Comparison of 3D Packages with $20 \mu \mathrm{m}$ bump pitch using reflow soldering and thermal compression bonding

Abstract

Moore's law has been slowed down in transistor scaling, but the demands of high computing processing does not cool down and the data rate transmission is growing exponentially in each day. The surge in high data rate causes present DDR4 memories succumb to this horrendous needs. High bandwidth memories (HBM) are then developed to fulfill these requirements. The upended trends drive up chiplets integrated package evolution. Presently most of these chiplets integrated packages are either homogeneously or heterogeneously packed together using 2.5D or Fan-Out-Multi-Chip Module. And these packages contain of micro bump pitches at the range of $40\sim 55 \mu \mathrm{m}$ . While HBM package are either stacked by 4 or 8 heights of memories dies with bump pitch of $40 \mu \mathrm{m}$ . Hybrid bond are then developed for much higher I/O density. However, hybrid bond technology has better economic values for bump pitch which is equal or below than \mu \mathrm{m}$ . There is a grey area of bump pitch between $10 \mu \mathrm{m}$ to $55 \mu \mathrm{m}$ . Shall this package be assembled by hybrid bonding, reflow soldering, or thermal compression bonding (TCB)? The main challenges of this fine micro bump interconnections come with bump coplanarity, Kirkendall void formation and capillary underfill flow between the chip to chip spaces. This paper provides the answers to these challenges. A 3D package with $20 \mu \mathrm{m}$ bump pitch using chip to wafer stacking is developed. There are two test vehicles: TV 1 and TV 2 under the same conditions of $6.5^{\ast} 10.1 \text{mm}^{2}$ in top chip, $10.1^{\ast} 11.1 \text{mm}^{2}$ in bottom chip, $13 \mu \mathrm{m} (\text{diameter})/20 \mu \mathrm{m}$ (pitch) bump. TV 1 uses solder reflow as stacking technology while TV 2 uses TCNCF (Thermal Compression Non Conductive Film.) Two types of bonding technologies are applied for comparison, which are solder reflow and thermal compression bonding (TCB). The test vehicle consists of 2 stacked chips, with die thickness of \mu\mathrm{m}$ each. Top chip is first singulated and then placed on second chip at a wafer formed. Prior to the placement, the bump coplanarity is collected prior to placement. The chip-to-wafer bonding are then established by reflow soldering and TCB. Underfill is dispensed to the chip to chip gap is characterized. Optimized underfill materials are then applied to subsequent process. Followed by molding and grinding top chip to $70 \mu \mathrm{m}$ thick. The stacked up chip modules are then placed on substrate using conventional flip chip process. The final packages are first electrically tested then stressed up to 10 times reflows and high temperature storage at 150 °C for 1000 hours. At the interim of readouts, the units were electrically verified prior to cross section. With the optimized conditions, 3D package has been successfully passed above thermal stress without Kirkendall void formation. In this paper, results of the bump coplanarity, characterization of underfill flow, and Kirkendall void inside the micro bump are presented and discussed. Cross-section of the micro bump before and after reliability tests will be shared.