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October 10, 2017

SMT Target Component Placement

I'm having trouble obtaining accurate placement for an IR SMT LED on a PCB during the SMT soldering process. Despite meeting the IPC standard, which allows for some shift of the component with respect of the pad, I would like to reduce the shift, and obtain a maximal displacement (post reflow) of less than 0.2 MM in both directions, X and Y.

Can you advise about ways to improve the placement accuracy for the location of the IR LED compared to the ideal target placement?

J.K.

Experts Comments

The key question is whether the location of the LED is shifted prior to reflow, or shifting during reflow. You state that your goal is to control X-Y location within + 0.2mm (+0.008") in both directions. If your current displacement is larger than that, then I'd surmise that the shift is occurring during reflow. Your placement system should be capable of maintaining accuracy of +0.001" or better.

Shift of chips during reflow is not uncommon, and LEDs can be especially problematic, because of the way their terminations are manufactured. The solution to controlling this shifting during reflow is to optimize the land geometry and paste deposit size to greatly reduce unbalanced forces that are present when parts of a termination wet before others. This usually means using a smaller pad and smaller paste deposit than may be optimal for inspection. A second factor to examine is the reflow profile; if there are strong thermal gradients during the time that the paste is melting, unbalanced wetting forces will be maximized.

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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
In this case the ideal placement is determined by your pad size. Pad size should not exceed the specifications recommended by the LED vendor by more than 10 mils.
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Edithel Marietti
Senior Manufacturing Engineer
iDirect
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.
First you should make sure your pick and place machine is placing the part accurately. Depending on the size of the package, you may have an issue of the part floating too much during reflow. If the package is small enough, you may be able to decrease your solder volume. This would also depend on the pad layout on your pcb. If the pad layout is too large for the component footprint, it may be difficult to keep the part straight. One option we have used in the past is to add a small dot of chip bonder to the center of the package out line. (between the pads) This would cure in the reflow oven before reflow and help keep the part in position where it was placed.
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Brien Bush
Manufacturing Applications Specialist
Cirtronics Corp.
Mr. Bush has 20 years experience in electronics contract manufacturing. Major areas of expertise include through hole, SMT, wave and selective soldering.

We are currently manufacturing Set-to-boxes and LED Televisions which use SMT IR sensors. I have increase the amount of solder paste on the 2 outer pads to increase the surface tension during reflow. It can be done by over printing on the solder mask, which is typically done for PIP(Pin-in-Paste) devices.

To test which works best for you, make up a test PCB, with different pad designs and different solder paste volumes, different parts behave differently. The key is to work out what is the best surface tension with respect to the mass of the device.

Alternatively you could ask your solder paste manufacturer to recommend a low surface tension solder paste, or use a more optimized reflow profile, which will reduce movement once the paste starts become liquids.
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Kishan Sarjoo
Process Engineering Manager - Electronics
Altech UEC, South Africa
Currently with Altech UEC and responsible for technology road map in PCBA electronic manufacturing and technical support for PCBA electronic manufacturing for Altech UEC and its JDM's. Over 7 years in SMT, Radial Insertion, Wave solder & Test Applications.

Is the placement poor and you are looking for the paste to correct it or is the placement good and the parts are floating during reflow? Either way, everything can be fixed with chip bonder it's the duct tape of the electronics world.

If adhesive isn't the answer you were looking for... try changing the size of the paste deposit can keep components from "floating" during reflow. If they are 90deg mounted LEDs you will have to be careful as they can be pushed by the reflowing solder deposit. Changing the shape of the deposit might help with this.

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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.
Without changing the shape of the pads on the board you might consider redesigning the stencil aperture to provide positional accuracy 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.
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