What is the difference between cycle time and throughput? Or t = ct + o.
Let's start with cycle time. For the printing process, cycle time is the total time required to perform a specific set of activities to achieve the primary function of the process. In this case, it includes board load, raise to vision height, fiducial alignment, raise to print height, squeegee lowering (if required), lower to transport height and board unload. The total time to move the board into the machine, perform the required tasks and move the board out of the printer is cycle time--and it varies greatly depending upon the type of printer selected.
Cycle time is a common specification for benchmarking printing systems because it can be used as a quantitative comparison between platforms. But it is really just a term that marketing folks use to say their product is faster than the competition's; it really does not reveal anything about how the production process will run or the final quality of the printing process. If you believe the marketing hype, faster is always better. But will a machine with a faster specified cycle time really produce more good boards?
Before we answer that question, let's review the term throughput. Throughput is a measurement of the total processing time to print the printed circuit board and optimize the quality of the print. This is a sum of the cycle time (all the activities noted above), the print stroke and overhead functions such as wiping, paste dispensing and inspection functions. In other words, throughput is the sum of all the required steps to turn a bare PCB into a quality board that is ready for the next step of the manufacturing process.
So, which is more important? Since the goal of every production line is to produce high quality boards, throughput is actually the metric that should be used to compare printing systems. Cycle time is only a component of what is important; throughput is the measurement that will really tell how quickly a line will actually produce quality boards. The overhead variables of wiping, paste dispensing and inspection functions have a huge impact on the quality and the pulse rate of the process: maximizing throughput rate should be the ultimate goal for every production line.
To accomplish this task, the focus of all electronics manufacturing operations should be to identify, evaluate and implement methods, procedures and tools that will optimize the amount of "good products" that are built each day. There is a definite balance between product cycle time and the quality of that product. Producing defective products to achieve a specified cycle time is certainly not acceptable, nor is slowing the process to an unacceptable rate to minimize every possible defect. We must understand and optimize this balance to optimize the throughput of quality products.
What equipment features are available to help us achieve this goal? On sophisticated solder paste printing equipment, there is two-dimensional (2-D) and three-dimensional (3-D) post print inspection, stencil wiping and stencil vacuum cleaning, automatic board support placement systems, board-to-stencil alignment systems, and other options that reduce process defects and cycle time. These features can help improve the final product quality but, with current printing solutions, impact the final throughput.
When considering the purchase of new process equipment, take into consideration the balance between those two important goals: throughput and quality. Remember: cycle time is important but it is just a subset of throughput. What really matters is how many good boards can be produced over a period of time. Maximize throughput, and you maximize profitability.
Joe Belmonte is project manager, advanced process development, at Speedline Technologies (speedlinetech.com); firstname.lastname@example.org.
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|Title Annotation:||Screen Printing|
|Date:||Apr 1, 2005|
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