FATIGUE PROPERTIES OF BGA SOLDER JOINTS: A COMPARISON OF THERMAL AND POWER CYCLE TESTS
Author: Hans-Jurgen Albrecht Company: Siemens AG Date Published: 9/10/1996
Surface Mount International
Abstract: To meet the state of the art requirements of BGA assemblies necessitates direct coupling of field conditions, simulation tools for life time study and advanced experiments for the assessment of physical degradation. For conventionally soldered SMD-components transformations between test and field conditions are still not completely known. Now, for new types of array components, the answers critically depend upon “component age” and change in fatigue mechanisms. The increasing complexity of microelectronic assemblies and the hidden joints of BGAs lead to an increase in reliability problems in this field. Therefore, to describe failure free times for different applications, fatigue relevant parameters of the ball solder joints need to be studied. With regard to the thermal coefficient of expansion, BGAs are mainly asymmetrical, consequent y, residual strains and stresses are generated in the solder joint array. The level of strains and stresses depend upon the global and local mismatch, the applied operating conditions and the temperature distribution in the ball solder joint array (chip location, ambient and operating temperature). For thermo-mechanical cycling procedures, hold and ramp times at upper and lower temperatures (e. g. -20” C/ + 100”C) are used to initiate strains in materials and interfaces. BGAs and PCBS show comparable thermal levels with regards to the test procedures mentioned before and the resulting stress conditions in the ball solder joints are a function of package size, DNP etc. The test results with regard to the generation of cracks are not directly comparable to the fatigue behavior under operating conditions. Therefore, different types of degradation tests were developed: thermo-mechanical, mechanical, electrical and / or corrosive procedures. Depending upon the chip location in the BGA package (symmetrically: PBGA, TBGA, CBGA; asymmetrically: MCM-BGA) frequencies, lateral and vertical temperature distribution under simulated power dissipations, and the internally generated heat will be used to induce stresses in the ball solder joints. For different values of power dissipation and ambient conditions thermal measurements were performed, screening the top to the bottom side of the BGA and the array field: The resulting information is a precondition in order to define power cycle parameters. For different test procedures locations of defects, crack initiation and growth in ball solder joints were studied by metallographic analysis. The practical measurements serve as analytical input to compare thermal and power cycle tests and they are a necessary step to perform a lifetime prediction.