Clive Ashmore

If the tooling block is too high, the print can be affected.

Several of my colleagues and I were conducting a printing evaluation recently and noticed some very uncharacteristic results, particularly for the board we were printing. The errors, picked up by the solder paste inspection (SPI) machine, were massive variations in standard deviations – as much as 20%. While some of the area ratios were a bit tricky, ranging from about 0.55 to about 0.66, they were nothing outside of what’s normally capable with our company’s platform. It should also be noted that the tooling block, stencils, solder paste and PCBs were supplied for this evaluation; the BoM was not selected by our team.

Let the troubleshooting begin! With deviations this large, we started running through the normal adjustments one would make to encourage a positive reaction. We tried investigating the paste condition. No luck. We changed the print stroke speed. Still coming up short. Then we had a look at the tooling. Culprit found!

Our printing platforms have a stringent tooling specification and a tight tolerance of -0, +50µm. The tooling block supplied for the evaluation was out of tolerance by a whopping 700µm. Machined 700µm (0.7mm) too tall, the tooling block was far beyond our specified 81mm. Unfortunately, this is precisely what can happen when a machine shop that isn’t familiar with the application or the importance of the required tolerances is charged with fabricating the tooling.

As with most printing inputs for today’s more challenging dimensions, accuracy is nonnegotiable, and this was most certainly illuminated with the tooling variation.
So, what happens with a tooling block that’s too high? When the board sits too high because of the tooling block height, everything starts to become non-coplanar. Depending on the rail system employed, the board can be “driven” from the transport belts. The board is pushing into the stencil and can actually begin to distort the metal. The rail system and the chase (stencil loading) architecture can be compromised. The machine can become damaged, and, of course, the actual print result is less than stellar. When the location of the Z axis – the location of the top of the board – changes, the board is pushed far into the stencil. During stencil release, the stencil wants to follow the board down for a distance that is the delta in specified tooling height versus actual tooling height – in this case an additional 0.7mm – to the point at which it is level again and release can occur. Often referred to as “stencil follow,” this condition results in a completely uncontrolled material release and, therefore, sub-par print outcomes.

Once our team discovered that the tooling was most likely to blame for the poor prints, the tooling block was altered to the proper manufacturer specification. The process for the area ratios of 0.55 and 0.66 was returned to normal, and we began to see standard deviations of less than 10% and mean volumes of 70% or greater, which are completely acceptable results.

But, that’s only part of the story! This experience got me thinking about some of our customers that use pallets. There are many reasons to use pallets: The boards being produced can’t be transported down the rails; boards have been depanelized and are being run singularly to accommodate for stretch issues, or there may be no perimeter on the boards. Pallets are pretty rough and ready, and there can be a lot of variation. Generally speaking, with pallets the process tends to be on the high side based on the way the board sits on the pallet, and if tape is used to secure the board, this adds topography and increases the height. As in the case of the tooling block that was too high, the same issues can result with pallets: The board is pushed further into the stencil than it should be because it is on a pallet and not on the machine’s rail system.

There are two solutions to this: produce better, more precise pallets or dictate the board thickness based on the offset of the pallet. Refabricating pallets may not be an economical option, of course; therefore, a workaround for production is to use the machine software to adjust for the additional thickness. Instead of a 1mm-thick board, for example, the operator can program a 1.2mm thickness for the board, telling the machine not to go as high and, therefore, placing the board 0.2mm less into the stencil. This protects the machine structure, the stencil integrity and, in the end, produces a better print. (Although I submit that using either tooling or pallets that are within specification is a wiser choice!) Remember that tooling is your foundation for a great printing process.

Tooling tolerances matter and process engineers should take note: inspect the components of your BoM before you begin. Measure your tooling and triple-check all the inputs. Trust me; you’ll save time, money and a heck of a lot of frustration.

CLIVE ASHMORE is global applied process engineering manager at ASM Assembly Systems, Printing Solutions Division, DEK (asmpt.com); clive.ashmore@asmpt.com. His column appears bimonthly.

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