For REWORK you need a minimum of 4 independent top and bottom zones for heating and one zone for cool down.

I guess you are assuming the use of convection reflow for your lead free application when you mention "how many zones". Convection isn't your only alternative for Pb free applications.

Interesting as this question is, the profile is based upon the ability of the equipment to provide enough thermal energy to reflow the solder paste within the length of the equipment. I’ve seen it done with a 4 zone oven, not very fast but it was done.

One has to look at the overall performance of the equipment and the equipment should not be running at its maximum performance limits. A larger oven, perhaps 8 zones would allow a more robust process and yet still have the capability to provide the robustness of the process demanded by the lead-free process.

A concern with the lead-free process is not only the thermal ramp up to reflow the solder paste but also the cooling rate as the lead-free solder alloys do perform differently than the 63/37 leaded alloys, so we need to concern ourselves with the cooling rate to prevent stresses from building up in the laminate and solder joints not to damage the components.

So my bottom line is go with an 8 zone unit at a minimum, which would provide the capability and capacity needed for reflowing lead-free alloys.

The more zones you have, the better you are able to sculpture and hence control the profile. The process window is tighter with lead-free and the more zones the better. Of course the more vertical zones, the longer the process tunnel and the faster you can run.

We recommend a minimum of 5 vertical zones for lead-free reflow. For more complex PCBAs, in terms of component mass, density and surface geometry, think in terms of at least 8 zones as sometimes having the flexibility of having two zones for reflow can be very helpful.

This is an interesting question.

Profile attainment normally has two factors – amount of heat and time in the heat.

Theoretically, we could obtain a ramp to peak leaf free solder profile in a single zone by adjusting the belt speed and zone length to obtain the time in the heat and the set point and convection rate to provide the amount of heat transferred. Likewise, a profile with a soak and spike could be done with two zones that were sized proportionally for the two times. But neither of these approaches would be flexible or practical.

Today's reflow process needs the ability to process different boards with an assortment of profiles. In general a 6 zone oven with 75 inches of heat can be used if the board is small, the production rate is low, and the profile has a wide process window. But as the production rates and board sizes increase, more zones are required so the profile can be tailored to required process. This tailoring is the relationship between the number of zones used for the soak and the zones used for the spike at the desired belt speed.

Most people are finding that an oven with 100 inches of heat (7 or 8 zones) is good for belt speeds of 30 to 40 IPM. An oven with 125 to 150 inches of heat (10 zones) is good for belt speeds of 45 to 60 inches per minute. Although ovens with 200 inches of heat and 14 zones have been made for even higher belt speed, the costs and footprint usually have people looking at multiple ovens instead of a single big oven.

The number of zones in a reflow oven is only one of many aspects of the oven that will influence a successful lead-free process. The oven stability, zone separation capability, heat transfer rates and uniformity, max heater capability, and many other variables are also critical.

To generalize, additional zones, both heating and cooling, are very helpful when running lead free production, and they are required as you mention to increase throughput. I have seen companies run lead free production in a small 5 zone oven, but that is rare.

The goal with any thermal process is to create an in-spec profile at the required throughput. As you know, the lead-free process window is both much tighter than the eutectic process, and it requires higher temperatures. An oven with fewer zones may require you to really fine tune the oven recipe in order to produce the required profile, if the oven is capable of an in-spec profile at all.

There are profilers and process optimization software on the market that enable you to identify those hard to find suitable oven recipes in a few seconds. Our company KIC is one of the suppliers for such products.