|Ask the Experts|
June 17, 2021
LED Component Shift During Reflow
|Expert Panel Responses|
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.
Vice President Technology
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.
Esterline Interface Technologies
Check your profile. There is a possibility that outgassing is causing the shifting of LED's.
Senior Manufacturing Engineer
Probably floating on too much flux or alloy. Try cross hatching pad.
Technical Sales Manager
BLT Circuit Services Ltd
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.
Technical Support Engineer
There many root causes for components drift during SMT reflow process:
Director New Product Development
Metallic Resources, Inc
In looking at the picture and blowing it up, I note there are many unplated holes all around these parts. Are some underneath the parts? If so, air currents would be moving the parts.
Also, this does not appear to be a bottom terminated component, as a large terminal is apparent on the lower end and two smaller terminals on the "legs" on the upper end of the component, so I don't think any of the comments regarding overprinting, etc, apply.
If it does have a bottom termination, then they do. The vias along the top edge show wetting to the pads. Possibly getting the pads out from under the part may help.
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