Along with the advancement in miniaturizing of mobile devices, typified by smart phones and tablet PCs, the semiconductor PKG substrate installed in these devices is demanded to be thinner and higher in density. As one of the most innovative solutions, the PoP (package on package) technology, which has the three-dimensional construction, has been expanding rapidly in recent years.
However, thinner PKG such as PoP tends to warp at the assembly process and cause the decrease in the connection reliability. Therefore ultra low CTE (coefficient of thermal expansion) core materials have been needed as a key solution for the reduction of the warpage for PoP.
We have already developed and mass-produced ultra low CTE core material named E-770G (CTE value: 1.8 ppm/°C) for this application. However, the demand for lower warpage is still stronger especially for thinner PKG applications. To meet this requirement, we confirmed that not only low CTE but also lower resin shrinkage control gives big impact to warpage.
In this paper, we would like to introduce new ultra low CTE and low shrinkage core material, named E-777G for next generation thin CSP. At the same time, we would like to explain the relationship between the warpage and material properties (CTE and resin shrinkage).
E-777G has ultra low CTE and low shrinkage. These properties were achieved by applying our original resin system designed by hard segments and soft segments. Hard segments have a stack structure of aromatic ring and the strong intermolecular force between them, which lead to the ultra low CTE and low shrinkage. Soft segments of low elastic modulus can follow approximately the thermal behavior of glass fabric when heated and cooled. Consequently, the material can show the ultra low CTE which is similar to that of glass fabric itself. As a result, E-777G has achieved the ultra low CTE of 0.7 ppm/°C by TMA method which leads to significant reduction of the warpage.
In the cooling process after the reflow, the bottom PKG of PoP shows a convex warpage. It is caused by resin shrinkage of substrate in the cooling process. As the method of decreasing resin shrinkage in the cooling process, we focused on decreasing free volume of the resin. By using molecular dynamics simulation, we designed the most suitable resin composition.
Confirming the warpage property, we evaluated the warpage behavior of the bottom PKG before/after assembly process. The bottom PKG using E-777G material showed lower warpage than E-770G. We confirmed that not only the ultra low CTE but also low shrinkage of material effectively contributes to the decrease of the warpage.
As the demand for the portable handheld products and devices, such as smart phones and tablet PCs, still continues to increase, higher density packaging technologies have been required to reduce the assembly area of substrate. From this point of view, three-dimensional packaging is a key technology to minimize the total size of products and devices.
PoP technology, which enables us to achieve three-dimensional packaging, has been accelerated in adoption particularly for smaller systems. However, because of its thinner construction in PoP, warpage of the substrate at soldering process, which may result in poor connection reliability, has been one of the key challenges to overcome. In many cases, the main factor of the warpage is the mismatch of CTE between the substrate and the silicon chip, and therefore, ultra low CTE core materials are required for thinner PKG applications.
We have already developed ultra low CTE core material named E-770G (CTE value:1.8 ppm/°C) for this application, and it is now in commercial mass production. However, the demand for ultra low CTE core materials is still growing for thinner PKG applications, and the CTE needs to be lower that of E-770G in these days.
In this paper, we introduce new ultra low CTE core material named E-777G for next generation thin CSP, and confirmed the relationship between the warpage and material properties (CTE and shrinkage).
Warpage in Assembly Process by Simulation
The basic idea of the warpage behavior of PKG substrates is understood by the CTE of the substrate and the silicon chip as follows. In the assembly process, the PKG substrate shows a concave shape at reflow temperature, since the thermal expansion of the substrate is larger than that of the chip. On the contrary, when the substrate is cooled down to the room temperature, it shows a convex shape due to the shrinkage of the substrate.
In order to investigate the relationship between the warpage and material properties (CTE and modulus), mathematical simulations were done. The PKG model and the substrate construction are shown in Figures 1 and 2. The thicknesses of core material and prepreg for the simulation are 100 μm and 40 μm, respectively. The simulations of the warpage behavior of PKG substrates were done by a three-dimensional elastic analysis method at the temperatures of 25 °C and 260 °C.
The simulation results are shown in Figure 3. From the results, the low CTE core material was confirmed to decrease the warpage at both temperatures. We have also confirmed that the contribution of the low CTE core material to reduce the warpage is more effective than that of the high modulus core material.
Molecular Design of New Resin System
As the simulation result demonstrates, lower CTE core materials can reduce the warpage of thinner PKG substrate effectively. The CTE formula is shown in Figure 4. We have the three approaches for ultra low CTE. One is to apply the low CTE resin. The second is to apply the low elastic modulus resin. The third is to apply the low CTE glass (S-glass) and close-packing of low CTE filler.
We have already developed two types of technical concepts for the further ultra low CTE (type A and type B) .
In the technical concept of type A, the resin system has a planer stack structure of aromatic ring and the strong intermolecular force between the stack can bring ultra low CTE and low shrinkage. The resin system of type A has been adopted in E-770G. On the other hand, we adopted another resin system for type B, introducing soft segments in the system to realize the low elastic modulus. By applying the soft segments, the elastic modulus of the resin itself becomes much lower than that of type A. Therefore, the resin can follow approximately the thermal behavior of glass fabric when heated and cooled. Consequently, the material can show the ultra low CTE which is similar to that of glass fabric itself. Moreover, the low elastic modulus of resin also shows the low residual stress.
Recently, we have developed new resin system for ultra low CTE core material (E-777G), featuring low shrinkage and low residual stress. The polymer of E-777G is formed by our original resin having hard segments and soft segments as illustrated in Figure 5. Hard segments of the new resin system lead to the ultra low CTE and low shrinkage. And soft segments of the new resin system lead to the ultra low CTE and low residual stress. Moreover, the resin system contains higher amount of inorganic filler to attain ultra low CTE and high modulus by applying the FICS (filler interphase control system) technology [2–4].
General Properties of New Ultra Low CTE Core Materials
The general properties of our newly developed core material (E-777G) are shown in Table 1. As shown, we also evaluated the high density (HD) and low CTE glass (S-glass) in addition to the standard glass (E-glass).
From the table, E-777G had ultra low CTE, high modulus, and low shrinkage. The CTE of E-777G (3.9 ppm/°C with E-glass, 1.3 ppm/°C with S-glass, 0.7 ppm/°C with HD S-glass) was lower than that of E-770G. The flexural modulus (33 GPa with E-glass, 36 GPa with S-glass, 38GPa with HD S-glass) at room temperature was similar to that of E-770G. The resin shrinkage of E-777G (0.08 %) was considerably lower than that of E-770G (0.14 %). Moreover, E-777G had excellent dielectric properties of low dielectric constant and low dissipation factor, at high frequencies. Especially, the low dissipation factor (Df: 0.005 at 1 GHz) of E-777G can be applicable to high speed PKG applications.
Influence of Resin Shrinkage
Generally, in cooling process after the reflow, the PKG substrate shows a convex shape due to the shrinkage of the substrate which largely depends on the core shrinkage.
So, the contribution of the resin shrinkage to the core shrinkage was examined following figure 6.
Figure 7 shows the relationship between the shrinkage of resin and core. As obviously shown, the decreasing of resin shrinkage effectively lowers the core shrinkage.
Figure 8 shows the CTE in cooling process when varying core shrinkage. In this evaluation, core materials that have the CTE of 0.7 ppm/°C in heating process were used. As shown in the figure, the decreasing of core shrinkage brings the ultra low CTE of the substrate in the cooling process which was confirmed to reduce the warpage.
Warpage Property of Thinner PKG Substrate
The warpage property of thinner PKG substrate with simple design using E-770G and E-777G with HD S-glass was examined. Figure 9 shows the overview and cross-section of the PKG substrate (4-layer PKG construction). In this evaluation, the prepreg was used as build-up layers. After the patterning, the chip was assembled and underfilled. The sizes of the PKG substrate and the chip were 14×14×0.2 mm3 and 7.3×7.3×0.15 mm3, respectively. The thickness of the core material and the prepreg were 0.20 mm and 0.04 mm, respectively. The warpage at the initial state of substrate, after underfilling and after dicing were measured by Shadow-Moire system. The overview of the measurement by Shadow-Moire system is shown in Figure 10.
The results of the warpage measurement at the solder ball area of PKG substrates at 25 °C and 260 °C are shown in Figure 11. As shown, the warpage of thinner PKG using E-777G core were much smaller than those of E-770G at both temperatures. The delta warpage of E-777G material was approximately 65 % of that of E-770G material. That means the CTE and shrinkage of material affect the warpage strongly. These results are well consistent with the simulation results of the warpage and the evaluation of shrinkage.
We have developed new ultra low CTE core material (E-777G) and examined the warpage property of the material.
(1) The ultra low CTE and low shrinkage was achieved by applying our original resin system having hard segments and soft segments .
(2) The new ultra low CTE core material showed much lower warpage than the conventional ultra low CTE core material.
(3) From the results of the simulation and the evaluation of shrinkage and measurement of the warpage, it was demonstrated that ultra low CTE and low shrinkage lead to smaller warpage in thinner PKG substrate.
Note: The contents of this paper are based on the results of experiments and do not represent a guarantee of the values for each property.