LED lifted soldering defect

It seems there isn't right connection between solder and solder-legs. Maybe check that the LEDs get huminidity, have some oxidation on the legs, try different solder paste, check the reflow solder profile.

On the photo I can not saw properly reflowed solder. For me, it seems to be there is to cold in your oven.

Our company place around 300-500K LEDs from 3014, 3528, 2835, 5050 casing and there weren't problems like yours - never ever. We never place opened bag or if we have opened bag we are storing them in huminidity storage.

That can be a problem the PCB is huge, if the customer doesn't handle them properly - curve them and make extra mechanical stress -, can be also a source of problem. Make mechanical test about find out how many issues you can make from not-acceptable handling.

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We also have a few customers that seem to buy the cheapest LEDs available and provide them for the build. We've had complete reels that had obviously been exposed and oxidized.

You can try a solder with a more aggressive flux. How long is your profile? Our solder paste is meant to be in process for a maximum of 4.5 minutes. I have the belt speeds set for a 4 minute run. I know some companies crank up the speed and try to push product through reflow faster.

Remember, at 30 inches, that board is in at least two different zones at any given time. We do many 48 inch boards, and so our boards are in 3 zones at once. You may need to get the boards a little hotter. I'd suggest increasing the temp set point in the front half of your recipe. That way you increase TAL with affecting PEAK.

Also, are these actually 30 x 12 inch boards, or are they strips that get separated? I've seen LEDs pop off when arrays are broken out and the board "twists" a little. Depending on your method to break them out.

Is this LEAD or LEAD FREE?

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Thank you both.

We are using EP256 Solder Paste Sn63Pb37 no clean from Kester. I attached our profile as well as Kester's target profile. We used a populated demo board for the profile. We tested along the width, and was less than a 2c difference.

It is a single 30"x12" board. We do many smaller boards with tabs for this client as well. Some with the same LEDs, some with different. This is the only board that has ever had this issue. The largest panel for the smaller boards is around 19" x 11", with 10 small strip LED boards within the panel.

PS: Please ignore the massive temp drop off at the end of the profile, we grabbed the board out from the oven early, to do multiple tests faster!

Attachments:

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The solder looks a little grainy for Sn/Pb solder but that might just be the picture quality.

There appears to be plenty of solder on the pads, so I'd rule out anything to do with the stencil.

One possibility that springs to mind; how long is your paste exposed to the air? Since you are hand squeegeeing, I'm guessing your stencil and paste are not enclosed, so maybe your flux is drying out. Depleted flux on components that are marginal to start with, might be the source of your problem.

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We currently use Kester's ep256 as well and it seems to be a bit more grainy than one would expect from lead, especially if the temperature is too low. That paste says it's specialty is being robust over a wide range of profiles and stencil conditions, so to me that means it lacks when you have the capability of fine tuning the process. We're looking at trying something slightly more active like ep256ha or preferably hm531 if a cleaning process is present. I'm interested in seeing what the problem ends up being, oxidized parts from open bags makes sense as much as a profile that's slightly too cool.

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Hmm, OK thanks for the input guys. Yeah, I'm surprised because our profile seems pretty solid, and it is such an isolated issue.

I like the thought of the flux drying out as a potential cause. It was definitely exposed to the air, and while we tried to add fresh paste every few boards, it definitely could have been too dried out. But I would expect more than just 3-4 diodes on two boards to have been affected no?

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Over the years I have gotten the best support from Aim and Alpha and the least support from Kester. If you can, you might want to switch to a solder supplier that gives good support.

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We agree with your statements. 1. Having manually printed board will compromise your print and you don't know how much paste you have on each pad. 2. LEDs falling off that easily might be related to contamination, considering that your profile is notch on.

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You might want to try baking the LED's in an oven for 24 hours prior to build. We keep our LED's in a humidity cabinet all the time, as we have noticed poor solderability if they are left out for too long. This may be even more important when buying from a less reputable source.

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