Issue link: https://iconnect007.uberflip.com/i/1489778
26 DESIGN007 MAGAZINE I JANUARY 2023 first blind via at the bottom of the stack as shown in Figure 1. As can be seen in Figure 2, at this point, the blind via has not been filled with copper. To stack another blind via on top of the first one, the void must be filled with copper by employing an operation called "button plating." is results in tiny bumps of copper sticking up above the surface at each blind via. ese bumps are removed by sanding to make the surface flat again. is is followed by lami- nating another layer of prepreg and copper thus creating a new outer layer. e blind via laser drilling is repeated above the existing blind via and the plating steps are repeated to create the next blind via and then the third stacked blind via, resulting in a set of stacked blind vias like that shown in Figure 1. e problem with reaching down to layer 3 is that the copper bond between the lowest blind via and the copper in the layer it attaches to is very weak. As the operating temperature of the device containing the PCB increases, the resin in all three laminate layers containing these stacked vias expands, pushing up on the copper pad on layer 1. is stress pulls the cop- per in the bottom via away from the layer it is in contact with, creating an open circuit. When an assembly with this kind of defect is returned to the factory for repair, it has cooled off, the resins have contracted, and the cir- cuit works properly. e assembly is returned to the field where it fails again. is process repeats until someone discards the assembly. Sometimes these PCBs are referred to as "rub- ber band" PCBs because they keep bouncing between the factory and the field as if they are connected to the factory with a rubber band. Unfortunately, there is very little that can be done during fabrication to increase the bond strength between the bottom blind via and the layer it contacts. But there is an alternative that can imple- mented early in the design cycle: e use of staggered blind vias as shown in Figure 3. Stag- gering vias precludes the above problems. Engineers and designers from around the globe are looking into this problem, includ- ing the IPC's Weak Interface Microvia Failures Technology Solutions Subcommittee. But in the meantime, designers—particularly in the aerospace segment—should consider using staggered vias for complex PCBs like this. With the demand for increasingly dense electronics in avionics and other military products, this kind of failure is going to appear more frequently unless designers find other ways to make connections deep into a PCB using blind vias. DESIGN007 Lee Ritchey is the founder of Speeding Edge and a signal integrity instructor who esti- mates that he has taught more than 11,000 engineers during his career. Figure 2: Close-up of an unfilled laser-drilled blind via. Figure 3: Staggered blind vias.