Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Isolating QFN Ground

Views: 1969

#68437

Isolating QFN Ground | 15 March, 2013

Hello All.....wondering if anyone out there has needed to isolate a QFN ground pad from the device and how you may have gone about it. We are thinking about using a micro drill to remove the pad, but if anyone out there has a better idea I would be very interested in hearing what you may have come up with. Thank you in advance....anvil

reply »

#68438

Isolating QFN Ground | 15 March, 2013

Let me clarify that we are looking at removing the gnd pad from the PCB not the device.

reply »

#68439

Isolating QFN Ground | 15 March, 2013

Hi,

Wouldn't be easier just ti cut the trace(s) going to that pad?

reply »

#68440

Isolating QFN Ground | 15 March, 2013

I agree that it would be easier to cut the trace or traces, but the issue is that there is a 1.2v line that is tied to that specific ground (design issue) and if we cut it then the 1.2v wont be available to the several hundred traces that are involved with that line.

reply »

#68442

Isolating QFN Ground | 15 March, 2013

Is this for something permanent? Where is the ground pad located (mid-bank, bank-end etc.)? What is the lead pitch? Are you looking on rerouting this pin some place else? Can you provide a picture of the pad in question?

reply »

#68443

Isolating QFN Ground | 15 March, 2013

Mask the pad on the PWB using liquid solder mask. Cure the mask and away you go.

Use a mini stencil and a squeegee to maintain planarity and minimize issues at paste print.

We have used this technique before. In other cases we have masked the bottom of the component to better match the pad design on the PWB.

You see all kinds in contract work. Say true.

'hege

reply »

#68444

Isolating QFN Ground | 15 March, 2013

I've seen 1-mil Kapton tape. Would that work, or would it be too thick? I've never tried it...

How thick is the average solder mask?

reply »

#68445

Isolating QFN Ground | 15 March, 2013

We avoid kapton in this situation due to the thickness. 1 mil additional thickness can equal up to a 20% increase in paste volume at adjacent pads if you are using a 5 mil stencil. But then again, that is not always bad. But must be taken into consideration at the printing of the solder paste. 'hege

reply »

#68446

Isolating QFN Ground | 15 March, 2013

Wow, I didnt realize that there was .001" Kapton tape....we will most likely try that if i can locate some over the weekend otherwise early next week....Thanks, great idea...anvil

reply »

#68447

Isolating QFN Ground | 15 March, 2013

We will try both the solder mask and the .001" Kapton and see where we end up with each. This is a fine pitch dual row QFN and we cannot afford too much paste on the .004" stencil we are using. We have experienced shorting in the past with .005" stencil. Thank you for the fine "Brain Work" on this you saved me some time for sure...regards, anvil

reply »

PCB Soldering Tools

Real-time PCB X-ray Inspection System