Prediction of Board Level Pad Cratering Strength with the Pre-Defined Failure Criteria From Joint Level Testing

Conventional reliability tests for the evaluation of pad cratering resistance are mainly classified into two categories: the board level test and the joint level test. The board level test is to imitate the loading conditions during normal operation. However, this type of test is expensive and not flexible. The joint level test is used extensively in the industry because it has the advantages of lower cost, higher throughput, and more quantitative results. It also allows the elimination of confounding factors such as PCB and component stiffness. Therefore, it is always desirable to predict the board level performance by a joint level test. In order to achieve this objective, the correlation between the joint level and the board level tests must be fully understood. Nevertheless, a precise correlation between the two types of tests for pad cratering evaluation is yet to be defined. This study investigates the pad cratering failure mode for the correlation of critical failure factors between joint and board level tests. An intermediate critical failure factor could be taken as a failure criterion in board level testing for failure detection. For verifying the validity of such a failure criterion, an experimental study should be performed. The 4-point bending test is chosen as the board level test for critical failure factor validation. In addition, an innovative pin shear test method is developed as the joint level test for failure factor detection. Both test methods are assessed by a series of parametric studies with an optimized process to ensure the accuracy of the results. From the results of the experimental study and simulation, the critical failure factor correlation is established between the board level 4-point bending and the joint level pin shear test. Using finite element analysis (FEA), the critical failure strain is identified from the pin shear test model and will be employed as the board level failure criterion. Subsequently the obtained failure criterion is verified by a 4-point bending model. As a result, this indirect correlation method can predict the board level failure with various geometric parameters.

Testing and analysis of solder joint reliability for BGA assembly under flexural loading

The solder joint reliability for BGA (Ball Grid Array) assembly is becoming a more concerned issue as these packages are featuring higher density interconnections, multiple functionality and higher speed combined with smaller size. The traditional test methods for second level PCBA (Printed Circuit Board Assembly) mechanical reliability monitor the electric resistance changes of Daisy chains in the test samples under 4-point bending. The method has been documented by Interconnecting and Packaging Electronic Circuits and Joint Electronic Devices Engineering Council in PC/JEDEC 9702 standard. The effectiveness of the test has been questioned when applied to the new lead-free soldered packages. Due to the failure mode shift from solder joinicopper pad interface cracking in Tin-lead PCBAs to pad-cratering cracking in leadfree packages, the electrical continuity monitoring becomes ineffective in detecting the interconneCt failure. On the other hand, the strain gauges recorded PCB strains during bend tests show little increase that would be indicative of an onset failure. This project applies Fiber Bragg Grating (FBG) strain sensors to detect the pad-cratering failure. FBO have been employed widely in different areas of engineering due to its advantages of small size, light weight and high sensitivity. In this project the FBG sensors are laid to the vicinity of the BGA substrate comers. By detecting and recording the solder joint fracture induced strain release, the onset of pad-cratering is explicitly revealed. The study has demonstrated that the FBG sensors are much more sensitive than electric resistance strain gauges in detecting the substrate strain release in BGA assembly 4-point bend testing due primarily to the sensor's much smaller geometric size. By placing the sensors very close to the comer solder joints, the new test obtains accurate strain information related to the first solder joint cracking. Furthermore, the recorded strain release enables the detecting, understanding and analysis of the critical load of the solder joint fracture, the brittle and ductile fractures and related strain relaxation phenomenon during the PBGA flexural loading, etc.

Testing and analysis of solder joint reliability for BGA assembly under flexural loading

The solder joint reliability for BGA (Ball Grid Array) assembly is becoming a more concerned issue as these packages are featuring higher density interconnections, multiple functionality and higher speed combined with smaller size. The traditional test methods for second level PCBA (Printed Circuit Board Assembly) mechanical reliability monitor the electric resistance changes of Daisy chains in the test samples under 4-point bending. The method has been documented by Interconnecting and Packaging Electronic Circuits and Joint Electronic Devices Engineering Council in PC/JEDEC 9702 standard. The effectiveness of the test has been questioned when applied to the new lead-free soldered packages. Due to the failure mode shift from solder joinicopper pad interface cracking in Tin-lead PCBAs to pad-cratering cracking in leadfree packages, the electrical continuity monitoring becomes ineffective in detecting the interconneCt failure. On the other hand, the strain gauges recorded PCB strains during bend tests show little increase that would be indicative of an onset failure. This project applies Fiber Bragg Grating (FBG) strain sensors to detect the pad-cratering failure. FBO have been employed widely in different areas of engineering due to its advantages of small size, light weight and high sensitivity. In this project the FBG sensors are laid to the vicinity of the BGA substrate comers. By detecting and recording the solder joint fracture induced strain release, the onset of pad-cratering is explicitly revealed. The study has demonstrated that the FBG sensors are much more sensitive than electric resistance strain gauges in detecting the substrate strain release in BGA assembly 4-point bend testing due primarily to the sensor's much smaller geometric size. By placing the sensors very close to the comer solder joints, the new test obtains accurate strain information related to the first solder joint cracking. Furthermore, the recorded strain release enables the detecting, understanding and analysis of the critical load of the solder joint fracture, the brittle and ductile fractures and related strain relaxation phenomenon during the PBGA flexural loading, etc.

Assessment of Fatigue Induced Pad Cratering With a Universal Expression of Printed Circuit Board Fatigue Resistance

Repeated loading is an important reason to cause pad cratering fatigue failure in ball grid array (BGA) device in printed circuit board (PCB) assembly. For industry application, the board level drop test is commonly applied to evaluate the pad cratering fatigue strength under the repetitive drop loading. Although this testing method is consistent with the actual service condition of BGA-PCB assembly, it is extremely time consuming in the testing operation and expensive in costs. Another fatigue evaluation testing method for BGA-PCB assembly is the board level cyclic bending test. Compare with the board level drop test, this testing method can be handled by universal testing machine automatically without manual operation during the testing process. In consequence, the cyclic bending test has the merits of simple, fast, and low costs, and it is always desirable to evaluate the repeated drop life of pad cratering with cyclic bending test. This research proposes a correlation between the cyclic bending and repetitive drop test in BGA-PCB assemblies. With assistance of finite element method, the equivalent cyclic bending testing conditions of drop tests are developed. The experimental validation is also conducted to prove accuracy of the correlation. From the analysis of finite element method and experiments, both cyclic bending tests and repetitive drop tests agree with the same strain–number of cycle (S–N) curve. This means the S–N curve can be treated as a generalized failure criterion of fatigue induced pad cratering. The conclusion is crucial for reliability design phases to prevent the pad cratering fatigue failure.

Non-Destructive Visualization of Bond Pad Defects using Acoustic Microscopy in the GHz-Band

GHz scanning acoustic microscopy (GHz-SAM) was successfully applied for non-destructive evaluation of the integrity of back end of line (BEOL) stacks located underneath wire-bond pads. The current study investigated two sample types of different IC processes. Realistic bonding defects were artificially induced into samples and the sensitivity of the acoustic GHz-microscope towards defects in BEOL systems was studied. Due to the low penetration depth in the acoustic GHz regime, a specific sample preparation was conducted in order to provide access to the region of interest. However, the preparation stopped several microns above the interfaces of interest, thus avoiding preparation artifacts in the critical region. Cratering related cracks in the bond pads have been imaged clearly by GHz-SAM. The morphology of the visualized defects corresponded well with the results obtained by a chemical cratering test. Moreover, delamination defects at the interface between ball and pad metallization were detected and successfully identified. The current paper demonstrates non-destructive inspection for bond-pad cratering and ball-bond delamination using highly focused acoustic waves in the GHz-band and thus illustrates the analysis of micron-sized defects in BEOL layer structures that are related to wire bonding or test needle imprints.