464
L. Chen et al.
Fig. 15.25 Comparison of moisture concentration, deformation, and stress at different time in
preconditioning process [17]
moisture distributions can be applied as body loads. The stress state depends on the
temperature state and instantaneous moisture distributions. (3) Combined thermal,
hygroscopic swelling and vapor pressure loads. In this case, Eq. (15.22) is used to
apply a body load which comprises both hygroscopic swelling and vapor pressure.
Numerical results from the above three types of analysis are summarized in
Tables 15.6, 15.7 and 15.8. Table 15.6 shows the maximum warpage, maximum
von Mises stresses in copper and mold compound, respectively, at three different
times during reflow: 200, 260, and 260 °C after 30 s hold. At 200 °C, hygroscopic
Table 15.6 Maximum warpage (W ) and von Mises stresses (σ e ) in copper and mold compound at
200 °C [17]
W
(mm)
σ e , MC
(MPa)
σ e , Cu
(MPa)
Thermal stress only
0.4e − 3
2.4
8.7
Thermal + hygroscopic
0.7e − 3
4.9
17.3
Thermal + hygroscopic
+ vapor pressure effect
1.0e − 3
6.3
22.9
Table 15.7 Maximum warpage (w) and von Mises stresses (σ e ) in copper and mold compound at
260 °C [17]
W
(mm)
σ e, MC
(MPa)
σ e , Cu
(MPa)
Thermal stress only
1.3e − 3
6.0
21.9
Thermal + hygroscopic
1.5e − 3
7.2
27.5
Thermal + hygroscopic + vapor pressure effect
2.1e − 3
11.2
38.9