Many of you will have an issue with “bottlenecks” in your process. This can happen at any point in the SMT Reflow process. Depending on the product that you are manufacturing, it is also likely that the “bottleneck” will jump from equipment set to equipment set. For our purposes, let’s look at the reflow process when it is identified as your bottleneck.
I have seen several methods that address a reflow bottleneck. The obvious solution is to increase the conveyor speed of the reflow oven. This is a task that requires a bit of skill. Your profiler becomes the single most effective tool to improve the Throughput Time (TPT) of the reflow process.
Let’s look at the fundamental changes of your profile with an increase in conveyor speed. First, the PCB will spend less time in each zone. Also, your process will move toward the shorter end of your spec as defined in seconds. For example, if soak time is defined as 30 to 90 seconds, your actual process will be perhaps in the 30-50 second range as opposed to a comfortable 50-70 second range that was established at slower conveyor speeds.
The longer the oven, the more wiggle room you have for increasing conveyor speed without having to make significant changes to your profile. Having a longer oven suggests that the PCB stays in the reflow process for a longer period of time, but keep in mind that it really comes down to how many products per minute exit the oven. Whether the oven is 10 feet or 30 feet in length, a higher conveyor speed will increase the number of products that exit the oven per minute.
Work In Process (WIP) is determined from the time the lot enters the SMT process to the time it is ready for shipment as a finished product. This duration is stated normally in hours and can add up to a few days to weeks, depending on the product. Having a longer oven does not mean increased TPT nor does it violate your WIP objectives.
It is tricky working with shorter ovens with fewer zones since they do require higher temperatures per zone to reach the desired specifications, as compared to longer ovens. I have found that using solder paste that uses a Ramp to Spike (RTS) profile works better in ovens with fewer zones. The RTS pays less attention to the soak and more to the overall length of the profile (the soak, of course, is often not listed in the solder spec for an RTS profile). Also, shorter ovens impact the slope in the RTS profile. For example, if you have a 5 zone oven, the first zones will need to be set at a fairly high set point in order to process the rest of the profile. At this point, the slope becomes very steep as the PCB moves from ambient to 160°C. 160°C could be the set point of the first zone! Also, in a distance of just a few feet, the product will need to rise in temperature to greater than 217°C in PB Free and above 183°C in eutectic solder. Many specifications will call for a peak of 200 to 240°C, which puts further demands on your shorter oven. In this instance, it is desirable to calculate the slope over the entire profile, setting it from 130°C to 180°C over a shorter period of time. Again, you need to look at how long (in seconds) the PCB stays in each area of the profile when designing your spec.
Now that we have looked at oven length when considering an increased throughput, how do we develop a profile that will remain within specification at higher conveyor speeds? Some profiling software will allow you to make a desired change to conveyor speed and then return a predicted profile. This typically can be completed in seconds, before even running your first profile.
A clever feature of some profiling software is that you can set a range of allowable conveyor speeds while maintaining acceptable limits to your process window. For example, when factoring the variability (drift) in your oven, you can comfortably run your process using only up to 70% of your available process window. In practice, anything over 70% is risky due to drift that can push your process out of spec. To eliminate this concern, run a “what if” scenario, where you define your minimum and maximum range for conveyor speed and maximum allowable PWI (in this case, set to 70%).
The profiling software will literally search billions of possible combinations, giving you the maximum possible conveyor speed without violating your 70% PWI threshold. Of course, you may find the conveyor speed to be too slow still. What can you do? Bump up your allowable process window or buy a new oven. In either case, you are in control of your predictive modeling (this sure beats hundreds of hours of trial and error and possible board destruction). This process is similar to the prior section on Getting your Product Deeper in Spec and it will take as long as it takes to heat up or cool down your oven and re-profile if verification of your new predicted profile is required. In practice, I find that it takes less than one hour.
