In case you missed it.

Board Coplanarity 

"PCB Dynamic Coplanarity at Lead-Free SMT Temperatures"

Authors: John Davignon, et al; [e-mail].

Abstract: The iNEMI PCB Coplanarity Working Group generated a snapshot in time of the dynamic coplanarity of several PCB designs from four market sectors. This paper addresses the question as to whether room temperature coplanarity measurements can predict the coplanarity at Pb-free assembly temperatures. This paper investigated the trends in dynamic coplanarity between market sectors, board thickness and global versus local area of concern measurements. It also shares the learning and issues of undertaking dynamic coplanarity measurements of PCB motherboards. (Journal of Surface Mount Technology, vol. 25, no. 2, April-June 2012)

Solder Joint Reliability

"Low Cycle Fatigue Behaviour of Multi-joint Sample in Mechanical Testing"

Authors: Dr. C.P. Hunt, 0. Thomas, D. Di Maio, E. Kamara and H. I.u; [e-mail].

Abstract: This paper explores the behavior of a copper test vehicle with multiple reflowed solder joints, which has direct relevance to ball grid arrays and high-density interconnect structures. It explores the relative stress conditions on distributed joints and sensitivity to ball joint shape. Joints were exposed to 1 isothermal fatigue, produced by a mechanical load that induced a cyclic shear stress across all the joints. The same structures were modeled using finite element analysis. The loading response distribution profile through the joints was analyzed. Regions of likely failure were identified to be along the shear band and at the stress concentration areas in the corners of the joints. Failure of the individual joints was analyzed by quantifying the accumulated creep strain per cycle. Solder joint models of three different shapes were investigated: rectangular, convex and concave shapes. This analysis has shown that less damage is found in concave shaped joints, indicating that BGAs would have more damage than the rectangular joints tested here. Results have also shown that more damage occurs in the outer joints as a vertical component appears due to a turning moment on the copper test vehicle. UPC Apex Expo, March 2012)

"Stress Analysis of Multilayered Printed Circuit Board under Mechanical Loading"

Authors: Jia-Shen Lan and Mei-Ling Wu

Abstract: This research demonstrates that the assessment approach can be successfully applied to the design of a BGA package with a more accurate assessment model for fatigue life under mechanical loading. Previous research has mostly focused on thermal cycling analysis; however, most modern portable electronic products have to endure extreme environments that involve not only thermal but also mechanical loading conditions. This research shows the assessment methodology is able to estimate BGA solder joint fatigue life under mechanical loading. Journal of Surface Mount Technology, vol. 25, no. 2, April-June 2012)

Surface Finishes

"Investigation of Factors that Influence Creep Corrosion on Printed Circuit Boards"

Authors: Dr. Haley Fu, et al; [e-mail].

Abstract: Test PCBs with various surface finishes were subjected to a mixed flowing gas environment with gaseous composition adjusted to achieve a tarEleted 500-600nm/day copper corrosion rate. Surface Finishes included immersion silver, organic solder ability preservative and lead-free hot-air-solder leveling. PCB test specimens were prepared by reflowing Pb-Free solder paste on the top side and wave soldering the bottom side with Pb-free solder using no-clean organic acid flux on some specimens and no-clean rosin flux on others. The solder paste contained rosin Flux with low activity and zero halides. Copper creep corrosion was observed mainly on the ImAg-finished boards wave-soldered with no-clean organic acid Flux. Pb-free HASL finished boards wave soldered with no-clean organic acid flux experienced some severe but localized creep corrosion due to exposed topper metallization and possibly flux residues from the Pb-free HASL or assembly operations. Creep corrosion was most severe in the wave soldered boundary areas where flux residue was present. PCBs wave soldered with no-clean rosin flux had insignificant creep corrosion. (Journal of Surface Mount Technology, vol. 25, no. 2, April-June 2012)

This column provides abstracts from recent industry conferences and company white papers. Our goal is to provide an added opportunity for readers to keep abreast of technology and business trends.