TABLE OF CONTENTS The organic based PWB has been and continues to be the key enabler within the interconnect hierarchy that is electronic manufacturing. It is clearly the most dominant Level 2.0 interconnect substrate and is found in most electronic products. Historically, extensive use of organic substrates began during World War II. Obviously its complexity as an interconnect substrate has increased significantly over the years.
The traditional PWB is basically an organic substrate with a copper foil clad on one side or both sides of the board. The clad copper is patterned or circuitized using the photoresist/subtractive etching process. Early PWBs were simple single layer conductor boards. Later, double-sided PWBs with conductor traces on both sides of the board were introduced. Top and bottom layers were interconnected using wires or metal clips inserted into through holes drilled into the substrate. These boards served exclusively as the Level 2.0 interconnect through the 1950s and into the late 1970s at which time the multilayer PWB appeared. Double sided multilayer boards (MLB) contain three or more conductor levels, consisting of the two outer layers and x-number of inner layers totally embedded within the substrate. The key to this technology was the plated through-hole (PTH). First mechanically drilled then plated, the PTH provides connection to the various embedded inner layer conductor circuitry. A PTH is illustrated graphically in Figure 15-1(a). A metallurgical cross-section of a PTH via is shown in Figure 15-1(b).
