Grid hole pitch

The relationship between the grid hole pitch, Lh, and the number hole density (holes per unit area of the bed), Nd, depends on whether the holes are laid out in triangular or square pitch. 

Figure 4. The relationship between hole density and grid hole pitch for triangular and square pitch.

Hole Layout. To increase the uniformity of fluidization, it is common to lay out the holes in triangular or square pitch as shown in Fig. 4. All the holes in a grid with triangular pitch are equidistant. This is not the case for a grid with square pitch. Triangular pitch will also result in more holes per unit area. 

The density of IC packaging technologies such as ball grid array (BGA) and chip scale packages continues to increase. In turn, this requires greater intercoimection density in the printed circuits onto which they are assembled. The need for increased density in the PCB impacts each of the components of the base material as well as the way in which they are manufactured. To achieve high levels of interconnection, component pitch densities result in smaller and more closely spaced plated through holes and circuit features. As the space between these holes and features decreases, the potential for conductive anodic filament (CAE) failures increases substantially. 

Many HDI structures can be tested with advanced examples of tilt-pin fixtures. Using such techniques as test pins that taper (see Fig. 39.1) to a very fine diameter at one end, fixtures have been built for devices with pitch down to about 0.010 in., with research and development (R D) examples at about 0.008 in. These fixtures contain additional plates to support the very thin pins (so they do not buckle under pressure or short to adjacent pins). Those fixture plates very close to the product must be fairly thin, or the closely spaced holes will break out into one another. Such fixtures increase the demand for higher grid densities.These fixtures and pins are moderately more expensive to prepare than standard fixtures, but use similar technologies and processes. Excellent process control is important, as is excellent pin tip symmetry. 

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