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Crystalized Flux under BGA

Dreamsniper

#29819

Crystalized Flux under BGA | 3 August, 2004

Hi,

What contributes to the above? I've got some BGA solder joints with a crystal like flux attached to it like a mini solder ball but do not have a perfect round shape. I had my aqueous cleaner's DI water sent for analysis and everything looks okay except for Sodium, which is 12.2 mg/L. Does this contribute to the problem or the said figure is considered bad for my Cleaner?

Thanks and regards

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#29821

Crystalized Flux under BGA | 3 August, 2004

Improper washing of water soluable flux residues can be a major contributor to flux residues remaining on the board surface. ;-O

That you appear to be only complaining about the pathetic state of the ability of your washer to clean under BGA, it may indicate that some adjustments can be made in the washer performance. [Assuming of course that the BGA has enough stand-off to allow wayer to penetrate under the BGA. If the BGA ball diameter is 0.030" or larger, most modern in-line aqueous cleaners do a fine job.]

Otherwise, it is an issue of surface tension, pressure and flow rate. With straight water alone, surface tension of water makes it difficult to get under low standoff parts. If, however, you add a suitable surfactant agent, so that your cleaning solvent gets under the parts, then it becomes an issue of how much fluid delivered, how fast. There are cleaning agents that will work for low standoff devices, without destroying the materials of construction.

We've talked about surfactants perviously on SMTnet.

It's not surprising that your BGA ball is not perfectly round. We expect a kind of flattened bulbous disk [what the word inventors describe as "the collapse"].

Ion exchange resins have a sodium-containing salt that readily gives off the sodium that binds calcium, magnesium, and other heavier metals, in an "exchange" process. We're not sure what is an acceptable level of sodium in DI water.

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Shean Dalton

#29825

Crystalized Flux under BGA | 4 August, 2004

Additionally, increases in temperature can increase the water's rate of reaction with the flux. Sometimes, only a 5 degree change can make the cleaning difference.

If the flux inconsistently presents itself, then the likely cause is something in the system which is varying.

If temperature is varying, monitor the incoming DI water temperature. Likely the Final Rinse cascades to the wash reservoir. If the final rinse temperature drops, this will have an initial effect on the wash reservoir temperature. If it drops greatly, the wash reservoir heaters in your system may not be able to compensate, and, the wash reservoir temperature will drop too.

If pressure is varying, check the filters.

Inspect for clogged / partially blocked nozzles along the manifolds.

Measure the flux loading in the wash reservoir, is it approacing a maximum limit? Along that line of thought, verify the incoming (final rinse) flow rate is greater than the outgoing flow rate for correct cascading. Note: Sometimes this is monitored by pressure (rather than flow rate). Pressure readings are effected by clogged / partially blocked nozzles; therefore, this can be a misleading indicator for verifying correct cascading.

Hope this is useful,

Shean Dalton Austin American Technology www.aat-corp.com

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Shean Dalton

#29826

Crystalized Flux under BGA | 4 August, 2004

and one more thought about your original question, if the flux does look really strange and out of place, then inspect the reflow oven.

A great amount of flux condensation in the reflow oven's cooling zone can cause crystalized flux to become airborne and land on the board. Perhaps, this is being washed off by the cleaner in most locations; but, not in the less exposed areas.

Shean Dalton Austin American Technology www.aat-corp.com

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Dreamsniper

#29839

Crystalized Flux under BGA | 4 August, 2004

Hi,

Thanks for your reply.

I run 2 boards, using the same paste, thermal profile and the same method of aqueous cleaning, 1st is a Topline test board with topline BGA (BGA 272 with 1.27 Pitch) and the 2nd is our product PCB with our BGA from Motorola (BGA 272 with 1.27 Pitch). The test board BGA seems to be fine, clean and without any milky looking Joint. Our product's BGA looked milky and the white residue is noticeable below the solder joint as if there's a gap between the BGA ball and the solder paste ball. Our PCB is FR4. Can you help me figure out the problem? I can provide photos if you need them. Just leave me your e-mail address.

Thanks,

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#29840

Crystalized Flux under BGA | 4 August, 2004

We assume this is different discussion that the discussion about "crystallized flux under BGA". Correct?

Milky residue with water washing makes us think "noclean flux res that just got washed". We know what you said, but we can't help ourselves.

Tell us about the solderability protection of the pads of board boards and about the ball attach on the components [BGA].

Send pix.

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#29842

Crystalized Flux under BGA | 4 August, 2004

Hi dave,

how can I send the pix?

Thanks,

Dreamsniper

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#29844

Crystalized Flux under BGA | 4 August, 2004

Left click on the blue underlined "davef" [just left of the Lou Reed head] on one of my postings. An email dialog screen should popup and you should be able to paste [or attach] your pix there.

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#29857

Crystalized Flux under BGA | 5 August, 2004

Hi davef,

Can't attach pix. No attachment option.

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Shean Dalton

#29861

Crystalized Flux under BGA | 5 August, 2004

You can e-mail a picture to sdalton@aat-corp.com

Thanks,

Shean

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#29880

Crystalized Flux under BGA | 5 August, 2004

Try: dave5252 at hotmail.com

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Shean Dalton

#29934

Crystalized Flux under BGA | 6 August, 2004

Thanks for providing the pictures. We evaluated what we could see and have the following idea's.

1) Because of different thermal charatoristics between the topline and your product, your Motorola BGA solder joints may have experienced a higher reflow temperature. Therefore, a cleaning process which works for the lower temperature application may not work for the worse case scenerio of the higher reflow temperature.

2) Your product looks like it may have non-solder-mask-defined pads. If so, consider the following. A) HASL flux residues remaining around the pad may be interacting with the flux used to attach the BGA. B) Your product may have a lower standoff because the available total solder volume is applied around the non-smd pad's base and outer side walls.

3) Possibly the solder mask top layer, "butter coat", is not fully cured, and, may be interacting with the flux.

Does there appear to be any residues on the bare boards, and/or, BGA components prior to applying the solder paste?

Shean Dalton Austin American Technology www.aat-corp.com

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#29959

Crystalized Flux under BGA | 9 August, 2004

Hi Shean,

Thanks for the analysis.

"Your product looks like it may have non-solder-mask-defined pads."

Yes. It is a NSMD PCB.

"Does there appear to be any residues on the bare boards, and/or, BGA components prior to applying the solder paste?"

No. But the HASL flux residue looks the similar or the same as that of the BGA's. Is this acceptable or not?

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Shean Dalton

#29982

Crystalized Flux under BGA | 10 August, 2004

Because the HASL process most deal with bare metals, HASL flux residues are generally more active than solder paste residues. Typically a Rosin Active (RA) flux is used in the HASL process. These flux residues should be removed from the bare board. Though with a really thorough examination, some level of HASL flux residue is identifiable on many boards. For NON-SMD pads, the solder mask does not cover up the HASL flux residues which may remain at the base of the pads. Therefore, any HASL residue present around the pad would be exposed to your solder paste process.

Combining HASL flux residues with your solder paste residues can result in complex residue formulations which may contribute to your current situation.

If your HASL flux is RA, and, your solder paste OA, and your cleaning process water only, your cleaning process would likely need a chemistry additive or semi-aqueous chemistry to remove the final resultant residue. Inspecting for clean bare boards, or, cleaning your bare boards may be a preferable approach.

Shean Dalton Austin American Technology www.aat-corp.com

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#29991

Crystalized Flux under BGA | 11 August, 2004

HASL Fluids are generally not RA based fluxes. They are generally fluxes based on hydrochloric acid or hydrobromic acid or a mixture of the two, plus some very strong wetting agents that are very hygroscopic. Doesn't the residue of hydrochloric go white when mixed with water and hydrobromic goes yellow cant remember exactly which way round but anyway you should have none of these on the PCB or it will eventually fail.HASL Fluids are a no no as far as reliability is concerned The other concern is that some wetters used have a tendency to gell when cleaned in cold water and become very difficult to remove this happens at HASL stage not assembly. As before use a saponifier with the cleaning process if the resist is porous then the saponifier will bleach the resist and give a clear indication all is not right. Hope it helps Cheers Greg BLT Circuit Services Ltd

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