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Cooling
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2 % Strain in barrel (referenced to 25 C)
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1 1 Laminate Tg PTH barrel Heating 0
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Cu free expansion 0 25 50 100 Tg 150 200 Temperature ( C) 250
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FIGURE 57.4 Strain vs. temperature for FR-4 (epoxy-glass), copper, and a PTH barrel in an FR-4 board during a single thermal cycle from 25 to 250 to 25 C. While the thermal expansion of the individual materials is fully reversible, much of the strain in the Cu PTH barrel is plastic, so most of the strain is not reversed during cooling. Note that the rate of thermal expansion of the FR-4 increases sharply at Tg. Results from Ref. 1.
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This relation will significantly underestimate life for high cycle fatigue which can occur after repeated thermal cycling in service. The strain e can be estimated by finite element modeling or analytically. If no other data are available, ef for electroplated Cu can be approximated as 0.3. The number of cycles to failure can be increased by increasing ef / e, primarily by decreasing e by:
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Decreasing or eliminating thermal shocks by preheating the board before hot-air leveling, wave soldering, rework with a solder pot, etc. Decreasing the size of the thermal cycle (see Fig. 57.52). Decreasing the size of the thermal cycle is the single most effective measure for increasing the life of the PTH, especially if the thermal cycle exceeds Tg. Decreasing the free thermal expansion of the laminate over the thermal cycle. The free thermal expansion can be reduced primarily by choosing a laminate material with a higher Tg, but also by choosing a laminate material (e.g., with Aramid fibers) with a low CTE below Tg (see Fig. 57.6). Decreasing the PTH aspect ratio (usually quoted as board thickness divided by finished hole size) by decreasing the board thickness or increasing the hole diameter (see Fig. 57.7). Aspect ratios tend to be higher in boards with eight or more layers because of their thickness and via density; aspect ratios greater than 3:1 require good-quality plating and aspect ratios higher than 5:1 are not recommended, in part because of the difficulty of achieving adequate plating thickness in the center of the barrel. Increasing the Cu plating thickness (see Fig. 57.82). Increasing the plating thickness also increases the distance a fatigue crack must propagate to cause an electrical failure. Using Ni plating over the Cu (see Sec. 57.5.1.3 for more discussion). The ratio ef / e can be increased by:
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Increasing the Cu ductility (increases ef) and yield strength (decreases e). Cu strength and ductility are often inversely related, so these two factors must be balanced against one
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% Strain in barrel (referenced to 250 C)
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FR-4 free thermal expansion
RELIABILITY OF PRINTED CIRCUIT ASSEMBLIES
104 Barrel cracks Corner cracks
103 Number of thermal cycles 1 2 102 Calculated curves
Test conditions: 10 1 MIL-STD-202 DM107C 65 C/30 min 125 C/30 min 2 MIL-STD-202 DM107C 65 C/30 min 150 C/30 min 3 Oil dip 25 C/5 s 50 100 260 C/5 s 200
150 Temperature ( C)
FIGURE 57.5 Peak temperature vs. number of cycles to failure by PTH barrel (solid bars) or corner (dashed bars) cracking in three different tests. Calculated lines are shown for comparison. Results are for pyrophosphate copper and FR-4. Other parameters are total strain energy required to cause fracture, 50 J/cm3; hole radius, 0.45 mm; distance from hole center to free end, 0.8 mm; plating thickness, 0.02 mm; distance from hole center to pad edge, 0.8 mm; board thickness, 2 mm. Results from Ref. 2.
another. However, the strength-ductility relationship can be altered by the choice of plating bath and plating conditions. The number of cycles to failure can be dramatically decreased by defects in the hole wall or Cu plating in the hole or PTH knee that act as stress concentrations (increasing the local stresses and strains) and/or facilitate crack initiation. Because of the importance of this failure mode, it has been extensively studied experimentally and with analytical modeling techniques and more quantitative models are available.3,4,5 Laminate and Cu/Laminate Adhesion Degradation. When a PCB is exposed to elevated temperatures for long periods of time, the adhesion between the Cu and the laminate and the flexural strength of the laminate itself will gradually degrade. Discoloration is usually an early symptom. Several standards tests are used to compare the thermal resistance of different laminate materials. Cu adhesion is measured using a peel test.6 Adhesion at elevated temperatures or after elevated temperature exposure gives some insight into the ability of the material to withstand rework and other high-temperature processes. Flexural strength stability is compared by measuring the times at 200 C before the flexural strength decreases to 50 percent of
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