Getting a close-up look at board quality.

A printed board microsection is one of the best ways to examine the quality of boards and any faults or failures.

The microsection (FIGURE 1) shows a plated through-hole that has been soldered with the nickel layer and through-hole copper visible. Normally, customers would accept the plating standards offered by the fabricator, or define their own, which may or may not impact the price. The nickel layer is part of the nickel/gold surface finish with the very thin gold of less than 1µm consumed during soldering and not visible. The remaining nickel is 5µm, and the copper is around 32µm. This is generous on many circuits board produced today and soldered very easily in production.

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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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First differentiate between rigid-flex and true flex.

As is often the case with flex circuits, knowing which solder mask to use on flexible circuits is somewhat of a trick question, one with several answers. The decision boils down to circuit construction and design intent.

To start, there are several ways to insulate circuits in the flex world. These include solder mask, coverlay and coverfilm. In most cases, the designer may simply note solder mask per IPC-SM-840 and leave the rest to the fabricator. This allows the fabricator to use the proper mask in the proper setting.

When making a design decision, first differentiate between rigid-flex and true flex circuits.

Let’s cover the easiest one first: rigid-flex. Typically, a rigid-flex construction will have solder mask applied to the external rigid layers to insulate all external traces, as well as define surface mount or BGA pads. It may also provide mask dams between pads to reduce the potential of solder shorts at assembly. This solder mask usually is classified under IPC-SM-840 as a type H solder mask, which denotes a high-reliability solder mask. These are the most common solder masks. Normally green in color, they can be modified for other colors, as desired. It is worth noting that if the color deviates from the as-formulated green option, there may be feature resolution and web size tradeoffs. This is because the additives used to change the color impact how the mask material absorbs light energy during the imaging process. As a result, the fabricator may need to ask for some relief for other colors.

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We can make more (money) by making less (product).

Arthur C. Clarke once said, “Before you become too entranced with gorgeous gadgets and mesmerizing video displays, let me remind you that information is not knowledge, knowledge is not wisdom, and wisdom is not foresight. Each grows out of the other, and we need them all.”

Today, we’re all familiar with gorgeous gadgets, and not only those we carry in our pockets, wear on our wrists or help us drive our cars. The factories we work in are dripping with sensors and automation, which is increasingly robotized, bringing a level of dexterity, efficiency, and reprogrammable flexibility that previous generations could only dream of.

We are fortunate to live in this period we now call the fourth industrial revolution, although we should recognize our predecessors have been working toward this for generations. It’s simply human nature. Since the beginning of industrialization, people have been making analyses – of processes, end-products, and how things are done – to achieve some improvement. Often, the goal is to increase productivity and quality but also to ensure safety and reduce environmental impacts. Recently, of course, reducing pollution and energy consumption, while addressing issues like recyclability, has become increasingly important.

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