Moisture expansion can cause more stress than thermal expansion
BY BONGTAE HAN, ERIC STELLRECHT AND MICHAEL PECHT
Polymeric mold compounds absorb moisture and, thus, swell when exposed to a humid environment. Hygroscopic stresses arise in plastic encapsulated microcircuits (PEM) when the mold compound swells upon absorbing moisture, and the lead frame, die paddle and semiconductor chip do not experience swelling.
A new experimental procedure to measure hygroscopic swelling was reported in the previous issue (see “Hygroscopic Swelling of Encapsulated Microcircuits, Part I,” June 2003). The procedure used a real-time whole-field displacement measurement technique called moiré interferometry to conduct accurate measurements. In this article, the technique investigated the stress-induced deformation of an actual package caused by the mismatch in hygroscopic swelling. The deformation produced by the mismatch in coefficient of thermal expansion (CTE) also was measured using this technique. The hygroscopic deformation is compared to the thermal deformation and implications are discussed.
Hygroscopic vs. Thermal Stress
A plastic quad flat package (PQFP) was selected for the test. The package was prepared as shown in Figure 1(a) to examine the interaction between the mold compound and the chip. The opposing sides of the package were trimmed and ground using a precision grinding machine until the silicon chip was exposed on both sides. This specimen configuration preserved the symmetric boundary conditions. After the existing moisture was removed by baking, a specimen grating was replicated onto the package surface at 85°C. The specimen grating was replicated from a special grating mold fabricated on an ultra-low expansion (ULE) glass. The ULE grating mold has a virtually zero CTE. This negligible CTE allowed the ULE grating to be used as a reference to set a null field at the measurement temperature (25°C). This technique is called bithermal loading, implying two discrete temperatures.
The package specimen was then cooled to 25°C and the resulting thermal deformations were measured. Fringe patterns, which represent in-plane displacement maps, induced by ΔT of -60°C, with a contour interval of 0.417 &etam, are shown in Figure 1(b).
null
The package was next subjected to 85°C/85 percent RH until the “saturation” state was achieved. The package was installed in the real-time moiré system and the deformations caused by hygroscopic swelling at the saturation state were measured. The results are shown in Figure 2. This measurement was made at the grating replication temperature (85°C) and, thus, the fringe patterns shown in Figure 2 represent deformations induced only by hygroscopic swelling and do not contain any thermally-induced deformations.
The displacement fields shown in Figures 1 and 2 represent total package deformation, which include the thermal (Figure 1) and the hygroscopic (Figure 2) part of the deformation and the stress-induced part of the deformation. Mathematically, the total strain of the package is:
 |
null
where εT is a total strain, εf is the free expansion/contraction part of strain, εσ is a the stress-induced part of the strain, ΔT is a temperature excursion, σ is the CTE in ppm/°C, C is the moisture content percentage and β is the coefficient of hygroscopic swelling (CHS) in (percent εh/percent C). The subscript of α and β denote the cases of thermal deformation and hygroscopic deformation, respectively.
 Figure 2. Moiré fringes resulting from moisture sorption at the virtual equilibrium state for the U or X field (top) and V or Y field (bottom) fringes. |
null
 Table 1. CTE and CHS values for the mold compound used in the PQFP package and a comparison of the stress-induced strains caused by a thermal excursion and a moisture gain. |
null
The values of α and β of the mold compound were determined from the regions sufficiently far away from the chip (regions marked by dashed boxes in Figure 1(a), where the deformations represent only εf of the mold compound. The stress-induced strains (εx) of the mold compound at the chip/mold compound interface were calculated using Equation 1. These results are summarized in Table 1.
At the chip/mold compound interface, the stress-induced strain caused by hygroscopic swelling, ε&simga;β, was nearly twice as large as that produced by the CTE mismatch, ε&simga;&alpha, with ΔT of 60°C. Although the magnitude of ε&simga;β is not large, a significant strain gradient and, thus, a large stress gradient at the interface arises because the strain of the chip is virtually zero.
Conclusion
This study shows that the hygroscopic swelling-induced deformations can be even larger than thermally-induced deformations in some packages. Numerical analysis such as finite element analysis has been used extensively to assess reliability of microelectronics devices. The analysis must include predictive capabilities of hygroscopic swelling if there are changes in relative humidity in the field condition.
References
For a complete list of references, contact the authors.
BONGTAE HAN, Ph.D., associate professor, ERIC STELLRECHT, graduate research assistant, and MICHAEL PECHT, professor, may be contacted at CALCE Electronics Products and Systems Center Department of Mechanical Engineering, University of Maryland, College Park, MD 20742; E-mail: [email protected], [email protected], [email protected].