With understencil cleaning now a more frequent part of the print process, quickness really counts.

While I often bemoan the necessary evil of understencil cleaning (you know my philosophy: if the process is optimized, super-frequent cleaning shouldn’t be the rule), the fact is it’s an integral part of the printing process, especially as the industry collectively shifts to finer pitches and smaller devices. In fact, cleaning frequency is on the rise. In the days of standard SMT, we cleaned between every 10 prints, on average. Today the average number of prints between cleaning is an aggressive five. And, while certainly not the norm, I know of a couple high-volume mobile device manufacturers that clean after every print. That’s right, every print!

With today’s manufacturing realities, the drive to lower costs while increasing yield and the constant demands of ever-smaller dimensions, amping up the understencil cleaning process can go a long way toward helping achieve operational objectives. First among these goals is high throughput, so a fast and effective understencil clean is critical. Using the proper cleaning chemistry and understencil cleaning fabric is central to the thoroughness of a clean. If the chemistry doesn’t break down the solder material in the apertures, and the paper roll isn’t absorbent (or leaves lint behind), then the clean is subpar, and subsequent print yields could suffer. Likewise, vacuum power in the age of miniaturization is also important. New vacuum pump technology with 35 liter per minute capability can handle the features of today’s stencil designs. Stencils manufactured for mobile phone boards, for example, don’t have much metal. The high-density boards used to accommodate all the componentry for mobile devices means tight interspaces and numerous small apertures, leaving a lot less metal and a lot more air. Soon we will be adding metric 0201s and higher-density CSPs to the mix. There is now a far greater requirement on the vacuum capability during understencil cleaning in order to shift enough air to get an ample draw, pull paste out of the apertures and deposit it firmly in the fabric to avoid contaminating the process.

Thoroughness is only part of the throughput equation, however. Speed, of course, is the other critical element and, with understencil cleaning now a more frequent component of the print process, quickness really counts. Conventional understencil cleaning regimens follow a three-sequence protocol: a wet process where cleaning chemistry is applied to the stencil, a vacuum step that pulls solder material out of the apertures and onto the fabric, and a dry process during which the fabric traverses the bottom side of the stencil. These are sometimes managed in a different order, but three sweeps with the cleaning head are generally the norm. A few years ago, a new cleaning technology was developed to manage all this in just two sweeps, integrated oscillating (side to side) action to deliver a more robust clean and effectively cutting cleaning time in half, while also reducing paper changeover time to a mere 20 sec. Indeed, this advancement has helped reduce print process cycle time. Now, as manufacturers are faced with the reality of pushing boards through the process every 20 to 25 sec., understencil cleaning speed is more critical. Extending the previous cleaning system progress, a faster technology has been engineered to deliver thorough understencil cleaning action in a single traverse. With this new technology, the three steps of wet/vac/dry can now be carried out in one pass. Compared with traditional understencil cleaning processes, this new approach can reduce cleaning time more than 60%.

In addition to the speed element, there are also considerable cost savings to be had by utilizing novel, faster understencil cleaning options. With a single pass, significantly less understencil cleaning fabric is required, contributing to a 50% reduction in consumables when integrating this more resourceful cleaning system. And, naturally, other overhead factors are also decreased as a byproduct of speed and efficiency improvements.

In the world of high-volume assembly, speed and yield reign supreme, and a spot-on printing process is often identified as the key to defect reduction. Faster and more effective understencil cleaning that improves speed by 60%, reduces cost, raises cleaning effectiveness and eliminates unnecessary overheads is a small – but important – part of enabling next-generation high-volume SMT manufacturing. It’s time to accelerate those print process necessary evils!

Clive Ashmore is global applied process engineering manager at ASM Assembly Systems, Printing Solutions Division (; His column appears bimonthly.

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