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RELIABILITY OF PRINTED CIRCUIT ASSEMBLIES
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to increase the strain in the Cu during thermal cycling, further accelerating failure. If the total time for part removal and replacement is kept under 25 s, little dissolution is measured.32 Weakening of the PTH by copper dissolution during PGA rework can be essentially eliminated by using NiAu plating. Although the thin Au coating that protects the Ni dissolves almost instantly during soldering, Ni dissolves quite slowly and effectively prevents thinning of the PTH metallization. Damage to Adjacent Components. Rework can also damage components adjacent to the one being repaired or replaced.The hot cracking phenomenon during wave soldering can also occur in solder joints near the rework site if they reach the melting temperature of the solder. At slightly lower temperatures, rapid intermetallic growth can occur. Temperature-sensitive components can also be damaged. To prevent these problems, localized heating and shielding should be used and the temperature of adjacent components monitored. The generally recommended maximum temperature is 150 C. There are wide variations in the amount of heating of adjacent components among different types of rework equipment and between process protocols.33 Other Rework Concerns. Rework can cause a host of moisture-related problems including measling and package cracking. Both of these problems can be prevented by baking the PCA beforehand to drive out moisture and by minimizing the peak temperature and time at elevated temperature during rework. Rework temperatures also weaken the adhesive bond between the copper conductors on the PCB and the laminate material; use of force to remove components when the solder is not completely molten can cause the pad to lift off the board. The latter can be a particular problem when using a soldering iron.21,31
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INFLUENCE OF MATERIALS SELECTION ON RELIABILITY
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PCB 57.5.1.1 Substrate. Difunctional FR-4 is the workhorse material for high-reliability PCBs because its moderate z-axis expansion and moisture uptake characteristics are available at relatively low cost. Alternative substrate materials (see Table 57.2) are generally selected for more favorable properties in one or more of the following three areas: thermal performance, including maximum operating temperature and glass transition temperature; thermal expansion coefficient; and electrical properties, such as dielectric constant. Thermal performance characteristics and thermal expansion coefficient can have a significant effect on PCB and solder joint reliability. Other characteristics of these materials, such as moisture absorption, can also affect reliability. PTH reliability can be improved by selecting a laminate with a lower z-axis CTE or a higher Tg. The damage caused to the PTH during a thermal cycle depends on the total z-axis expansion during the temperature change. Since the CTE is much lower below Tg than above,
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TABLE 57.2 Physical Properties of Some Printed Circuit Board Laminate Materials CTE, x,y ppm/ C 14 18 14 16 6 8 6 12 CTE, z ppm/ C 180 170 66 35 Tg, C 125 135 140 150 125 188 250
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Material Epoxy glass (FR-4, G-10) Modified epoxy glass (polyfunctional FR-4) Epoxy aramid Polyimide quartz
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Source: After IPC-D-279.
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the PTH strain can be reduced by increasing the Tg so that more or all of the cycle is below Tg (see Fig. 57.18a). Figure 57.18b shows that the increase in life can be quite significant. The strain imposed on the PTH can also be reduced by decreasing the CTE at temperatures below Tg, but the effect on total z-axis expansion is much smaller.
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5 Free expansion (%) 4 3 2 1 0 100 200 Temperature ( C) Tg , C 125 200 240 300 z-axis CTE, ppm/ C 17 73/300 50/230 50/230 Driving force for failure
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Material Copper FR-4 FR-4/CE blend CE
100 Cumulative failure probability FR-4 Cyanate ester/ FR-4 blend FR-4
10 1
10 Thermal cycles
FIGURE 57.18 (a) Effect of differences in CTE below Tg and Tg on the free z-axis expansion of FR-4 (epoxy-glass), cyanate ester (cyanate ester-glass) and a cyanate ester/epoxy blend. Tg and CTE for each material below/above Tg are indicated. Cyanate ester is abbreviated CE in the figure. Cu is shown for comparison. (b) Weibull plot of PTH failures for these substrates during thermal shock cycling between 25 and 260 C. PTHs are 0.029-in diameter on 0.100-in grid on 0.125-in-thick laminate. (After Fehrer and Haddick.4)
A variety of specialty resins with increased Tg is available, albeit at higher prices. Modified FR-4 materials with higher functionality offer the best combination of improved Tg at a reasonable price. Further improvements in Tg and other characteristics can be obtained with bismaleimide triazine (BT), GETEK, cyanate ester, and polyimide, but at greater price penalties.
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