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Consider This: Designing Via-in-Pad for Higher Density Flexible Circuits

For flex circuit designs with high layer counts utilizing high density outer layers, the extra area utilized for separate pads and SMT components severely limits the available space for trace fan out. By designing via-in-pad in flex and rigid-flex circuits, it can significantly increase density, utilizing vias as mounting pads. The copper- or silver-filled flat vias allow for soldering components directly on via holes.

Utilizing via-in-pad will release extra surface area for routing of traces. However, like any new technology, there are a few points to be aware of when using this type of construction with flex. When manufacturers fill rigid PCB vias for SMT pads, they are typically filled with a conductive epoxy, copper plated, and then planarized flat. The final sanding leaves a very smooth pad surface allowing the assembler to process the circuit board normally through the pick-and-place cycle.

The via-in-pad technology has progressed so that via fill is very common on rigid PCBs. The same equipment utilized on rigid PCBs to perform fill and planarization will crumple a flex panel. A flex panel will not be held stiff enough in the screen fill machine nor will it survive the screening fill process, which presses hard on the surface to drive the thick ink into the shallow hollow of the microvia. Also, the flexible panel will bend and crinkle in the tough sanding planarization machine.

Rigid PCB vias are conductive ink filled after drilling and electroless plating, but before imaging and etching. After an oven cycle to cure the conductive ink, a sanding process is used to flatten the surface and remove any of the conductive fill material from the surface. The resulting surface is quite smooth and ready for dry film. The rigid panel has dry film applied, imaged, developed, and electroplated with copper plating and tin. The dry film is removed, the base copper etched, and then tin stripped. The resulting via surface hole is copper filled and is now reasonably flat copper with a slight dimple, if any.

The via fill operation for flex is quite different. It utilizes a horizontal, flat screen-printing process to fill the vias with conductive ink. Inevitably, some fill material, usually a small bump,is left on the surface of the panel where it is not wanted. This over-screening residue needs to be removed. The residue will be plated over during cap plating, resulting in a possible short when final etch is performed. It is very difficult to remove this unwanted residue on a flex PCB panel. They are too thin and crumple easily when run through a planar sanding machine, which is typically used to remove the hardened fill residue and a small amount of surface copper on a rigid PCB. Due to these difficulties, this method is seldom used on flex microvias or laser vias.   

A new and improved process for a flat via-in-pad on multilayer flex is to fill the vias with a special type of copper plating. On thin flex layers that are laser-drilled, the new copper fill plating chemistry is designed to fill the laser via from the bottom up, creating a reasonably flat top on the via. Though some small dimpling can usually be seen, it presents no problem for assemblers. This plating technology is used on a wide range of flex and rigid-flex circuits.

Top flex circuit manufacturers have specially designed copper plating lines which preferentially plate inside the via holes, effectively filling them without plating any significant amount of copper on the panel surface. Excess electroplated copper on the surface is not desired as it would be a less ductile copper where, typically, RA copper for the traces is required for flexibility. Also, any extra copper plating on the trace surface will make it very difficult to etch very small features on outer layers that are typically used on high density designs utilizing via-in-pad technology.

Because the vias are selectively fill-plated with only copper, and resin fill is not utilized, the surface is smooth enough to forgo a subsequent sanding operation. The entire sanding and conductive screening process with the uneven surface topography of the flex circuit is therefore not a problem. The only negative to the copper plating fill process is that the copper-filled vias are sometimes not completely filled flat or have a small outward dimple. The dents and small dimples are not a problem for the assembler. The use of filled copper vias in flex can greatly increases routing area by freeing up via pads and spaces. 

Small dimples on very small pads can cause some assembly headaches since the dimple occupies a large portion of the pad surface. The small bump is usually not an issue, but on very small pads such as 01005 chips and small BGAs, the dimple can pose challenges for the assembler. Sometimes the component assembler must modify their tools and processes to deal with the small depressions by adding slightly more solder paste to pads that have a dimple.

Everything I have covered to this point assumes multilayer flex. Rigid-flex can also utilize via-in-pad, and depending upon the application and material stack, the fill method can vary by design. For instance, if the rigid cap layers on a rigid-flex are relatively thick (~0.010” +), they need to be filled with conductive resin and planarized like a rigid PCB. Thinner outer cap layers (0.003” or less) may use the copper plating fill option. For everything between 0.003” and 0.010”, it is probably wise to get your flex supplier’s input on the best option for your application.  

This column originally appeared in the May 2021 issue of Design007 Magazine.