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48 The PCB Design Magazine • November 2017 the commercial use of larger and much higher I/O die elements. Regarding flip-chip mounting, interconnection from die element to the PCB is commonly achieved with alloy bumps, spheres or, for very fine pitch applications, raised cop- per pillar contacts that, although very small, are compatible with a conventional reflow solder- ing processes. Achieving Higher Circuit Density For many applications, the cost of high-den- sity printed circuit boards has remained a de- tractor. Although PCB complexity has increased, however, the prices for HDI have declined and analysts expect this trend to continue to decline further each year. This is due in part to increased competition but the trend can be attributed to diligence in refining fabrication process meth - odology and controlling material utilization. Process refinement includes the development of more efficient imaging capability, improved etching and plating chemistry and refinement in base materials and lamination methods. Many user companies have already established a business relationship with these key suppliers and have qualified their level of expertise and capability. Regarding specifying narrow copper conductor width and spacing, the IPC-2226 has defined three HDI complexity levels for both ex - ternal and internal locations (Table 1). Some companies can produce copper con- ductors as narrow as 25 μm (1 mil) but they likely rely on using dielectric materials that have a very thin copper foil or utilize base ma- terials prepared for a semi-additive copper plat- ing process. But, before committing to adopt- ing any level of HDI technology, the designer should confirm that the PCB suppliers selected are able to meet the required complexity level with acceptable process yields at the anticipated production quantity. A key contributor to enabling higher density circuits is in the advances made in circuit pat- tern imaging. Circuit pattern imaging tradition- ally relied on first digitally imaging the circuit pattern onto polymer-based film masters then, using contact imaging to transfer the circuit pattern onto photo-resist emulsion applied to the copper-clad panel surface. Many fabricators have streamlined their processes by transferring the circuit image directly from the CAD file onto the panels resist coating using laser technology. Both laser direct imaging (LDI) and digital im- aging (DI) systems have become mainstream technology for a wide segment of the PCB fab- rication industry. Direct imaging eliminates ex- tensive process steps required in preparing the film masters and avoids physical distortions at- tributed to varying thermal conditions and hu- midity on the polymer-based film masters. Hole and Via Forming Methodologies Mechanical Drilling Drilling systems are manufactured by a broad number of companies worldwide and can range from a single spindle head system for low vol- ume applications to multiple spindle head con- figurations to accommodate very high-volume fabrication requirements. Mechanical drilling is the most economical and efficient method for providing holes in circuit boards. The current generations of precision NC drilling systems are designed for accuracy and high throughput and many systems support post lamination profile routing capabilities. Regarding specifying via hole size, mechani- cally drilled and plated vias can be furnished as small as 100 μm to 150 μm (4−6 mils) by some, however, the drill bit selected to furnish a finished STRATEGIES FOR HIGH-DENSITY PCBS Table 1: Complexity levels for HDI technology as outlined in IPC-2226.