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24 PCB007 MAGAZINE I SEPTEMBER 2018 second pulses with picosecond or femtosecond pulses. These offer not only smaller vias but do so with a much smaller HAZ, meaning mi- crovia pitch can be greatly reduced and as la- ser absorption is improved, the resulting vias are of a much better quality with less surface debris or "splash." As these laser systems be- come available, the capability of microvia den- sity will take a step improvement, enabling an- other jump in HDI capability. Once the microvias are formed, the next is- sue becomes plating, and while vias have not had dramatic changes in aspect ratio, typically maintaining 0.7-0.8, they have steadily reduced in size, which has led to challenges in plating. One important development has been the abil - ity to fully fill vias with copper, which has en- abled the stacking of vias directly on top of each other. This has been so influential that stacked vias are an integral part of HDI design as they enable space savings, especially over the older staggered vias, improved thermal and electrical management, and their superior surface planar - ity also contributes to final assembly yields. One recent plating development has been the release of processes for through-hole filling (THF). Solid columns of copper will improve thermal conductivity in heat-critical areas al- lowing for improved cooling and THF poten- tially supports increased I/O density as via-in- pad is still possible. BGA Design Rules As in many cases, the critical design param- eters for an item are often linked to other as- pects within that design, and HDI PCBs are no different. In this case, some of the controlling factors are the BGA pad size and pitch. Figure 4: Typical microvias formed with nanosecond laser (left) and USP laser (right). (Source: Orbotech) Figure 5: Stacked microvia and through-hole filling.