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Screen Printing

Clive Ashmore

Getting machines to talk to each other enhances output and yields.

It’s common sense, really, and probably one of the most familiar sayings of mankind: Communication is key. Good communication is central to productive relationships, effective business strategy … just about everything, honestly. It’s not just communication, however, but communication quality and transparency that result in informed decision-making. This is especially true in the manufacturing setting and is the basis for Industry 4.0. Machines have been cranking out data for decades, but applying them in a meaningful way is, at the core, what Industry 4.0 is all about. Until recently, however, data exchange was largely supplier-specific; proprietary equipment system software could manage tasks rather seamlessly, but communication among disparate equipment brands in relation to PCB movement and traceability was challenging. The IPC-SMEMA-9851 standard provides a solid foundation and is still successfully employed, yet enhancements are required to progress toward a nimbler, automation-friendly solution that permits open and uniform machine-to-machine communication.

How it started. While several stencil printer platforms and everything within their respective ecosystems – board handling equipment, SPI and closed-loop feedback tools – are data rich, self-correcting and optimized for the printing operation, the data generated by printers relating to the PCB characteristics must be passed down the line. That, of course, means the data must be vendor-neutral. Moving beyond simple board recognition from one system to the next, true traceability is required for smart factory effectiveness. Consequently, the Hermes Standard Initiative (IPC-HERMES-9852) was born as the result of more than a dozen equipment vendors unifying behind the cause for an improved open communication protocol, which speaks volumes for the requirement and the customer desire for such a solution. By simplifying the transfer of PCB data between machines regardless of supplier, efficiency and productivity improvement are a given. And, with scalability options, board data can be customized so that when the PCB is passed from, say, the printer to the placement machine, each machine is compelled to recognize the data set, potentially add to it, and transfer that record through the assembly line, making for a more holistic view of the PCB. In fact, these data represent the digital twin of the physical PCB, and Hermes transports the PCB and its digital twin consistently down the SMT line. Integrating Hermes with IPC’s Connected Factory Exchange (CFX) standard broadens this line efficiency and communication transparency to the factory, while other MES systems can extend that to the global enterprise.

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Clive Ashmore

Our expert troubleshoots three common problems.

Regular readers of this column and certainly most process engineers are acutely aware of the multitude of problems that can arise in the stencil printing process if it is not optimized. With numerous inputs and variables – shown in the fishbone diagram (FIGURE 1) – the number of things that can go wrong are many, but that shouldn’t portend that things will go wrong. Stencil printing, as I’ve said before, can be simplified into having the right amount of material at the right time in the right place. Therefore, too little or too much material, or improper timing or location, can result in defects. On the flip side, knowing how to avoid or correct the most common printing defects can mitigate against proliferation and secure successful results. This month, we look at the basics, discuss the top three printing-related defects and the problems they cause, and share advice on how to resolve them. (Caveat: There are multiple potential causes and cures; here, we discuss the most common.)

Problem: Insufficients (too little material).
Potential result: A dry joint or a faulty/unreliability interconnect in the field can cause a broken circuit, often brought on by temperature or vibration stress.
How to avoid or correct:

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Clive Ashmore

For 01005 parts, some apertures are better than others.

Continuous reduction in component size has been at the forefront of electronics product innovation, assembly process development and the industry conversation for years. Readers will no doubt recall the papers presented, tools developed, and processes modified to accommodate the “coming soon” metric 03015 and 0201 components. That preparation is essential. In my opinion, however, it is more likely than not that widespread use of these ultra-small chips is far in the future; it will come, but probably not in the next generation.

Another reality presents, perhaps, a more immediate challenge: increasing component density beyond current norms. Realistically, for next-generation mobile phones and wearables, the primary consumers of the most miniaturized components, board designs will continue to incorporate the 01005 chip (metric 0402). There are a gracious plenty of reasons for this, not the least of which are cost and component availability. The challenge for product designers is how to get the most function from chips that may be larger than they would prefer. What’s the solution? Squeeze the 01005s closer together, of course!

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Clive Ashmore

An RFID tag can log everything from storage location to print strokes.

Outside of sheer printing machine capability, the stencil is arguably the next most important element of the printing process. Stencil material, thickness, aperture integrity, sidewall smoothness (or lack thereof), and tension all play a role in the quality of the solder paste deposit. And, like all consumables, metal stencils have a lifetime: They do not last forever. Unless a stencil is damaged, tension loss is the factor that most often determines when a stencil has run its course. A properly tensioned stencil enables a good, solid release of the paste deposits onto the board. Alternatively, a stencil that has lost tension and has begun to “sag” may result in defects such as “dog ears”1, bridges, or insufficient paste on pad, to name a few.

Today, stencil tension is more important than ever. Historically, when stencil thicknesses averaged 200µm, one was far more likely to retire a stencil from damage than from wear. Now, however, with the exceptionally thin 60µm foils required for miniaturized designs, tension loss can occur sooner, as repeated stencil pressure during the print stroke eventually reduces stencil elasticity. As has been addressed in this column, there is a proven correlation to changing tension and the output of the printing process.2

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Clive Ashmore

Multitasking platforms are becoming the standard.

While productivity – manufacturing more product, more efficiently in less time – has been center stage in electronics assembly for decades, today’s razor-thin margins, coupled with the requirement for limited human intervention, have put an exclamation point on managing output proficiency. (This is especially true as the world restricts building access and maintains safe personal distances.) An optimized stencil printing process, as I’ve said many times, comes down to depositing the right volume in the right place at the right time. These are the three pillars of the print operation. Ultimately, for maximum productivity, a manufacturing operation needs a stencil printer that is always available and, when it is available, efficient and reliable.

It wasn’t long ago the bottleneck on the production line was usually the placement machine, so the stencil printer was generally available and had plenty of time to run the print routine. With recent modular approaches to manufacturing line setups, however, this is no longer the case. Placement platforms have exponentially improved speed. The printer now must maintain a much faster pace; this starts with mechanics and cycle time. In mass production settings, getting a printer down to a core cycle time of five seconds has become a necessity.

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Clive Ashmore

Do thinner boards require a different transport mode?

Just when we think we have reached the limit on shrinking substrate thicknesses, tighter pad spaces and higher component densities, the industry says, “Not so fast!” Today’s mobile phone boards average a remarkable 0.6mm thickness, with as many as 1,000 components packed into a 20mm x 80mm space. Over the past five years, advanced equipment sets have accelerated transport, tooling, vision systems, inspection capabilities and platform controls, all of which have certainly made producing high-quality products with ultra-small dimensions possible. However, in the stencil printing world, even more may be required to ensure maximum board stability during the print operation.

Traditionally, the mode of transport – bringing the PCB or pallet into the machine – has been achieved on some form of rubberized belt. This will no doubt continue as the solution for the assembly line. Inside the printer, however, not only is the board brought into the machine on the belt, but the substrate is clamped to the belt to hold it stationary, present it to the stencil and print. This has worked very well for years and is fine for multiple product builds. For mobile phones and other handheld products, however, current and future dimensions dictate a new paradigm. What are 600µm-thick phone PCBs today likely will continue to get thinner and, even at their current architectures, are susceptible to any type of undulation or extra pressure. Clamping thin, small boards or pallets to a rubber belt can result in movement, twisting or bowing at the substrate edges and potential print accuracy issues. There are flat belt options, which have been the interim solution for thin board printing, but the belts are still constructed from rubber and not completely rigid. Finally, belts are subject to wear; they eventually lose elasticity and require replacement. Without proper maintenance, even greater instability can occur.

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