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Halogen-free Solder Reflow Quality Issues

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Halogen-free Solder Re-flow Quality Issues | 24 August, 2015

I've recently pushed the company I work for to switch to a halogen-free solder that our supplier carries, Kester's NP 505 HR, prior we were using Kester NXG1.

NP 505 HR performs great on the screen, we have a lot of down time in between prints (we do small runs, of a variety of different products), and it seems to stay usable longer in comparison. One of the contributing reasons we made the switch was because some of our finer pitch IC's were not printing successfully. The NP 505 HR was appealing because of it's smaller mesh size.

However, some of our customers have noticed a difference in appearance in solder joint appearance in contrast to the NXG1. The surface appears more grainy, and dull. The flux looks more viscous as it appears to remain on top of the joint more, adding to the uneven appearance. The flux also appears more transparent, almost milky/cloudy at times, the NXG1 on most of our boards looks almost amber tinted, while still being somewhat transparent. On top of that our inspect team says they can see cooling lines, (which I should say, I have yet to see).

Our board substrates are typically .075" FR4, but they can vary down to .031". We also use aluminum, which can be as thick as .125". We are experiencing these results on all substrates.

I understand they are different products and they will have different characteristics and appearances. But I was just looking for assurance that they look appropriate, or a point in the right direction from someone who has experienced similar results with halogen-free solders.

The spec'd reflow profiles are identical for the two products. I've MOLE'd my ovens with on-board thermal couples and verified that they are functioning properly and within spec. I've tried adjusting my oven temps up, and playing with the conveyor speed to see they were cooling too fast or too slow. Nothing seemed to produce the results we had hoped to see. We've even ran both solders side by side to see if the NXG1 changed in appearance as well, it didn't.

Obviously, I know I could include more information, so if there is anything in addition to what I've provided that you would like to know feel free to ask. I was just trying to keep this somewhat short.

These are the data sheets for each product:

Had issues attaching the images, but these are images of both types, labeled accordingly:

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Halogen-free Solder Re-flow Quality Issues | 24 August, 2015

Your solder looks like it hasn't had time to reflow. Are`the pictures of an aluminium substrate ? So just positing a theory your boards need more soak time, the solder seems to have formed little hills/mountains and then cooled off before the surface tension had enough time to form that one little hump.

good luck sarason

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Halogen-free Solder Re-flow Quality Issues | 25 August, 2015

Both pictures are from .031" FR4.

According to my MOLE it's actually staying above the liquidus temperature for close to too long. Datasheet says 90 secs, I'm in the high 80's.

Not ruling it out as a possible cause, I suppose the data I'm getting could be inaccurate, as everything you said makes sense based on the pictures I've provided. But I don't know if we can really get the boards any more hot than they are already getting. Our boards have to maintain a certain physical appearance because they are visible in our customers' final product.

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Halogen-free Solder Re-flow Quality Issues | 29 August, 2015

When I made my previous reply, I don't think I was very clear in what I said, so like any politician I will rephrase my response to the question. My intent was that the board didn't have enough soak time leading up to final reflow. So what was happening at the final reflow, was that the heat was being sucked out by your substrate, so it didn't have time to reflow correctly. Mayne slow down your conveyor a tad. If you get the heat into your components(even though this wasn't part of your test, I am assuming you intend to put components on your substrate),before the final heating phase all should run well.

regards sarason

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Halogen-free Solder Re-flow Quality Issues | 29 August, 2015

And I might add the immortal words of Lord Kelvin "To measure, is to know" and again reiterated by Deming in a slightly more acidic way "In God we trust, All others must bring data"


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Halogen-free Solder Re-flow Quality Issues | 4 September, 2015

Maybe look at reducing the time above liquidus to @ 60 seconds. HF materials tend to want tighter reflow profiles...halogen activators are great at higher temps and longer reflow profiles..but HF prefer shorter profiles. The HF pictures look like the flux has been exhausted whilst the solder is still liquid so it tends to start to oxidise and not look smooth..

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Halogen-free Solder Re-flow Quality Issues | 1 October, 2015

Would be nice to see your profile, as I would imagine you could be too cool in reflow especially if seeing milky residues in Paste could be Organics that havent volatised out properly

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Halogen-free Solder Re-flow Quality Issues | 2 October, 2015

The following is representative of on-board temps of a FR4 at .069" thickness. The thermal couples were soldered directly to the board using high-temp solder. Again, the halogen-free solder requires an identical profile to our old solder. We had great results with the original variety. This profile is slightly different than what we used for the old solder, the changes were made since we noticed the difference in appearance between the two.

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Halogen-free Solder Re-flow Quality Issues | 2 October, 2015

After looking at your profile, some general thoughts....

Your profile looks like what is commonly called a ramp-spike profile and not a ramp-soak-spike profile. Going off of Sarason's comment, you may not have a good soak period before the spike into reflow temps.

It also appears you have a 3 zone oven. Most people doing lead-free reflow use an oven with at least 7 zones. It will be hard if not impossible to get a true ramp-soak-spike profile with only 3 heat zones in your oven. That link shows the difference in the 2 profiles and has some good general information on reflow soldering.

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Halogen-free Solder Re-flow Quality Issues | 9 October, 2015

Thanks for the profile Ive also managed to get onto a proper computer and look at the images again. Where has all the flux gone?? You arent over pressurising the blades too much causing the flux to shearaway from the joint are you?? Do you have good areas and bad ones on the same PCB?

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Halogen-free Solder Re-flow Quality Issues | 28 October, 2015

That's my main concern, the patchy/splotchy appearance doesn't look as pretty, especially under a microscope. When it was mentioned that my oven may be too hot in the ramp-to-faux-soak it was the first thing I tried adjusting. As far as our blade pressure goes, I don't think it was ever something we considered adjusting when changing solders. I believe all of our printer's default to 3 kg and that's typically what they stay at, (possibly up to 5 kg).

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Halogen-free Solder Re-flow Quality Issues | 3 November, 2015

how long are your squeegees for 3 - 5Kg pressure?

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Halogen-free Solder Re-flow Quality Issues | 6 November, 2015

They're probably 12 - 18 inches, maybe slightly longer.

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Halogen-free Solder Re-flow Quality Issues | 6 November, 2015

Also, for anyone who's taken an interest in this thread:

We just received samples of Henkel/Loctite's room temperature stable(yes, you read that right), halide-free, lead-free pastes, both the GC 10 and the GC 3W (which is a water soluble/wash paste).

I have to say we're very, very impressed with the boards that we ran on both. After looking through the product materials, we decided to use our existing profile because they deem a pretty wide (in my opinion) range of profiles to produce acceptable results. While our profile wasn't perfect, this stuff is the bee's knees. Our profile was probably on the upper end of the spectrum in terms of max temperature, and the leftover flux looked a little dark but once cleaned the joints looked superb.

Due to an inability to really get outstanding visual results with the Kester halide-free product, and the fact that Henkel's is room temperature stable I think we'll be changing solders again.

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