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Reflow Oven Zone Separation Challenges



Reflow Oven Zone Separation Challenges
We recently purchased an oven profiler. Some of the zones are competing with each other to get to the correct temperature. How do I find prevent the oven from having competing zones while getting an accurate process?
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Phil Zarrow
Phil Zarrow
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.
Jim Hall
Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

Transcript


Phil
And welcome to Board Talk with Jim Hall and Phil Zarrow of ITM Consulting, on this forum as The Assembly Brothers. We’re here to help you with your process, equipment, methodology, material, questions and whatever else may be bothering you.

Today we a note from T.G. What is bothering him is his reflow profiling situation. He says we have an eight-zone reflow oven in our SMT production line. We recently purchased an oven profiler. We have been having some issues with our tin-lead process. There is a fairly large list of paste options within the oven profiler software. However, the part number of our pastes doesn’t match any of the options.

The issue we have is that some of the zones in the oven are competing with each other to get to the correct temperature. How do I find the best way to prevent the oven from having competing zones while also getting an accurate process?

Jim
There are two real issues. Let’s start with the oven. I have been an oven designer for 30 years. The issue that T.G. is talking about is what we used to call thermal spill. In any given oven, there is a limit to how much difference in temperature you can set two adjacent zones. Think about the way a reflow oven works, you have these recirculating zones sitting right next to each other with an open tunnel and a conveyor moving, and ultimately boards moving between them.

All ovens are designed to give you as much separation as possible. But as you start to make one zone much, much hotter than the zone next to it, there at some point will be a tendency for some of that heat to be conducted, convected and any mechanism transferred from the hotter zone to the colder zone, following the first law of thermo-dynamics. There are going to be limits when you set up your profile temperature, your recipe temperatures, your zone temperatures, how much you can set that. That varies from oven to oven.

Most ovens have gas management techniques that will allow you to enhance that. First thing is, talk to your oven manufacturer to find out about the zone separation issue and see if there are techniques that are built into the oven to help you with that.

Phil
That’s the oven aspect situation. As Jim said, to avoid the spill-over of parasitic heating different ovens in your design. You may look down at the panel on the inside of the oven and see a plate with a bunch of holes in it.

As Jim will attest to, he spent over 30 years of his life designing these things, there are some really interesting dynamics going on there. So, back to the oven aspect Jim, apparently he is using Brand X solder paste and the prediction software didn’t list it there.

Jim
People who make oven profiling hardware and software try to make it as simple as possible and they try to put in most of the common solder paste specifications. But you should always be able to go in and set your own specifications. I always recommend to start with a paste that is listed in there.

As you optimize your process and you find out what actual profile within the solder paste manufacturer’s process window, what specific profile works best for your board, that you set the specification limits in the profile software specific for that board. And in this case, you can start by going in to that part of the software where you manually enter in the solder paste specifications, maximum ramp rate, minimum ramp rate, maximum peak temperature and so forth.

Start with just the data sheet from the solder paste that you are using and then optimize from there, tightening the process window, the specifications, to get to the specific profile that works best for your board. In terms of minimizing zone separation, most people are using a straight ramp profile, where you start out and each zone down the tunnel gets a little hotter, giving you a relatively constant heating rate from ambient up to peak temperature.

Particularly with an eight-zone oven, you shouldn’t have great temperature differences between adjacent zones and that should minimize the tendency for zone thermal spill, for cooler zone overheating due to having a hotter zone next to it. Typically, where you run into zone separation is when you need to put a soak, or worst case a shoulder into your profile. Where you have a relatively cool zone next to a hot zone.

If you are using a soak profile, you might want to consider moving to a straight ramp, eliminating those big differences between adjacent zones. Most pastes work perfectly well with a straight ramp profile, although there are other reasons such as thermal uniformity that can necessitate the need for a soak zone. But those are the general rules of thumb that I would suggest. Manually enter the solder paste specification into your profiling software. Try using a straight ramp profile to minimize zone separation issues.

Phil
Yeah, as Jim said you have a number of things going for you, you have an eight-zone oven. Unless it is a bad oven, hopefully it is not, eight-zones should give you a pretty good ability to sculpture your profile. The other thing is you are doing a tin lead profile with a tin-lead alloy so you have that going for you. The only other thing is, it is good to know how to profile.

I know the prediction software that oven profilers have come out with. We have worked with them. We are old school. We had to profile, no only back in the days when there was no such thing, we had to profile back in the days of infrared. You guys have it easy these days. It is kind of like, yeah I use a GPS all of the time but I certainly remember how to use a map. It is kind of the same thing. We are depending on the automation.

The oven company could certainly tell you, as Jim implied, what kind of zone separation you can expect. If you need help with the profile, certainly, look at the data sheet for your solder paste. That is a prerequisite anyway regardless. Understand that if you have any questions, call the tech support of the solder paste company. Unless it really is Brand X and they don’t have a tech support. But do understand how to profile an oven, what all is involved.

Jim
I think you summed that up very well. It is a shame you can’t solder that well. Don’t solder like my brother.

Phil
Yeah, I was going to say whether you are using a prediction or not, no prediction can help you solder like my brother. Thank you for listening to Board Talk.

Comments

A point of differentiation on solder paste specifications vs. recommendations. The solder paste manufacturer will supply a recommended profile or process window. These are not pass/fail specifications, where by you will be making bad product if you don't meet them. The profile specifications need to take into account component heat sensitivity, solder paste, cosmetic and warpage concerns, etc.. It is made up of more than just the solder paste recommendations.
Mark Waterman, ECD
Oven recipe (AKA: Profile) generating software tools are great but are not fool proof and they should not be relied upon as the only source for oven recipes. They are a good starting point to help generate a recipe for a new paste or assembly. Common sense should always prevail. Only through "real" thermal profiling (measuring the temperatures on the assembly through the reflow process) can you be assured that the oven recipe is adequate, meeting the needs of the paste and the limits of the components.
Paul Austen, ECD
Zone separation (temperature delta from zone to zone) is difficult to achieve in linear zones unless the gas circulation to upper and lower zones is segregated and is not linear. I think a few oven manufacturers are onto this function of gas flow and how it affects temperature zone to zone. Plus, some ovens are designed to control static pressure from zone to zone, linearly as well as top to bottom. Control of static pressure in each zone via fan speed gives an engineer a much better chance to create a delta from zone to zone. Complex assemblies that rely on creating zone-to-zone temp delta in order to successfully reflow are major sales points for ovens that can create the delta.
Russell Claybrook, MicroCare, LLC
Additionally, there may be smart features available in the profiling software that can help overcome these types of obstacles. For example, automated prediction software can "learn" what the maximum temperature delta between zones is in specific areas of the oven. It then can suggest predicted, and optimized, recipes that the oven is more likely to be able to control to while getting your profile in-spec. You may want to contact tech support from the profiler company to see if they offer those tools.
Tom Bergeron, KIC
If you think zone segregation is tough on bending the rules of physics, try adding in top to bottom temperature differential as well.

Believe it or not we actually had it work in a unique application where very long boards running pretty much back to back created the separation between the top & bottom heaters.

There were a few other tricks involved but I’d have to kill you if I told you.
Ray Chartrand, Chartrain Consulting

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