|Ask the Experts|
March 7, 2018
Lead Free Reflow Oven Zone Count
We are a small EMS company and process a large variety of boards. We have a 4-zone oven that is not rated for lead-free soldering and are in the process of replacing it. We have looked at ovens with as few as 5 zones and as many as 10 zones. How many zones are really necessary to do a good job of reflow for lead-free. Will too many zones just over-complicate the process?
|Expert Panel Responses|
From my experience, most companies are quite comfortable with a 7-zone reflow oven, which I think has been the standard configuration for mid-to large volumes for quite a few years. Many smaller EMS companies have done quite well with a 5-zone also. However, you may wish to consider a 7-10 zone for a number of reasons. I will try to touch on some of them. Do you use nitrogen? If you do, and you gain significant reduction in your overall DPMO as a result, then the first thing you may wish to consider is the nitrogen use of a new oven. Certain models use nitrogen much more efficiently than other models, and being a small EMS you certainly don't want to jump onto a great big honkin' nitrogen cost. So keep that in mind. The cool-down rate is just as important, actually more important, than the ramp-up rate especially with lead-free temperatures. Remember as a child when you fried marbles and dropped them into ice water to get all of that cool cracking inside? Well, if you cannot control the cool-down portion of the reflow profile and the CCA is allowed to cool too fast, fractured solder joints, pad cratering, and other defects can occur the same way. A 7-zone or 10-zone oven can effectively "stretch-out" your process window and allow you to control both ends of the spectrum. A generally accepted rule for rate of cooling is between 1 and 2 degrees (C) per second. However, you do not want to drag it out less than that, either. In terms of throughput, a larger oven is less susceptible to temperature loading. Here we go with another analogy: If you have a small glass of hot water, and you drop a lot of large ice cubes into it, the temperature of the water is drastically reduced. Likewise if you have a 5-zone oven it may only be able to accept a single large CCA every 10 minutes and keep that CCA within the process window needed to achieve a good reflow of the paste throughout the entire CCA including under the BGAs, which are somewhat insulated from the heat. So you may even need to slow down the conveyor. This drastically affects your rate of production and can put a bottleneck in your work in process (WIP). On the other hand, a larger oven can (typically) accommodate a heavier load and recover faster, allowing you to reflow perhaps even two or three CCAs spaced next to each other and followed closely by many other CCAs and yet remain fully inside of the process window throughout the entire spectrum of the reflow profile. Granted, the larger oven may consume more power but that is not always the case because a smaller oven struggling to remain at the set temperatures under heavy loading can actually be less efficient. Another advantage of a 7 to 10 zone oven is that it will allow you to "tailor" your profile much better than a 5-zone oven. This can be critical especially for lead-free paste, as you do not want to burn off the flux in the paste too rapidly prior to achieving temperatures above solidus. It also allows you to have an elongated TAST without having the CCA peak at even higher temperatures than what is required, which can lead to reduced reliability of both the CCA and the components. Having said all that, there are other factors that come into play. Can the components on your CCAs withstand lead-free convection temps? Do you build a large number of massive CCAs and backplanes, in which case you may want to consider a vapor-phase system in conjunction with your current convection reflow oven. However, there may be LEDs, LCDs, or other components that cannot be immersed into the vapor. Even a stand-alone batch oven such as those manufactured by APS, as an example, can be a very effective solution. It all depends on what your needs are. In order to choose, you really need to understand your present and future needs. Be prepared to qualify your machine selection before throwing production boards at it. To understand the Process Window concept, go to http://en.wikipedia.org/wiki/Reflow_oven. It has and excellent description of the process window for reflow ovens. Good luck in your selection. Feel free to contact me.
More zones offers more flexibility in the board or solder paste target thermal profile "shape" because you have more places along the length of the oven (or furnace) to introduce a different set point, at each new zone. This also can add a little more "complication," as you suggest, but may well be worth the added flexibility. Another reason to consider an more zones is to run faster conveyor speeds. This is because the average reflow solder profile takes about 5 to 7 minutes no matter how long how many zones you have. The more zones, the faster you must run the conveyor to get the boards through in that time. So if your line speed is an issue, more zones is the only answer. If you do not need to fight a fast moving line, than a lower zone count can work fine. Then there is space. More zones means longer machines. Do you have the space (length) in the room. Don't forget facility requirements as well: power, compressed air, ventilation, nitrogen (if needed), etc, all of which may be a little more in larger zone count machines. Another consideration is energy distribution. More zones means each zone does less work when driving heat into the board. So you are spreading the work of heating the board over more zones, which can be more efficient, because you can use fewer Watts per zone to get the job done. A good compromise zone count is seven heated zones, which offers reasonable flexibility and a lower cost than a 10 or more zone count machine. Please remember, no matter what zone count you choose, don't forget to measure the boards thermal profile using a profiler. This allows you prove the oven is doing the correct job of meeting the thermal needs of the solder and avoiding the costly danger of damaging the components at the now higher process temperatures of lead free solders.
Senior Project Engineer
Electronic Controls Design Inc
Short answer, greater number of zones give you greater control over your process. Let's not forget, the purpose of a reflow oven is to create a profile for your product. Lead free specs are tighter and less forgiving than lead processes, often requiring ramp soak spike (RSS) profiles and not just the straight ramp to spike (RTS) that you are used to. You have certainly heard about the rise of defects in lead free processes as engineers struggle to find the right balance between tight ramp rates, limitations of the oven and throughput requirements. Consider this, the shorter the oven, the higher your ramp and less wiggle room to create for example a soak to mitigate perhaps defects such as tombstoning. If you run into trouble and have a shorter oven, the only option you will have is to slow down your conveyor speed perhaps well below your throughput requirements. Also you will likely face higher deltas between zones to achieve your desired profile, which may exceed the specs of your equipment.
Global Account Manager
The number of zones allow for more control of the profile shape, and usually higher production speeds. The minimum number of heated zones I would consider is 7 (only my opinion), so that at least two zones could be used for a soak area. Some assemblies my not require a soak, indicating they could be produced on an oven with only 5 heated zones. The bottom line is the more zones you have, the more control you have over how your product is heated.
M.O.L.E. Line Product Manager
Electronic Controls Design, Inc. (ECD)
A small EMS with the potential for multiple changeovers a day, high mass and low mass PCBs would be as well to consider Vapor Phase. The issues of using a large reflow system capable of dealing with the difficulties of high reflow temperatures for low volumes of products and then having to deal with changeovers and delays in production as the reflow system stabilizes, are considerable.
There are differences between oven brands in terms of thermal capabilities, but in general more zones tend to make it easier to set up a lead-free process. The larger ovens with more zones, however, are often purchased for high throughput considerations rather than the need for processing PCBs in the narrow lead-free process window. 5 or 7 zone ovens are usually adequate for "normal" lead-free production at medium throughput. The use of profilers with process optimization software tends to make appropriate oven set up both easy and quick, even for tight lead-free processes.
inspīre solutions LLC
For the most part it really depends on your board density, type of material etc. Assuming it is just a standard FR4 board without massive heatsinks for the most part a 5-7 one would be preferred. The main advantage to a larger oven is that you can run it faster than a say 5 zone oven. Typically our biggest seller personally is a 5 or 6 zone reflow. 90% of our customers are running lead free with no issues at all.
It depends on the volume you need and size of PCB. ESSEMTEC offers a 4 zone lead free oven that has 300mm width, and for example can produce 60-70 PCB per hour operating on 7kw. Proven, more 1,000 in the field. Ok our 5 zone lead free oven that has 400mm width, and for example can produce 110-120 PCB per hour operating on 7kw. Proven, more 1,000 in the field. Bottom line is lead free can be done in good, smaller ovens. You do not need 7 zones. 7 zones will give you more through put.
The number of zones that you require is more dependant on the thermal complexity of the boards, and the efficiency of the oven than the alloy. The more thermally demanding the board the more zones will be required to meet the profile requirements of the solder paste and board (ramp rate, peak temperature Delta T, etc). Since the limiting factor is the board for any given oven you need only look at your most difficult product and make sure that you can achieve your required profile. If you have a thermally simple board and an efficient oven the 5 zones may be sufficient. 7 and even 10 zones will give you more flexibility if your boards change.
Senior Applications Chemist
Depending on the complexity of the board (PCB itself) as well as the population, more zones will offer you the flexibility. Fewer zones ovens work for lead free assemblies but there is no guarantee that you will not have assemblies that will be pretty challenging to reflow - heavy copper layers, very small components like 01005 and very large components like ceramic or metallic BGAs on the same board. With larger machines the profiles can be designed so unnecessary stress will not be applied to your assemblies (quick ramp to reflow and/or cooling). Other important factors to be taken in consideration are the throughput and the elevated temperatures that the machine has to produce. In this case, from my perspective, more is better.
Engineering and Operations Management
The biggest problem I see in our customer base with profile set-up problems is not having enough zones to effectively control the profile. The temperature difference between adjacent zones is the enemy of the reflow oven, large delta's zone to zone are not easy to maintain, with mixing at zone boundaries etc. Every machine has a maximum zone to zone delta, modern machines are excellent but still it is good practice to not be running on the upper edges of these spec's if you can avoid it.
A higher zone count means you can achieve a desired profile using more controlled steps and minimise any large temperature jumps between zones, this will allow profile setups that run nice and stable through the working day.
Ovens with seven zones give an excellent level of control, and unless higher throughput speeds are required is more than enough for most size manufacturers, you do not buy a new oven every day and you are never sure what's around the corner!
A lot of manufacturers do use smaller zone counts successfully, it really is down to the range of complexities of your electronic assemblies.
If you really feel that a 4 or 5 zone oven is enough given an appraisal of your assembly shapes/sizes and some profiling trials, you could also help yourself out by selecting a paste model with an wider working process window.
Founder / Director
For lead-free soldering, I will recommend at least a 7-zone reflow oven, which will give you flexibility to properly design and adjust the reflow profile. When it comes to SMT manufacturing processes, many defects or quality issues (such as bridging, solder balling, poor wetting, tombstoning, solder beading, voiding, dewetting, intermetallic formations, copper erosion, components cracking, and PCB delamination or charring, among others) can be minimized or eliminated by optimizing the reflow profile.
To help you understand the correlation between reflow profile and defects better, I recommend that you read an article I published on this topic which is available at https://metallicresources.com/wp-content/uploads/2017/02/n1.pdf.
Director New Product Development
Metallic Resources, Inc
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