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:
http://www.kester.com/download/NXG1%20Data%20Sheet.pdf
http://www.kester.com/download/NP505-HR%20Solder%20Paste%20Data%20Sheet.pdf
Had issues attaching the images, but these are images of both types, labeled accordingly: http://imgur.com/a/cJ9Tl
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