoloured or bleached after cleaning. It is possible to absorb flux and saponifier into the resist and this will definately cause poor SIR or electromigration especially in humidity or 0C environments. Just a thought as this is very common over here, one test that a customer did was to breath over the board while under test and this would be enough to fail?

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