Issue link: https://iconnect007.uberflip.com/i/1369942
94 DESIGN007 MAGAZINE I MAY 2021 same equipment utilized on rigid PCBs to per- form 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 shal- low hollow of the microvia. Also, the flexible panel will bend and crinkle in the tough sand- ing planarization machine. Rigid PCB vias are conductive ink filled aer drilling and electroless plating, but before imaging and etching. Aer 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. e resulting surface is quite smooth and ready for dry film. e rigid panel has dry film applied, imaged, developed, and electroplated with copper plating and tin. e dry film is removed, the base copper etched, and then tin stripped. e resulting via surface hole is copper filled and is now reasonably flat copper with a slight dimple, if any. e via fill operation for flex is quite differ- ent. It utilizes a hor izontal, f lat screen-print- ing process to fill the vias with conductive ink. Inevitably, some fill material, usually a small bump , is le on the surface of the panel where it is not wanted. is over-screening residue needs to be removed. e residue will be plated over during cap plating, result- ing in a possible short when final etch is performed. It is very dif- ficult to remove this unwanted residue on a flex PCB panel. ey are too thin and crumple easily when run through a pla- nar 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 difficul- ties, this method is seldom used on flex microvias or laser vias. A new and improved process for a flat via-in-pad on multi- layer 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 chem- istry is designed to fill the laser via from the bottom up, creating a reasonably flat top on the via. ough some small dimpling can usu- ally be seen, it presents no problem for assem- blers. is 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 preferen- tially plate inside the via holes, effectively filling them without plating any significant amount of copper on the panel surface. Excess electro- plated 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 flexibil- ity. 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 sub- sequent sanding operation. e entire sand- ing and conductive screening process with the uneven surface topography of the flex circuit is therefore not a problem. e only negative to the copper plating fill process is that the cop- Figure 1: Copper-plated filled via showing the small dimples typically created from copper fill plating.