INTERCONNECT DESIGN AND RELIABILITY OF LAND GRID ARRAY CSPS
Author: Z.P.Wang Company: Gintic Institute of Mnfctr. Date Published: 9/12/1999
Abstract: For land grid array (LGA) packages, there are no solder balls on the package soldering pads. The geometry and size of solder joints formed during the reflow process depend on the amount of printed solder paste and pad size on the PCB. In this paper, the finite element method was used to examine the effects of pad size and stencil thickness on the stress and strain distributions in the solder joints of LGAs. Two types of pad, dogbone pad and blind via pad, were designed with two different soldering pad diameters in each case for the assembly of a land grid array chip scale package (CSP) with 284 I/Os. Stencils with different thickness were used to print the solder paste for the assembly of another package with 117 I/Os. The solder joints of each package assembled were daisy chain connected. Temperature cycling test of the assembled packages is performed. The condition of the test was from -40oC to 125oC with 15 minutes dwell time at each of the extremes. The electrical resistance of the daisy chain in each package was monitored continuously in the test. Some samples were taken out for failure analysis at regular intervals. The Weibull distribution function was applied to analyze the failure data. An elastic-plastic-creep analysis was performed to study the non-linear strain distribution in the solder joints during the temperature cycling test and to predict the fatigue life. It is found that the prediction is close to the first failure of the packages in the test.