|Ask the Experts|
July 2, 2018
Board Spacing During Reflow
When running boards through a convection reflow oven, should there be a gap between boards of one board length? Or will it typically make no difference to have boards running end-to-end in a continuous train to save time and energy? What do you advise??
|Expert Panel Responses|
It depends, different ovens have different thermal capabilities, and different boards will have more or less thermal demands. As a rough rule of thumb, always have a gap between boards to ensure that the oven is given enough time to recover between boards as there will always be a temperature drop when a new board enters. This effect occurs due to the thermal capacity of the board soaking up some of the heat. Whereby in most modern oven, this will not influence the heating function of the oven extensively. However, a more practical solution is to have a small gap between boards in order to give the stacking system or conveyor at the end of the oven time to move the cured board off before the next one arrives; as this could cause the boards to start pushing each other and damage could be caused to the boards. Imagine 10 of you friends drive as close as possible next to each other going through a toll bridge or tunnel, the idea of the game is not to stop or slow down, and see what happens when the first car reaches the toll gate???
Convection reflow requires heated air pass around the board for proper heating. If the boards are run edge to edge, it may negatively affect the circulation of heated air causing unexpected soldering defects. Also board loading can cause undesired changes in an ovens temperature, so you would want to strive to keep boards entering the oven at a consistent rate, rather than large numbers all at once.
M.O.L.E. Line Product Manager
Electronic Controls Design, Inc. (ECD)
All ovens behave different depending on the amount loading. It is more important to load the oven consistently for a given run. Keep whatever space between the boards as close as possible to the same between each board. The oven will adapt to the loading presented. Change the loading throughout the run and you will see more variation in the thermal profile.
Senior Project Engineer
Electronic Controls Design Inc
Idon't know whether or not spacing will have an impact on energy savingsand I am certain you will find the answer to this question from theoven manufacturer, but you should be aware that the spacing betweenboards can have an influence on your ability to produce quality productor not. Keep in mind whatever you choose to do, be sure to profile andprofile again! If you choose to subject your oven to a heavy load(many tightly spaced boards), a profile prior to your run is likely notgoing to give you an accurate profile. Best practice is to drop yourprofiler right in with the run. Also be sure to give your oven plentyof time to stabilize before starting your run. I would wait 15-30minutes even after the oven appears to be stable, not knowing the ovenyou are using. Lastly, if you want a cheap and easy way to saveelectricity, KIC has a software option called Power that will set up aprofile recipe for your product that minimizes the energy needed toproduce your product in spec. Savings up to 15% have been determinedwith three independent studies and it does not require you to messaround with spacing/loading while ensuring you are in spec.
Global Account Manager
Spacing between boards passing through the oven will have certain influence on the quality of solder finish.Mahesh V Draksharapu, Aristos EMS Pvt. Ltd, India
Initially when using a thermal profiler to measure the on board temperatures on a board, there are no boards in the oven and the oven is in its optimal performance condition. Obviously, one gets different results when using a thermal profiler on a bare board versus a populated PCB.
Heat absorption by a PCB is normal and depends on variables such as number of layers, board thickness, # of ground planes, width of tracks, component density, thickness of components and their thermal absorption characteristics.
All these variables compounded will have some influence on the quality of the solder, so, a gap which is approximately a little over 50% of the previous board already on the conveyor will probably result in much more consistency in quality of solder between boards.
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