A Combined Analysis of Moisture Diffusion with Interface Fracture Mechanics on the Interface Delamination in the Plastic
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interfaces between the die pad and EMC were calculated. From this information, crack tip parameters such as the energy release rate, stress intensity factor and phase angle were derived. NUMERICAL PROCEDURE The following analysis applies the methods developed by Kitano et al.[I], and Tay and Lin [2] in which heat transfer and moisture diffusion are analyzed only in a direction perpendicular to the die pad (x-direction) using the finite difference method. Thus, the heat and water diffusion is essentially 1-dimensional and the schematic diagram of the die pad and surroundings is shown in Fig. 1. The coordinate center was located on the top of the EMC layer as shown, and the x axis was set normal to the die pad. The epoxy resin containing water molecules was treated as a thermodynamic solution obeying the Henry's law, and water molecules were assumed to diffuse in EMC according to the Fick's law [1,2,3]. Thickness of the EMC layer below the die pad was varied from 1.2 to 1.6 mm. Temperature and Moisture Distribution during the T/H Preconditioning and Reflow Soldering Processes The die pad/EMC interfaces were assumed to remain uncracked during the 85°'C/85%RH preconditioning process which lasted from 1 to 7 day and the delamination of interfaces was 31 Mat. Res. Soc. Symp. Proc. Vol. 515 ©@1998 Materials Research Society
Vapor
a
1 Die pad
Wma.x
assumed to take place at the onset of the reflow soldering process. Then, from the Henry's law, the moisture concentration C on a delaminated interface at a given time t is given by C(t) = P(t) * S(t)
EMC
at x = 0
(1)
where S is the solubility of water molecules in the EMC solution [1,2,3] and P is the vapor pressure inside the crack opening which is to be determined. The reference time t was set to be Fig. 1. A schematic diagram showing heat transfer and moisture diffusion along the x axis.
zero at the start of the reflow soldering process.
Vapor Pressure at Delaminated Interface during Reflow Soldering For perfectly rigid die pad and much more compliant EMC, bending of EMC, due to the vapor pressure build-up can be obtained by applying the thin plate theory [4] to the EMC bending, Fick's law [1] to the water diffusion in EMC and the ideal gas law to the water vapor inside the crack. Then, the maximum deflection of EMC plate wmax, and accumulated moisture mass m and water vapor pressure inside the crack P are given respectively by [5].
192(1-
+b2)
p Eh3
(2)
m=mo + A D
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