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BGA Rework - flux dispensinx issue

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#57160

BGA Rework - flux dispensinx issue | 4 November, 2008

Hi All, Anybody knows good, repetable method of dispensing flux for micro BGA rework? 6x6 mm(0.5 mm pitch) The flux dispensing amount by brush may vary depending on operator etc. If operator dispense too much flux the BGA "swim".Around the BGA component there are many small resistors and capacitors very close (within 1-2mm distance) to the BGA.

Regards, Martin

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#57164

BGA Rework - flux dispensinx issue | 4 November, 2008

Sure. Here's what you should do to apply repeatable amounts of flux on BGA during rework: * Squirt some gel flux on a plate or something like that that has a flat bottom and raised sides to corral the flux. * Cut notches into one edge of an old credit card. The number and depth of the notches control the flux thickness. We call it a comb. * With the BGA in one hand and the notched credit card in the other, swipe the notched edge of the credit card through the flux. * Dip the BGA into the flux as the card passes through, leaving a just perfect amount of flux on the solder balls of the BGA. * Place the BGA and get to soldering.

We get the impression from the way you posed the question that you're using a liquid flux. If that's true, it's not good practice to squirt liquid flux of any type on rework areas, because the liquid spreads and flux on the outside edge does not receive the proper amount heat for activation, leaving harmful resides.

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#57165

BGA Rework - flux dispensinx issue | 4 November, 2008

Hi Dave, Thanks for the recommendations. Have you tried this method for the micro BGA as well? Would you mind attaching some pictures to get better understanding how you do this? At the moment my understanding is that this method is very similar to taking small cup, putting flux into it - the height of the flux in the cup should be lower than BGA ball diameter, next deep the BGA into the cup and pick up the component from the cup by pick up tube during pick&palce sequence. I only wonder if the flux will do its job (remove residues from pads) if we apply it to BGA balls instead of pcb pads as ball will touch the surface only in small point not across whole pad area... But on the other hand the flux should spread from ball across whole pads during heating.

Regards, Martin

P.S> We are using tucky flux Alpha UP78

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#57171

BGA Rework - flux dispensinx issue | 4 November, 2008

Hi Martin,

Have you considered printing the flux with a stencil? Stencils can be used for printing more than just solder paste. Quite regularly people use stencils for printing thermal grease, fluxes, conductive epoxy and more. The thing I like the most about a stencil is that it gives me a consistent print from board to board no matter what operator is performing the process. I typically just print 1/1 with my smt pads, place the bga, and reflow it.

I have found these stencils for $80 here http://www.stencilsunlimited.com/product_info.php?products_id=81

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