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Water Residue Stains - how to remove?

ianchan

#18481

Water Residue Stains - how to remove? | 19 December, 2001

Hi Experts,

I need help here, advice appreciated from all...

Production is running a Lot of flex-circuit PCB, through the SMT line, with 0402 components. The flex-PCB is taped to FR4 support "pallets" when processing thru' the SMT P&P m/c.

We use WS (water soluble flux) process, and so require Aqueous water wash thru' a Electrovert Aquastorm 90 m/c.

Subsequent oven-bake is required to dry off the water residue (on the side facing in contact to the FR4 pallet side) that wasn't dried up by the Electrovert Air dryers.

Problem is production "forgot" instructions to peel off the flex-PCBA from the pallet before oven-bake drying process. Now the side facing down in contact to the FR4 pallet, is "spotty" with water-stain marks. As this affected side is the contact pads for the end-product "reader" unit, my customer says the water stains are not healthy, and require us to clean up the stains...

we already tried prozone/IPA, and this only "cleans" the pads temporary, after the chemicals "dry-up" the pads have darker-yellowish water-stains again...

I think the flex-PCB pads are either gold/copper in primary content, and my thinking is if its predominent copper, then the water-stains are stuck there for life? (Cu has lousy resistance to itchy finger prints and water stains?)

Any suggestions what chemicals can help us out? Anyone else encountered this problem, and what/how did you resolve it?

Help!

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ianchan

#18482

Water Residue Stains - how to remove? | 19 December, 2001

Did I mention, we tried to use the same WS flux formulae to brush-cover the affected flex-PCB pads, and subject it to another round of water-wash, followed by oven-baking, and it still doesn't help abit?

Oven bake : 100 deg-C, approx 15~30mins (manual visual check for water stains, prior to exit from oven-bake)

The death rate of the water stains infection onto the flex-PCB pads, stands approx 43% occurence. All occuring after the oven-baking process.

We have solved the repeated occurence by un-tape one side of the Flex-PCB to the pallet, as this permits water to "drip-dry" assist the oven-baking....

We still need help to clean off the water-stain marks, so we can ship the little buggy flex, ASAP....

Any suggestions?

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Michael Parker

#18488

Water Residue Stains - how to remove? | 19 December, 2001

Have you tried to polish the flex with an ESD camel hair brush? The residual films - are they organic, left over from the water? Maybe try some TSP - tri-sodium phosphate, it's a mildly caustic detergent. A thorough rinse would be needed after using the TSP.

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ianchan

#18499

Water Residue Stains - use toothpaste remover | 20 December, 2001

I called a cuppa of indutrial friends, and they concluded that the flex-PCB in question, was being subject too high a oven-baking temperature of 100deg-C...50~60deg-C oven-baking temperature would be more appropriate for flex-PCB, due to its very pad plating content, in comparision to rigid boards plating makeup...

To resolve the flex-PCB pads tarnish, we looked into gold tarnish angles, and found that goldsmiths use "cologate" toothpaste to polish gold and its sister whitegold...we tried using "darlie" toothpaste (avoid the new-fab toothpaste with "improved" calcium..etc...) coz of its "original" toothpaste formulae minus the potential corrosive chemicals added over the years...the "darlie" toothpaste worked wonders! the gold pads are hey-O clean, welcome to the sunshine club!

This is my two cents worth contribution to helping fellow SMT community folks...cheers!

PS: Do comment if anyone has any data/knowledge on any reprecussions by using toothpaste to remove gold pad stains?

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ianchan

#18500

Water Residue Stains - how to remove? | 20 December, 2001

Hi M.Parker,

thanks for the suggestion, and tips shared from you...

Merry Christmas!

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Michael Parker

#18533

Water Residue Stains - how to remove? | 2 January, 2002

Thanx and the same wish for you as well. and I did think of the toothpaste angle after my last posting, glad you found something that worked.

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