In case you missed it.

Laminate Reliability

"Reliability and Failure Mechanisms of Laminate Substrates in a Pb-free World"

Author: Kevin Knadle; [e-mail].

Abstract: This paper surveys the most significant via and via-related laminate failure mechanisms from past to present using data from current-induced thermal cycling (CITC) testing, failure analysis and other sources. The relative life and failure modes of thru vias, buried vias, and microvias (stacked vs. non-stacked) are compared, along with the effect of structure, materials, and peak temperatures on the above. The origin of via-induced laminate failures such as "eyebrow cracks" and Pb-free-related internal delamination is also explored. Laminate coupons during Pb-free reflows are shown, including examples of failure mechanisms as they occur, to vividly illustrate the challenges involved and to help reveal the root causes. Finally, an extrapolation to future technology trends for laminate substrates is attempted to address the question--what might be the failure modes of tomorrow, and will via/laminate reliability be better or worse? (Apex, April 2009)

Solder Joint Reliability

"Analysis of Voiding Levels under QFN Package Central Terminations and their Correlation to Paste Deposition Volumes and Propensity for Device Standoff and Poor Joint Quality"

Authors: David Bernard, Ph.D., Bob Willis, Martin Morrell and Matthew Beadel; [e-mail].

Abstract: The central terminations of the quad flat-pack no-leads (QFN) leadless package, also known as land grid array (LGA), are prone, in most practical experience, to exhibit a high, or even excessive, level of voiding when seen under x-ray inspection. Such excessive voiding not only affects the package's thermal performance during operation, but it can also increase the standoff height from the board, making the QFN float higher on the solder surface. Such action can apply stresses to the device outer terminations, causing them to no longer remain planar and affect joint quality. Therefore, monitoring central termination voiding provides a valuable method to qualify the presence of unsuitable standoff heights, which, in turn, may increase the propensity for open joints during production. This paper discusses results of experiments undertaken on identical QFN devices, where the quantity of solder under the central termination was varied and the ensuing voiding level calculated by x-ray inspection. The results are discussed inline with correlating these data as a method to provide a suggested upper limit for QFN central termination voiding, so as to minimize the possibility of open joints in production. (Apex, April 2009).

"Comparison of Thermal Fatigue Performance of SAC 105 (Sn-1.0Ag-0.5Cu), Sn-3.5Ag, and SAC 305 (Sn-3.0Ag-0.5Cu) BGA Components with SAC 305 Solder Paste"

Authors: Gregory Henshall, Ph.D. et al; greg. [e-mail].

Abstract: In this study, the thermal fatigue performance under accelerated test conditions is compared for three common BGA hall alloys: SAC 105, SnAg3.5 and SAC 305 as a control. Accelerated thermal cycle (ATC) testing was performed using 676 PBGA components with 1.0 mm pitch and electrolytic Ni Au finished component pads. These components were assembled to high-temperature rated Cu-OSP coated boards using SAC 305 solder paste, which represents one of the most common assembly practices in industry. ATC testing was performed using TPC-9701A TC1 condition of 0/100[degrees]C with 10-min. dwells (nominal); three different failure criteria were used in constructing the Weibull failure curves. The data indicate that SAC 105 has the lowest thermal fatigue resistance among the alloys tested, with SnAg3.5 and SAC 305 having similar and superior performance. The impact of failure criterion on the Weibull curves is also presented. The implications of these findings and areas for further study are discussed. (Apex, April 2009)

"Reliability and Microstructure of Lead-Free Solder Joints in Industrial Electronics after Accelerated Thermal Aging"

Authors: Francesca Scaltro, Mohammad H. Biglari, Alexander Kodentsov, Olga Yakovleva, Erik Brom; [e-mail].

Abstract: This work investigated the reliability of Pb-free solder joints in surface-mounted components after thermo-cycle testing. Kirkendall voids were observed at the interface component/solder, together with the formation of fractures. The evolution, morphology and elemental analysis of the intermetallic layer were evaluated before and after thermal treatment. Voids produced by the release of volatile species during the soldering process due to the application of flux were present. Compared with SnPb systems, Pb-free joints are characterized by larger and a higher amount of voids. In several electronic joints (BGA, SMDs, etc.), fractures developed after the thermal stresses generated during accelerated thermal aging. PCB warpage also was observed. Backward and forward compatibility of SnPb and Pb-free BGA connections was performed on pads with an ENIG finish. The effect of the reflow peak temperature on the structure of the intermetallic layer is assessed. (Apex, April 2009)

Circuits Assembly provides abstracts from recent industry conferences and company white papers. With the amount of information increasing, our goal is to provide an added opportunity for readers to keep abreast of technology and business trends.