Printed Circuit Design & Fab Online Magazine - Current Issue

Current Issue

Flex and rigid-flex can handle all via types – with some considerations.

We have talked before about vias, but some situations can trip you up in flex and rigid-flex, and some tricks can be exploited to overcome them. When it comes to rigid-flex and flex, keep in mind these things.

To start – yes, you can put a microvia in a flex laminate. In fact, I would argue that in an apples-to-apples comparison, a microvia in flex laminate will be more reliable than in rigid reinforced laminate. There are a few reasons for this. One is, the thin flex cores enable low aspect ratio vias, which has been proven to improve reliability.

Flex microvias can also take advantage of certain material attributes. Because flex laminate is a homogenous film without any reinforcement, it lasers very cleanly and the side walls of the hole are uniform. Without any glass weave, the laser cuts through the dielectric evenly, reaching the target pad at the same time with the same power level across the pad. This results in a consistent target pad surface for plating.

Read more: Via Viability

Proper design and handling can minimize the risk of rips.

I have a potential new application for a flexible circuit for use in an inspection device. The flexible circuit will see some light pulling and tugging forces when extracted. Should I be concerned the circuit may be prone to tearing? If so, are there different materials that can be used that will not tear?

The short answer is yes; flex circuits can tear if not properly designed or handled. You did not mention the number of layers or the thickness, so I will assume that your application is only one or two layers and is on the thin side. As the layer count goes up, so does the thickness, and therefore the strength and tear resistance. Once a flex is four-plus layers with standard thickness materials, it should be very durable, and tearing should not be a concern if the extraction does not generate extreme shearing forces.

Read more: Tear Resistance in Flexible Circuits

Coupling buried vias with microvias can solve many manufacturing challenges.

When it comes to designs, we all make choices – material selections, feature sizes, via structures, components and more. Often, we also make tradeoffs.

Maybe the 0.8mm BGA you want allows standard vias, but the 0.5mm BGA takes up less space. Or you need impedance-controlled signals (which seems to be a requirement on almost all new designs), and line widths must be balanced with dielectric thickness and Dk values.

As packaging challenges mount, so does part thickness and layer count. For many designs, two of the biggest cost and risk drivers are aspect ratio and annular ring. These two attributes are often at odds with each other. There are ways to help them coexist.

Aspect ratio, or board thickness to drilled hole diameter, can be a limiting factor for a few different reasons. Keep in mind that there are two aspect ratios to consider. The first and most commonly thought of is the mechanically drilled through-hole. The second is the blind microvia.

Read more: Aspect Ratio and Annular Ring

White mask is possible, but careful of overexposure.

I am designing a flex circuit that will have LEDs in one area. I would like that area to be white in color instead of amber. What is the best way to do that?

Answer: A flex circuit can have a white outer surface, and this can be achieved in several ways. I will cover the different methods with the pros and cons for each.

Screen printing. For a "quick and dirty" solution, simply have that area of the circuit screen-printed with white ink (over the top of the standard amber polyimide covers). The downside to this method is that if you also want screened legend, a separate process using a contrasting ink color is necessary. Any LEDs must have clearance to ensure no ink ends up on the solder pads. This method will produce a white surface, but there may be rough edges and color tone variations. I would not recommend attempting any tight bend radius forming in areas with screen-printed ink. Some inks can crack if bent sharply, and if a crack forms in the ink it can, and probably will, propagate through the underlying polyimide film over time. And even if the cracks do not propagate through the polyimide film, the ink around the cracks will flake off and end up as FOD (foreign object debris) in the system.