Printed Circuit Design & Fab Online Magazine - Current Issue

Flexperts

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.

Read more: Relieving the Strain

Voltage, heater size and material costs all factor into the decision process.

I would like to use a flexible heater in a product I am designing, but I am concerned about cost. Flexible heaters typically seem a lot more expensive than regular (copper) flexible circuits. What is the advantage to using a resistive metal? Why not just use copper for all flexible heaters?

Copper can certainly be used for the heating element material in a flexible heater in many applications. When a design permits copper to be used for the heater, that will usually be the most cost-effective way to go. But how do you know if copper is a good choice for an application?

Cost impacts of copper heating element. The first hurdle to clear to ensure copper is a good cost-effective solution for a heater is the electrical resistance requirement. The biggest problem with using copper as the heating element is that it has very low coefficient of resistance. To get any appreciable resistance in the heating area, very thin copper must be used and element traces made very narrow in order to pack as many lineal inches into the heating area as possible. Both these will work against your goal of reducing costs. Also, base laminates that are clad with less than 0.5oz copper thickness are typically more expensive due to their fragile nature, and industry use of materials this thin is far less than use of 0.5oz or 1oz copper thicknesses. Another drawback of narrow traces is that the manufacturer's yields will be less optimal, which in turn equates to a higher selling price of the end-product. Another downside to incorporating very narrow traces that many designers fail to take into account is the trace width variations from part to part due to etch tolerances.

Read more: Copper vs. CuNi for Flexible Heater Elements