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Reflow help needed, Hotflow 5

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Reflow help needed, Hotflow 5 | 9 September, 2011

First of all, if you are going to tell me to throw my crappy equipment away, and buy a better oven because 3 zones isn't enough for lead free soldering, please find another thread to troll in. We have been using a Hotflow 3 for several years for lead free solder with pretty good results. If you know how to get the most out of what you can afford, like us, please continue reading...

We have a Hotflow5 that is our "backup" when our Hotflow 3 is down. Beleive it or not, we have better results with our 3. The 3 has the belt option, the 5 does not. Would that make the 3 work better, as you are setting the board on a belt that is already warm?

Anyway, on to my real question, The Hotflow 5 has 2 of the 6 "sections" of air nozzles blocked off. I determined that part of our issue is the boards with bigger parts are not hot enough long enough. If I turn the temperature up hot enough to reflow the big parts, the leads on the resistors turn gold. And at just about every setting our SOT23s look terrible. Slower belt with lower reflow temp didn't work, as I think we were burining off the flux before it hit the reflow zone.

Anyway, I took those covers off, and it didn't seem to help, now I'm cooking boards even worse. So was there a reason they blocked those off? With them off, I have 2 preheater zones of 22" each, and the reflow zone at 22". With those blocked, I have a only about 14" of reflow zone. My thought was remove them and get more time above reflow temp, and I might be able to lower the temperature.

Anybody know what length each zone should be? Should I put the shields back on the Reflow zone?

Ideally I want to find a profile that works on a wide range of boards, as we run 4 Pick and Place machines, and are down to 1 oven. Even with the other fixed, we only have 2. So Ideally I have one set for Lead, the other for Lead Free. It used to work well with the Hotflow 3 running.

Does anybody have a good resource for profiling? I have been attaching a thermocouple to a through hole pad on the board as it runs through, and trying to tweak it with that. Do I put one on the biggest part, and one on the littlest? Do I use a board that has already been run through, or only a fresh board? How do you attach the thermocouple without it pulling the part off the board?

Any suggestions for better (And easier) methods would be great.

We are trying to find a used "waverider" or something similar, but they are expensive, and our datalogger with thermocouples seems to work pretty well (until they get caught in the belt, that is).



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Reflow help needed, Hotflow 5 | 9 September, 2011

Wow, a lot of questions here...and you are definitely doing some "tough sledding".

To the point. If you have 3 - 22" zones you'll have to calc your belt speed and temperatures to match the solder paste requirements with regard to flux activation and timing, as well as reflow peak temperature and the TAL (time above liquidus). If you know all this, then you understand how multiple zones can be more flexible and effective in this regard. I'm not sure that an optimal profile can be had with only three zones, but you might be able to get into the ballpark at least. Depending upon your board size and mass, and depending upon your solder paste spec, you might try calculating backwards from something like a basic 5 to 6 minute lead free profile. 100 to 120 seconds in each zone should give you a workable belt speed, set it and leave it. From there get your thermocouples attached using ChipBonder, if you have 4, place one next to the largest part, one next to the smallest,(far apart from each other) one on the backside middle, and one in the air slightly forward of the leading edge of the board. That way you can see your oven performance when the PCB hits the respective zones.

Now with all things in consideration, Flux activation temp and range, peak solder temps at 242C +/- 5deg. 1:00-1:30 TAL, start [playing with your zone temps until you get close enough to what you want. You may want to consider "adding" a zone by preheating the PCBs before placing into the oven. Maybe preheat to 90C, then send through your three zones at 145, 205, and 265.

Unplug any restrictors that someone may have placed in the airflow, for sure. If it wasn't designed with the blockage, remove it.

And with three zones, I wouldn't try too hard to come up with a profile that will work for many boards. That might take just about forever. Reflow is a function of PCB and Component mass. In lead free soldering the window is even tighter.

Best of luck, I understand your position, and I empathize with you. I know (that you know) you've got to get a better oven, so I won't beat that horse.

There are tons of references to reflow issues in these hallowed pages....research, research....

Regards `hege

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Reflow help needed, Hotflow 5 | 16 September, 2011

Reflow Oven

reflow oven profiler