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Problem with shielding component

Views: 4576

aj

#46565

Problem with shielding component | 9 January, 2007

All,

We have ran a few revisions of a New Product which is a tracking device using satellite navigation.

The protos went well but now they have added a shielding can. We place 2 of these to cover the Frontend of the PCB.

The test guys are having difficulty getting the same performance out of the boards now.

The shields are roughly 20mm square approx 3mm in height. They are not covered on top at this stage but have a pickup point to allow for automated placment.

I have profiled the board and I am happy that we are getting enough heat even with these cans but it looks as if it is directly related to these.

Anyone have a similiar experience - oh yeah we are running the board with PB paste due to one of the 2 bgas being Non Rohs.

aj...

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#46566

Problem with shielding component | 9 January, 2007

Not unless it's a case of "the only difference is..." What I mean by that is more than once I've been told the only difference in a new rev is one component or one change. But the truth was that was the only reason for the change, and in fact there were several changes made because of the primary change. For example a diode was changed to a much smaller package, and the kit was issued with the old rev parts but caps and resistors had changed to match the new diodes.

Make sure that the shielding was the only change.

Also can you hand solder the shielding on later? If so try a few without it, test them and add them, then test again.

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Guest

#46576

Problem with shielding component | 9 January, 2007

Dear AJ,

Try removing shield ,test it, verify if it would perform as the previous runs . If not, there maybe something wrong with device or as one of our collegue mentioned that there is possibilty the your customer change some components.If the assembly performance is the same as the previous runs , problem may be related to the shield, either poor grounding or your customer may need to fine tune the RLC portion of the circuit.

Good Luck

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#46577

Problem with shielding component | 10 January, 2007

Shields can carry a large thermal mass. If you ever tried removing one in repair, you'd find out how hard they can be. Also, they do deflect air flow. So in a convection oven, your air flow will be turbulent, which also will affect your reflow.

I suggest re-profiling with shields on. My bets on the thermal mass.

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aj

#46580

Problem with shielding component | 10 January, 2007

We ran 4 boards without shields and they work fine.

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aj

#46581

Problem with shielding component | 10 January, 2007

We also took the shields off one of the boards and retested and it still fails...

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Guest

#46616

Problem with shielding component | 12 January, 2007

Your evaluation results confirms that Real Chunks is right on thermal mass absorption of the shield, that probably brought cold spots during reflow . I agree with his suggesttions as well.

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aj

#46617

Problem with shielding component | 12 January, 2007

All,

I ran a failed board back thru reflow with shields removed and it works fine. I think that tells me something ?

Thanks all who contributed to the thread.

aj...

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#46640

Problem with shielding component | 12 January, 2007

I think she might be more on track about the air flow. I would think they not only have thermal mass but also emit thermal energy well which would add to the dificutly of removing them. I think a Dpak would have more local thermal mass than the shield. We are talking about the kind that operators and inspectors call a "fence" aren't we? The kind you snap a lid on sometime after reflow.

But in any case I think Real Chunks has nailed it.

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aj

#46643

Problem with shielding component | 13 January, 2007

Hi all,

What length of time is recommended for the "soak stage"?

aj...

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aj

#46695

Problem with shielding component | 16 January, 2007

All,

The component that seems to be effected most is a :

NemeriX NJ1006A

QFN type device.

If heat isthe problem what exactly is happening to the part -cold spots? joints not soldered?

aj...

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#46716

Problem with shielding component | 16 January, 2007

How close is the "fence" to the part in question?

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aj

#46730

Problem with shielding component | 17 January, 2007

2-3mmm from edge of component

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#46744

Problem with shielding component | 17 January, 2007

2-3mm would seem close enough to affect the air flow and I was thinking it could be displacing the qfn. However, if you ran a failed board back through reflow and it was functional after that, I'd put my money on insufficient reflow.

I suppose it's possible that the part centered itself but that seems unlikely for something with such small terminations/pads.

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aj

#46794

Problem with shielding component | 18 January, 2007

All,

apologies for dragging this thread up again but after some xray analysis it looks like there is no issue with the soldering of any components namely 2 x bga and 1 qfn device which are placed within the shielding area..but yet they fail to perform like the boards that run thru without shielding.

?

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#46835

Problem with shielding component | 19 January, 2007

Is the failure just out of tolerance or is it non functioning? For tolerance problem, compare the x-rays of the good and bad units. It will take a very high resolution x-ray to spot the differences. Changes in capacitance values can occur based on how much the solder wets out.

For not functioning, are the parts ruined by overheating? For the reflowed units, was the fence removed mechanically reflowed and it started working?

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