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PTH diameter, mm 0.25 5000 2000 Number of thermal cycles 1000 500 200 100 50 20 10 0.010 0.020 0.030 PTH diameter, in 0.040 Tg 94 C Tg 152 C Tg 116 C 0.50 0.75 1.0
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FIGURE 57.6 Effects of substrate Tg and PTH diameter on mean number of cycles to failure. The thermal cycle was 2-h cycle with extremes at 62 and +125 C. Multilayer printed circuit board thickness 0.10 in (2.5 mm); Cu in unfilled PTH is 1.2 mil (30 m) thick. Results from Ref. 33.
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FIGURE 57.7 Cycles to failure vs. PTH aspect ratio for 65 to +125 C thermal shock cycles. Various hole diameters, board thicknesses, and board constructions. (After Ref. 3.)
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RELIABILITY OF PRINTED CIRCUIT ASSEMBLIES
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FIGURE 57.8 Effect of PTH plating thickness on number of thermal cycles to failure for thermal cycles with the indicated peak temperatures. For acid sulfate copper and FR-4 boards. Other hole parameters are the same as Fig. 57.5. (After Ref. 2.)
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its original value. The quality of the bond between the resin and reinforcement is compared by measuring the time it takes for copper-clad laminate to blister during a solder float test at 290 C.7 57.2.1.2 Mechanically Induced Failures. PCBs may be mechanically loaded by test fixtures or processing equipment, when PCAs are loaded into card cages or fixtured into place with brackets, or when the assembly experiences mechanical shock or vibration in use. In general, once the PCA has been assembled, the interconnects to the components are the weak links in mechanical loading situations, not the boards themselves. 57.2.1.3 Electrochemical Failure Modes. The primary function of a printed circuit board is to provide electrical connections with the desired, stable, low-impedance and highimpedance insulation between them. A high surface insulation resistance (SIR) value is usually assumed by the circuit designer. Exposure to humidity, especially when ionic contaminants are present, is a common cause of insulation resistance failures that is accelerated by elevated temperatures and electrical bias. The impedance often decreases slowly over a long period of time. If the SIR value falls below the designed level, there will be cross talk between circuit elements that should be isolated, and the circuit may not function properly. Insulation
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resistance deterioration is particularly harmful for analog measurement circuits. If these circuits are used to measure low-voltage, high-impedance sources, changes in circuit impedance can result in the deterioration of instrument performance. Medical products that use sensors attached to a patient also pose special concerns, because deterioration of insulation resistance has the potential to cause electrical shock. For general applications, the surface resistivity is usually specified to exceed 108 / , but for these specialized applications higher values may be required. Electrochemical failures are usually accelerated by temperature, humidity, and applied bias. High humidity is a significant cause of reliability problems because many corrosion mechanisms require water to operate. A humid environment is an excellent source of water, even when it is not condensing. Polymers commonly used in PCBs are hygroscopic; that is, they absorb moisture readily from the environment. This phenomenon is reversible; the moisture can be driven out of the PCB by baking it. The amount of moisture absorbed and the time to reach equilibrium with a humid environment depend on the laminate material, its thickness, the type of solder mask or other surface coating, and the conductor pattern. The moisture absorbed by the PCB and ionic contaminants on or in the PCB play a role in a number of failure modes. Because the permittivity of water is much higher than that of most laminate materials, the increased water content can significantly affect the dielectric constant of the laminate, and thus may affect the electrical functionality of the board by increasing the capacitive coupling between traces.Absorbed and adsorbed water can lower SIR values, especially in the presence of ionizable contaminants (often from flux residues) and DC bias. The introduction of no-clean assembly processes has significantly increased the importance of measuring SIR values because contaminants left on the PCB remain there after assembly. Industrial pollutants can also be a source of ions that accelerate corrosion. In addition, typical industrial pollutants such as NO2 and SO2 can damage many materials used in PCAs, particularly elastomers and polymers. Some important mechanisms that cause failures due to low insulation resistance include dendritic growth and metal migration, galvanic corrosion, and conductive anodic filament growth. Whiskering can also cause electrical shorts, but neither electrical bias nor moisture is required. Conductive Contaminant Bridging. Bridging of circuits by conductive salts may occur if plating, etching, or flux residues remain on the board. These ionic residues are good conductors of electricity in a moist environment. They tend to migrate across both metallic and insulating surfaces to form shorts. Corrosive byproducts, such as chlorides and sulfides formed in industrial environments, are chemically similar and can also cause shorting. An example of this type of failure is shown in Fig. 57.9.8 Dendritic Growth. Dendritic growth occurs by the electrolytic transfer of metal from one conductor to another; consequently, it is also termed electrolytic metal migration. It is also referred to as electromigration, although it should not be confused with the process that occurs in aluminum conductors in integrated circuits, which has a different mechanism. An example of a dendritic growth failure is shown in Fig. 57.10. Dendrites form on surfaces (including the interior surfaces of cavities) when the following conditions are met:
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Continuous liquid water film, a few molecules or more in thickness Exposed metals, especially Sn, Pb, Ag or Cu, that can be oxidized at the anode Low-current dc electrical bias
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It is significantly accelerated by the presence of hydrolyzable ionic contaminants (for example, halides and acids from flux residues or extracted from polymers). Delaminations or voids that promote the accumulation of moisture or contaminants can promote dendritic growth. Conductive anodic filament growth (discussed later) is a special case of dendritic growth. Time to failure is inversely proportional to spacing squared and voltage. The failure mechanisms in accelerated tests have been reviewed.9 Dendritic growths usually form from cathode to anode. Metallic ions formed by dissolution at the anode are transported along a conductive path and reduced and deposited at the
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