Electronic Packaging Moisture Interaction Study

Transcription

1 Electronic Packaging Moisture Interaction Study Yung Hsiang Lee, Ian Chin, Wei Keat Loh Intel Technology Sdn. Bhd. Lot 8, Jalan Hi-tech 2/3, Kulim Hi-tech Park, Kulim, Kedah, Malaysia Abstract Reliability defects associated with thermal humidity environments are not new to the electronics packaging industry, yet to this day it remains a key concern even for our latest technologies. Moisture absorbed into electronic packaging can impact package warpage, cause corrosion, underfill crack and interfacial delamination. Fundamental studies are needed to better understand the effect of moisture interaction with different package designs. This paper summarizes the lab experiments and Finite Element Analysis (FEA) that have been performed to study package moisture absorption-desorption, room temperature (RT) warpage and dynamic warpage, on packages post exposure to thermal humidity environment. This has yielded good fundamental learning and identified areas for future work. Keywords moisture diffusion, experiment, finite element modeling, elctronic packaging I. INTRODUCTION Electronics have become an indispensable, integral part of our daily lives. From portable electronic products to autonomous vehicles, humans are become increasingly dependent on electronic technologies. So it s not surprising that reliability defects are a critical concern for the electronic industry. One commonly underestimated cause of product failure is moisture. Though moisture related failures are not new, yet they are often not realized until product failure occurs. Moisture can affect all non-hermetic electronic package because they are made up of polymer based hygroscopic materials in the solder resist, dielectric, epoxy, mold and core. These materials act like sponges, absorbing moisture from surrounding relative humidity (RH), until an equilibrium is reached [1-2]. Upon moisture absorption, hygroscopic materials experience volumetric expansion or so-called hygroscopic swelling. The bi-material beam study from Shirangi M.H. et al showed that hygroscopic swelling has a direct impact to the stress state of the beam and changes its RT warpage post moisture exposure [3]. Interfacial delamination or pop-corning occurs during the reflow process when entrapped moisture, due to blockage by non-hygroscopic material such as copper, silicon and solder, causes localized vapor pressure to build up due to heat, leading to the separation of the weakest interface. Typically this delamination propagates from the die shadow region, to die edge and can even cause underfill fillet cracks. In order to investigate such internal failures, optical microscopy inspection and Through-mode Scanning-Acoustic-Microscope (TSAM) were used to detect Manufacturing Exposure Time (MET) induced defects, followed by destructive cross-sectioning to investigate the failure mechanism in more detail. The primary defects observed is delamination which occurs at the weakest link in the package, typically the SR-to-Copper (SR-Cu) and ABF-to-Copper (ABF-Cu) interfaces. This delamination could subsequently evolve into secondary or tertiary defects such as substrate blistering, pop-corning or underfill (UF) fillet cracks with severe solder bump bridging or solder bump open failures which are more visible, devastative and sensitive to functionality and reliability performance of the electronic devices. The transition to lead-free solder, which require higher reflow temperature, exacerbates the problem. Therefore it is crucial to define and control MET for packaging assembly to avoid such failures. IPC/JEDEC define a standard to classify packages according to their moisture sensitivity levels (MSL) risk. In this paper, we study the moisture absorption-desorption rate of a variety of electronic package types, form factors and under different environmental conditions. Lab scale experiments were conducted to understand the moisture diffusion characteristic of these assembled packages using a thermal humidity chamber. Saturated packages were subsequently subjected to simulated reflow, using Shadow Moiré to measure the package dynamic warpage response, and also surface potential delamination or blistering. Finite element analysis was used to model these packages, study their moisture absorption and desorption characteristics, and compare results with experimental data. The model generation uses a meshed mapping technique which captures the package design structure while maintaining mesh uniformity and keeping the model size manageable. We explore vapor pressure stress analysis and show an example of it being used to identify high risk locations in a design. Future work to improve the current modeling prediction capability on moisture diffusion and structural stress analysis is also discussed. II. MATERIAL AND METHODOLOGY A. Test Vehicle Four packages were selected for the experimental study, which includes: cored, coreless, molded and multi-chip package (MCP) configurations. TV1 is a 12x12mm coreless package with Package-On-Package (PoP) design. TV2 is a 12x12mm over-molded Flip Chip Chip-Scale Package (FCCSP) with thick core. TV3 is a 17x17mm cored FCBGA package with thin core and thin substrate. TV4 is a 40x28mm cored FCBGA package with 2 dies, thick core and thick substrate. The packages in this study cover a range of package

2 sizes or form factors (FF) from the small FF TV1 to a large FF package like TV4. Key package attributes have been summarized in Table 1. Pack age TV1 TV2 TV3 TV4 Attribute Coreless, PoP Core, Overmold FCCSP Core, FCBGA Core, FCBGA, 2 dies TABLE I. FF (mm) PACKAGE ATTRIBUTE Die Thick ness (Nor m.) Core Thick ness (Norm.) Substrate Thicknes s (Norm.) Mold Encaps ulation 12 x No 12 x Yes 17 x No 40 x No B. Experiment Setup A thermal humidity (TH) chamber was used to provide a controlled test environment at the desired temperature and RH levels. Two thermal humidity conditions were evaluated: 30 o C/60%RH, which is the floor life condition suggested by JEDEC, and an accelerated condition of 85 o C/85%RH. The changes in package weight and RT package warpage were monitored periodically throughout testing. A microbalance, with 0.01mg readability, was essential to detect minute package weight increment due to moisture uptake. For package warpage measurements, shadow moiré interferometry was used [4]. This metrology tool is commonly used in the electronics packaging industry to study out-of-plane warpage. This tool has been equipped with a thermal chamber which enables package warpage measurement from RT up to a maximum of 300 o C. The experiment test flow is shown in Figure 1. All samples were baked at 125 o C for 48 hours prior to testing to ensure they were completely dry. Samples were then split into two groups, one for package weight measurement and the other for RT warpage measurement. This is to reduce noise in package weight measurement from the white spray paint process required for shadow moiré measurement and also minimize the duration samples taken out of the chamber for each readout. Multiple interval readouts were taken until the packages weight change stabilizes, reaching fully moisture saturation state. After that, half the samples were subjected to bake condition, followed by a simulated reflow using the Shadow Moiré tool on all units. Post reflow, packages were inspected for signs of blistering on external surfaces. They were then sent for TSAM inspection while some samples were used for further failure analysis. C. Failure Analyis Methodology Legacy FCBGA cored packages that go through the MSL Level 3 evaluation, require Visual Inspection (VI) and TSAM acoustic scanning, to check for any delamination or cracks. These metrologies worked well on package technology with substrate core, to detect popcorn or blistering, as well as UF fillet cracks. However, for coreless packages, VI and TSAM are insufficient to detect marginal delamination that initiates at the SR-Cu interface. Early stages of delamination at this interface is extremely small and extremely difficult for the acoustic scanning to detect, as compared to the larger secondary or tertiary defects such as pop-corning and blistering. Hence, a more detailed but time consuming and destructive physical de-layering technique is needed to effectively reveal delamination in coreless packages. These analysis techniques include cross-sectioning and Secondary Electron Microscopy (SEM), and were carried out on limited samples when results were confounded or unexpected MET performance were observed. These in-depth root cause analysis were required to rule out potential outliers and clean up the overall experiment data. Figure 1. Moisture experiment test flow. D. Finite Element Analysis In this study, a commercial FEA software has been used to simulate moisture diffusion and hydro-thermo-mechanical effects on the package stresses. For the moisture diffusion analysis, the Mass Diffusion approach was used as the FEA solver technique. Unlike the pseudo thermal analysis approach, the Mass Diffusion analysis procedure can address different saturation levels across different material sets defined in the model. This is especially useful for complex electronic package designs. The model generation leverages both the actual package design file and a general mesh mapping technique to define design details of the package without the typical meshing associated with fine features of a highly structured design [5]. The moisture models were generated with the substrate assigned with individual material property to capture the internal design details of the substrate. The analysis process flow chart is as shown in Figure 2. The Mass Diffusion analysis needs both temperature dependent diffusivity (*Diffusivity) and humidity dependent solubility (*Solubility) properties defined. Utilizing the Fickian diffusion law and the Arrhenius theory, individual material diffusivity properties are defined by equation 1: D = D 0 exp E (1) kt

3 where: D(T): Temperature dependent Fickian diffusion coefficient D 0 : Initial diffusion constant E: Activation energy k: Boltzmann s constant T: Testing temperature Solubility is assumed to have a linear relationship to RH and therefore defined as the saturated moisture content (Csat) by the RH. When exposed to 0%RH condition, Csat would be 0 or no moisture diffusion happens. While at 100%RH, Csat will be the maximum level. = C a (2) RH% Most polymers are hygroscopic and have temperature dependent diffusivity. For hydrophobic materials, like silicon and metals, they have zero values for both diffusivity and solubility. The model uses a fine mesh size and materials assigned to designated element sets, so that most design details of the package can be captured. The moisture concentration output is then used as an input to a swelling and vapor pressure numerical model, this is to examine warpage changes and hygroscopic-stress developed. Specific moisture densities can be determined using moisture concentration from the diffusion model, along with swelling, thermal expansion coefficients and material void fractions of hygroscopic materials. The moisture phase condition and vapor pressure developed in material structures can then be determined at various temperatures using the well documented Pressure-Specific Volume (P-v diagram) relationship of water,. III. RESULT AND DISCUSSION A. Moisture uptake All four TVs mentioned were subjected to 85 o C/85%RH condition to characterize the package level moisture absorption. The moisture absorption curves for these different TVs are shown in Figure 3. TV1 absorbed the least moisture mass, while TV4 absorbed the most. The total absorption level shows strong dependency on package size. When normalizing moisture mass gain to its package dry weight, the moisture gain for all TVs ranged from 0.25% to 0.4%, as shown in Figure 4. TV2 showed the highest moisture mass gain per package weight, as over-molded packages typically have a higher volumetric ratio of hygroscopic material to total package buildup mass. TV1 has the highest moisture absorption rate under 85 o C/85%RH condition compared to other TVs. This is because coreless packages have higher moisture absorption rate compared to cored packages, as the moisture travels through the dielectric layers faster than the substrate core layer. Besides package stack-up, the temperature and RH condition inside the chamber also affect the package moisture diffusion rate and saturation level. Figure 5 shows the percentage moisture mass gain, with reference to its package dry weight, at different temperatures and RH conditions, for TV1 and TV2. These packages showed a consistently higher moisture diffusion rate and saturation level under 85 o C/85%RH. The higher moisture diffusion rate is driven by the higher exposed temperature, while the higher moisture saturation level is driven by higher relative humidity inside the chamber. Figure 3. Moisture absorption curve at 85 o C/85%RH. Figure 2. Moisture diffusion analysis process flow chart. Figure 4. Moisture absorption at 85 o C/85%RH by normalizing to its package dry weight.

4 Figure 7. Illustration of moisture swelling effect on package RT warpage. Figure 5. Moisture absorption comparison for 85 o C/85%RH and 30 o C/60RH. To evaluate moisture desorption characteristics, the packages were subjected to 125 o C bake for 24 hours. As shown in Figure 6, all desorption curves in the first 8 hours are steep, indicating a rapid desorption rate, where more than 70% of the moisture content is released from the package. TV1 had the fastest diffusion rate, reaching a dry state within 10 hours. However, TV4 still had about 10% moisture remaining even after the full 24 hour bake process. This experiment has showed that coreless packages absorb and desorb moisture at faster rate compared to cored packages. As the soak time increases, package RT warpage reduces until it is saturated with moisture. Although TV1, TV2 and TV4 have different RT warpage to begin with, their RT warpages reduce by about 30% at the end of the moisture soak as shown in Figure 8. Figure 9 shows the normalized package RT warpage reduction with reference to moisture content. These datasets show that the increase in moisture content drives higher substrate material swelling till a certain point where the moisture content saturates and warpage change plateaus. Figure 8. Normalized RT warpage across soak time for 85 o C/85%RH. Figure 6. Moisture desorption curve. B. Moisture swelling For FCBGA packages, the thermal strain mismatch between silicon and substrate during the package assembly process typically drives the package to warp in a convex shape at RT. During moisture uptake, the hygroscopic materials in the substrate such as SR, dielectric and core material experience volumetric swelling. The swelling of the substrate counters the thermal strain mismatch between silicon and substrate during assembly which eventually reduces the convexity of package. This phenomenon is illustrated in Figure 7 and is subsequently validated. Figure 9. Normalized RT warpage and moisture content for 85 o C/85%RH. C. Dynamic warpage After the packages reached saturation, the next logical step is to establish the high temperature dynamic warpage behavior. Half the samples were baked at 125 o C for 24 hours prior to this warpage measurement using shadow moiré, while the remaining samples were sent directly for shadow moiré without bake. Figure 10 to 13 are the measured package land side Peakto-Valley (P-V) warpage across simulated reflow profile temperature. The post baked packages for all TVs consistently had higher RT warpage than the non-baked packages (moisture saturated). As the temperature increases, package warpage reduces and changes shape from convex to concave. As the temperature reaches 220 o C and beyond, a sudden spike or steep increase in warpage magnitude was observed for those non-

5 baked samples, which indicated potential delamination within the substrate layers. Physical inspection on these post-measured packages revealed blistering on the substrate surface. TSAM also revealed dark region which confirmed internal delamination. Although baked samples from TV4 did not show sudden increase in warpage magnitude, however TSAM did capture three samples showing minor delamination which suggested prolong exposure of moisture can reduce the adhesion strength and weaken the material. Cross-section image confirmed that the delamination surfaces were at SR-Cu and ABF-Cu interfaces as shown in Figure 14. The delamination propagates from the die shadow region out to the die edge and causes UF fillet crack. Figure 13. TV4 dynamic warpage response. Figure 10. TV1 dynamic warpage response Figure 14. Delamination at Cu-SR and Cu-ABF interface which later propagates from die shadow region out to the die edge Figure 11. TV2 dynamic warpage response. Figure 12. TV3 dynamic warpage response. D. Finite Element Analysis Correlation with Experiment Data FEA on moisture diffusion is next logical step to study the package moisture uptake. The diffusivity and solubility used in the FEA material properties dictate the moisture diffusion rate and saturation level respectively for the model. The material property set used is based on generic materials that have been characterized in the lab. Figure 15 shows the predicted moisture absorption curve for TV1 compared to experiment data. For TV1, the FEA result shows higher diffusion rate and under-predicts the total moisture mass gain by 10%. This difference is within the modeling acceptable range looking at the generic material property used and also potential measurement noise in the experiment. The FEA prediction for TV4 is closer to the actual total moisture mass gain although it also shows higher diffusion rate as shown in Figure 16. There are significant challenges to get the right material properties and identify other factors that potentially cause the delta between FEA and experimental data. During absorption, the surrounding exposed substrate surface of TV4 is saturated with moisture as shown in Figure 17. Red and blue color in the contour plot indicates the highest and lowest normalized moisture concentration respectively. Moisture diffuses at a slower rate at the region underneath the die due to the longer moisture travel path from sides and

6 obstruction from copper layers as well as silicon from top and bottom. After prolong exposure, TV4 reached 70% moisture saturation level post 24 hours soak under 85 o C/85%RH condition and fully saturated post 240 hours soak. Subsequently, the model was subjected to reflow simulation to capture the desorption behavior. Post reflow, there was about 70% moisture saturation level remaining inside the package. The reflow process takes place over a very short period of time and is insufficient time for all the moisture to escape due to impediment by non-hygroscopic material within the package. E. Finite element analysis case study In order to demonstrate the effect of moisture on package reliability, a case study package model, namely TV5, was modeled. TV5 is a 22x25mm coreless with 2-die MCP package shown in Figure 18. Figure 19 shows the cross-sectional view of the package model. Most of the key design structures are captured in order to ensure the moisture diffusion is being simulated as close as possible to actual conditions. Figure18. TV5 package with 2-die coreless substrate. Figure 15. Predicted TV1 moisture absorption curve. Figure 19. TV5 mapper model cross-section view. Figure 16. Predicted TV4 moisture absorption curve. In this numerical study, transient moisture diffusion was simulated by first subjecting the package to moisture preconditioning at 85 o C/85%RH for a 10-day exposure followed by a typical reflow temperature profile that peaks at 260 o C. Figure 20 and 21 show the package normalized moisture mass absorption and desorption during the preconditioning and reflow process respectively. Notably, the figure shows moisture in the package reaches saturation after about 7 days of moisture exposure. Figures 22 and 23 illustrate the normalized moisture concentration distribution for the package post-soak and reflow respectively. The results suggest that even after being subjected to a reflow cycle, the package still contains 36% moisture saturation level while the exposed surface of the substrate was relatively dry. The remaining moisture is concentrated at the die shadow region and close to the upper layers of the substrate where the moisture is difficult to desorb due to longer diffusion paths and obstruction of nonhygroscopic materials. This residual moisture can reduce interfacial adhesion strength and generate vapor pressure inside the substrate during the reflow cycle. Figure 17. TV4 normalized moisture concentration contour.

7 Besides internal stress generation, substrate swelling also has an effect on package warpage behavior as shown in Section III (B). Thermal coefficient mismatch between the die and substrate materials cause the package to warp convexly when cooled to RT. However, hygro-swelling in the substrate actually reduces the thermal strain which eventually make the overall warpage less convex as illustrated in Figures 25. The RT warpage can reduce by around 40% after saturation which is quite close to what has been recorded in Figure 9. Since this advance vapor pressure and swelling modeling technique is still new, a more comprehensive study of vapor induced stresses and package design sensitivity is still being investigated at the time of writing. Figure 20. Normalized moisture concentration during 85 o C/85%RH preconditioning. Figure 24: TV5 at peak peel stress condition red-highest stress, blue-lowest stress. Figure 21. Normalized moisture concentration during simulated reflow. Figure 22: TV5 full saturated post 10days at 85 o C/85%RH soak. Figure 25: Effect of moisture on TV5 RT warpage. Figure 23: TV5 at 36% saturation post reflow. High moisture concentration at die shadow. Figure 24 shows the effect of the vapor and swelling induced stresses on the die shadow region. As observed, the peak peel stress (S33) are consistently located below the die shadow region where moisture concentration remains highest during reflow as shown in Figure 23. These peak stresses are also coincident with the location of stacked vias. This could be due to two reasons: firstly the stacked via regions tend to act as escape path for moisture between two Cu planes which accumulate higher concentration. Secondly, Cu vias resist the swelling and vapor pressure induced expansion effect of the hydroscopic ABF layers which generate higher peeling stresses on the vias. As a result of all the complex stresses developed, delamination and blistering could happen between the laminates as depicted in Figure 14. IV. CONCLUSION In this work, we have summarized the fundamental effects of moisture on FCBGA cored and coreless packages by leveraging lab experiment, FEA modeling and complementing them with failure analysis result. Coreless packages have higher absorption and desorption rates compared with cored packages as coreless packages have shorter diffusion paths through the substrate thickness. Higher moisture diffusion rate is driven by higher temperature setting while the higher relative humidity increases the moisture saturation content in the package. During moisture uptake, hygro-swelling takes place inside the substrate which reduces the package RT warpage as it works against the initial substrate thermal strain generated during assembly. The dynamic warpage of the package was slightly altered at elevated temperature and delamination is induced when vapor pressure is higher than the adhesion strength. Cross-sectional data revealed moisture induced defect

8 signatures and provided good visualization on the delamination mechanism. Extensive package design details have been incorporated into the FEA model which showed good correlation with experiment data. This study covered a wide range of package moisture interaction behaviors such as absorption-desorption, bake and reflow. A preliminary FEA case study was presented, showing the vapor pressure induced peel stress on stacked via during reflow and RT warpage reduction due to hygro-swelling effect. Future work is still required to look into more comprehensive FEA study on vapor pressure induced stress which relates with package design features and material choices. ACKNOWLEDGMENT The authors would like to thank Shaw Fong Wong, Yi He, Jenn Seong Leong and various technical groups on the experiment planning, support and technical discussions. Finally, special thanks to Assembly & Test Technology Development management for their trust, support and encouragement. REFERENCES [1] Yi He and XJ Fan, In-situ characterization of moisture absorption and desorption in a thin BT core substrate, Electronic Components and Technology Conference 2011, pp [2] Y. He, In-situ characterization of moisture absorption-desorption and hygroscopic swelling behavior of an underfill material, Electronic Components and Technology Conference 2011, pp [3] Shirangi M.H., Fan X.J. and Michel B., Mechanism of moisture diffusion, hygroscopic swelling and adhesion degradation in epoxy molding compounds, International Symposium on Microelectronics IMAPS 2008, pp [4] Chee Kan Lee, Wei Keat Loh, Kang Eu Ong and Ian Chin, Study of dynamic warpage of flip chip packages under temperature reflow, International Electronic Manufacturing Technology Conference, Kuala Lumpur, [5] WK Loh, YH Lee, KE Ong, Ian Chin, JS Leong, Advancement in modeling vapor pressure Induced stresses in electronic packaging, International Conference on Electronics Packaging Conference 2015, pp