Spray flux through vias!!

I have had some field failures returned recently, the boards had flux contamination under the components in a sensitive area, Nothing really visible until the component was removed, and the board had no evidence of rework of any type. Is it possible for flux to come through the via holes which are present under the component. (there is a spray fluxer used in the wave solder process) If so I suppose 'tenting' the vias with solder resist would prevent this. Anyone had similar experience?

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| I have had some field failures returned recently, the boards had flux contamination under the components in a sensitive area, Nothing really visible until the component was removed, and the board had no evidence of rework of any type. Is it possible for flux to come through the via holes which are present under the component. (there is a spray fluxer used in the wave solder process) If so I suppose 'tenting' the vias with solder resist would prevent this. Anyone had similar experience? Hi Clive! Abso-tively!! The designer that did this should have his mouse hand spanked and sent to bed without desert for doing that to you, he violated one of basic design rules for via placement, never between passive component pads. If he gets in a habit of doing that, he coud really hose you if you have to epoxy any passive components on the bottomside. Depending on the diameter of the via, the epoxy could flow down the via which would provide little, if any, holding force when you wave that puppy...hasta la bye-bye to that component when it hits the wave. Now, with all that being said, I know that rules are made to be broken, and what with the demand for everything being smaller, lighter, faster, cheaper...ya' sometimes does what ya' gotta do...so I do understand. You knew the answer, tent them bad boys...do it from the bottom though. If you tent the topside, you'll wind up with little "Blobbies of solder" (that's my high tech term) hanging from from the via. What happens is the via fills with flux and outgases when it hits the wave, but solder will have wet to the via pad...you'll see these little hollow solder bubbles attached to the via's. I just wanted to ask, hows your washer? You got plenty of water pressure and volume? Nozzle angles? Do you do any sort of cleanliness testing? I'm pretty sure that if there were flux beneath the passives like you described, it would have shown up in a ionograph. Hope this helps... -Steve Gregory-

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Thank you Steve for your input it was very usefull. I think I skipped a few pertinant details on my initial message though, the component which we found the flux contamination on was on the top side of the board, I was curious if the flux could come up through the via and sit there on top of the board waiting to screw us up in the field! The via between passives on the underside makes absoloute sense, I'll make sure they avoid this from here on in. Your point about the via tenting was good, I was considering just tenting the top as I require the bottom side for probing at the ICT stage. However in view of your comments I think I'll hold back on this, the thought of 'solder blobbies' (an industry standard term I think) hanging off the underside is not pretty !! As for the washer, well there is no cleaning process at present. ( though this may change if we cannot guarantee no flux under components on the top!) The process is supposed to be low residue, and indeed on the wave soldered side it looks good, but the flux stuff we found on the top is not. Thanks once again Clive

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