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September 21, 2016

LED Component Shift During Reflow

LED Component Shift During Reflow
What can be the root cause of SMT LED Component Shift during reflow? See the photo.


Experts Comments

Shift can result from insufficient channels for out gassing of the solder paste vehicles during reflow. One way to correct this is to window pane the stencil aperture which leaves channels for outgassing during reflow.
Bill Coleman
Vice President Technology
Photo Stencil
For over 18 years, Dr. Coleman has been the vice president of technology for Photo Stencil, working closely with customers to understand their printing requirements. His efforts have resulted in several new stencil products.
One of the most common reasons for a component to shift during reflow is that it is "floating." The reason a component could float is by applying too much solder paste during the stencil printing process. My recommendation is to verify both the pad and aperture design. I would use the manufacturers recommended design as a start, and IPC recommended in case the manufacturer doesn't have any processing details in their datasheet. Also a consideration is the thickness of the stencil.  

Once you establish a baseline using a good starting point, if you still have the shifting you can adjust the aperture design by doing a reduction. Either a straight percent reduction or pulling paste away from the part to reduce float. When doing this, it is always best to record and document your experiments for future reference.
Walter Pierowski
Process Engineer
Esterline Interface Technologies
Mr. Pierowski is currently a Process Engineer for SMT and Electrical Assembly with 20+ years of experience. A majority of the experience was based in screen/stencil printing for electronics manufacturing including application support for SMT, LTCC, HTCC and thick film.
Check your profile.  There is a possibility that outgassing is causing the shifting of LED's.
Edithel Marietti
Senior Manufacturing Engineer
Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.
Probably floating on too much flux or alloy. Try cross hatching pad.
Greg York
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has twenty two years of service in Electronics industry. York has installed over 350 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.
Depending on the amount of solder on the pad, the LED can become buoyant and "float around" on the surface of the molten solder. This can further be aggravated when reflowed in a convection reflow oven where the fans can actually blow parts around.  

I hope that it goes without saying, but I was also check the placement before reflow, make sure the boards are not getting bumped and that operators did not place or reposition these parts manually.
Eric Bastow
Senior Technical Support Engineer
Indium Corporation
Eric is an SMTA-certified process engineer (CSMTPE) and has earned his Six Sigma Green Belt from the Thayer School of Engineering at Dartmouth College. He is also a certified IPC-A-600 and 610D Specialist. He has an associate's degree in Engineering Science from the State University of New York and has authored several technical papers and articles.
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