|Ask the Experts|
September 22, 2020
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?
|Expert Panel Responses|
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.
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.
Senior Manufacturing Engineer
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.
Manufacturing Applications Specialist
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.
Process Engineering Manager - Electronics
Altech UEC, South Africa
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 90° 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.
Technical Support Engineer
Without changing the shape of the pads on the board you might consider redesigning the stencil aperture to provide positional accuracy during reflow.
Vice President Technology
All of the suggestions from the experts are worth investigation, but you may have an issue with the tolerance of the artwork with respect to the PWB reference datum points, and or with respect to the box the card mounts into.
So if the required positional tolerance of the LED is .008", but the tolerance of the artwork to the PWB is +-.003" (standard tolerance in the industry), then you may be fighting the wrong problem with regards to the aperture size, placement accuracy, etc, etc.
Look at those other things, but make sure you look at the whole picture, so to speak.
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