Introduction: Simple QFN Placement

By Soldertools.net
About: I am principal of BEST Inc. Had 18+ years in running small businesses. By trade I am an electrical engineer and holds patents in various fields including industrial controls, surface science and PCB rework/rep…

This instructable will describe the easiest method for placing QFNs and other leadless devices. the materials required will consist of:

1. Device to be placed

2. Matching StencilMate(TM) bumping and matching board stencil

3. Low end reflow source (no split vision required)

4. Paste flux (water solubale is recommended)

5. Cleaning brush

6. Kim wipe

7. Squeegee

8. Solder paste-based on requirements or specifications

9. Flat ceramic plate

In practice you should also x-ray these parts after placement. For such an x-ray inspection service look here.

It should take about 15-20 minutes for this process.

Step 1: Align and Place StencilMate(TM) Stencil

Fixture your part so that you have free hands to work on the part stencil. Align QFN rework stencil followed by putting pressure on to adhesive-backed material.

Step 2: Roll Solder Paste in to Apertures

Roll solder paste using miniature squeegee slightly larger than the part dimension through the apertures. Squeegee paste back and forth until all of the apertures are filled up. Clean off excess solder paste using kim wipe.

Step 3: Inspect Paste Printed Leadless Device Stencil

Make sure all of the apertures are filled up. Make sure solder has not been "scooped" out of the apertures. Make sure remnant solder is not on the surface of or on the edges of the QFN stencil.

Step 4: Reflow

Reflow solder paste as per the solder paste manufacturer's suggested profile. No split vision system needs to be part of the reflow source.

Step 5: Peel Off Stencil, Clean and Inspect

Peel off the QFN stencil. Clean using isopropyl alcohol. Inspect the bumped part.

Step 6: Prep Site Where Part Will Be Attached

Inspect site where bumped replacement part will be going. Make sure site is clean and wicked as flat as possible.

Step 7: Place Board Stencil

Place the StencilMate(TM) board stencil in place. Activate adhesive after alignment by applying pressure to the surface of the stencil.

Step 8: Roll Solder Paste Through Apertures

Roll solder past through the apertures using the properly-sized squeegee. Make sure solder has not been "scooped" from the center ground. Clean off remnant solder in and around apertures

Step 9:

Step 10: Place Bumped Device

https://www.instructables.com/editInstructable/edit/EO1WEMEHVTLVCD1/step/10#Add paste flux to the bumps on the device. Place the device in to the board stencil apertures, They will tactally fit in to those apertures. Reflow and inspect.

Step 11: Inspect

See here for more information on these stencils that will make this QFN rework job the easiest.