Electronics Assembly Knowledge, Vision & Wisdom
5 vs 8-Zone Ovens
5 vs 8-Zone Ovens
We are setting up an assembly line for small line production. We prefer to purchase a 5-zone oven but our concern is that five zones may not be enough to ensure a reliable reflow process compared to an 8-zone oven. Jim Hall and Phil Zarrow, The Assembly Brothers, share their insight.
Board Talk
Board Talk is presented by ITM Consulting

Phil Zarrow
Phil Zarrow, ITM Consulting
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, ITM Consulting
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.

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Transcript
Phil
And welcome to Board Talk with Jim Hall and Phil Zarrow, the Assembly Brothers. Pick and place. Also, by day known as ITM Consulting. We are here today high atop Mount Rialto.

Jim, we have a question from B.B. B.B. says we are a small electronic design company setting up an assembly line for small line production. We prefer to purchase a 5-zone oven, as cost is lower. But our concern is that five zones may not be enough to ensure a reliable reflow process compared to an 8- zone oven.

Our boards are 5x7", components on both sides, the largest thermal mask is an inductor which is 10 x 6mm. Most components are .5 mm pitch BGAs, .5 mm QFN and a .65 mm TQFP. Passives are 0603 or larger. With this board, would we see an actual difference in manufacturing process reliability between 5-zone and 8-zone ovens?

I think I'll start by saying not from a reliability standpoint, although there is a caveat out there, please don't buy a cheap oven. There is some really suspect looking things out there, particularly if you start getting down to tabletop where they think a beanie propeller is an idea of convection.

Better you buy a decent oven, even an older one, something that has been maintained and calibrated properly since there is no fixed tooling on it. But from my perspective 5-zone vs 8-zone, I don't see a reliability issue, I see a capability situation.

Jim
My feeling is that sooner or later that capability is going to affect your reliability. My general feeling is if you are going to do nothing but tin-lead, a 5-zone oven is okay. If you are going to process lead-free, it really helps to have an 8-zone oven. The reason is because with lead-free you are going to higher temperatures. Your process window at peak temperature is much smaller than it is with tin-lead.

Most people prefer to use the last heating zone in their oven to control the initial cooling after peak temperature, that addresses principally warpage characteristics and grain formation within the solder alloys. To hit that tight process window between your 230, 235 minimum temperature and your 250 peak temperature, to do that with one zone can be problematic. That I think could challenge your reliability.

Most people like to use two zones for the reflow section of their oven. A common set-up for profiling an oven with eight zones would be to use the first five zones for your preheat, maybe a soak, maybe a shoulder. Then to use zone 6 and 7 for reflow, usually with the first one a little hotter and the second one a little cooler to prevent overheating your small components. Then to use zone 8 at a lower temperature to control your initial cooling to manage warpage and grain formation in your SAC alloys.

That is important because they mention that they mention they are doing .5 mm BGAs, so head and pillow issues due to warpage could be an issue. With a 5-zone oven with lead-free I think you could be strapped to really control the profile to get a reliable process.

Phil
Right. And that isn't to say that you can't do lead-free with a 5-zone but more is better. This is one case where more is better. We like the idea that the more vertical zones you have the more you can sculpture the profile and get everything where you want it.

Yeah, an 8- zone although it doesn't seem to be a concern in your operation, an 8-zone wonder oven does allow for faster processing speed. I totally agree with Jim. Go ahead, splurge for the 8-zone. As I said, even if it is a used one you never look back and regret.

Jim
Unless you're sure you are going to be using only tin-lead and then I feel that a 5-zone is good. I agree with Phil. You can reflow anything anywhere really. There are people who reflow boards in toaster ovens. But that doesn't mean that is a good choice as a reliable manufacturing process.

Phil
And make it easy on yourself. You have a lot of other stuff to worry about, especially in that printing process. I would go easy. So, if that is your choice, well I hope you will consider the wisdom we have imparted here.

Hopefully we answered your question and gave you some guidance. In the meantime, regardless of whether you are using a 5-zone, an 8-zone, a 20-zone or a toaster oven please don't solder like my brother.

Jim
And please don't solder like my brother.

Comments
The number of zones in your oven depends on your process. If you have done the profile calculations correctly you will know this before you even start making the decision. You can not make the speed of your process in the oven fit the speed of your line.

Once you have done the calculations for your profile based on the solder paste manufactures specifications and the actual heating zone sizes in your oven, (not just the heating process length). Of course you need to include the components manufacturers specifications as well. You can then calculate the best profile for your oven meaning the best fit of Actual Profile to Ideal profile. This is where the more zones you have gives you a better fit based on the delta form actual to ideal, it also gives you better flexibility. Once you have the best Actual profile, you will then calculate the conveyor speed. From the 3 possible speeds you choose the one that best meets the paste and component manufacturers specifications. Then the conveyor speed is fixed.

The number of Zone sizes for each stage (Ramp, Soak and Reflow) of the profile will be known as well as the time In each stage. This is what will determine how many zones are used for reflow. It make no sense to use a heating zone as a cooling zone, that is why it is a heating zone. Once the alloy has passed back below the liquidus point its job done.

The thermal mass of the PCB is far greater than any component on the PCB and usually the oven heating size if far greater than the actual panel size so there is plenty of heat available.

The warpage of the PCB depends on how it is conveyed by the oven till it passes it Tg point typically for FR4 ROHS boards around 150 to 170C

The only issue with cooling is the rate of cooling must not exceed the maximum rate of the components you are using.

Your conveyor speed again will determine you process speed of the line. It has to be faster than the speed of your Pick and Place machine. If your placer is pushing a board out every 20 seconds but your oven can only accept a board every 25 seconds. Then you have purchased the incorrect oven for your process.

If your process calculations work on a 5 zone oven then you will have no problems with the quality. However as pointed out in the article, it must be a decent quality oven with good heat transfer. When you measure the profile a good oven will give you a delta of about 5 to 10C between your set point and the actual temperature measured on the board,a bad oven will be 45+.
Les Watts, Testerion
A lot of it also depends on heated tunnel length. There are a lot of short 5 zone ovens that you would have to set the belt speed so slow it would barely work for lead free. I personally have a lot of customers using 5 zone ovens for lead free and doing it successfully I might add. Of course if you have the power and budget more is always better for flexibility and throughput speed but do not discount all 5 zone ovens. It is always good to send your boards to whoever you are looking to buy an oven from and let them run a profile on it to prove how well it does work. If the manufacturer will not do this then I would absolutely shy away from them.
Chris Ellis, Manncorp
I would definitely go with an 8 zone oven, especially if you are a contract assembler, to allow you more flexibility with preheating certain thicker substrates, and allow the use of lead free solder for the future. This gives you some margin, and the option for increased throughput for the future. I would also recommend a convection oven with top and bottom heating; more zones = more flexibility.
David Brand, Henkel Corporation
Until recently we had 4 SMT production lines all running SAC since 2006. The oldest line was a 5 zone unit and de-com'd a month ago. It performed well however there are severe limitations. Specifically line speed 45cm/min compared to 7 zone 76cm/min for the same job. The major restriction you will have is board mass / density. Its unlikely you will achieve a suitable profile with a PBA of > ~ 0.5g/cm^2, no coins or large Electro's. The area measurement excludes waste strips. e.g Z1:140, Z2:175, Z3:165, Z4:210, Z5:255. Note Z3 is lower. Your oven is going to last you +20yrs go for 8 or nine zones if capital is available. 5 zones - limited wriggle room.
Rob Hills, Tait International
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