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

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


QFN standoff, industry standard

Views: 2543

#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 »

#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.

reply »

#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

Regards

reply »

#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.

reply »

#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.

reply »

#82235

QFN standoff, industry standard | 28 March, 2019

Davef,

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

Regards

reply »

#82925

QFN standoff, industry standard | 1 July, 2019

Yes, No standard.Normally QFN supplier suggest 50-75 um (2-3 mil), but it's very hard to achieve and hard to make consistent stand-off in prodution.

reply »


SMTA-Pat

#82951

QFN standoff, industry standard | 3 July, 2019

We have seen that vias included in the PCB under the thermal pad will give less voiding.

This message was posted via the Electronics Forum @ SMTASMTA

reply »

#82954

QFN standoff, industry standard | 3 July, 2019

Pavel Murtishev: Please click the link under my avatar [to the left of this message] and send me your email address.

reply »

Electronics Experts Needed - Contract Jobs - We have Electronics Auditor positions available now!

Polymeric Workmanship PCB for Training

Led testing color sensors