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QFN standoff, industry standard

QFN standoff, industry standard

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Hello, Could you please tell me if there is any industry ... - Mar 26, 2019 by Hypnotia

There is no "industry standard" that I am aware of. Ideally,... - Mar 26, 2019 by charliem

Charliem, Thank you for the input. Following this logi... - Mar 27, 2019 by Hypnotia

200uM seems high. I say this because typically our solder ... - Mar 27, 2019 by charliem

There is no industry standard for BTC stand-off height. "... - Mar 28, 2019 by davef

Davef, Thank you for the comprehensive reply. I do really... - Mar 28, 2019 by Hypnotia

Yes, No standard.Normally QFN supplier suggest 50-75 um (2-3... - Jul 01, 2019 by sssamw

We have seen that vias included in the PCB under the thermal... - Jul 03, 2019 by SMTA-Pat SMTA

Pavel Murtishev: Please click the link under my avatar [to t... - Jul 03, 2019 by davef

Hypnotia

#82222

QFN standoff, industry standard | 26 March, 2019

Hello,

Could you please tell me if there is any industry standard specifying QFN to PCB standoff after soldering? Is any gap allowed? Browsing IPC-A-610, I have found nothing.

Any input would be appreciated.

Regards

reply »

charliem

#82223

QFN standoff, industry standard | 26 March, 2019

There is no "industry standard" that I am aware of. Ideally, after reflow you want 100% coverage of all pads. Unfortunately this virtually impossible particularly if a large center pad is involved. Flux residue /voids come into play. We try to get >80% on edge pads and >60% on center pad(s). So with this in mind the component body should stand off a bit.

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Hypnotia

#82225

QFN standoff, industry standard | 27 March, 2019

Charliem,

Thank you for the input.

Following this logic, in case if there are no soldering issues, pad coverage and voiding levels are within acceptable limits and a gap between BTC and pad let us say 200um, this could be accepted, right?

IPC-A-610 and J-STD-001 do not mention BTC gaps at all.

Regards

reply »

charliem

#82226

QFN standoff, industry standard | 27 March, 2019

200uM seems high. I say this because typically our solder stencils in this application are between 80uM and 100uM thick which would leave the component height less than that after reflow. However if your not experiencing any shorting between pads I would think your ok. You need to have some gap otherwise you have little or no solder which obviously is bad.

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davef

#82234

QFN standoff, industry standard | 28 March, 2019

There is no industry standard for BTC stand-off height.

"IPC-7093A Design and Assembly Process Implementation for Bottom Termination SMT (BTC) Components" just finished "Final Draft for Industry Review" in December 2018. It mentions stand-off height several times, but no definitive "standardy" statements:

* Stand-off should be consistent between assembly runs

* Activate NC fluxes properly, since cleaning is difficult with low stand-off BTC

* "Package standoff is a parameter affecting the reliability of BTC solder joints." ... "an increase in stand-off will result in lower strain in the I/O solder joints upon a given temperature change, so increased board-level reliability under thermal cycling is expected."

* In a fatigue life study, bumped pads [200um] are more reliable that non-bumped [100um] pads ..."resulted in a 2X increase in printed board reliability of the package as shown in Figure 8-5."

* Take away: Very low stand-offs: < 30 μm). Typical Stand-Offs: 50-75 um.

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Hypnotia

#82235

QFN standoff, industry standard | 28 March, 2019

Davef,

Thank you for the comprehensive reply. I do really appreciate your help.

Regards

reply »

sssamw

#82925

QFN standoff, industry standard | 1 July, 2019