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baked on water soluble flux residue betwwen fine pitch comp.

Jim M.


baked on water soluble flux residue betwwen fine pitch comp. | 1 December, 2000

My company currently uses water soluble paste for our SMT process. We were having trouble retaining hot water in our in line, closed loop DI cleaner. The cleaner kept shutting down when the water temp. dropped below 125C.

As a result, the conveyor shut down and the boards stayed in the hot area of the cleaner consequently any flux residue has been baked on the board.I've recreated this scenario with great success during my experiments to find out where the white material was from. Most of the dried flux residue is located between or on the pads of a 15 mil pitch QFP (near the back of the pads)so i am unable to manually clean (Ie: with a brush as the bristles will not make it throught the pitch.

I've tried three cleaning solutions recommended by solder manufacturers without any success. I've also tried IPA and acetone without success. I have two more samples on the way.

Other then removing the QFP and manually cleaning, (which is the option i don't want to travel) Does anybody in the industy have a cleaning solution that would remove the baked on water soluble flux residue between the pads of the 15 mil QFP?

The boards are high temp FR4 with components on both sides including plastic connectors.I will need to do SIR testing for any residue remaining as the boards go into a medical device.

Your response would be greatly appreciated.

Thanks in advance.

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Dason C


Re: baked on water soluble flux residue betwwen fine pitch comp. | 1 December, 2000

Jim, you may need to look with different paste instead of the cleaning solvent. Please advise what is the paste which you currently using?

Beside, when you talking about the board stayed in hot area, is it a drying area or the cleaning zone. If it is a cleaning zone then the board should be wet becos of the vapor. If the board is in drying zone then the board should completely clean w/o residue. Please also check your cleaning machine resistivity as well and the board cleaningless after cleaning by Omega meter.

Good Luck

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Jim M


Re: baked on water soluble flux residue betwwen fine pitch comp. | 1 December, 2000

Thanks for your reply. I 've tried to clarify and answer your questions.

The water soluble paste used is WS3060, type 4. The boards are sent through a inline di cleaner after reflow. There is a hot Di waterwash, rinse and then hot air to dry the water. Boards are ran on carriers through the smt line into the cleaner on the carriers. The boards were in the hot di wash when the the connveyor stopped. The water is 130C. Boards were probably in this area for 1/2 hour before operators removed them. So esentially the flux is off all area's except the hard to get area inbetween the pads at the back of the QFP. The wash is now working properly, with the same boards and water soluble solder paste. I am lazy by nature and was just looking for a cleaning solution that will remove the baked on flux without all the rework invloved. The boards with the flux residue fail the cleanliness test.

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Re: baked on water soluble flux residue betwwen fine pitch comp. | 1 December, 2000

Curious, very curious ...

Residues � Where are these residues? [solder & laminate / mask, solder only, laminate / mask only] What is the result of your analysis to the residues? How do you know it�s a flux residue, rather than a chemical by product of the flux and metal oxides? Why isn�t this residue forming during the soldering process when the temperatures are presumably higher? Or could it be: the residues are forming during soldering and they are just becoming noticeable during cleaning? Have you monitored the temperature of the component leads where the residue is forming throughout the process?

Flux � What flux are you using?

Recommended cleaning solutions � What were the three flux manufacturer recommended cleaning solutions? How were these solution applied? What were the results?

Washer � What washer are you using? What is your hot box temperature? Why doesn�t the hot box shut down when the conveyor shuts down?

If these are baked flux residues, � * Water soluble fluxes can form water insoluble decarboxylated and polymerized substances when heated. * Mild alkaline solutions such as dilute saponifiers can help in removing oxidized and decomposed water soluble flux acids.

Remember: A decrease in wash water temperature of 10�C can reduce the cleaning rate of your washer by 50%. So, you may have a bigger problem than you think.

So, here's the scoop ... find a saponifier salestype ... find out if it thinks its materials will clean whatever you think you've got [of course it will say it can do the job] ... talk to a technical type to make yourself feel comfortable ... tell the salestype you want assess it's chemicals by running a coupla boards through their lab ... get your boards cleaned ... say pfew!!! Live well live long.

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