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www.us-tech.com


Nov/Dec, 2022


SAC305 Interconnects at LTS Temperatures Using Supercooling


By Ian Tevis, Ph.D., CTO and President, SAFI-Tech


driving force for innovation in solder metal technology. The supercooled liquid state repre- sents one of these innovations, and is an opportunity for the process temperature of the metal to be decoupled from the melting temperature of the metal. SAFI-Tech’s supercooling


D


technology can be used to encap- sulate RoHS-compliant solder alloys inside a thin shell that keeps the metal in a metastable supercooled liquid state to as low as ambient temperatures. The shell can be mechanically broken or chemically dissolved to release the liquid metal, which then rapidly solidifies without requiring heat. Using this technology,


SAFI-Tech has created SAC sol- der paste that can be used at LTS temperatures. SAFI-Tech has created a new platform to keep metal alloys metastable liq- uid state as they cool to tempera- tures well below their melting point. This supercooled liquid metal particle, or microcapsule, is a new material that can make soldered interconnects at dra- matically reduced temperatures.


Supercooling Supercooling is a phenome-


non that all materials experience at a phase transition. As a metal in a molten liquid phase cools below its melting point, the atoms enter into a thermody- namically unfavorable, kinetical- ly trapped state. Here the metals are thermodynamically driven to be solids but the path to solidifi-


SEM analysis of solder joint and intermetallic compound.


energy makes it the most com- mon pathway. The second path- way, homogenous nucleation, requires the spontaneous and stable formation of an arrange- ment of atoms into a nucleation catalyst. It has the highest acti- vation energy needed and there- fore is a rarer process. SAFI-Tech’s supercooling


technology consists of micropar- ticles of liquid metal surrounded by a thin oxide shell, similar in concept to a water balloon. The particles can be made by emulsi- fying a molten metal ingot in the presence of shell-forming chem - istries, which upon cooling yields microcapsules in a supercooled liquid state. The shell protects the metal from the outside envi- ronment and prevents heteroge-


point of the metal and the tem- perature at which the metal freezes. The emulsion-based technology used to make the microcapsules utilizes a carrier fluid, oxidizing agent, a shell cre- ating chemical, and a shearing system. Each alloy has a differ- ent melting point and often dif- ferent surface chemistries. The production process


involves heating a SAC305 ingot in a heat stable carrier fluid above the 428°F (220°C) melting point of the alloy, in the presence of a shell forming compound, and then applying a high shear force to produce fine droplets of metal with an oxide/organic shell. This process creates a Type 6 (5 to 15µ diameter) powder supercooled from 428 to 167°F (220 to 75°C).


emand for low tempera- ture processing of known and trusted materials is a


cation requires a nucleation cat- alyst that can trigger crystalliza- tion.


There are two pathways for


crystallization. Heterogeneous nucleation requires a small impurity in the material or a con- tainer surface to trigger crystal- lization by lowering the activa- tion energy needed for the phase transition. The low activation


neous nucleation. Since the metal core inside


the microcapsules is a liquid, it can flow after the shell is removed, either mechanically by crushing particles or chemically using flux.


SAC305 Supercooling Supercooling is the differ-


ence between the normal melting


Prototype Application Process


SAFI-Tech can currently


supercool SAC305 to 167°F (75°C). As a result, the SAC305 is solid at ambient conditions. In order to recover the supercooled liquid phase, the microcapsules need to be briefly heated to the normal melting point of SAC305, before being used to solder below the melting point of SAC305 (known as reconstitution). The SAC305 based paste does not contain an active flux ingredient. There are no organic acids or resins to dissolve the shell. This means that throughout the entire process the shell remains intact.


The PCB with printed paste


was mounted in a BGA rework station and then the microcap- sules in the paste were then sub- jected to the reconstitution process. After reconstitution, the microcapsules were held at 356°F (180°C), well below the freezing point of SAC305. A Topline BGA169 test vehi-


cle was mounted to the top part of the BGA rework station. The BGA169 was aligned over the reconstituted microcapsules and brought up to 356°F (180°C). The aligned and warm BGA was then dipped in the flux to ensure an even coating of the SAC305 sol- der balls on the package. The BGA was then brought into con- tact with the warm supercooled liquid microcapsules on the PCB to accomplish the formation of solder joints at 356°F (180°°C).


Solder Joint Analysis The soldered daisy-chained


Continued on next page


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