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QFN84 Solder Printing Issue (QFN with Inner LGA Pad)

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Hi, we are experiencing insufficient solder and bridging so... - Nov 10, 2017 by myke_SMT  

#79319

QFN84 Solder Printing Issue (QFN with Inner LGA Pad) | 10 November, 2017

Hi, we are experiencing insufficient solder and bridging solder. Any recommendation on the inner pad (LGA) and stencil thickness. Any suggestion will be very helpful. link: http://www.latticesemi.com/~/media/LatticeSemi/Documents/ApplicationNotes/PT/PCBLayoutRecommendationsforLeadedPackages.pdf?document_id=46898

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#79320

QFN84 Solder Printing Issue (QFN with Inner LGA Pad) | 11 November, 2017

One suggestion I have seen for packages like this is to have the device balled first.

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#79323

QFN84 Solder Printing Issue (QFN with Inner LGA Pad) | 13 November, 2017

Just a few thoughts, as we use similar quite frequently without issues.

Ensure correct support when pasting the board, especially if it is thin.

Correct support when placing the part. Is it placed correctly by the machine? If it is very slightly off, it should pull in OK. Is the bridging coming from the LGA paste? If so, then maybe your stencil is wrong. If it is bridging across the QFN pads, placement is most likely the issue.

Can you provide us with a picture of a faulty part?

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#79338

QFN84 Solder Printing Issue (QFN with Inner LGA Pad) | 15 November, 2017

IMO – Look into nano-coatings for your stencil and radius all square apertures This should help out with solder paste releasing from the stencil helping with insufficient solder

QFN LGA aperture design, try a stencil cut with a few different aperture designs to test.

I have had success with aperture reductions in “pad” width and increase in “pad” toe (almost 15%) to take care of bridging

For the larger ground center “pad” window pane aperture should aid paste to allow outgassing. Also reduce the ground aperture about 40%, check the volume of paste under the QFN ground, if the volume is too much it will lift/tilt the component too high and could cause opens or poor solder joints on outer “pad”

I hope this helps

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#79416

QFN84 Solder Printing Issue (QFN with Inner LGA Pad) | 1 December, 2017

Hello. We have a problem with QFN type. Component is elevated. Stencils are reduced. Vias is without holes. What are the solutions to fix the fault. Look at the photo.

Attachment: https://drive.google.com/open?id=1OIgaXUz2yoKP3fWZg2rUsrmZMBWidk6_

Thank you in advance.

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#79417

QFN84 Solder Printing Issue (QFN with Inner LGA Pad) | 1 December, 2017

It seems that it will work with 5 mil stencil. To be safe use 4 mil if the rest of the design allows it. 10x20mil apertures for both rows. If you have bigger footprint in the corners, make bigger apertures there. For the ground plane, I would use version B(3x3 square array). Make sure there is at least 15 mil distance between inner row leads and ground plane(you didn't mention where you see the shorts).

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#79418

QFN84 Solder Printing Issue (QFN with Inner LGA Pad) | 1 December, 2017

Stencil thickness is 4 mil. Via's are plugging from bottom.

Gas may be developed under the component?

Take Reflow Ovens Thermal Profiling changes?

In your opinion?

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#79419

QFN84 Solder Printing Issue (QFN with Inner LGA Pad) | 1 December, 2017

Brikainis,

it looks like these vias are not plated and closed. I would say that thickness of your paste is lifting the part. Are you willing to try another stencil with ground pad apertures matching the via holes? It is probably worth trying. Another approach will be to apply less paste with thinner stencil. Good luck and please keep us posted. Emil

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#79420

QFN84 Solder Printing Issue (QFN with Inner LGA Pad) | 1 December, 2017

EMIL,

The first stencil was with 4 squares. then there was a worse percentage. Changed to the circles. Is better, but still not perfect. Stencil thickness did not change. I added 2 types of stencil openings.

Pictures of watch:

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#79421

QFN84 Solder Printing Issue (QFN with Inner LGA Pad) | 1 December, 2017

Very interesting pictures. It shows paste height of 200um - which is at lest 6mil stencil deposit. That on the other hand posts another question. Can you measure the ground pad level in relation to the solder mask level with the SPI? I had assemblies in the past, where the the plating is way lower than the solder mask. If that is the case you might be getting a ton of paste in there, that has nowhere to go, because of the closed via holes. My suggestion is to measure that level and if you discover any difference in height, you should change the approach with the stencil. Make circles with the size of the holes or slightly bigger and print them over the holes, so the height level remains lower in the center of the part.

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#79422

QFN84 Solder Printing Issue (QFN with Inner LGA Pad) | 1 December, 2017

How about step down in the middle of thermo pad? I mean 0.075mm

This can help?

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#79423

QFN84 Solder Printing Issue (QFN with Inner LGA Pad) | 1 December, 2017

Rework said that with hot air gun, heated the solder paste comes out of the middle part out to the side terminals and soldering extending beyond is perfect.

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#79425

QFN84 Solder Printing Issue (QFN with Inner LGA Pad) | 1 December, 2017

I am not sure that is doable. Ground Aperture is very small(for the stencil blade to get in there) and very close to the other apertures. I want to say the minimum requirement for step is 6mm clearance from other apertures for the stencil houses that I worked with. There might be a requirement for minimal size of the step as well. You can always ask your stencil house, but I can't foresee a visible effect doing that. Most likely is to stick these paste into the holes given and see what you end up with as coverage under the part on the X-ray. Another solutions will be PCB design related. I would ask PCB designers if these holes could be drilled all the way or be plated, so this paste can stick to something and go somewhere. Part datasheet might give you some answers, too.

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#79426

QFN84 Solder Printing Issue (QFN with Inner LGA Pad) | 1 December, 2017

Recommended land and solder pattern says: VIA wall plating, via holes should be tented with solder mask on the backside and filled with solder.

Solutions at this moment ?

Thanks for all :)

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#79428

QFN84 Solder Printing Issue (QFN with Inner LGA Pad) | 2 December, 2017

Have you ever heard of "Solder Mask Defined Thermal Pads"?

I wrote a short blurb on this here - https://www.pcblibraries.com/forum/ipc7093a-btc-qfn-solder-mask-defined-thermal-pad_topic2154.html

This technology saves fabrication costs by not having to fill or Tent the vias (extra charge) and improves assembly results by reducing voids.

This technology will be available in the upcoming release of IPC-7093A - Design and Assembly Process Implementation for Bottom Termination Components.

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#79429

QFN84 Solder Printing Issue (QFN with Inner LGA Pad) | 4 December, 2017

If you are looking to rework (after all of the previous suggestions are taken and tried....you have some solid ones in there).

When a device like this is placed and is hard to get to we use the 7721 bumping process using a controlled heat source (not hot air gun+operator) After removal site prep. Take new component and bump with 4 mil Kapton(TM) stencil. Place using paste flux, clean and inspect.

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#79430

QFN84 Solder Printing Issue (QFN with Inner LGA Pad) | 4 December, 2017

A new stencil has been purchased. to be continued.

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SMT Stencil Cleaning Process

Led testing color sensors