RELIABILITY OF MIXED ALLOY BALL GRID ARRAYS UNDER THERMAL FATIGUE AND DROP SHOCK TEST
　　Ranjit Pandher and Ashok Pachamuthu
　　Cookson Electronics
　　South Plainfield, NJ,USA
　　rpandher@cooksonelectronics.com
　　ABSTRACT
　　Near-eutectic SnAgCu solder alloys(SAC305, SAC405) have largely been used as a replacement for the SnPb solder alloy in electronics industry because of their good wetting and reliability performance and also due to the fact that the conventional wave and reflow soldering equipment can be used because their melting temperature is only slightly higher than SnPb. The thermal and mechanical shock properties of these alloys (Generally referred as high-Ag alloys) are very well understood. Even though these alloys show very good thermal fatigue performance, their mechanical integrity under high strain rate stresses is not as good as SnPb. Presence of Ag in these alloys results in Ag3Sn type of IMCs in the bulk of the alloy, the net result of which is an increase modulus of the alloy. Over the last decade industry has developed a range of other SnAgCu alloys with Ag level ranging from 0.3 to 1.2% with the purpose of improving the solder performance in high strain rate situation, primarily by decreasing the bulk modulus of the solder. In addition to good resistance to mechanical shock, these alloys cost much less as compared to SAC305 or SAC405 because the lower Ag content. Most of the studies on these Low-Ag alloys agree on their mechanical properties but there are some disagreements on their thermal fatigue life. Also, in the real world scenario, there exist unavoidable situations where the components have high silver content solder alloys and the solder paste used to assemble the boards contains a low silver content alloys or vice versa. Therefore it is important to understand the reliability of the solder joints formed with a combination of different solder alloys. In this study a number of low-Ag alloys and SAC405 have been used as spheres alloy as well as in solder paste to assemble the test vehicle. The test vehicle used is a drop shock test board designed per JEDEC standard having 15 BGA84 components. The same TV was used for temperature cycling and drop shock testing. In the temperature cycling test the boards were cycled between -40°C and 125°C with 10 minutes dwell at each temperature. The failures were monitored by in-situ measurement of the resistance with a failure threshold set at 900 ohms. The drop shock tests were performed as per JEDEC standard JESD22-B111. The failure fraction data from both the thermal shock and drop shock test were plotted as a function of number of thermal cycles and drop shock respectively. The failed solder joints from these test were cross sectioned, analyzed for failure modes and intermetallic layer formation.

　　INTRODUCTION
　　Solder joint is the primary interconnection that provides both an electrical path and mechanical integrity in the electronic devices. The weakest leak in an electronic package is the solder joint susceptible to failure due to various reasons. Therefore the solder joint reliability is of prime importance which is affected by system design (e.g., process parameters, pad finish, intermetallics, bulk solder properties etc.) and also by the service environments (e.g., mechanical shock, thermal shock, and vibration, etc.). Portable electronic devices such as cell phones, laptops etc. are constantly subjected to variety of stress due to mechanical and thermal shock while the device is in use. For example, repeated push of the keys in cell phones or laptops, accidental drop of the devices, and power on/off cycles causes lot of mechanical and thermal stress that causes the solder joints to fail.
　　Solder joint is formed by the reaction between the pad materials and bulk solder leading to the formation of various intermetallic compounds (IMCs) depending upon the composition of the solder alloy and the pad material during reflow in the assembly process. For example intermetallic compound Cu6Sn5 or Cu3Sn is formed for SAC based solder and Copper pad. A reliable solder joint is formed when the molten solder completely wets the pad during the reflow process and forms uniform intermetallic layer(s). The reliability of the solder joints is also affected by the change in the structure and thickness of the intermetallic compound because of the ambient and operating temperatures during service life [1].