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42 The PCB Design Magazine • November 2017 If there's enough space on the board and the component is available with different pitches, we also recommend selecting a component with a larger pitch since it reduces the complexity of the board and saves costs. Smaller components may be less expensive to purchase or more readily available, but this approach might render the board unnecessar- ily expensive in relation to its end application. Opting for small components, usually, increases the complexity of the circuitry, and therefore the board will increase in cost also. This is where the customer should work with their PCB supplier to determine if the design needed for such components is a cost-effective one: Is the reduced cost associated with buying more readily available, but more complex com- ponents, balanced with a potentially more ex- pensive PCB? Consider if, for example, it is to be used in mobile phones destined for the con- COMMON HDI DESIGN PROBLEMS PRODUCTION PROBLEMS DEPENDENT ON: BEST SOLUTION Dielectric too thick for laser vias. Too small microvia size. Too tight geometries in the form of too small capture and target lands for the microvia. Too tight demands on permitted dimple on copper-lled microvias. Too tight demands on the thickness of overplating of plugged vias. (POFV or VIPPO). Epoxy via plugging demands for too many dierent sizes of vias, this applies to both buried and through vias. Microvia placement. Too small of a distance between the staggered holes and the microvias, microvias or microvias/buried vias. Increased time for laser drilling, lower productivity. High risk for voids in the plating process, especially in the bottom of the microvias. Increased price for the PCBs due to reduced yields. Increased risk for the microvia to be blocked by unknown material and therefore won't be plated satisfactorily. High risk for poor plating of the microvia, especially in the bottom. Increased price for the PCBs due to reduced yields. If the target land is too small, the risk will increase for partly missing it (so called overshoot), and material adjacent to the pad will be burnt down to the next layer. If the capture land is too small, it is a risk for the land to be broken, which is not accept- able to any class in IPC-6016. Increased price for the PCBs due to reduced yields. Aects the ow of the process, at a reasonable thickness of the overplating all the vias can be drilled in the same operation, which makes the process much easier. If the overplating is too thick this will reduce the possibilities to produce outer layers with thin racks/small isolation. Hard to control that bubbles don't occur in the nal plug, and that there won't be a problem with complete lling. If microvias are placed directly into SMD surfaces, unnecessarily voids can arise in the solder joints at reow soldering. The price structure increases if the microvias are copper-lled. If the staggered microvias are placed too close to each other, there is a risk that the overlaying hole can intrude on the underlying one with bad plating as a consequence. This can be solved by copper lling of underlying microvias or overplating if buried vias, all this means increased cost and risk. Use an aspect ratio under 0.8:1. Use microvias of 100 m with an aspect ratio under 0.8:1 for microvias intended for copper lling. If possible entry and capture pads should be 200m greater than the microvia. For tighter geometries consult NCAB. Place the requirement of dimple to a maximum of 25 m. Set the requirements according to IPC-6012 class II and demand only ≥ 6 m as overplating thickness. Only one size of the plugged vias are preferred, if more sizes have to be plugged, keep them within a range of 0.15 mm. Pull the microvias from the SMD surfaces if possible. If there is no place to do alternative 1, place the microvias right into the pad and demand for them to be copper-lled. Where possible we recommend 300m between microvia holes, 220m at lowest to ensure no processing issues. EIGHT HDI PCB DESIGN PROBLEMS AND SOLUTIONS HDI PCBS: MAKE THE RIGHT CHOICE FROM DESIGN TO VOLUME