Printed Circuit Design & Fab Online Magazine - Current Issue

Flexperts

Nick Koop

If the clearance is not at least 10 mils, yields may drop.

One of the often-overlooked aspects of a board design is the moat. Perhaps this conjures up images of Monty Python’s Holy Grail, but moat does not refer to the ring around a castle. Instead, this is the clearance between pads and a surrounding copper plane, sometimes also referred to as embedded clearance.

These clearances often are 0.004" to 0.005" wide. This may seem like plenty of room, but Pareto analysis tells us this can lower overall manufacturing yield. These clearances often lead to unexpected yield loss, depending on certain design and processing factors. Believe it or not, etching these moats or clearances is difficult, due to the closed-ended, circular nature of the clearances. They do not image or etch well and are prone to shorting.

One reason is that driving the energy into the resist can result in bleeding and create an imaging short. But etching is also more difficult, as the etchant flow is trapped in a dead-end donut. These can conspire to create unintended image/etch shorts.

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Read more: How Big is Your Moat?

Mark Finstad

It might be factually correct, while also being completely impossible.

Question: I am looking to add a flex circuit supplier to our vendor base and requested its technical roadmap. After review, it appears almost exactly as the two vendors we currently have. Is this a coincidence, or do most (or all) flex suppliers have the same technical capabilities?

A technical roadmap is basically a document that outlines what a company can and cannot do from a technical standpoint (e.g., minimum trace/space, layer count, pad and via size, overall circuit size, etc.). I have never been a fan of technical roadmaps. Virtually every flex (and rigid) PCB supplier is compelled by their customer base to provide their capabilities, and therefore also their limitations. The problem with roadmaps is twofold. First, every supplier advertises the absolute best it has ever done in every single category. This is true even if it only did it one time, on one circuit, and in a beaker. This is not a fair representation of what the fabricator can or cannot do on production quantities. The second problem is the real answer for what a supplier can or cannot do is “it depends.” Let’s look at a few examples.

Read more: How to Really Read a Technical Roadmap

Nick Koop

Yes, but there are performance and cost tradeoffs.

You have started a new design. The chassis is defined, and you are thinking about how everything could be connected. Unlike the past, you are thinking about the interconnect strategy early in the design process, rather than at the last minute. Now you must decide what will be connected as an integrated rigid-flex and what might need to be done separately. So many design options are available right now. Here is where the question comes up: What if my layer counts are not consistent everywhere?

When faced with this, don’t worry. Rigid-flex allows us to design in almost any configuration. Each has performance and cost tradeoffs. Let’s review few of the more common design styles.

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Mark Finstad

Material choices are often based on the planned assembly.

Many different materials are used to rigidize flexible circuits. Likewise, the reasons for stiffening an area on a flex board are many. The “best” stiffener material is tied to exactly why you are stiffening your flex circuit.

Rigidized SMT or through-hole component areas. Providing a rigid, stable surface for mounting components is probably the most common reason for stiffening an area on a flexible circuit. If components are mounted on a flex, which is then bent in that area, there is a very good chance the solder joints or solder pads will be damaged. The industry standard is to rigidize any area on a flex that has soldered components. If components are all SMT, install the stiffener on the side opposite the components. If through-hole components or connectors are used, mount the stiffener on the same side as the components. If components are on both sides, rigid-flex construction is probably needed, but that is a topic for a future column. By far the most common (and least expensive) stiffener material is epoxy-glass laminate (FR-4). This inexpensive sheet material comes in a range of thicknesses and is machined to size and shape by the flex circuit manufacturer. The machined stiffeners are then applied with either a pressure-sensitive or thermosetting adhesive (see below). Another material for stiffening a component area is 0.003" to 0.005" polyimide film. This material is common and cost-effective, since these stiffeners can often be added in panel form. This option is typically specified when overall thickness is a concern. The material is a bit more expensive than FR-4 but offers significant time savings during stiffener mounting. This material will not provide the same level of stiffness as a thicker FR-4 stiffener, so operators must exercise care in handing and forming during installation.

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