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Screen Printing

Clive Ashmore

Automating paste application saves time and money.

Increasing productivity through process automation, software intelligence and multitasking capability are the foundation of Industry 4.0. Executing manufacturing tasks with exponentially more efficiency and precision ultimately drives cost lower and quality higher. This is proven across operations within numerous industries. As one of the critical sub-processes in SMT manufacturing, stencil printing is an area where substantive gains in quality and cost-efficiency can be made. Often overlooked is the smart factory approach to solder paste replenishment, though it is integral to a true closed-loop system.

Obviously, in order to print, material must be on the stencil in front of the blade. Currently, the predominant method for achieving this condition is manual application. A line operator physically scoops paste out of the jar and places material on the stencil. This seems like an appropriate use of operator resources, as they are positioned on the line anyway, but, in the “little and often” methodology for screen printing processing, this approach is counter to process stability, optimized throughput and cost-efficiency. Manual application of paste could be carried out more frequently to comply with “little and often,” although that would require a machine stop, which may impact throughput and, therefore, cost. Conversely, the operator can apply a large volume of paste on the stencil to accommodate more prints, which may alleviate some of the throughput concern, but could have an adverse effect on process stability.  

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Clive Ashmore

Six areas to consider for optimal print quality and consistency.

In the stencil printing process, the squeegee blade often fails to get the recognition it deserves. Yet the squeegee is the item that does all the work and is the unsung hero. Consider a squeegee running in high volume on a 300mm board may put in between five and 10 miles per day of grueling aperture filling, and it becomes clear close attention to squeegee attributes may result in higher-quality results. With that said, here are my top squeegee awareness tips.

Material. In the early days of SMT, squeegee blades were predominately made from polyurethane (rubber), as the very first surface-mount printing processes used mesh screens. As the industry transitioned to metal-etched stencils and then laser-cut, stainless steel squeegees became standard. However, there are applications – such as heavily stepped stencils (say a 75µm step down on a 150µm-thick stencil) – where the compliance of a polyurethane squeegee is beneficial. The vast majority of squeegee blades today, though, are stainless steel. And not just any stainless steel; to be sure, a tremendous amount of IP and proprietary alloy formulation is in today’s sprung steel compounds used to manufacture high-quality blades. They keep a good sharp edge and provide excellent consistency for the pressure and force applied, which delivers the aperture filling necessary for a repeatable process.

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Clive Ashmore

Reconfigurable with dedicated-like support.

As board complexity has increased with decreasing pitches, thicknesses and component sizes, ensuring support for thin, high-density substrates – essential to cost-effective, pinpoint accuracy stencil printing – continues to pose challenges. Using vacuum to secure miniaturized assemblies is, for the most part, a successful technique but requires the use of dedicated tooling plates, which can be costly. Considering the quantities of dedicated tooling blocks needed in a high-volume manufacturing environment, finding a suitable, lower-cost alternative has been a longstanding ambition. And, while commercialized automatic pin-based tooling systems are a good option for some applications, they are not as effective for high-density, thin boards.

How, then, do we bridge the gap and provide similar quality substrate support without requiring a dedicated tooling plate for each product and each SMT line? One solution lies in a high-flow vacuum system that supports the PCB – no matter how densely populated – through an almost counterintuitive use of airflow, low-pressure vacuum and reconfigurable metal plates (FIGURE 1). The plates – which are tooling height, approximately 2.0mm thick and constructed of different lengths – can be configured and overlapped to form a box, the top of which is constructed slightly smaller than the PCB perimeter so the edges of the substrate sit on the frame. The rising table contains a vent, and support pins are placed for stability. Once positioned, the tooling cube creates a semi-sealed environment where the vacuum pulls air through the table vent to create substrate stability during the print cycle. Unlike a conventional vacuum connected to a tooling plate, which uses a sealed technique to generate incredible pull (trust me, don’t get your finger anywhere near the vacuum pipe!), this new approach floods the area with tremendous amounts of air, allows for leakage (unlike dedicated plates) and securely holds the PCB with low vacuum. While there is upfront time to set the plates in the desired location, this system provides the support needed for thin, high-density, heavily routed PCBs without the expense of dedicated plates, and it can be reconfigured for an infinite number of board sizes.

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Clive Ashmore

Precision mass-alignment of singulated substrates.

Discussion around smaller devices, complex designs and manufacturing challenges as a result of miniaturization: a never-ending story, isn’t it? Truth is, just when it appears the industry has hit a wall in terms of capability, we find a way forward. Yes, miniaturization is rolling on, and the industry continues to overcome perceived obstacles, this time enabling a higher accuracy approach to mass processing of singulated substrates.

Several years ago, the general thinking was components would keep getting smaller. The prevailing view was that by this time, the metric 03015 and the metric 0201 would be working their way into mainstream production. Although the processes to accommodate these small devices have long since been developed, it will likely be some time before they appear on a majority of BoMs. What is happening, though, is manufacturers are trying to eek out slightly more with standard 01005s by placing them closer together, creating a much narrower gap from the edge of one component to the edge of the next. (See “Screen Printing,” December 2020.) These narrow gap designs – which today see pitches of approximately 100µm with 75µm on the horizon – in combination with the other elements of miniaturization require much tighter alignment tolerances in the stencil printing to ensure solder paste hits the pad target.

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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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