The Flexperts

Nick Koop

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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Mark Finstad

A little information goes a long way – but can carry added cost.

“My company has traditionally specified the finished thickness for each flex printed circuit (FPC) layer, and total thickness. This is because it’s understood some material layer thicknesses (i.e., adhesives) change during the manufacturing process due to compression and curing. As a purchaser of FPCs, we are less concerned with the initial raw material thickness than the finished thickness.

“We have received feedback, however, that the FPC market in general specifies the raw material thickness used in FPC fabrication, and not finished thickness. The assertion was nearly all customers purchasing FPCs follow this rule to minimize miscommunication. Is this common practice?”

Answer: The level of detail we see on customer drawings is all over the map, but the majority of customers that do specify individual materials will indicate the raw material thicknesses and then the overall finished circuit thickness.

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Nick Koop

Strategies for vias and routing.

It seems every new design has at least one BGA component on the board. The 1.0mm pitch BGA has become vanilla. Even the 0.8mm pitch BGA is commonplace. These components are not limited to rigid PCBs; BGAs of all shapes and sizes are implemented in flex and rigid-flex designs as well.

The rules for BGAs are much the same whether the board is rigid or rigid-flex. Due to some of the material differences in a rigid-flex, however, extra care is recommended when it comes to the artwork and the trace routing in the BGA field.

Let’s start with pad and via design. For microvias, many suppliers recommend staying at or above 0.005" diameter vias for reliability reasons. Much experience tells us vias smaller than 0.005" tend to have a much lower mean time between failure (MTBF) than vias at or greater than 0.005". In more benign applications, smaller vias may be an option. If the product will experience temperature extremes, however, the conservative bet is to stay above 0.005" diameter microvias. Depending on the design and manufacturer, the associated pads may range from 0.010" to 0.012". Smaller pads risk a via sliding off the edge of the pad. If it does, the risk is the laser may cut through the dielectric and down to the next copper layer.

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Nick KoopCan via-in-pad be used on a flex or rigid-flex circuit with SMT parts?

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Mark FinstadFlex circuits can run 10+Gb/s signals, but many factors need to be met.

Can flex circuit boards run 10+Gb/s signals? Answer: Multiple factors must be juggled to successfully run signals that are 1Gb/s and above on flexible circuitry. I will address each of them individually.

Controlled impedance. Just like any high-speed rigid PCB, a successful high-speed flex design will have to incorporate a target characteristic impedance. To do so, match the characteristic impedance of the flex to the rest of the system to ensure minimal reflections and crosstalk. This can have negative consequences for mechanical performance, however. Elevated impedance requirements typically equate to thicker dielectrics, thereby making the circuit much less flexible.

The impedance value of a circuit is driven primarily by the signal trace width, the layer-to-layer spacing between signal trace and reference plane, and the dielectric constant (Dk) of the insulating material between the signal and plane. For most flexible-circuit manufacturers, yields start to drop when trace widths fall much below 0.003" (0.0762mm), so any significant trace width reduction beyond that can have a hefty cost impact. Also, traces under 0.005" are fragile and may develop cracks in tight bend-radius applications.

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Nick KoopMany flex designs perform well with panel plating for countless bend-to-install applications.

When copper-plating vias and through-holes, there are several process options in the PCB manufacturer’s toolbox. Typically, they fall into three buckets: panel plating, pattern plating and button plating.

Panel plating (FIGURE 1) means the entire panel surface and all the holes will be electrolytic copper plated to the full plating thickness requirement. The etch process will etch down through the base and plated copper, leaving a pattern with features comprised of both the base and plated copper. Pattern plating is accomplished by creating a pattern of all the circuitry on the two exposed layers with a plating resist, then plating up the pattern of the outer layers. After stripping the resist, the etch process will etch away the base copper between all the plated patterns, leaving a pattern with features comprised of both the base and plated copper. Panel and pattern plating essentially result in the same end-product. For this discussion, we will compare button plating and panel plating.

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