LGA vs. BGA: WHAT IS MORE RELIABLE? A2 nd LEVEL RELIABILITY COMPARISON

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1 LGA vs. BGA: WHAT IS MORE RELIABLE? A2 nd LEVEL RELIABILITY COMPARISON Ahmer Syed and Robert Darveaux Amkor Technology 1900 S. Price Road Chandler, AZ (480) ABSTRACT A recent trend in portable electronics application is towards the use of Land Grid Arrays (BGAs without balls). While the Land Grid Array (LGA) package, with solder joint height of only 2 to 3 mils, offers the advantage of reduced mounted height for the package, the primary driver for its usage is the potential improvement in board level bend and drop performance. However, because of the reduced solder joint height, it is also expected that the thermal cycle performance of this package would degrade substantially compared to a similar package with balls. This paper examines the effect of this ball-less package on the bend and thermal fatigue reliability. Board level reliability tests (bend and thermal cycle) as well as simulations were performed comparing the LGAs with their BGA counterparts. INTRODUCTION Cyclic bending of circuit boards due to repeated key press and drops of portable electronic products during their actual use is becoming a major concern for portable system designers. In both cases, the out of plane displacement of motherboard causes significant stresses in solder joints which may mechanically fail after a few drops or thousands of bend cycles. This is a real failure mechanism which may occur right after a product is purchased or after one or two years of use, as opposed to a more traditional thermal cycle failures which may never occur in the normal use conditions. This has led the portable system designers to use underfill between the package and the board to reduce the stresses on the solder joints. This, however, is considered a temporary fix rather than a long term solution, which will require improvement in both package and motherboard designs. One of the package design improvements being actively considered is Land Grid Array (LGA) package. A Land Grid Array or LGA is an area array package similar to the BGAs but without solder sphere or balls attached to the package. The package is then mounted on the motherboard by printing and reflowing solder paste on the board resulting in solder joint height of 2 to 3 mils as opposed to 8 to 15 mils for a fine pitch BGA package. This not only reduces the overall mounted height of the package (a plus of slim portable electronic products) but has also been thought to improve the board level reliability for cyclic bending and drop. It is also expected that the thermal cycle reliability of the package will decrease because of reduced solder joint height. Although this is a secondary issue, the designer still needs to make sure that the life reduction in thermal cycle is not significant. LGA JOINT RELIABILITY FOR BEND CYCLE Although Land Grid Array packages result in lower mounted height of the package, they are primarily being used or considered for enhancing the board level reliability performance for mechanical loading encountered during key press and drop. To determine if LGAs really improve the joint reliability, finite element analysis was performed on a 8mm-64 lead PBGA type package for both BGA and LGA versions. The ball size for the BGA version was 0.46mm which resulted in a mounted joint height (Cu to Cu) of 0.31mm. For the LGA case, the joint height was reduced to 0.070mm. A linear stress analysis was performed simulating a 3-point bend, as shown in Figure 1, with a span of 90mm and the deflection of 3 mm at the package center. 250 mm 150 mm 200 mm Figure 1: Schematics of 3 and 4-Point cyclic bend test setup. The simulation showed a 2.4X reduction in joint stresses for the case of LGA. The lower stresses in the joint will result not only in improved performance under drop testing when a cell phone is dropped parallel to the circuit board orientation, but also longer life in cyclic mechanical bending resulting from repeated key press actions. To further confirm this behavior, a number of cyclic bend tests were performed comparing both LGA and BGA versions of the same packages

2 In the first test a tape based 6mm, 56 lead, 0.5mm pitch TapeArray package was used. While the standard package had 0.3mm diameter eutectic Sn/Pb balls, the LGA version had no balls attached to the package. However, because of 0.050mm thick tape it was expected that a good solder joint may not be formed during circuit board assembly. Therefore, a paste printing process was first used to fill the openings (via) on the tape, and the package was then reflowed to form solder bumps flushed with the tape surface. This bump x-section is shown in Figure 2. The bumped LGAs as well as the standard BGAs were mounted on a 62x114x0.8mm motherboard using 100 microns thick stencil. The mounted joint x-section is also shown in Figure 2. The 5 assembled units of BGAs and LGAs were then tested in a 3-point bend configuration with a span of 100mm, deflection of 3mm, and the cyclic frequency of 1 Hz. Although the test resulted in trace failures in the circuit board for most units, one LGA package resulted in up to 3X higher joint life, as shown in Figure 3a. failure analysis that the two versions were assembled with a different die size; 13x13mm for the LGA vs. 9.5x9.5mm for the BGA. As is shown in reference [1], this increase in die size will result in about 2X reduction in life under bend cycling. Accounting for this difference, it seems that the joint reliability of LGAs in this case is about the same as their BGA counterpart for this particular package. 100,000 10,000 1,000 BGA (a) LGA Open Symbols = Trace Failures Closed Symbols = Joint Failures 100,000 Pad Solder Bump Tape Figure 2: Non-bumped and bumped section of tape substrate based package. Also shown are the solder joint x- sections after board mount for BGA and LGA. A similar test was also performed on another tape based 12mm, 132 lead flexbga package. Again the openings on the tape were filled by using the via fill process described above. The results of this test, however, showed that LGA joints failed much earlier than the BGA joints, as shown in Figure 3b. As the via fill process was not optimized in these cases, it can be argued that these results may not represent the true behavior of LGA joints in bending. In order to eliminate this effect, a bend cycle comparison was also done on a 15mm,208 lead ChipArray BGA, a laminate substrate based package. Although the solder mask surface is not flushed with the pad surface in this case either, the LGA version was directly mounted on the motherboard without using the via fill process on the package. Also, a 4-point bend configuration was used in this case as shown in Figure 1, and described more in detail elsewhere [1]. The 15 parts for each version (BGA and LGA) were mounted on 0.8mm thick motherboards and were tested using 5mm deflection of inner anvils with the cyclic frequency of 1 Hz. Figure 4 shows the Weibull plot of solder joint failures for both versions of the package, showing about 2X reduction in life for the case of LGA. However, it was later found from 10,000 1,000 BGA LGA BGA LGA (b) Figure 3: Bend Cycle Reliability Comparison for (a) 6mm- 56 lead TapeArray and (b) 12mm-132 lead flexbgas. The above results for bend cycle show that the solder joint reliability of an LGA is dependent on the package type and size and maybe as good or worse than that for a similar package in BGA version. The data gathered so far shows life improvement for a smaller package only. LGA JOINT RELIABILITY FOR TEMPERATURE CYCLE Although temperature cycle failures for a portable products where LGA is most likely to be used are not of primary concern for system designers, the system still needs to be designed to meet the minimum reliability requirements for temperature cycle environment. This raises a concern for LGA reliability because of the reduced solder joint standoff height. In order to investigate their effect on thermal cycle performance of solder joints, a number of simulations and tests were performed comparing LGAs with their BGA counterparts

99.0 15mm-208, 0.8mm Pitch Packages Weibull CABGA This assumption has been shown to be valid as it does not significantly effect the creep response of the critical joint [2]. Cummulative % Failed 50.

The dimensions are either measured from the cross-sections, wherever available, or calculated by a mathematical formula with the conservation of volume as the input.

3 mm-208, 0.8mm Pitch Packages Weibull CABGA This assumption has been shown to be valid as it does not significantly effect the creep response of the critical joint [2]. Cummulative % Failed F=15 S=0 CA-LGA F=9 S=6 In the second step a very refined model of the critical solder joint is generated accounting for the correct shape and dimensions. The dimensions are either measured from the cross-sections, wherever available, or calculated by a mathematical formula with the conservation of volume as the input. A representative package and solder joint model is shown in Figure β1=3.9, η1= , ρ=0.9 β2=6.4, η2= , ρ=0.9 Figure 4: Weibull Plot of failure distribution for 15mm,208 lead ChipArray BGA and LGA from 4-point bend test. Life Prediction for Thermal Cycle The simulations were performed using a life prediction approach that involves 3 dimensional finite element modeling including creep of eutectic Sn/Pb solder. For any package, either a 1/8th or a 1/4th symmetric, 3-D model is generated using solid elements including all the relevant details of the package. Temperature dependent and nonlinear material properties are used wherever applicable. However, the material properties are assumed as linear, temperature independent if the material behavior is only slightly nonlinear or temperature dependent within the range of loading conditions. This assumption improves the analysis efficiency as it allows the use of sub-structuring technique. In the first step of the analysis the model of the package as mounted on the circuit board is first analyzed for 100 o C rise of temperature to determine the location of joints with maximum shear strain. Since the purpose of this model is only to locate the critical joint, a relatively coarse mesh is used with rectangular column shape for the solder joints. Once this joint is located, the elements associated with this joint along with the interfacing package and board elements are removed from the model and the rest of the model is analyzed as a substructure with unit temperature increase. This creates a super-element which is subsequently used in the detailed nonlinear analysis of the critical solder joint. The use of sub-structuring results in greater than 10X reduction in analysis time and is a very efficient technique if most of the model contains materials with linear material properties. It should be noted that any material with significant temperature dependent behavior within the temperature range of interest is not a part of the substructure. Although solder behavior is very temperature dependent also, the non-critical joints are included in the substructure. The modulus of these joints is artificially lowered to 1000 MPa to account for the creep behavior. Figure 5: Finite element representation of a typical global package model and local joint model. Since creep occurs in eutectic Sn/Pb solder even at 40 o C, the following constitutive equation is used to simulate this behavior [3, 4]. ε s. s. cr. εs. s. cr. = = εgbs + εmc σ σ x D + x D E( T ) E( T ) The above equation describes two different steady state creep mechanisms and is not a standard form of creep equation in ANSYS or ABAQUS finite element analysis software. However, it has been successfully implemented in both of these software as a user subroutine, and can be made available by contacting the author. The detailed solder joint model was then analyzed using the relevant temperature cycle condition. The analysis results in accumulated volume averaged creep strain, separated into two parts described by the above equation. Finally, the post processing of results for a 25 microns thick layer of solder at package or PCB interface results in the predicted cycles to mean failure by using the following equation for both interfaces. N f ( 0.02ε ε ) 1 = gbs mc Where N f = Cycles to Mean Failure or N50, and ε gbs and ε mc = Volume Averaged Accumulated Creep Strain per Cycle for the two mechanisms. The life prediction model described above has been correlated with more than 60 test data points from leadless ceramic chip carriers to leaded QFPs as well as flip chips 00003

4 and BGA packages. The prediction accuracy is mostly ±25%, as shown in Figure 6. Cycles to Mean Failure (Predictions) PBGAs LCCCs QFPs Flip C hips * PBGAs - Motorola TS OPs * Master 2X Above 2X Below 25% Above 25% Below Open Symbols : Model Development Closed Symbols : Model Validation * Unpublished Test Data Test Data & Predictions Scaled by the Same Factor Cycles to M ean Failure (Test) Figure 6: Life Prediction Model Correlation. Junction Temperature Rise, o C FEA model Test Time, sec Figure 7: Transient Heat transfer response for 7mm,MLF package, 0.75W, 0m/s. Life Prediction Method for Power Cycle The approach can easily be implemented for power cycle applications also. The only difference is that while in temperature cycle case a uniform temperature is assumed for the whole model, the temperature distribution for power cycle needs to be determined by first performing a transient heat transfer analysis on the model for a given power dissipation. The temperature distribution is then used in stress analysis to determine the mean life as described above. The accuracy of prediction for power cycle is expected to be the same as for thermal cycle if the heat transfer simulations are accurate. Figure 7 shows a comparison of transient response of a package from test and simulation. As can be seen, the model predictions are very accurate with an error of only 2%. Clearly with this much accuracy in thermal analysis the time temperature response of the package as well as the reliability of solder joints under power cycle conditions can accurately be predicted. The life prediction methodology described above was used for a number of packages to investigate the effect of LGA version on solder joint reliability. BGA/LGA Comparison A 8mm, 64 lead ChipArray package with 3.2mm square die was used as the first case to compare the solder joint reliability under temperature cycling. The solder joint height was 0.31mm (for 0.46mm ball) and 0.07mm for BGA and LGA case respectively. The simulations were performed for a 40 to 125 o C, 2 cycles/hour temperature cycle. The mean life for the BGA case was predicted to be 2860 cycles for a 0.8mm thick motherboard which compares very well with the actual test data of 2740 cycles. The simulations for the LGA version of the same package surprisingly predicted the mean life to be 3560 cycles, a 25% improvement compared to the BGA version! The above result is contrary to the conventional thinking that the solder joint life is proportional to the joint height. To investigate this further, a number of analyses were performed where the joint height was decreased while keeping the pad sizes and joint shape constant. The results are plotted in Figure 8 which shows that the life remained almost the same for joint height of 0.31, 0.23, and 0.15mm respectively but started to increase as the height is further reduced. To further understand this phenomenon, a linear stress analysis was performed on the BGA and LGA versions to determine different strain components. The results, listed in Table 1, indicate that while the shear strain component remained almost the same for both cases, the tensile component of strain was reduced by 25% for the case of LGA. More importantly, it should be noted that the tensile component of strains was significantly higher than the shear component, indicating a dominance of bending over shear during temperature cycling of this package. Table 1: Strain Components for BGA and LGA Joint Height (mm) Tensile Strain Shear Strain 0.31 (BGA) 0.50% 0.17% 0.07 (LGA) 0.37% 0.18% Figure 8: Solder joint life prediction as a function of joint height. This is an important result which cannot be predicted if only pure shear assumption is used in solder joint life prediction methodology. In reality, both bending and shearing occurs 0.15 Joint Height - Cu to Cu (mm) Predicted Mean Life (Cycles) 00004

5 during temperature cycling. Depending upon the assembly stiffness (die/package size ratio, modulus, thickness) of a package, the bending component may actual dominate over shear. As bending adds compliance to the system the reliability will be higher if bending is allowed to occur compared to the case where only pure shear condition exists. A further proof of this is the reliability of single sided vs double sided mirror assemblies such as in Memory Cards. It has generally been observed and published [5] that the solder joint reliability for symmetric mirror assemblies (pure shear) is reduced by 50% compared to the single sided assembly (bending and shear) of the same package. The dominance of bending, as well as the lower tensile stress in case of LGA resulted in higher predicted life for the LGA version of this package. The higher life for the LGA was also confirmed through actual testing of a 6mm,56 lead TABGA (tape based) part. Figure 9 shows the Weibull plot of failures for 40 to 125 o C cycle for both LGA and BGA version of this package. Although the first failure occur slightly earlier in case of LGA (1214 vs 1296 cycles), its mean life is slightly better than that of BGA. Also note that the second failure for the LGA occurred at 1545 cycles, a gap of 330 cycles between the first and the second failure. As this tape based package was solder bumped by a non optimized via fill process, it is conceivable that the first failure may occur later for a controlled process and the reliability may actually be better for LGA from the comparison of first failure also. Tests are being conducted on the 8mm,64 lead package also and as of this writing 1500 cycles have been completed for both BGA and LGA version without any failure. Cummulative % Failed mm-56 TA BGA vs. LGA Weibull BGA β1=8.9, η1=1921.4, ρ=1.0 β2=5.5, η2=2149.0, ρ=1.0 F=14 S=16 LGA F=5 S=8 die size of 8.5mm sq. For a 0 to 100 o C temperature cycle, the reliability of LGA joints (Mean Life = 1625 cycles) was predicted to be about 50% lower compared to the BGA version of the same package (Mean Life = 3310 cycles). This indicates that the LGA joint is not more reliable than its BGA counterpart in every case and the actual reliability is a strong function of package stiffness. Again, this was confirmed by actual testing of a 16mm, 280 lead flexbga package. The Weibull plot, shown in Figure 10, indicates a 40 percent reduction in life for the LGA version of the same package with the same cycling condition. It should also be noted that higher life for LGA was also observed on a ceramic substrate based package [6] where a trend reversal has also been predicted depending upon the stiffness of package. Cummulative % Failed mm-280 LGA vs. BGA β1=28.5, η1=2102.8, ρ=0.9 β2=11.3, η2=1447.0, ρ=1.0 Weibull 16mm-280-BGA F=8 S=22 16mm-280-LGA F=24 S=1 Figure 10: Weibull plot of thermal cycle failure distribution for BGA and LGA versions of 16mm-280 lead flexbga package. SMD vs NSMD Pad Typically in BGA packages the solder pad on the package is defined by the opening in the solder mask. This is commonly referred to as the solder mask defined (SMD) pad configuration. One of the reasons for this pad configuration is the higher solder ball shear strength. The ball shear values are typically lower and less repeatable in case of metal defined or non solder mask defined (NSMD) pads due to lifting or tearing of the pad itself in some cases. For LGAs, however, this is not a factor as there is no balls in these packages, and NSMD pads can be used. Figure 9: Weibull plot of thermal cycle failure distribution for BGA and LGA versions of 6mm-56 lead TABGA. To investigate the effect of package assembly stiffness on the solder joint reliability of LGA, further predictions were performed on a 17mm, 256 lead flexbga package with a Figure 11: Cross-sectional joint shape for SMD and NSMD defined pads on LGA package substrate

6 The NSMD pad can significantly increase the thermal cycle reliability of LGAs and was predicted for the 8mm,64 lead ChipArray LGA. For the same pad size, the joint resulting from NSMD pad, shown in Figure 11, was predicted to have a mean life of 12,650 cycles. This is about 3.5X higher than that for SMD Pad. This level of increase has also been shown from actual testing for BGA packages with NSMD pads [7]. [7] Mawer, A., et al, The Effect of PBGA Solder Pad Geometry on Solder Joint Reliability, proceedings of SMI 96, pp It should be noted that the NSMD pad may not result in higher life for bend cycle if the failure occurs on the board side of the joint. It is also possible that excessive bending may result in pad tearing or lift off from the package substrate for NSMD pads. SUMMARY AND CONCLUSIONS The data from both simulations and testing presented in this paper indicates that the solder joint reliability of LGAs is a strong function of package stiffness. It is shown that the 2 nd level reliability of LGA packages for bend cycle may not be higher than that of its BGA counterpart in all cases. More surprisingly, it is shown that the thermal cycle performance may actually be better for LGA joint for some package configurations. The test and prediction data presented in this paper also shows the robustness of the prediction methodology and how it can used for different design and test conditions. ACKNOWLEDGEMENT The authors wish to thank Larry Barker and Ezekiel Bucheit for making samples and conducting the actual tests. Special thanks go to Billy Oyler and Andrew Mawer of Motorola for some of the tests reported here. Finally, the help of Sean Duggan, Xing An Wong, and Bruce Guenin for thermal test and analysis is greatly appreciated. REFERENCES [1] Darveaux, R., and Syed, A., Reliability of Area Array Solder Joints in Bending, to be presented in SMTA, [2] Syed, A., A Review of Finite Element Methods for Solder Joint Analysis, proceedings of Experimental/Numerical Methods in Electronic Packaging, Vol. 1, SEM, 1996, pp [3] Wong, B., Helling, D. E., and Clark, R. W., A Creep- Rupture Model for Two-Phase Eutectic Solders, IEEE CHMT-11,No.3, pp , [4] Syed, A. R., Factors Affecting Creep-Fatigue Interaction in Eutectic Sn/Pb Solder Joints, Advances in Electronic Packaging 1997, InterPack97, pp , [5] Darveaux, R., Optimizing the Reliability of Thin Small Outline Package (TSOP) Solder Joints, Advances in Electronic Packaging, ASME 1995, pp [6] Maeda, K. et al, The Application of HITCE Ceramic Material for LGA-type Chip Scale Package, 50 th ECTC, 2000, pp

7 ( search... All Go SMTA International Conference Proceedings LGA vs. BGA: WHAT IS MORE RELIABLE? A 2ND LEVEL RELIABILITY COMPARISON Authors: Ahmer Syed and Robert Darveaux Company: Amkor Technology Date Published: 9/24/2000 Conference: SMTA International Abstract: A recent trend in portable electronics application is towards the use of Land Grid Arrays (BGAs without balls). While the Land Grid Array (LGA) package, with solder joint height of only 2 to 3 mils, offers the advantage of reduced mounted height for the package, the primary driver for its usage is the potential improvement in board level bend and drop performance. However, because of the reduced solder joint height, it is also expected that the thermal cycle performance of this package would degrade substantially compared to a similar package with balls. This paper examines the effect of this ball-less package on the bend and thermal fatigue reliability. Board level reliability tests (bend and thermal cycle) as well as simulations were performed comparing the LGAs with their BGA counterparts

Measurement and Prediction of Reliability for Double-Sided Area Array Assemblies

Measurement and Prediction of Reliability for Double-Sided Area Array Assemblies Anthony Primavera, Ph.D. Mike Meilunas Universal Instruments Corporation Binghamton, NY James M. Pitarresi, Ph.D. Shiva