The manufacture and assembly process for PWBs and CCAs can represent several reliability concerns. The concern related to manufacturing and assembly is the introduction of defects. The implication of these irregularities to long-term service reliability is crucial. Most defects fall into three major categories: intrinsic material defects (including improper material properties and cracks), improper geometry (including incorrect size, incorrect thickness), and improper location (including misregistration errors).38 The presence of defects can increase the susceptibility of a CCA to potential failure mechanisms. Defect magnitudes can be addressed in terms of their effects on time to failure or on performance parameters. While the basic geometry, material, and operation parameters determine a nominal design, the worst-case defect magnitudes determine the defect-related reliability risk. While some defects exist in all CCAs within the limits of the statistical process control, the allowable defect magnitudes can be derived based on the life-cycle stress profile and mission life requirements.

Efficient application of screens can detect defects introduced by the manufacturing and assembly processes. Although it should be noted, some defects may not be screened. Defect types and locations are needed to design effective product screens. CALCE Electronics Products and System Center has a detailed on-line resource38 for defects related to the assembly of PWBs. Cost-effective screens are determined by assessing the effects of the worst-case defect magnitudes during accelerated tests. The effectiveness of a screen is directly related to the defect for which the screen is designed.