Thin boards are prone to vacuum suction, increasing the risk of over-prints.

Despite the provocative headline, this column is not a rant about how much I despise defects or the havoc they wreak on yields and profits; that’s already well established! Rather, this month’s space will focus on dedicated tooling and print platform vacuum control.

 

Most printing specialists are very aware of the fact that today’s thinner substrates (defined here as 1mm or less) are generally supported by dedicated tooling plates, rather than other automatic board support mechanisms, simply because of the flexibility of the boards. Though often referred to as “rigid” substrates, the PCBs used for the majority of today’s handheld devices are far from rigid by definition, as they have quite a bit of flex in them. In order to secure substrates of any thickness to a tooling plate to create a nice flat and stable surface on which to print, vacuum is used to hold the board in place on the tooling fixture. If the tooling plate is manufactured incorrectly and isn’t flat to a tolerance of +/- 25µm, several problems could result, particularly for thinner PCBs. For the purposes of this discussion, however, it is assumed that the tooling plates used are properly manufactured and flat to within tolerance. Even if the tooling fixture is perfectly produced, though, things can still go wrong on the tooling front. The culprit? Vacuum strength.

This is something many assemblers have caught on to, but plenty still haven’t. If the vacuum is on, the suction of the board to tooling plate is seen and the immobility of the board felt, one would assume that all is right with the world. That’s not always the case. If the vacuum is set up with too much suction, several causations for defects can occur. In fact, I’ve witnessed this in a manufacturing situation. At this particular customer, we noticed a pattern of transfer efficiency errors on certain areas of the board where the solder paste volume had increased. Using SPI, we mapped the pattern and identified that the over-printed locations aligned perfectly with the location of the vacuum cups in the dedicated tooling plate. In those areas, the very thin board was being pulled down – or dimpled – by the vacuum force and compromising the stencil to board seal, resulting in over-prints on those areas.

Most printing machines are shipped from the manufacturing facility with a pre-set vacuum rate, which is generally set for boards that are thicker (2mm and greater) substrates that do require quite a bit of vacuum to flatten them. When producing assemblies for mobile devices, the vacuum settings for a 2mm thick PCB may be a bit overzealous. Not only can high vacuum create the dimple effect, as mentioned above, but a stronger-than-necessary vacuum can also cause the vacuum to leak between substrate and stencil, thus triggering the solder paste to leach from apertures and cause smear and bridging.

Controlling vacuum rate is, in most cases, a manual exercise and relies heavily on the expertise of the operator. Some machines have sensors that issue a warning if improper vacuum is detected, but setting the vacuum initially is managed on a product-to-product basis. Honestly, the manual nature of vacuum setting is often the cause of its oversight; operators simply forget about it. As the industry moves to thinner substrates, controlling vacuum settings needs to become routine practice in order to avoid defects like those mentioned above.

My recommendation for achieving the proper vacuum suction? It’s not written in any engineering manual, but my formula for success is that less is more. It’s really all about surface area and spreading the suction – even a small amount – over a large area for a good, tight seal. Simply back off the vacuum setting until the board isn’t gripping the tooling plate, turn it up slowly until it starts to grip, and then just add a little bit more suction for good measure. Controlling vacuum suction on dedicated tooling is a bit instinctual but, then again, so is much of what printing experts do.

Clive Ashmore is global applied process engineering manager at DEK International (dek.com); cashmore@dek.com. His column appears bimonthly.

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