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May 8, 2018

LED Component Shift During Reflow

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

M.M.

Expert Panel Responses

Shiftcan result from insufficient channels for out gassing of the solder pastevehicles during reflow. One way to correct this is to window pane thestencil aperture which leaves channels for outgassing during reflow.

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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.

Oneof 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 muchsolder paste during the stencil printing process. My recommendation is toverify both the pad and aperture design. I would use the manufacturersrecommended design as a start, and IPC recommended in case the manufacturerdoesn't have any processing details in their datasheet. Also a considerationis the thickness of the stencil. Onceyou establish a baseline using a good starting point, if you still have theshifting you can adjust the aperture design by doing a reduction. Eithera straight percent reduction or pulling paste away from the part to reducefloat. When doing this, it is always best to record and document yourexperiments for future reference.

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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. Thereis a possibility that outgassing is causing the shifting of LED's.

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Edithel Marietti
Senior Manufacturing Engineer
Northrop Grumman
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.

Probablyfloating on too much flux or alloy. Try cross hatching pad.

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Greg York
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has over thirty two years of service in Electronics industry. York has installed over 600 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 becomebuoyant and "float around" on the surface of the molten solder. This canfurther be aggravated when reflowed in a convection reflow oven where the fanscan actually blow parts around. I hope that it goes without saying, but I was also check theplacement before reflow, make sure the boards are not getting bumped and thatoperators did not place or reposition these parts manually.

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Kay Parker
Technical Support Engineer
Indium Corporation
Kay Parker is a Technical Support Engineer based at Indium Corporation's headquarters in Clinton, N.Y. In this role she provides guidance and recommendations to customers related to process steps, equipment, techniques, and materials. She is also responsible for servicing the company's existing accounts and retaining new business.

There many root causes for components drift during SMT reflow process:
  1. Ramp up rate too high at reflow stage. High ramp up rate will cause excessive flux outgassing and pushes the component away from the original position.
  2. Vibration or sudden speed change during the reflow process.
  3. Component or board oxidation. Heavily oxidized components and boards will cause uneven wetting during the reflow.
  4. Insufficient flux activity on solder paste, which will also cause uneven wetting on two sides of the component.
  5. Excess solder paste deposition (stencil too thick). High volume of molten solder will cause component floating at reflow stage.
  6. Solder paste misregistration during the stencil printing or inaccurate component placement.
  7. Uneven heating on two sides of the component. This can be caused by either asymmetrical heat sink effect or ramp up rate too high at reflow zone.


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David Bao
Director New Product Development
Metallic Resources, Inc
David Bao has more than fifteen years of experience in developing new solder paste, wave soldering fluxes and other SMT consumables. He currently serves as the Director of New Product Development at Metallic Resources Inc. He received a Ph.D. in Chemistry at Oklahoma State University.