You are putting in a new assembly line to assemble some large boards for which your company just received a 3-year contract. The boards are 45 cm long and you expect the cycle time from the component placement machines to be 40 seconds per board. Your boss is pressuring you to get another 5-zone oven, as they are cheaper and take up much less space than a 7- or 10-zone oven. But, you are concerned that a 5-zone oven may not have the capacity that is needed to keep up with the component placement machines. Let’s make some calculations and see if your concerns are justified.
Table 1 shows some typical reflow oven metrics:
Let’s assume that you will be using a typical modern SAC solder paste. By studying the reflow profile in above, we see that the amount of time needed in the heated zone is about 4.5 minutes or 270 seconds.
So if we choose the 5 zone oven the belt speed will be:
Belt Speed = BS= Heat Tunnel Length/Time in Heated Tunnel = HTL/Time = 180 cm/270 seconds 0.66 cm/sec
The component placers will be presenting a 45 cm board every 40 seconds, so the belt speed needs to be:
BS = Board Length/Cycle Time = BL/CT = 45 cm/40 sec = 1.125 cm/sec
So clearly a 5-zone oven won’t work. What about a 7-zone oven? Let’s calculate the belt speed for this oven.
BS = HTL/Time = 250 cm/270 seconds 0.926 cm/sec
Now we can see that the 7-zone oven won’t do the job either.
How about the 10-zone oven? Let’s see if the belt speed is greater than the 1.125 cm/sec needed.
BS = HTL/Time = 360 cm/270 seconds 1.33 cm/sec
Success! Since 1.33 cm/sec is greater than 1.1125 cm/sec, this 10-zone oven will work. The extra belt speed will allow for a small amount of spacing between the boards. Let’s calculate what it will be:
BS = (BL + Spacing)/CT = 1.33 cm/sec => BL+ Spacing = BS x CT => Spacing = BS x CT – BL
Spacing = 1.33 cm/sec x 40 sec – 45 cm = 53.2 cm – 45 cm = 8.2 cm
To summarize: for our 45 cm board that has a cycle time of 40 seconds, we need a 10-zone oven with a heated tunnel length of 360 cm. There will be an 8.32 cm spacing between the boards in the oven.
If you would like an Excel spreadsheet to make these calculations send me an email at firstname.lastname@example.org.