25.1 INTRODUCTION

As circuit density continues to increase, along with the demand for higher and higher accuracy in hole location, the drilling machines and the environment in which the machines operate must be tightly controlled in order to achieve success in the drilling operation. High-density interconnect (HDI) has been defined as referring to holes with diameters of ?0.006 in. HDI holes generally are expected to be made by nonmechanical means, such as lasers, plasma, or photoimaging. However, the bulk of holes continue to be created by mechanical means, and as the hole size approaches, or in some cases goes below, 0.006 in., special challenges face the mechanical drilling process. This chapter addresses these issues.

In this chapter, holes created by traditional mechanical drilling methods are referred to as drilled, and the process is referred to as drilling. Holes created by laser, plasma, or photoimaging are not really drilled, even though the term is often used to describe them. Where there is a reference to nonmechanical via hole creation processes, these are referred to by their specific type.

25.2 FACTORS AFFECTING HIGH-DENSITY DRILLING

PCB technology requires drilled holes as small as 0.002 in (0.05 mm) with extremely high accuracy, particularly when drilling dense hole patterns. The processes and machines used to drill these holes now constitute a highly developed science.

As the drilling process approaches these HDI dimensions, numerous factors become increasingly critical, such as:

• Hole...