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FIGURE 10.5 Decomposition curve for a phenolic high-Tg FR-4.
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Decomposition curve for an alternative high-Tg/high-Td material.
resin decomposition when the peak temperature reaches 235 C. Figure 10.8 presents the same results when the peak temperature is increased to 260 C. The increase in temperature to 260 C has a severe impact on resin decomposition for the traditional FR-4 materials as they experience multiple thermal cycles, especially the conventional high-Tg material (product C). The rapid degradation in material C after only a few thermal cycles highlights why this material is not recommended for lead-free assembly applications. Indeed, as more data are gathered, thermal cycling seems to have a much greater impact than even one cycle to a very high temperature, even if the cycling occurs for a long time. As an example, review Fig. 10.9, which plots decomposition versus time at different isotherm temperatures for a standard dicycured high-Tg FR-4 material (C in Figs. 10.7 and 10.8). In Fig. 10.9 it can be seen that relatively low levels of decomposition are experienced for extended time periods even at the 260 C and 275 C isotherm temperatures. This same material, C in Fig. 10.8, exhibits rapid decomposition when cycled up to 260 C and cooled to room temperature multiple times.
5 0 % Weight loss 5 10 15 20 1 2 3 4 5 Cycles 6 7 8 9 A B C D
FIGURE 10.7 Decomposition through multiple cycles to 235 C.
THE IMPACT OF LEAD-FREE ASSEMBLY ON BASE MATERIALS
5 0 % Weight loss 5 A 10 15 20 1 2 3 4 5 Cycles
FIGURE 10.8 Decomposition through multiple cycles to 260 C.
B C D 6 7 8 9
Moisture Absorption As pointed out in Table 10.2, the vapor pressure of water at lead-free assembly temperatures 260 C, for example is much higher than at eutectic tin-lead assembly temperatures such as 230 C. Figure 10.10 plots the vapor pressure of water, in both mm Hg and psi, versus temperature. At 230 C the vapor pressure of water is near 400 psi. At 260 C, it is close to 700 psi. Therefore, any absorbed moisture within a PCB during assembly can have a much greater impact in lead-free assembly, as the greater pressure stresses the adhesion between the base material components and can also create small voids within the resin system. This means that much more care must be used in selecting materials for their moisture absorption properties. However, all common base materials absorb some level of moisture.
100.2 100 99.8 Weight (%) 99.6 99.4 99.2 99.0 98.8 0 10 20 Time (min) 30 40 50 Universal V4.1D TA instruments 370std 230C 370std 245C 370std 260C 370std 275C
FIGURE 10.9 Decomposition versus time at different isotherm temperatures..
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70,000 60,000 50,000 mm HG 40,000 30,000 20,000 10,000 0 0
FIGURE 10.10
1,400 Pressure, mm Hg Pressure, PSI 1,200 1,000 PSI 800 600 400 200 0 25 50 75 100 125 150 175 200 225 250 275 300 Temperature, C
Vapor pressure of water versus temperature.
So the storage of these materials, both during manufacturing of the PCB as well as prior to assembly, must also be examined. Additional drying or baking steps may be needed in some applications to drive off any absorbed moisture prior to exposure to high temperatures. Additionally, it can be difficult to correlate actual performance with the moisture absorption data commonly found on material data sheets. One reason for this is that it may not be just the moisture absorption that is important, but the rate at which moisture can be driven from the material as well. As an illustration, consider the data in Fig. 10.11. These materials were conditioned for 60 minutes in 15 psi steam and then solder shocked for 20 seconds at 288 C. Their moisture absorption was measured and compared to their
1.0 Dicy cure 0.8 0.6 Epoxy/PPO 0.4 0.2 0.0 Hydroxyl cure Halogen free
Solder resistance rating (20 seconds at 28 C)
Moisture uptake (% wt.) (60 minutes in 15 psi steam)
5 4 3 2 1 1 2 3 4 5 6 System ID 7 8 9 10
FIGURE 10.11
Moisture absorption versus solder shock resistance for several materials.
THE IMPACT OF LEAD-FREE ASSEMBLY ON BASE MATERIALS
1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0.1 0.2 0 40 80
Moisture absorption (%)
24 hour bake 120 Time (hours) 160 200 240
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