Sridhar Canumalla

Resolution of broadband transducers in acoustic microscopy of encapsulated ICs: transducer selection

The lateral resolution of broadband transducers commonly used in acoustic microscopy is discussed in the context of selecting optimum transducers to make images for the best possible resolution of defects in plastic encapsulated integrated circuit (IC) packages. A predictive model to accurately calculate the effective lateral resolution afforded by a transducer is proposed. The model, which explicitly considers the measured frequency dependent attenuation behavior in water and in encapsulant materials, is a practical tool for the acoustic microscopist interested in selecting a transducer with optimum characteristics for inspecting a particular IC package. Experimental data from plastic quad flat packages (PQFPs) molded with two different encapsulants-Nitto MP8000CH and Sumitomo 6300HJ are presented to demonstrate the validity of the modeling approach. Recommendations for optimum transducer selection are presented based on a parametric study of the effect of various transducer and material parameters on the effective lateral resolution and signal loss.

Package to board interconnection shear strength (PBISS): effect of surface finish, PWB build-up layer and chip scale package structure

The quality of the interconnection in a fine pitch, area array chip scale package (CSP) is evaluated at the system level using a new test method, the package to board interconnection shear strength (PBISS) technique. The influence of printed wiring board (PWB) sample finish, build-up layer and package structure are quantified after surface mount assembly. Clear differences were evident in the shear strength and fracture location data indicating that the PBISS method is sensitive to the presence of black pad, weak build-up layer and package stiffness. The PBISS value of the CSPs with OSP/RCC-FR4 pad/build-up layer combination were measured to be 37 /spl plusmn/ 6 MPa and 38 /spl plusmn/ 2 MPa for the two different structures investigated. The PBISS method is demonstrated to be a viable candidate technique to quantify interconnection quality at the system level due to issues such as black pad, etc. and this method can help identify weaknesses in the interconnection chain for effective assessment of supplier or product quality.

Effect of intermetallic phases on performance in a mechanical drop environment: 96.5Sn3.5Ag solder on Cu and Ni/Au pad finishes

Mechanical drop test reliability risk due to incompatibility between component and board surface finishes of thin (30-100 /spl mu/m) 96.5Sn3.5Ag solder joint assemblies is examined in this study. Failures in test components with ENIG plated interposers occurred owing to inhibition of any Ni-Sn(Cu) intermetallic phase growth at the interposer/solder interface on account of Cu migration from the solder/ceramic interface, i.e. a coupling effect between the two interfaces of the solder joint and consequently the lack of a metallurgical bond between solder and interposer. Results of this study demonstrate that interface reaction coupling phenomena play a critical role in determining the performance of thin lead-free solder joints under mechanical loading.

Package to board interconnection shear strength (PBISS) behavior at high strain rates approaching mechanical drop

Interconnection shear strength tests performed on three different package constructions at two different test speeds indicate that the deformation of the package to board interconnection is sensitive to these variables. The faster deformation rate used in this study approaches that observed in mechanical drop or shock loading of portable electronic products. The failure mechanisms observed during high rates of deformation include build-up layer and trace fracture for the two ball grid array packages studied, while the land grid array package exhibited primarily solder shear. Results indicate that by choosing the appropriate package type and shear deformation rate, the PBISS test can be used to measure the strength of different elements of the interconnection. Thus the PBISS test can be used as a proxy for the performance of the package under different kinds of mechanical loading.

A study on ranking of commercial fluxes for electrochemical migration (ECM) propensity for Cu, Sn and Ag surface finishes

This study extends the water drop test to allow for quantitative measurement of the electrochemical migration (ECM) behavior of several commercial solder fluxes. The technique developed affords a quick and approximate way to rank and select fluxes based on ECM propensity in applications where condensing humidity or ingress of liquids occurs in the use environment

Package to board interconnection shear strength (PBISS): effect of surface finish, PWR build-up layer and chip scale package structure

Tlie package to board interconnection shear strength (PBISS) of two different area array packages with Sn62Pb36Ag2 solder balls are characterized as a function of pad finish, board build-up layer Rpe and package Structure. Results indicate that the measured PBISS value is sensitive lo the qualip of surface finish and build-up layer on the board: but tlie sensitivily is dependent on the package stmcture. In general, tlic presence of hypercorrosion of the Ni layer (blackpad) on the board results in lower mean strengths and wider variation in tlie strengths than for organic soldenbilip presewative (OSP) surface finish Further. the usage or anmid reinlorced build-up layer results in a lower measured PBISS values primarily because of a lugher incidence of fractures within the build-up layer. The combination of OSP with rcsin coated coppcr (RCC) build-up layer appears to yield the highest interconnection strength. Tlie finite element simulation results and fracture surface analysis together indicate that llie stress field nithm tlie solder joints is primarily shear. The PBISS lest method proposed here bas potential applications in emloation of tlic quality of interconnection of an area a m y package after final assembly. Introduction One of the priinary factors controlling the reliability of an electronic product subjected to adverse thermal and/or mechanical exposure is the robustness of the solder joints. Solder joint shear strength_ along with pull strength. is especially critical to tlie reliable operation of chip scale packages (CSP), ball grid amy packages (BGA). flip chips and other fine pitch leadless surface mount components. from thermal excursions while pull strength of the solder joints influences failure in mechanical drop or bending. In a package mounted on a board and subjected to shear forces. failure can occur at one of three locations: a) in the solder, b) at tlie padholder interface of the componenl or the board. and c) under tlie pad. Interconnection failure is defined by the failure that is most likely among the tluee Failures. Therefore, tlie weakest of the three strenghs will dominate the definition of the interconnection strength. Tlie test method to evaluate the Package to Board Interconnection Shear Strength (PBISS) described in tlus paper \vas developed to measure this interconnection strength for functional products and not just laboratory samples [I]. Instead of forcing failure to occur at one of the tluee locations. U= test allows failure to occur naturall!p and dictated only by the relative ranking of the three sucngths. The ability to measure the interconnection shear strength can potentially be used a) for production and supplier qualih. assessment, b) for materials/proccss selection in the development phasc, c) as a cost effective and scientlfc test method complernentaly to thcnnal fatigue tests and mechanical shock tests in research. A candidate test method needs to meet the following requirements to be useful in a high volume, manufacturing environment: 1. These measurements should be possible on actual because of sinall feature sizes. Shear strcngtli defines f a1 .I ure Figure 1. Representative examples of blackpad related failures from a fracture perspective a) fracture surfaces of solder balls still attached to the CSP showing the cleavage ppc fracture and the imprint of the pads on the fracture surface b) hgh magnification view of polished cross-section revealing the fncture between the solder ball below the IMC layer and the pad. The presence of bypercorrosion at the Ni grain boundaries is also visible. 0-7803-7991-5/03/

Thin layer acoustic image interpretation and metrology for microelectronics using a broadband model

17.0The quality of the interconnection in a fine pitch, area array chip scale package (CSP) is evaluated at the system level using a new test method, the package to board interconnection shear strength (PBISS) technique. The influence of printed wiring board (PWB) sample finish, build-up layer and package structure are quantified after surface mount assembly. Clear differences were evident in the shear strength and fracture location data indicating that the PBISS method is sensitive to the presence of black pad, weak build-up layer and package stiffness. The PBISS value of the CSPs with OSP/RCC-FR4 pad/build-up layer combination were measured to be 37 /spl plusmn/ 6 MPa and 38 /spl plusmn/ 2 MPa for the two different structures investigated. The PBISS method is demonstrated to be a viable candidate technique to quantify interconnection quality at the system level due to issues such as black pad, etc. and this method can help identify weaknesses in the interconnection chain for effective assessment of supplier or product quality.

Enhancing drop test efficacy for product quality improvement through interconnection shear tests

A broadband model is proposed to describe the nature of ultrasonic pulses in multilayered systems with a sub-wavelength thickness layer. Experimental results are presented to illustrate how delaminations and cracks with foreign material or moisture ingress can appear to be well-bonded and why acoustic images of interfaces with thin layers can sometimes give erroneous indications of the bond state. Applications of this model for delamination analysis of a geometrically complex package are demonstrated. The model can not only predict ultrasonic pulses in the time and frequency domain accurately (the forward problem), but can also provide a theoretical framework for solving the inverse problem, namely, measurement of the thickness and material properties of sub-wavelength thick coatings and layers.

Critical issues in thin layer acoustic image interpretation and metrology for microelectronics

Comparison of data from high speed shear and drop test of four different tin based BGAs, DIP 34, DIP 33, DIP 18C and DIP 18B show that there is a good correlation between these two tests. The test performance ranking of the alloys from the drop test is DIP 33 > DIP 18C > DIP 18B > DIP 34 and that from the shear test is DIP 33 ~ DIP 18C > DIP 18B > DIP 34. There was a good matching of the failure mechanisms observed during these two tests. In addition, this article shows that by increasing the shear test speed, a better correlation with the drop test can be achieved. For the shear tests, there was a clear difference in the failure modes between samples having the lowest and highest shear strength values. In all the sample sets, the weakest link appears to be the component side IMC layer. This article shows that the shear test can be used as a proxy to the drop test to evaluate quality of components that are assembled in a mobile electronic product

Resolution and signal loss in acoustic microscopy of encapsulated IC packages

A broadband model is proposed to describe the nature of ultrasonic pulses in multilayered systems with a sub-wavelength thickness layer. Experimental results are presented to illustrate how delaminations and cracks with foreign material or moisture ingress can appear to be well-bonded and why acoustic images of interfaces with thin layers can sometimes give erroneous indications of the bond state. Applications of this model for delamination analysis of a geometrically complex package is demonstrated. The model can not only predict ultrasonic pulses in the time and frequency domain accurately (the forward problem), but can also provide a theoretical framework for solving the inverse problem, namely, measurement of the thickness and material properties of sub-wavelength thick coatings and layers.