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??

K. D.

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???

Doug Dixon
Marketing
360-Biz

Douglass Dixon is the Chief Marketing Officer for 360 BC Group, a marketing agency with offices throughout the US. 360 BC specializes in consulting and implementing successful marketing programs that utilize the latest in marketing, sales and technology strategies. As an electronics veteran, Dixon has worked in the industry for over 30 years for companies like Henkel, Universal Instruments, Camelot Systems, and Raytheon. Dixon's electronics industry experience includes a broad skill set that includes engineering, field service, applications, product management and marketing communications expertise.

Your first objective should be to obtain consistent thermal repeatability or uniform heating across your boards. The ovens today are much more efficient and more capable of a higher heating capacity then the days before RoHS.

Normally your throughput requirement would determine the board spacing. You must consider the thermal mass of what you're attempting to reflow; to large of a mass or a high thermal mass differential on the board could affect the thermal repeatability.

The one board spacing is a conservative or safe distance to ensure a consistent profile. When changing from a one board spacing to edge to edge, a check or a new profile should be conducted to ensure proper temp and times are achieved.

Slight adjustments (conveyor speed, zone temps) might be needed to compensate for the additional load added in the oven. http://www.apsgold.com/learning/reflow

John S. Wasyliw
VP of Engineering
APS Novastar LLC

John Wasyliw is VP of Engineering with over 20 years of experience in complex mechanical and motion design including control and software development. He is currently responsible for all product engineering, new product development and commercialization for APS Novastar LLC.

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.

Mark Waterman
M.O.L.E. Line Product Manager
Electronic Controls Design, Inc. (ECD)

Mark Waterman is a trainer and field engineer with 17 years experience in service and applications specialties. Intimate knowledge of soldering processes and measurement systems. Six sigma and statistical process control generalist.

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.

Paul Austen
Senior Project Engineer
Electronic Controls Design Inc

Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 39 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, engineer, manufacture, and customer. His focus has been the design and application of measurement tools used to improve manufacturing thermal processes and well as moisture sensitive component storage solutions.

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.

Brian O'Leary
Global Account Manager
Indium Corporation

Mr. O'Leary is the Global Account Manager for Indium. He has and extensive global network of contacts in the electronics industry with expertise in SMT equipment and processes.

Reader Comment

Spacing between boards passing through the oven will have certain influence on the quality of solder finish.

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.

Mahesh V Draksharapu, Aristos EMS Pvt. Ltd, India

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