Interconnect stress test

In another test, three high-Tg materials were evaluated through Interconnect Stress Test (1ST) testing. This particular test provided insight into the effect of thermal expansion and decomposition temperature on long-term reliability, as assessed by the 1ST method. The 1ST test method uses an electric current to heat test coupons that contain a network of plated through holes. The test samples are generally preconditioned several times to simnlate exposure to... 

Freda, Michael, and Furlong, Jason, Application of Reliability/Survival Statistics to Analyze Interconnect Stress Test Data to Make Life Predictions on Complex, Lead-Free Printed Circuit Assemblies, EPC 2004, October 2004. 

ST interconnect stress test WEEE (European Union legislation)... 

A drawback of this method is that micro-Raman spectroscopy cannot generally be applied to metallic materials. Furthermore, stress estimation can only be made on the basis of Raman spectra gathered in the substrate and the dielectric on the sides of the interconnect lines. Such measurements cannot be made in the region immediately below the line, where the stress is more uniform and more sensitive to the stress in the lines themselves, if the substrate is nontransparent. Despite these limitations, the micro-Raman spectroscopy method has been successfully used to infer the average values of internal stress components in interconnect lines tested in the as-fabricated condition as well as after electromigration testing (Ma et al. (1995) DeWolf et al. (1999)). 

Encapsulated ICA Flip-chip Interconnection. Stress analysis of encapsulated ICA flip-chip interconnechon was also executed with 2D FEM calculahons (Ref 45). Corresponding to experimental tests, four chip dimensions, 3 by 3 mm, 6 by 6 mm, 9 by 9 mm and 15 by 15 mm, were modeled. Only one joint was considered in each model, and the distance to neutral point (DNP) is half the diagnostic length of the test chip. To consider its anisotropic properties, the PCB substrate was modeled as a sandwich structure consishng of hve epoxy layers and four hber layers. The mesh for the whole package with hne structure details is presented in Fig. 27. Since the real joints were peripherally located on the test chip, axisymmetric boundary condition was employed. 

A test method to evaluate the shear stress capability of a seal material is reported [36], An electrolyte-anode-electrolyte trilayer was glass sealed to two metal interconnect plates as shown in Figure 5.11. Shear testing was done in two different modes, constant loading rate and constant displacement rate, to determine the shear modulus and viscosity. 

Anotiier property of importance is the coefBcient of thermal expansion. Any mismatch in the coefficient of thermal expansion of the dielectric insulator and die metal interconnects can create stress on liners and metal lines during processing and reliability testing. This is especially critical for oiganic dielectiics that have significantly higher thermal expansion than the metal wiring. 

However, to determine the field operating life, one needs to know the life of the parts in lab testing. The raw data obtained from the lab test could be fitted with a failure distribution to determine the mean life of the parts. Typical failure distributions include Weibull, Normal, Lognormal, and Exponential. For wear-out type of failures, the Weibull and Lognormal distributions are usually used, with Weibull being the most common. Weibull distributions are lowest value distributions derived from the weakest-link theory. Solder joint interconnects can be considered as connected in series. Usually, the failure of one joint at a critical location could cause the entire device to fail. The joints that fail early are usually located at the highest stress locations in the package. Devices with more resilient joints would not fail early. A Weibull distribution captures the minimum solder joint life, and the shape parameter captures the quality of the joints as a function of their construction and the applied stress. There are different types of Weibull distributions one-parameter, two-parameter, and three-parameter. The three-parameter Weibull Probability Distribution Function (PDF) is as shown in Eq.59.1. 

On the other hand, relatively low stresses (e.g., below the yield stress) that are applied at reduced loading rates cause the deformation to shift into the solder material. These conditions characterize the service life environment of most solder interconnections as well as accelerated aging tests. As a consequence, the solder deformation will include time-dependent or creep deformation. In the case of cyclic loading environments, the solder deforms by a combination of creep and fatigue responses. Temperature vari-... 

Time-independent or plastic deformation refers to a material performance that results from relatively fast loading rates. In the laboratory, time-independent deformation is typically generated by the stress-strain experiments. The tests are carried out under either strain-rate control or stress-rate control, but most often, the experiments are performed under strain-rate control. An approximate boundary between time-independent deformation and time-dependent deformation for solders are strain rates of s . The test sample dimensions are typically large, relative to the microstructural features of the material. However, there is a growing need to understand size or length-scale effects on these properties as solder interconnections become increasingly smaller, particularly solder joint dimensions less than 100 pm. 

The ASTM method for creep testing is provided in Ref 53. Any one of the test methods described in Ref 4 to 7 can also be adapted for creep testing. Novel test procedures can be used to assess the creep behavior of soldered joints. Although time-dependent deformation is, in general, less sensitive to joint geometry that is the faster stress-strain test, it is not completely immune to the effects of joint dimensions, as well as possible size or length-scale effects that may occur for very small interconnections (e.g., flip chip joints). 

Given the strength of most Pb-free solders and the stress levels that often prevail in an interconnection as a result of service environments or accelerated aging tests, the solder will deform primarily in the primary or transient stage. Therefore, it is important to understand the underlying deformation mechanisms that are active at this stage. A brief synopsis follows, describing several proposed theories for primary creep. A detailed account of those theories can be found in Ref 54. 

Cracking. Due to the temperature fluctuation caused by the circuit power on/off cycles, ICA interconnects have to sustain cyclic stresses from thermal expansion mismatch between the substrate and component, and thermo-mechanical fatigue cracking is considered as one of the primary failure mechanisms. Based on temperature cycling tests and cross-section observations, the fatigue cracking behavior of ICA joints of... 

The interconnection of microhardness, determined according to the results of the tests in a very localized microvolume, with such macroscopic properties of pol5mieric materials as elasticity modulus E and yield stress Oy is another problem aspect. At present a large enough number of derived theoretically and obtained empirically relationships between E and Oj, exists [16],... 

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