No more friction: new board transport systems can aid printer throughput.

In my previous column, I talked about the importance of process audits and how critical it is to perform periodic assessments of the screen printing process. Inevitably, in the course of these audits and subsequent discussion about process improvements, it comes to light that certain essential elements for robust printing are purposefully eliminated to save valuable process time for the sake of producing the units per hour (UPH) required. In many cases, critical steps such as under-screen maintenance or total post-print inspection are the areas that suffer.

For years, this has been the dilemma: Do I meet the UPH necessary for volume requirements or do I sacrifice that so I can perform process maintenance or inspect all boards? The majority of assemblers choose the former option and reduce the time spent on process maintenance and post-print inspection. This strategy works fine if the process is in control (something all manufacturers strive for) but reducing these actions within volume manufacturing, which has the normal variation of man, machine, methods, materials and measurement, will inevitably lead to end-of-line defects.

So, how do we get around this issue and speed the printing process so that critical elements for maximum print performance do not have to be eliminated? Obviously, an incredibly fast and accurate printing system is essential and many advances in core machine speed have been made over the last several years. Other unique technologies can deliver even more time efficiencies. Board transport and post-print inspection are two areas that can enable even faster throughput. By incorporating innovative board transport technologies in tandem with an already fast system, board entry into the machine is accelerated, alignment is performed quickly and core cycle time is dramatically reduced--in some cases to as little as four seconds. Most board conveyor systems use friction to move the board into and out of the printing system. New frictionless board transport technology eliminates some of the instability and speed challenges of previous systems by gripping the board, adapting to its dimensions and quickly moving the board into its printing position (Figure 1). The time saved with this method then permits other essential printing functions such as underscreen cleaning or all-board post-print inspection to be performed, while still keeping up with the line beat rate.

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In addition to certain process maintenance essentials, sacrifice is also often made in post-print inspection. Post-print inspection is often performed on an every nth board basis or only on a selected site on the board, not the entire PCB. So if there happen to be print defects on a portion of the board that is not inspected or an entire uninspected board, end-of-line yield will be affected.

The ability to quickly inspect every board has, to date, been impossible without slowing production time. Excessive programming time, not to mention required operator training, has made only partial post-print inspection realistic for high-volume manufacturers. A new inspection philosophy changes all of that, however, and permits 100% of boards to be verified within the line beat rate. Traditional inspection is data-rich and the routines run by these systems take, on average, 1 sec. per 28 [mm.sup.2] to inspect a typical SMT assembly. A different methodology for print verification can accelerate this to 1 sec. per 1200 [mm.sup.2]. Instead of performing the historical quantitative inspection, this new method isolates faults through rapid good/bad verification. It analyzes streamed images instead of successive still images and records pass/fail per board against pre-set thresholds. This permits extremely fast verification, so that a number of sites on 100% of printed boards can be analyzed in real time. Bad boards are quickly isolated and removed, and end of the line yield and throughput are dramatically increased.

Clive Ashmore is global applied process engineering manager at DEK (dek.com); [e-mail]. His column appears semimonthly.

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