Device Packaging 2019

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Applicability of Selective Laser Reflow for Thin Die Stacking
Keywords: 3-D Integration, Laser Reflow, Interconnects
A 3-D packaging approach such as die stacking is an attractive way to package greater functionality and performance into a smaller footprint, often at a reduced overall product cost. Achieving such 3-D integration can however place significant demands on the manufacturing process, often requiring substantial production expense to bond multiple die in a sequential manner. One alternative, potentially offering significant throughput, is the use of selective laser reflow to bond stacked die. This process produces a localized heating of the stacked die sufficient to produce soldered interconnects at the bonding interface but with very short exposure times that minimize the heating of other parts of the assembly and decrease the overall duration of the bonding process. The use of a commercial infrared (IR) laser reflow instrument for the sequential attachment of thin die into a 3-D stack was considered. The bonding technology under study consists of an infrared laser coupled with a custom optics system that allows the laser beam to be shaped into a range of rectangular shapes and sizes. This allows for a single device or target area to be selectively heated and reflowed while other regions in the near vicinity are not. The target reflow area is exposed to a shaped laser beam for a combination of laser powers and exposure times according to the desired thermal profile. The top most materials absorb the IR energy of the incident laser and conduct that thermal energy down to the solder joints and into rest of the assembly. The rates of heat transfer realized depend significantly upon the sample geometry and material composition. Since this produces a highly accelerated reflow, typically 2-8 seconds, the entire process is highly transient in nature. This paper reports on the investigation into the utility of a selective area laser reflow process to sequentially bond thin die into a multi-chip stack. Thin silicon die (~60μm thick) were bonded to a 150μm thick laminate substrate using a selected area laser reflow process. The die interconnects were formed through the soldering of SnAg capped copper pedestals under the local laser heat. First level die were placed onto copper pads on the laminate and then bonded using the laser process. Subsequent die (2nd and 3rd level) were placed on and bonded to gold plated nickel pad sites. At each step of the process the quality of the solder interconnects was examined using optical and electron microscopy. Specifically, the impact of the laser reflow process on interconnect microstructural features such as secondary intermetallic precipitate and interfacial intermetallic structures was explored. Quantitative microstructural analysis of the intermetallic secondary precipitates was performed and correlated with the unique thermal history of the samples. A range of laser profiles were utilized with several different thermal parameters: peak temperatures ranging from 235-255C, time above liquidus between 1-3 seconds, and a fixed cooling rate of 40C/s. The impact of the laser reflow process upon non- conductive adhesives (NCAs) used to hold the die flat after placement and before bonding was also investigated. Control samples of stacked die were assembled using conventional manufacturing techniques to enable direct comparisons of the interconnects and die bonding characteristics.
Luke Arthur Wentlent, Process Research Engineer
Universal Instruments Corp.
Conklin, New York

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