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Motherboard become NO POST when humidity increase to 90%

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#60503

Motherboard become NO POST when humidity increase to 90% | 20 November, 2009

Hello,

This is interesting topic that I would like to share and discuss with you..

I came across few projects from various SMT factories where its product(Computer) reach field, experience No Post/boot defect. This problem become very obvious when there is heavy rain.

When the affected product returned to SMT factories for verification, they found that this problem cannot be duplicated. However, when they put these affected motherboard into humidity chamber where they set it at 90% humidity level, they can see the problem..When decrease the humidity(Less than 90% humidity level), the problem disappear..

This problem also can disappear if we clean the motherboard bottom side, especially near to the BGA bottom side...This action lead them to believe that this could be flux residue that induce this defect...

However,as far as I know, flux residue can cause this problem when its absorb enough moisture and become conductive which lead to low impedance. This problem should not go away even the humidity drop less than 90%..What I mean is that this problem will stay there unless we heat it up which make the flux residue vaporize..Am I right?

At the same time, we also found that wave flux solid content is out of specification. Its current value is 1.22wt% and spec call for 2.5% +/- 0.5wt%. Is there any relationship between low solid content and No Post issue??

Note: Surface Insulation Resistance (SIR) test for this flux is within specification.

Thanks, Sean

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#60504

Motherboard become NO POST when humidity increase to 90% | 20 November, 2009

Hi all,

Another thing that I would like to discuss here is that I used to come across few reports that mention about the flux residue has great impact on the PCBA impedance, especially when everything become smaller and smaller...I mean, when motherboard, IC, BGA designed to be smaller, then flux residue might potentially impact the PCBA impedance...Is this valid?

As a result, many SMT companies used auto-brush machine where its brush away the flux residue at the PCBA bottom side.

Thanks, Sean

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#60506

Motherboard become NO POST when humidity increase to 90% | 20 November, 2009

In 90% humidity it’s easy to get condensation. Condensation will create electrical failure when the board isn’t conformal coated. In your case the failure disappears when the humidity level drops so to me it’s unlikely that the flux has anything to do with the problem. If the flux would be part of the problem you would see permanent damage and no recovery.

When you have condensation on the board the CO2 dissolved in the condensation water forms carbonic acid and the acid level is enough to reduce the SIR causing impedance problems.

Water and electronics is a very bad combination!!! If your boards are exposed to those harsh environments you will need to coat the boards.

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#60509

Motherboard become NO POST when humidity increase to 90% | 20 November, 2009

One of our instruments has a high impedance front-end. We: * Passed functional test in the environmentally controlled shop. * Failed test, on the same unit, while carrying it to burn-in chambers in an uncontrolled part of the plant.

Solder flux residues and fairly high humidity were the culprit every time.

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#60510

Motherboard become NO POST when humidity increase to 90% | 20 November, 2009

Dave was that a no-clean flux residue? And if so alcohol or VOC free (water based)

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#60513

Motherboard become NO POST when humidity increase to 90% | 21 November, 2009

Hi Patric,

You are right...WE found that this problem disappear once we seal the PCBA bottom side (Intel's Northbridge area). Once we remove the seal, the problem is coming back..

In this case, I thnk conformal coating is necessary...

Besides, I also agree with you that flux residue always cause permanent damage unless we heat it up which cause its vaporize...

Thanks, Sean.

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#60514

Motherboard become NO POST when humidity increase to 90% | 21 November, 2009

Hi Patrick,

Sorry, it should be Intel's Southbridge BGA instead of Intel's Northbridge BGA..

Thanks, Sean

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#60515

Motherboard become NO POST when humidity increase to 90% | 21 November, 2009

Sean,

Thank you for the feedback.

Patrick

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#60516

Motherboard become NO POST when humidity increase to 90% | 21 November, 2009

Patrick: The flux was OA, but you can get the same result with low residue flux.

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#60518

Motherboard become NO POST when humidity increase to 90% | 21 November, 2009

Dave,

I would only agree with you when you talk about low residue (water based) fluxes. In low residue (alcohol based) fluxes you can use non water soluble acids, so in the event you still have flux residues on the board water has no effect on them.

And I assume with OA you mean water-soluble flux. These residues can do damage to the boards even without elevated humidity, these acids are all hydroscopic by nature and have to be removed from the board.

Have a great weekend, Patrick

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#60522

Motherboard become NO POST when humidity increase to 90% | 21 November, 2009

Patrick

I can only agree with you about low residue (alcohol based) fluxes, providing you apply the amount specified by the supplier. Amounts in excess of the specified amount can have harmful effects.

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#60523

Motherboard become NO POST when humidity increase to 90% | 22 November, 2009

Dave,

A low residue/No residue flux does exist that when an excessive amount of flux is applied it would only create a cosmetic problem not a reliability issue. The active ingredients are neutralized by the heat applied in the soldering machine and are NOT soluble in water or humidity/condensation.

As you know Dave I can’t be more specific without compromising the technical orientation of this forum.

But even if the original poster used this type of flux the problem he had under high humidity and condensation would still exist without the use of conformal coating. For reasons explained in my previous post.

The point I want to make is that flux is not “ALWAYS” the culprit.

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#60550

Motherboard become NO POST when humidity increase to 90% | 24 November, 2009

I agree Patrick many people blame the assembly fluxes when it is some other hygroscopic chemistry left on the bare PCB such as surfactents from the HASL process these will do exactly what the guy reported in his post fail then cool down and pass everytime. If it is this type of chemistry then Coating the PCB will also fail just slower in most cases. IS THE BOARD HASL finish? To cause this failure leaving a no clean or no residue you would need fairly significant amounts on the PCB and it would need to be a condensing atmosphere as MOST but not all no clean fluxes are not hygroscopic. Is the area around the BGA covered in white organic acid?

Cheers Greg

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#60560

Motherboard become NO POST when humidity increase to 90% | 25 November, 2009

Hi Greg,

The board is with immersion silver finishing..One of my friend had the same issue even with PCB OSP finishing..

Besides, no white residue found on the top side... Its only appear on the bottom side but no so obvious...

Rgds, Sean

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#60561

Motherboard become NO POST when humidity increase to 90% | 25 November, 2009

Is this the Nitric Acid Based Silver finish or the Alkali, I think most are Nitric. Have you tested the solder resist as well for porousity. Cheers Greg

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Plasma prior Conformal Coating