Selecting Reflow Oven Length

Folks,

You are putting in a new assembly line to assemble some large boards for which your company just received a three-year contract. The boards are 45cm 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 above, we see that the amount of time needed in the heated zone is about 4.5 min. or 270 sec.

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 sec. or 0.66 cm/sec

The component placers will be presenting a 45cm board every 40 sec., so the belt speed needs to be:

BS = Board Length/Cycle Time = BL/CT = 45cm/40 sec = 1.125cm/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 = 250cm/270 sec. or 0.926cm/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 = 360cm/270 sec. or 1.33cm/sec

Success! Since 1.33cm/sec is greater than 1.1125cm/sec, this 10-zone oven will work. The extra belt speed will permit a small amount of spacing between the boards. Let’s calculate what it will be:

BS = (BL + Spacing)/CT = 1.33cm/sec => BL+ Spacing = BS x CT => Spacing = BS x CT – BL

Spacing = 1.33cm/sec. x 40 sec – 45cm = 53.2cm – 45cm = 8.2cm

To summarize: For our 45cm board that has a cycle time of 40 sec., we need a 10-zone oven with a heated tunnel length of 360cm. There will be an 8.32cm spacing between the boards in the oven.

If you would like an Excel spreadsheet to make these calculations send me an email at rlasky@indium.com.

Cheers,

Dr. Ron