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LED lifted soldering defect

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Hi all, We are having a confusing defect with one of our cl... - Dec 03, 2019 by kyleh04  


LED lifted soldering defect | 3 December, 2019

Hi all,

We are having a confusing defect with one of our client's boards. It is a large board, 30" x 12". The client supplies the LEDs, they are cheap overseas diodes, and it is not uncommon to have reels with the packaging torn. We have warned the client about them, but he tells us to proceed with the process.

We have had this issue with 2 of the 40 boards, each board has around 500 diodes. We have used the same diodes for the same client on smaller boards, never having an issue. Boards are ENIG.

The problem is: The board tests perfectly, then after they are shipped, and assembled in the client's final unit, a couple diodes fall off of the board. The pads are soldered, and you can see the imprint where the diode was, but it is like it just didn't "stick". I have an attached picture.

On both boards that it happened, it was 2-3 diodes, right near each other.

We are using kester leaded solder, we profiled the oven to be correct with the larger boards as per kesters datasheet. The three pads that are on the same side are connected electrically, so the picture that shows the solder bridged is fine, as there is no solder mask.

I see 3 possibilities: 1) We are hand stenciling the boards, as the stencil is much larger than our printer can handle. It makes sense that the defects are in a group, as maybe too little paste was applied there. But, it appears like enough paste is applied in the picture.

2) Since the diode bags are sometimes torn, maybe some are getting oxidized, and hurting the solderability. This doesn't make sense why the issues are in close proximity to another.

3) Something is happening with the oven, not heating consistently across the board. This doesn't make sense, as we profiled the board, and I feel like it would be a more common defect if this was the case.

Any other thoughts?


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LED lifted soldering defect | 4 December, 2019

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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LED lifted soldering defect | 4 December, 2019

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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LED lifted soldering defect | 4 December, 2019

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!


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LED lifted soldering defect | 4 December, 2019

You have a peak of 216 and roughly 40 seconds TAL. So you are right in the middle of the given ranges. So there's room to get things a little hotter and for a little longer.

I am surprised you are having this trouble with TIN/ LEAD, as the process window is very forgiving. Your board could stand to be in the oven a bit longer as well. You seem to be about 30 seconds faster than the KESTER profile. But then you'd have to start over. So I'd try to get things hotter with the current belt speed.

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LED lifted soldering defect | 4 December, 2019

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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LED lifted soldering defect | 4 December, 2019

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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LED lifted soldering defect | 4 December, 2019

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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LED lifted soldering defect | 5 December, 2019

I know your application isn't BGA but the data sheet also mention about the peak temperature - 235 Celsius. For the LEDs you can reach higher temperature and I recommend to reach 225-235 Celsius. You will get significantly better results because the solder past seems to be cold on your first photo.

If I were you I make a new profile and test the soldering with mechanical stress and breaking down a LED. If the soldering isn't perfect, try a different solder paste with more flux. For example the paste what we are using (Senju M705) have 11% flux with very good wetting and soldering characterism, our customers love how it works.

If you have Type3 paste is very good, better as your application require type4 would be way cost effective for LEDs.

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LED lifted soldering defect | 7 December, 2019

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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LED lifted soldering defect | 9 December, 2019

All termination platings are not created equal (sic).

Some parts put up more of a fight against wetting than others, and in those cases you really have to dial it in.

It seems like that in most of those cases I end up reducing the temps in the middle of the profile in the hopes that I'll end up with more flux when I really need it.

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LED lifted soldering defect | 9 December, 2019

Did you solve this issue? It doesn't much look like the flux dried out, ti should be across whole board and has much more same defect if flux dried out.

Did you have failure ratio data to show the defect location, for example 10% at location A on bot side, 30% on location B on top side, etc.

Collect more data can show it more clear for analysis.

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LED lifted soldering defect | 17 December, 2019

Haven't run those boards again, so haven't had the issue..

Since it seems like we may want to start using more aggressive flux based paste soon, what are some good choices? Ideally I'd like to stay with Kester, since they are readily available.

EP256HA is the same that we use, but has a more active no-clean flux

R231 is a RMA based paste. We have never used any rosin based flux before. We don't have any washing capabilities as of yet. Kester says this flux doesn't have to be cleaned.

What are your thoughts on the RMA vs HA no-clean based pastes?

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LED lifted soldering defect | 19 December, 2019

if use more aggressive paste,be careful risk of electro-chemical migration

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LED lifted soldering defect | 19 December, 2019

True, but with rosin it should protect the joints pretty well right?

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LED lifted soldering defect | 20 December, 2019

Not really, if this is a normal consumer electronics, could be no big issue. Anyway flux residue provide ions and the substance absorb water or moisture.

If for automotive or space, could have high risk, it depends your application.

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LED lifted soldering defect | 24 December, 2019

Hmm OK. Even a NASA article describes that rosin is safe not being cleaned lol.

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LED lifted soldering defect | 26 December, 2019

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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LED lifted soldering defect | 2 January, 2020

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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