How to determine how long a reflow oven heating tunnel should be

Many electronic assemblers assume they need a longer reflow oven to meet their throughput requirements and to provide more precise control of their solder profiles, especially if they’re working with lead-free solders. As a result, many manufacturers find themselves spending more money—and sacrificing more floor space—than they really need to.

While long, 8- to 10-zone ovens and faster belt speeds may be the best solution in a high-volume manufacturing environment, our experience shows that, for the vast majority of medium- and lower-volume, high-mix assembly lines common in the U.S (and many other countries), shorter reflow ovens can easily keep pace. In fact, smaller, simpler, and more affordable 4- to 6-zone models are some of our best sellers, and do an outstanding job in handling pick and place throughput, meeting solder paste manufacturers’ reflow specs, and providing reliable, high-quality soldering performance.

But how can you be sure? How much volume can a 4-, 5-, or 6-zone oven handle? A few simple calculations based on data from your solder paste and equipment suppliers will give you a pretty good idea:

Solder Paste Heating Time

The first thing to consider are your solder paste manufacturers’ recommended profiles for the paste formulas you will be using. Solder paste manufacturers generally provide a fairly wide window (in terms of total heating time) for the various stages of the reflow profile — 120 to 240 seconds for preheat and soak, 60 to 120 seconds for reflow/time above liquidous. The average recommended heating times for eutectic and lead-free solders are typically close enough to make any differences negligible for these calculations. Given this range (3 to 5 minutes), we find that an average total heating time of 4 to 4½ minutes (240-270 seconds) is a good, relatively conservative estimate to use.

For this simple calculation, we recommend that you disregard the cooling portion of the solder profiles. Cooling is important, but typically doesn’t affect soldering quality unless the PCB is cooled too quickly. If the PCB is not cooled quickly enough, handling problems may result. These issues may necessitate controlled cooling in higher-volume production environments with larger ovens and faster belt speeds, but again, we find that 4- to 6-zone ovens with passive cooling do a more than adequate job.

Heated Length of Reflow Oven

The next consideration is the total heated length of the reflow oven. It is very important to differentiate between the oven’s total overall length and its heated length. Almost all oven manufacturers will provide the oven’s heated length, sometimes referred to as heating tunnel length, in their specifications. Determine this length (in inches) for each of the different sized ovens you are evaluating. We recommend that you not include the length of any cooling zones — in this simple calculation we are only concerned with the areas of the oven in which heating takes place.

Belt Speed

For each oven you’re considering, divide the heated length (in inches) by the recommended total heating time (in seconds). Then multiply by 60 seconds to get the belt speed (in inches/min) you’ll need to achieve that profile:

Formula to calculate reflow belt speed

For instance, if your solder heating time is 240-270 seconds and you’re considering a 6-zone reflow oven with an 80¾-inch tunnel, you’d divide 80.7 inches by 240 and 270 seconds. Multiplied by 60 seconds, this tells you that you’ll need to set the belt speed of that oven between 17.9 inches and 20.2 inches per minute.

Below is a chart comparing the required belt speeds for Manncorp’s 4, 5, 6, 8 and 10-zone reflow ovens using a typical solder paste profile of 240-270 seconds. Reflow oven manufacturers design their ovens to provide a range of belt speeds that accommodate typical board sizes and solder profiles, but it’s certainly worth checking to make sure the oven(s) you are considering can deliver the required belt speeds.

OvenHeated LengthRequired Belt SpeedOverall LengthApprox. Cost
4-zone52"11.5" - 13"/min6 ft, 7 in$25-35k
5-zone75"16.7" - 18.8"/min10 ft$35-45k
6-zone80.7"17.9" - 20.2"/mind13 ft, 8 in$40-65k
8-zone106"23.5" - 26.5"/min15 ft, 9 in$50-75k
10-zone134"29.8" - 33.5"/min18 ft, 1 in$60-85k+

Once you determine the required belt speeds for the ovens you are considering, you will need to determine the maximum number of boards per minute that can be processed in the each of those ovens.

Maximum Number of Reflowed Boards Per Minute

Assuming that at maximum capacity you would have to load boards end to end on the reflow oven’s conveyor, it is easy to calculate the maximum throughput.

Formula to determine how many boards per minute you can reflow

For instance, if your board is 7" long and the belt speed of the 6-zone reflow oven ranges from 17.9"-20.2" per minute, the maximum throughput from that oven will be 2.6 to 2.9 boards per minute. This means a soldered assembly would be exiting the oven about every 20 seconds. Now you can start to get the picture. Are PCBs coming from your pick and place machine every 20 seconds? You can also see that minor variations in heating times and belt speed are probably not going to have a significant effect on oven throughput.

Line Speed

The overall speed of your production line is going to be a significant consideration in choosing your new oven. Whichever process (stencil printing, SPI, pick and place, pre-reflow inspection, etc.) takes the longest will ultimately control your total throughput or line speed.

Typically, in a fully conveyorized line, the pick and place system will dictate the line speed. Our fastest high-mix pick and place machine with 3 heads is rated at approximately 10,500 cph or almost 3 components per second. On average, the automatic loading and unloading of the PCB to and from the pick and place with a three-stage conveyor takes a minimum of anywhere from 10-15 seconds. This means that even if the 7" long PCB we mentioned above has as few as 30 components, the pick and place operation (including transport) will take at least 20 seconds.

Formula to calculate the speed of your pick and place machine

Of course, a 7" long PCB with only 30 SMDs would probably be quite rare, but again this is only an example to illustrate the relative ease with which a reflow oven can keep pace with production needs. Even smaller boards with fewer components are likely to be panelized which would increase the total number of placements and the total time for the pick and place process.

Which Reflow Oven Best Fits Your Needs

Armed with these numbers, you can start to visualize the relative flow of different products through your SMT factory when using different size ovens. It’s not our intention to over-simplify the oven selection process — this is just a starting point and there are many other things to take into account. For instance, production of double-sided boards may necessitate two separate passes of the same assembly through the oven. Offline processes and manual assembly operations will also affect how much reflow soldering capacity is really needed. If your SMT assembly speeds are very fast, but other processes limit your factory’s throughput, the largest oven in the world isn’t going to do you much good. Another factor to consider is changeover time from one product to another. How much time does it take for oven temperatures to stabilize when changing from one profile to another? There are many different things to consider.

Nevertheless, as we mentioned at the beginning of this post, the majority of our customers usually find, and are often quite surprised, that 5- to 6-zone ovens are more than adequate to handle their needs. Companies will sometimes over-spend on larger ovens in anticipation that higher throughput may be needed in the future. If you fall into this category, keep in mind that smaller ovens use less energy, require less floor space, and their typically lower base price can free up funds that can be put toward options and accessories like nitrogen blanketing, water-chilled cooling, and/or thermal profiling equipment. We have even had customers elect to purchase multiple smaller ovens over a single large oven to give themselves more flexibility for running different temperature profiles simultaneously.

Use these quick calculations to get you in the ballpark. You may indeed find that a larger, 8- or 10-zone, higher-capacity oven is your best answer. We will be happy to supply you with one! Our bet is that if you’re like most of our customers, a 5- or 6-zone will serve you just fine. If, after running through these calculations, you still have questions, contact a Manncorp SMT assembly equipment expert.

East Coast Office
Chris Ellis. Text or call 215-869-8374, or email cellis@manncorp.com

West Coast Office
Ed Stone. Text or call 215-808-6266, or email estone@manncorp.com