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Vapour phase soldering - problem

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#59547

Vapour phase soldering - problem | 18 August, 2009

Hello, I have a customer who recentely installed a vapor phase soldering system. Everything goes pretty well, but from time to time the customer has the same problem with the soldering: one of the components on PCB (usually, it's a QFP package) is rotated after the soldering. Whereas the rest of the components are soldered perfectly. The interesting point is that the only one and the same component is rotated. If we look at the board before the soldering process, the placement accuracy of the component is good enough. Has anybody faced with such a problem? What can be a solution for that? Thank you!

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#59548

Vapour phase soldering - problem | 18 August, 2009

Hi,

Yes, I think this sounds like a form of tomb-stoning, and we used vapor phase until it ended up driving us totally nuts trying to deal with tomb-stoning. We tried everything, and all kinds of stencil apertures.

Vapor phase is an amazing process in theory, however in practice this tomb-stoning issue was impossible for us to overcome. On more complex boards, almost every unit of production had some kind of trouble like this.

So we went to regular convection. With vapor phase the surface tension of the solder is just way too high and it pulls components around.

However to qualify that, we never tried vapor phase with lead free solder, which I expect does not suffer from the same issue. We sold our oven before the lead free transition. Also, the company who purchased our vapor phase oven was soldering large passives with large power supply components all on the same board, and they really liked the process. Only Vapor Phase can solder massive components and small SMT all on the same board without almost any delta t, so thats a positive for them.

But for us it was a nightmare as we were using 0402.

I hope this helps, but I would recommend going to a convection oven, as even a cheap one will give a dramatic increase in reliability.

Regards,

Grant

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#59553

Vapour phase soldering - problem | 18 August, 2009

Sergey, Try increasing the cool down time if that is a recipe option. We have witnessed the same effect on components that have rather slow cool down rates and were able to eliminate the problem by extending the cooling time.

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#59554

Vapour phase soldering - problem | 18 August, 2009

Is the mechanics ok with the raising and lowering into the vapour ie no small movements that could be moving the part ?

Most Vapour Machines have a lower and raise mechanisim so movemnt of parts can happen if not smooth, especially when coming out of TAL - liquid to solid into cooling...

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#59557

Vapour phase soldering - problem | 19 August, 2009

Hi! Thanks to everyone!

Josh, I don't think it could be something wrong with the mechanics, as just one component was rotated. Anyway, we will check the mechanical parts of the oven.

LarryD, thank you for the advice. But what is the physics of the component rotation/tombstoning if the board is cooled down too fast? Any idea?

Just in case - I have attached two pictures with the rotated IC.

Attachments:

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#59563

Vapour phase soldering - problem | 19 August, 2009

We don't know dip about vapor phase soldering, but we can't help but wonder how well the leads of the component match with the pads on the board.

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#59566

Vapour phase soldering - problem | 19 August, 2009

I'd like to ask a question. Have you determined where all those solder balls are coming from? I even see them on top of U40 and U15.

This seems like a dumb question to me considering the size of the part (and having seen pictures of only one board), but are you sure you don't have something outgassing causing the part to move and spraying solder around? Are there a bunch of tented vias under the part?

I could see having a few solder balls on the PCB itself as a result of the leads being forced off the pads... but on top of other components?

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#59568

Vapour phase soldering - problem | 20 August, 2009

Hi,

I also paid attention on the solder balls.. I thought it was because of the pop-corning, but the customer told me the component was not damaged.. Regarding the bunch of tented vias, I need to ask the customer about that. Probably, they have unpopulated boards or gerber-files.

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#59570

Vapour phase soldering - problem | 20 August, 2009

This is a picture of the board without components.

Attachments:

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#59571

Vapour phase soldering - problem | 20 August, 2009

One diagnostic tool that I have found useful in the past is the "hot plate test". Place suspect components and/or PCB's on a hot plate and turn it on High. Then observe. Granted the conditions on the hot plate are not the same as in a reflow oven, but I can't ride along with the board in the oven. The hot plate I have used doesn't have enough power to exceed 2 degrees C per second, but it gets hot enough to reflow 63/37. I have been able to observe outgassing due to absorbed moisture in PSA used to adhere flex circuits to a couple of different substrates.

A non-destructive means to test for outgassing (assuming absorbed H2O is the issue) is to start to bake boards and parts prior to assembly. I would say 90 degrees C for a few hours. If the skewing problem goes away after baking one or the other then I think you could say that absorbed moisture/improper storage or handling of the parts is the issue.

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#59584

Vapour phase soldering - problem | 20 August, 2009

Sergey,

With that many leads and contact surface area points, it doesn't make sense that the part would move as it has without a gust of wind.... What is your confidence that you don't have an MSD issue on this part. Being that it is a TQFP, it's will be especially suceptible to potato chipping. That's what this looks like to me. Do you have the corners turned upward at all? Lay a feeler gauge across the corners as a quick check. If so, you might try a proper bakeout prior to soldering and see if your results change.

Hoss

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#59585

Vapour phase soldering - problem | 20 August, 2009

reduce the stencil aperture to prevent tombstoning in vapor phase process. from my experience a 6 mil thick must be at least between 20 to 30 % aperture reduction for 0805's and below.

cheers!

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#59643

Vapour phase soldering - problem | 25 August, 2009

Hi Sergey, Do you know the Moisture Sensitivity Level of the component ? This is normally recorded on the DRY PACK that the device comes in. It is not unusual for a device to POP CORN and still work in test. It depends how much damage there is to the package / silicon / bond wires in the package. If you are sure the device has been stored correctly then POP Corning probably won't be the cause. But if the device is full of moisture then it is a good possibility.

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#59654

Vapour phase soldering - problem | 26 August, 2009

Hi Graham,

the MSD level of the component is 3. I am not sure that my customer properly stored the component before the soldering process. And I am not sure that it was dried at all. The customer says they did everything that they had to, but I just can beleive or not and speculate on the reasons of the failure. What I think now is it was still the popcorning. Looking at the solder balls on the PCB surface and on the top of the surrounding components, I suppose that during reflow a micro-crack in the package could appear. And when hot vapour of moisture went out through the micro-crack, it could blow off some solder and move the component. I think it was 90% probable cause..

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#59657

Vapour phase soldering - problem | 26 August, 2009

Hi Sergey, I agree with your probable cause. Maybe if you still have a component you can bake it before reflow to DRY it. Then if the problem doesn't happen in reflow you have surely proved the cause. Consult IPC JEDEC 33B spec for storage and Drying times of MOISTURE SENSITIVE DEVICES, and ask your customer if he complies with the guidelines for storage of his devices as per this spec. Maybe suggest to him that an easy way to store MSDs and eliminate moisture problems is use a dry cabinet like those offered by TOTECH.

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