June, 2016
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Instant Reflow Oven Changeover in a World of Short Production Runs
S
preading beyond its traditional North Ameri- can and European markets, the industry trend characterized by short production runs
and frequent line changeover shows no signs of slowing. Improved technologies for line balancing, production planning, smart feeders, MES, and oth- ers are being used more effectively to allow for faster production line changeover and less down- time.
One obstacle tends to be the reflow oven.
Having a multi-million dollar production line sit idly for 30 minutes or more, while waiting for a $40,000 oven to stabilize on a new recipe, has the potential to put a crimp in profits. The goal should be instant oven changeover, and today’s technolo- gy means that this goal can be achieved in the ma- jority of applications. First, the purpose of a reflow
oven: we must ask, “What is the main job performed by the reflow oven?” The answers tend to be to melt sol- der, solder the components onto the board and create a strong solder joint. While all of this is true, there is more to it. Clearly, we do not want a strong solder joint that has cracked the component in order to achieve it. So, the reflow oven needs to solder the assembly within the tolerances and process window that is deter- mined by the solder paste, compo- nents and substrate.
Challenges of Reflow The way we measure and con-
trol this is by the time vs. tempera- ture profile of the solder joints, PCB, and most importantly, the compo- nents. In other words, the oven needs to produce the correct profile. We can even think of the reflow oven as a profile-producing machine. A reflow oven has literally bil-
lions of possible recipes. Each differ- ent set of zone temperatures and con- veyor speed will produce a unique profile on a given assembly. It is im- perative that the correct recipe is se- lected to establish the acceptable pro- file for all the different PCB assem- blies. Therein lies the challenge. It may take an oven 10, 20, 30, or even 40 minutes to stabilize on a new set of temperatures, especially if the new recipe is cooler than the previous one, since it takes longer to cool down an oven than to heat it up. The key to fast or instant oven
changeover is intelligent selection of the oven recipe. If two PCB assem- blies are similar and have the same process window, then it will be possi- ble to select a single recipe that will produce an acceptable profile for both assemblies. An experienced process engineer can find such a recipe man- ually.
But, what if the PCB assemblies
are quite different in their size, ther- mal mass and other requirements? This is a great challenge. It is not rea- sonable to perform trial and error ex- periments when there are billions of possible oven setups and each result- ing PCB profile must be examined in order to find a common recipe. This sort of challenge is a perfect data mining application for a computer, however. Predictive software, even running on an older operating sys- tem, is fast enough to do an exhaus- tive search on the recipes and to se- lect the optimal oven setup that will produce an acceptable profile for a wide range of different assemblies. This may seem complex, futuristic and expensive, but the fact is that
By MB Allen, Product Manager, KIC Thermal
several competing technologies have been able to do this for many years at low cost. The technology chosen, the kind of applications, and the reflow oven itself will affect success. For example, an up- to-date oven with 10 or more zones and a range of PCB assemblies that do not include extreme low or high thermal mass can be batched together with a single oven profile that accommodates them all. The fastest oven changeover is no changeover.
Keeping Reflow Flowing In practical terms, one method to achieve this
Predictive software matches oven profiles to decrease changeover time.
is to attach both the largest and lowest thermal mass PCB to the profiler and run this train simul- taneously through the oven. The data mining pre-
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