Depth-Controlled Drilling Methods

Depth-controlled drilling utilizes the top of the drilled stack as its reference point. Before this depth-control method can be utilized mechanically, the machine needs to know what the distance is from the pressure foot insert to the tip of the drill bit. 

There are three common methods of depth control used in the drilling process ... 

Several methods are available for the drilling and installation of monitor wells. The advantages and disadvantages relating to ease of construction, character of formations penetrated, well diameter and depth, risks of contamination, and intended use of the well have been reported by several investigators ( 5, 6, 7). All field personnel should be thoroughly familiar with well specifications, quality assurance/quality control procedures, and safety plans and requirements prior to installation of monitor wells. 

In order to test the protective efficiency of the surface treatments, a number of 0100 mm and 060 mm cores were later on drilled out from the concrete decks. For Structure 1, the cores were removed from the deck slab after approximately four years, while for Structure 2, the cores were removed from the deck beams after approximately two years of exposure. Shortly after all surface treatments, a number of 060 mm cores had also been removed for control of penetration depth of the hydrophobic agent. This control was carried out by use of visual observations based on splitting of the removed cores and spraying with pure water [9]. The control of chloride penetration later on was based on successively grinding off small powder samples, from which the chloride content was analyzed by use of a spectrophotometric method [10]. 

In standard PCB fabrication, metallized drilled holes provide an electrical connection between any two layers. However, through vias utilize board real estate on all layers whether an interconnection is required on a particular layer or not. The number of layers not connected by through vias can be significantly reduced through the use of controlled-depth drilled vias and the sequential lamination of pre-drilled core layers (blind internal layer vias). While the use of blind vias (vias that connect individual circuit layers) increases board utilization it may significantly add cost. Additive board layer processes and micro-via formation methods provide alternative solutions to mechanically-drilled holes. Figure 2A, shows an example of inner-layer (buried vias) and outer-layer (blind vias). 

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