Concurrent Processing in Selective Soldering Print E-mail
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Written by Alan Cable   
Thursday, 31 May 2012 14:39

What if we were able to solder one board while the next one was preheating?

In a high-speed, high-volume production environment, in simplest logic, if you double your capacity, you double your throughput, which also means cutting cycle time in half. But you also must double your manufacturing equipment footprint, as well as your investment.

For selective soldering, which consists of multiple steps – e.g., fluxing, preheating, and then soldering – we have long been wondering “what if”: What if we could somehow make the process faster, increasing or even doubling throughput, in the same machine? If we could make the process more efficient, save time somewhere, wouldn’t this make it possible to get more out of one machine without increasing its footprint and the expense of a second, or larger, machine?

Selective soldering consists of three basic steps, all incremental, but preheating and soldering are the biggest time-eaters, and we have discovered that both take almost equal amounts of time for a given assembly. We also subscribe to the idea to keep the board at temperature during the actual soldering process. This is done by incorporating a topside “maintenance” heater. This feature is an absolute must for any board that requires initial preheating, so two separate preheaters are required: initial and maintenance.

What if we were able to solder one board while the next one was preheating? That would nearly cut overall cycle time in half, but would require some creative machine design thinking and motion studies, so that the job could be done concurrently in a single machine with one lane rather than in incremental steps. The solution began by breaking up the selective soldering process into individual steps and controlling those steps specifically and tightly. It’s all about timing and control.

Productivity in selective soldering is, or has been, the combined processes of fluxing, preheating and soldering. In incremental processing, there isn’t much opportunity to shorten cycle time, with the exception of making the individual steps more efficient where possible. A certain board, based on its density, mass and other factors, will take a specific amount of time to reach the appropriate temperature for selective soldering.

But we still needed to shorten cycle time significantly through a concurrent processing scheme, and at the same time maximize the use of space within the machine, and cut down on board indexing. So we wondered, why couldn’t we combine the fluxing and preheating into one module? It took some doing, but the result was greater process control and efficiency. Whenever you can combine process steps into fewer modules, a more streamlined and controlled process is the result.

In this new approach, the printed circuit assembly is brought into the machine and flux is applied, as it always has been, but then the fluxer backs out of the process area, and we move in the top preheater that brings the board up to temperature. At that point, the board is indexed immediately downstream to the soldering module. As the board exits the preheater to be soldered, another board moves into its place, is fluxed, and heating begins.

By effectively splitting the processes of fluxing, preheating and soldering, this approach reduces TAKT by up to 50% versus incremental processing. This more efficient processing scheme effectively shaves time off the sequential waiting times that are unavoidable in linear incremental processing, and permits higher throughput and consequently higher productivity.

In some ways this could be labeled a multi-tasking approach to selective soldering. We’re making more efficient use of time, with more than one product being processed in one lane, while another is undergoing a different stage in the process. But it is important to note that maintaining temperature (i.e., process control) is key. If, for example, the soldering step for a particular board takes longer than the preheating stage, then the waiting board has to be maintained at proper temperature before soldering. This is where timing comes in.

In the sequence of events, Board “A” is positioned under preheater 1 and is fluxed; then preheater 1 is turned on. As soon as Board A reaches its target preheat temperature, it is sent to the second station and preheater 2 is turned on to maintain temperature, while Board “B” is positioned under preheater 1 and fluxed, and the preheater turned on. While Board B (now already fluxed) is preheating, Board A is soldered. So it’s extremely important to maintain board temperature between the preheating and soldering stages.

Alan Cable is president of A.C.E. Production Technologies (; This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Last Updated on Friday, 01 June 2012 14:40


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